Friday, February 10, 2017

I can't get my holding pattern timing to work!

As I was preparing for a lesson with an instrument student on holding patterns the other day, I got to thinking - why does the timing seldom seem to work out with several recent students? How bad could my teaching be?

As a refresher, for most non-RNAV holding patterns, you want the inbound leg to be 1 minute long.



To make it 1 minute, you adjust the outbound leg timing to be more or less than a minute depending on the wind. You know, you see it in all the training references, and they give examples like "if your inbound time is 50 seconds, then make your outbound leg 1 minute and 10 seconds" or similar examples where the time difference is reasonably small.

And it usually works out that way.

But I distinctly remember noticing with my last instrument student that it didn't seem to work out very well, and I couldn't explain why! He'd time the inbound leg, adjust the outbound leg, and still be quite off. It wasn't his airspeed control, that was just fine.

So I started digging into it and ran a few mathematical simulations, because that's the kind of thing I do.

Turns out, the "add or subtract the difference in time" only really works in light wind. Now, none of us expect that in 50 kt gale force wind any of this would work, but what I found is that the simple method really starts to break down at some fairly routine wind speeds, at least my part of the country (Oklahoma).

Remember turns around a point from Private Pilot training, and how you keep correcting for the wind to maintain that constant radius from the point? If there was no wind, you would maintain a constant bank angle and end up right where you started. Your ground track would draw a perfect circle.

However, what would happen if you had a headwind and maintained a constant bank angle? Your ground track would look roughly like this, the only thing that would change is the amount of elongation depending on the wind speed.


Can we calculate the distance between the beginning of the turn and the end of the turn? Of course we can, and it's especially easy when we're talking about instrument flight and using a standard rate turn of 3 degrees per second, or a "2-minute turn". Since the 360 degrees of turn will take two minutes, the downwind displacement in that amount of time is simply the distance that the wind will move in that amount of time.



For example, if the wind is 10 knots, then the distance between start and end points is 0.33 nm.

At the midpoint of the turn we are half that distance "downwind" from the start point.

Of course, in Private Pilot training we adjust our bank angle to keep that ground track looking like a nice circle. But we don't have that luxury in instrument flying (there being no ground references), so the same principles greatly affect our holding patterns.

And that's what was causing my student's (and my) confusion.

Here in Oklahoma, as with much of the central U.S., most of our holding is done somewhere around 3000 MSL (as in the example above). As I write this, the winds aloft at 3000 at 230 at 45 knots! Now, it is certainly a windy day today, but it is very commonplace here for the wind at 3000 to be about 20-30 knots. And this makes getting quality practice on holding patterns very tricky.

Let's assume a not-unusual wind from the north at 30 knots and a holding speed of 90 knots, also typical for airplanes used in instrument training, like a Cessna 172 or many of the Piper PA-28s. Our mind's conception of a holding pattern looks basically like this:

Yes, this is actually TO SCALE! That might be a first for this blog.
That's great for no wind. We know any wind is going to distort it some. But how much?

With a 30-knot headwind, our outbound turn adopts a shape more like this (in red):

Keeping the nominal holding pattern on there for reference. Everything is approximately to scale.

The end of the outbound turn is clearly quite some distance down the outbound leg - since this turn took 1 minute (standard rate) and the wind speed was 30 knots, that means the rollout point is 0.5 nm past the abeam point. Also, since we start timing the outbound leg when we are abeam the holding fix, this rollout point is somewhere about 17 seconds into the nominal 1-minute outbound leg (the wind is on our tail for this portion of the maneuver, and it's almost a complete tailwind, meaning our average ground speed is close to but not quite 120 knots which would cover the 0.5 nm in 15 seconds).

So we fly outbound for 43 more seconds (at a ground speed of 120 knots), traveling another 1.4 nm downwind, and begin our turn back to the inbound course. Of course, now we're turning into a headwind, so our ground track is a mirror image of what it was before, and our ground speed is slowing. We now have to claw back about 2.4 nm at a ground speed of only 60 knots, which will take about 2 min and 24 seconds!


Huh. Well, we want to get a nice 1 minute inbound leg, so we have to take some time off the outbound leg. But, the normal guidance is you subtract the amount of time from the outbound leg that you were over on the inbound leg - which in this case, is 2:24 - 1:00 = 1:24. Subtract 1:24 from our 1:00 outbound leg? That's a problem. My stopwatch doesn't work on "negative time".

About the best we can do is never stop turning - literally, make a aerial "circle" once we cross the fix the first time. What happens when we do this?

Our ground track looks exactly like the first image in this article!

It turns out that we now are left with the "perfect" case, where we roll out on the inbound course 1 nm from the fix, which at 60 knots ground speed will take right at 1 minute.

So, somewhere between a light breeze and 30 knots of wind, the rule-of-thumb for adjusting timing fails us. That's why it's a "rule of thumb" of course - an attempt to perform easy calculations that work most of the time. Unfortunately, as I have found, it's pretty far off at just 20 knots of wind as well, which is a normal day here in the plains states.

Interestingly, I was recently up with an instrument student on a day when the wind aloft was 40 knots. Pretty smooth actually, but a lot of wind. He was in the holding portion of the syllabus but I knew we weren't going to get much useful practice in that wind. However, for an even worse example of the situation discussed above, we flew a holding pattern directly into the wind. I had him do as stated above and just keep a standard rate turn going through all 360 degrees after hitting the fix. Sure enough, we rolled out on the inbound course and it took about 1:30 to get back to the fix!

It seems that once the wind speed at your holding altitude gets to 1/3 of your holding true airspeed, you would need to do the continuous turn. Any wind speed greater than that will cause you to not be able to obtain a 1-minute inbound leg using standard holding methods.

But the headwind is actually the easier case! At least you have time to adjust and get established on the inbound course.

What about a tailwind?

About this time you probably regret starting instrument training...

This is about what it looks like if you time a 1-minute outbound leg.

15 second inbound leg, that's not much time to get established on the course! That's basically entirely within the "cone of confusion" at these speeds.

For this one, I'll skip ahead to the answer.

To make a 1-minute inbound leg using this tailwind, we will need to head outbound for 2:30 before turning in. That's a lot longer than the rule-of-thumb would give you!

Disclaimer

Yes, I know. We fly real airplanes in real conditions and nothing works out perfectly like the math says - there are a million additional variables involved. But I did find it interesting to see how some of these scenarios would work even if everything was "ideal". And finding out that the headwind scenario doesn't work at all when the wind is greater than 1/3 of your TAS was pretty interesting!

What if the wind was a crosswind?

I think that will have to wait for another day!

Sunday, May 15, 2016

Dead Reckoning legs on Instrument Approach Procedures

I was looking for some unusual approach types to discuss with my current instrument student and came across the "dead reckoning" situation. You don't see this a whole lot, but if you face it while flying it could definitely be a little confusing.

What I'm talking about is an example like this, the Springfield, IL (KSPI) VOR/DME RWY 31:



Look at that leg starting at LATHA. It has the text "3100 NoPT to PUWGO 246 (7.9) and 296 (4.5)". It doesn't show a radial to fly off of LATHA, it just gives a heading. So what gives?

Instrument approach charts don't exist in a vacuum. Often to understand them, we must also refer to the appropriate enroute chart:


Notice that LATHA is sitting on V50 between AXC and SPI VORs, on the AXC R-276. To fly this approach, however, you are expected to fly heading 246 degrees from LATHA. This takes you OFF of the radial, and in fact you have no course guidance at all for this leg!

That's why it's called a "dead reckoning" leg. It's just a heading to fly. In this case, you will fly the heading 246 from LATHA until intercepting the SPI R-296 inbound on the approach. This should take about 7.9 nm according to the charted distance. Then you will fly another 4.5 nm on that R-296 until reaching PUWGO. The distances are approximate, of course, as wind drift will affect your actual track, but are there to give some idea of how long it should take for the intercept.

Notice there is a fix called (CFBVH) - with parentheses - at the intersection of the heading and the final approach course. This is known as a Computer Navigation Fix and is there solely for reference by GPS receivers and FMSes, helping them to align you on the proper course.

Now, you can imagine that with a 7.9 nm leg that has no course guidance, you could be pretty far off course if you have a strong crosswind - and you'd be right. Fortunately that is accounted for in the procedure design and the protected areas are HUGE, and get larger the farther away you get from the starting point.

You tend to find these more often on ILS procedures, as the localizer signal doesn't always point in a convenient direction. Two more examples are at Champaign-Urbana, IL (CMI) ILS or LOC RWY 32R (see the leg from NEWMY),


and the Salina, KS (KSLN) ILS or LOC RWY 35 (legs from both ANTON and GUTER).


There are many more examples, of course, but with ATC radar vectors, we fortunately don't have to fly them very often. Do you have any favorites? Let me know!

Tuesday, March 1, 2016

Procedure turns - when can you descend?

This blog is a tie-in with the Stuck Mic AvCast episode 115, available here!

On the podcast we talked about the procedure turn (PT) and hold-in-lieu-of-procedure-turn (HILPT), and specifically, WHEN can you descend when executing the maneuver?

This type of question comes up often in instrument rating checkrides, job interviews, and of course in real flying and it's important to know the proper point to begin your descent in all phases of flight. So let's get right to specifics!

The first procedure discussed was the Pocatello, Idaho VOR RWY 3:


This procedure has a fairly standard layout, with one exception which we will get to in a bit. But first, some definitions!




Let's say we are starting from somewhere east of the field, cleared direct to the PIH VOR, maintain 8000, and cleared for the approach. When can we descend? There is a "7200" minimum altitude depicted on the left of the profile view, so at some point we know we can descend to that altitude - but where to start?

From the FAA's Instrument Procedures Handbook, Chapter 4:

"The altitude prescribed for the procedure turn is a minimum altitude until the aircraft is established on the inbound course."

Also,

"Descent to the PT completion altitude from the PT fix altitude (when one has been published or assigned by ATC) must not begin until crossing over the PT fix or abeam and proceeding outbound."

The result of this is that we remain at 8000 (since that's what was assigned by ATC) until crossing the PIH VOR/DME. As we turn to that outbound course of 235, we can then begin descending to our procedure turn completion altitude of 7200. We do not have to wait until we're turned around inbound - in fact on some procedures, depending on the amount of altitude we need to lose, that might cause problems in itself! The descent gradients established within a procedure turn are based on the expectation that we will begin descending when crossing the PT fix - the longer we wait, the steeper and steeper we will have to descend to make the FAF altitude (or MDA if there is no FAF).

Once we are turned around and established on the inbound course, THEN we can continue our descent to the FAF altitude - 5600 in this case. From there on in, the procedure is flown like any other.

I skipped over something for a bit - that "PT Fix altitude" of 7800 in this case. Not all procedures with a PT have these. This is an established minimum altitude we must maintain until crossing the fix outbound. In our example, ATC cleared us to the fix at 8000 - so we're above the PT Fix Altitude and there is no problem. But maybe we're flying along an airway, say V21 southwest-bound:


Note the MEA along V21 is only 7000. If we don't plan ahead and are flying right at the MEA, we may find ourselves having to CLIMB to 7800 to meet the PT Fix Altitude! If there is no PT Fix Altitude (as is usually the case), then the PT Completion Altitude is the minimum for entry as well (of course - you wouldn't climb in a PT).

The method of indicating the PT Fix Altitude above is the current charting standard. However, you will still see approaches that have the altitude shown as in this example from Livingston, MT (LVM):


The next example we discussed on the show was the Twin Falls, ID (TWF) ILS OR LOC RWY 26:


This procedure incorporates a holding pattern in lieu of a procedure turn, often called a hold-in-lieu or a HILPT. Just like a procedure turn, the holding pattern is established as a means of turning ourselves around. The only expectation is that we perform the holding pattern entry (using any method, such as the three "standard" holding pattern entries). Then, when we are established on the inbound course we continue on with the procedure. ATC does NOT expect us to perform multiple circuits of the holding pattern, and if we need to do so (in order to get established or maybe to lose altitude) we are required to inform ATC prior to doing so. Again, from the Instrument Procedures Handbook, chapter 4:

"If pilots elect to make additional circuits to lose excessive altitude or to become better established on course, it is their responsibility to so advise ATC upon receipt of their approach clearance."

This procedure has a slight bit of an unusual twist, in that once we are established on the inbound course of 258, we can descend an extra 100 feet, down to 5900 for glideslope intercept.

The last procedure we discussed on the show was the Asheville, NC (AVL) ILS OR LOC RWY 35:


Here's the question - if we are inbound from the SUG VORTAC on the established feeder, where can we descend and to what altitude? The answer is that if we are cleared for the approach from over the SUG VORTAC, we can descend to 6200 while flying that feeder route to the BRA NDB. Once crossing the NDB (which is the HILPT Fix) we can further descend to 5200 while outbound on the holding pattern entry (which, using one of the three standard entries would be a parallel entry). We would stay at 5200 as we turned inbound and all the way back to the NDB. Once crossing the NDB we would begin our descent to 4000 for glideslope intercept.

Those were the three procedures we talked about on the show, but there are a couple more examples of unusual situations that I want to mention.

Some PTs even have a MAXIMUM altitude established at the PT Fix, like Twin Falls, ID (TWF) again, this time the VOR RWY 26:


Notice that we must cross the TWF VORTAC to begin our PT at no higher than 10,000! Maximum altitudes are rarely used on procedures but here one is. Often they are at the request of ATC, but when it comes to PTs they can also be used to limit the size of the evaluated area. For a given Indicated Airspeed, True Airspeed increases with altitude, and therefore turn radius does as well, so PTs above 10,000 have a larger area for obstacle evaluation than those at lower altitudes.

This procedure also has a stepdown fix along the inbound course at 3 DME (XULXU). Just like with any stepdown fix in final, if you can't identify it you have to use the higher set of minimums. In this case, if you find that once you get established inbound you're already inside 3 DME, then you can begin further descent right away. 

One last example, the Kremmling, CO (20V) VOR/DME-A (notice how all of the fun examples are in mountainous states?):


This one actually has a 15NM PT distance limitation, to give pilots more distance to deal with the high altitudes and descents involved. There are some 16,000 and 17,000 foot MEAs on nearby airways, so descent planning becomes a very real consideration!

Notice the PT Completion Altitude of 13,000 is also the first stepdown fix altitude at HADLA, 10 DME. Further descent is allowed to 11,800 at 4 DME, then crossing the VOR is the FAF at 10600. When is the best time to figure this all out? Obviously on the ground during flight planning!

I think that's enough about PTs for now. Thanks for reading (and listening to the show), and let me know if you have any comments or questions!



Monday, January 25, 2016

2015 Cirrus SR22T Review

I know it has been a long time between blogs recently. Been busy flying!

I saw an advertisement in the NAFI Mentor magazine for CFIs to take demonstration flights in Cirrus aircraft. Obviously this is intended to introduce CFIs to the capabilities of Cirrus aircraft so that we can make informed recommendations to clients. I figured, why not?

Well, a few days ago I had the pleasure of taking a beautiful 2015 Cirrus SR22T GTS Xi up on a demo flight with Jeff Sandusky, Regional Sales Director for Cirrus Aircraft. Based at Wiley Post Airport in Oklahoma City, this is a special airplane. Not only because it is the top of the line Cirrus model, it is also the 6000th aircraft Cirrus has produced, and therefore has some definite "appearance" upgrades (seats, trim, etc.). This aircraft has the G1000 panel with 12" screens, synthetic vision, infrared enhanced vision, air conditioning, built-in oxygen, ADS-B in/out traffic and weather, FIKI, dual AHRS, Envelope Protection, XM music and about a million other features. I was pretty excited to step on in and take it up!

My chariot for the morning!

Given that it was a breezy 32 or so degrees outside, Jeff gave me a brief (but still thorough) look at some of the exterior features of this airplane. I was especially interested in the stall protection designed into the wing, in a few distinct locations.

The wing root has a large vortex generator that controls airflow over the root of the wing. The first stall strip ensures that the stall starts there, inboard, and not further out.

I wonder how many people have accidentally stepped on that vortex generator? It's really perfectly positioned to be a step.

At about mid-wing there is a noticeable break in the leading edge that causes the outboard portion of the wing to have a lower angle of incidence than the inboard portion, so aileron control is maintained while the inboard portion is stalled. I got to test out the stall characteristics later in the flight.

Cirrus wings look funny, but it's all about stall control.

With all the custom appearance options, the interior was very nice. The seat was noticeably firm at first, but after some initial commenting, I did not notice or think about it for the rest of the flight. The 4-point harness made me feel secure, though it did have a tendency (as these do) to ride up if not adjusted tight enough. Maybe that's just a signal to tighten it? This harness was equipped with airbags in each shoulder strap.

Nice high quality leather and styling touches awaited me.

The back seat was the "60/40 Flex Seating" split seat designed for three passengers. Clearly for three to fit, these must be smaller passengers, children, or very friendly with each other. 

I cannot report on the comfort of the back seat!

As someone who flies "club" airplanes a lot, I really liked the ability of the G1000 to store up to 25 user profiles for screen setup and configuration. 

Want a different configuration for IFR and VFR? Local and longer flights? No problem!

I quickly figured out that this handle was not a door handle or something to pull on to help adjust my seat! This is, of course, the Cirrus Aircraft Parachute System handle overhead. The parachute has a minimum deployment altitude of 600 AGL. Above that, standard Cirrus training is for the parachute to be pulled immediately in the event of any serious malfunction up to 2000 AGL. Above 2000 AGL, Cirrus trains pilots to go ahead and troubleshoot before pulling the parachute.

Fortunately we didn't have to pull this, although that would have certainly made for a very interesting article!

What is this? An actual keyboard in a light single! No more twisting knobs to enter waypoints or frequencies. Most of the other radio and autopilot functions are replicated on this center console as well. Note the blue "LVL" button in the middle. More about that later. The keyboard would take some getting used to, as it's not a QWERTY layout. But it's still faster than turning knobs to enter airports or intersections.

Almost all of the controls you need within easy reach of your right hand.

Ah, standby instruments! Situated right above the pilot's knees. In this model, they are all digital. The altimeter setting can be slaved to the primary display, so you only need to set it once. Nice.

Previous versions had analog instruments, but these were all digital.

Okay, enough about the systems. You know that I was really ready to fly this thing! Due to a solid cloud layer from about 2800 MSL (1500 AGL) to 4000 MSL or so, we had filed IFR. The temperature was right around freezing so there was the chance of some ice - however with the TKS weeping-wing system this Cirrus is approved for Flight Into Known Icing. We quickly popped above the layer and tried to negotiate a block altitude and area for maneuvering from Oklahoma City Approach - but they weren't having any of that (unusual, I've requested and received this many times before). So we headed north 30 miles or so until we entered Kansas City Center's airspace and made the same request - no problem!

At this point Jeff ran me through a pretty thorough demo flight - explaining the capabilities, letting me experience the handling and systems, and stressing the myriad safety features on the airplane. 

First, cruise. At 10,000 feet, 30.3 inches of MP equaled 87% power (easy to set with the single-lever power control). This gave us 18.7 gph and a TAS of 183 kts, which is right at book value. As this is the turbo model, TAS gets faster up into the Flight Levels where 210+ KTAS is achievable. Obviously this is a high cruise power setting and 75% or 65% power settings will result in slower airspeeds but commensurately lower fuel burns.


On the MFD I need to point out the leaning procedure - you lean the mixture until the fuel flow is at the blue line (left side of the screen, 1/3 of the way down). That's it - simple.


Time for a little hand-flying, though. I found the stick forces and response to be both interesting and exciting. Gentle pressure on the controls resulted in equivalently gentle maneuvering of the airplane and it felt "normal". Move the stick a little more than normal, for quick maneuvering, and the whole personality of the airplane changed - response was quick, solid and immediate. More "aerobatic-like" than the traditional single-engine airplane feel. This is enhanced by the control system having a spring-return to neutral.

Handling and stability in the stall was fantastic. There was no tendency to drop a wing and the ailerons remained effective throughout the stall. This really felt like an affront to my traditional stall experience, as I teach using the rudders to keep the wings level in a stall for the usual reason of spin avoidance. But in this airplane, it was no problem. Sure felt weird though.

The Perspective system includes Garmin's "Electronic Stability and Protection" system, ESP, which I got to give a thorough workout. This system "helps" the pilot avoid unusual attitudes by assisting in returning the airplane to normal attitudes using the autopilot servos (even though the autopilot is off). With the ESP system on, banks of up to 45 degrees are normal. Past that the airplane pushes back, and keeps pushing until bank is at 30 degrees. This push is definitely noticeable but easily overcome if you want to - just push the stick a little harder and it will let you do what you're trying to do. However, there will be no mistaking that you are exceeding its built in parameters. The same is true for pitch, with different limits. The ESP system can be temporarily disengaged by simply holding the autopilot disconnect button on the stick, if needed for maneuvering.

Jeff did an interesting demo of the ESP system. While holding the plane level, he increased the aileron trim to full left, trimmed full nose down and added full power. When he released the stick, the airplane immediately rolled to the left and the nose pitched down as expected. (I say "as expected". I lie a little. Although he warned me what it was going to do beforehand, and conceptually of course I knew wat was going to happen, having an airplane roll hard to the left and dive with nobody holding the controls was certainly weird and a little uncomfortable.) Once it got past 45 degrees of bank, the airplane tried to right itself in bank. Simultaneously, as the nose lowered and the speed built up rapidly, the airplane tried to fix that too, using the only tool it had available - pitch (no autothrottles, yet). The aircraft pulled up surprisingly hard in an attempt to limit the airspeed gain - I'd say about 2 g's but can't find that in the POH. After a few oscillations it returned to a "normal" pitch and bank attitude and held it there. Not to straight and level flight, but within the established parameters for pitch and bank.

That's what this next thing is for - the GFC 700 autopilot has the "blue level button" which I got to press a couple of times. It does as advertised - returns the airplane to straight and level flight from whatever strange attitude you've managed to get into. More than just an emergency button though, I could see it as useful when hand flying and having to copy down an ATC clearance or other similar temporary distraction.

I really like these features from a safety standpoint. It would be very hard to not notice getting into an unusual attitude (for example through spatial disorientation), and the airplane would keep trying to get you back to normal, both helping you and giving you the tools to do it yourself. Great stuff.

At this point I really wanted to see the airplane on approach, especially the "Highway in the Sky" symbology since the last aircraft I flew with a G1000 didn't have that option.

Back into the cloud deck, we did pick up just the slightest trace of rime on the leading edges. Not enough to bother with engaging the TKS system, though of course we watched it closely for further accumulation.

Just the tiniest little bit of ice if you look closely.

Cleared for the RNAV (GPS) RWY 35R approach into Wiley Post (KPWA), we intercepted the glideslope and started on down. Of course the autopilot is fully integrated and can fly the whole procedure hands-off with the pilot only making power changes and then flaring to land. But I was most interested in the "Highway in the Sky". "Flight simulator" computer programs as far back as the 1980's had HITS depictions as a "futuristic" guidance option. Well, now it's the future, and HITS is here! When hand flying an approach, all the pilot has to do is keep the flight path marker (green circle with crosshairs) within the magenta squares, pointed at the runway and it will be a perfect approach every time.

With all these navigational aids, it would be hard to go wrong.

Short final was flown at 80 kts, and the landing was straightforward and uneventful (fortunately!) with a different sight picture than many single-engine pilots are used to. The nose drops away and the panel is low, so the impression is that you need to pull back more than you really do. It's like some twins in that regard - you feel like you're landing flat but you aren't.

A few takeaways:
- I can see why these airplanes are so popular. 
- The integration of all the aircraft systems was amazing to me. Like many pilots I am used to an almost random array of instrumentation from different eras and manufacturers in the airplanes I routinely fly. In this airplane, everything talks to each other.
- The handling was enjoyable. The sidestick took exactly zero time to get familiar with.
- The seating position felt a little odd at first (very high up for me). I did wish the seats had more adjustability, but I stopped noticing as soon as we started moving and promptly forgot about it, so apparently this wasn't as big a deal as I thought.
- I can't believe I forgot to test the enhanced vision system!
- Getting in and out of the airplane took a different routine than I am used to and I'm sure I looked funny doing it.
- This aircraft would be a great (and quick) way to travel. 180+ KTAS and long range will get you many places.
- I need to convince Cirrus to let me evaluate this aircraft on a longer flight - with my wife. Say to Florida. Or Phoenix. Or anywhere warmer than Oklahoma this winter.

Many thanks to Jeff and Cirrus for giving me this great look into the capabilities of a fantastic airplane!

Thursday, October 22, 2015

Why is the LNAV/VNAV DA sometimes higher than the LNAV MDA?

Sometimes this instrument stuff just doesn't make any intuitive sense, does it? You’ll see an approach chart with minimums like these:
  

The LNAV MDA and visibility are lower than the LNAV/VNAV DA and vis! We “assume” that because the LNAV/VNAV offers a glideslope, that it must be better than the LNAV. “Better” is a subjective term of course, but in this case it doesn’t mean “lower”.

Why?

Well, let me tell you. Here’s where it gets a little involved.

(Note: The vast majority of LNAV/VNAV procedures out there were evaluated using the criteria in FAAO 8260.54A. While this has been replaced by the 8260.58, the concepts and calculations are similar. I will use the 54A in my examples below, since that’s what most current procedures are based on.)

It’s really all a matter of WHERE the most significant obstacle in final is located. This is called the “controlling obstacle”, and is the one which causes the highest MDA or DA.

For a non-vertically guided approach, like an LNAV, Localizer, or VOR, the evaluation can be very simple. Find the highest obstacle in final, and add 250 feet to it, then round up:

Yes, that's the Eiffel tower. Why not? Note: not to scale!

That’s your MDA! (It’s not always as simple as this, but it can be. I’m leaving out some details for brevity.)

But what about a vertically-guided approach? It’s different for LPV and ILS than it is for LNAV/VNAV, and LNAV/VNAV has some serious handicaps. LNAV/VNAV was originally designed for use with barometric altimetry – meaning that the “glideslope” you would follow was calculated by your FMS using barometric pressure – basically an internal altimeter – NOT an electronic signal. For the most part, only business jets were ever equipped with this technology. Also, we know that altimeters have many errors as a result of non-standard temperatures.

See HERE and HERE for more discussion on that topic.

This was called “Baro-VNAV” and the formulas have to account for varying temperature limits. That’s why you’ll often see in the notes for an RNAV (GPS) approach procedure something like this:


The errors introduced here require a little more “cushion” when it comes to obstacle clearance, so instead of something nice and simple like the LNAV evaluation, the evaluated area is composed of two general regions:


That flat part of the dashed red line extends about a mile from the runway threshold, dependent on altitude and how cold it gets at that airport during the winter (yes, really). If there are no obstacles that penetrate that dashed red line, then the LNAV/VNAV will get great minimums. But if an obstacle DOES penetrate, then the DA is highly dependent on WHERE it penetrates and by how much. An obstacle that penetrates that flat area has a comparatively small effect. However, an obstacle that penetrates the sloped portion can have a significant effect on DA.

The new DA is determined by placing the DA at a point on the glidepath above where the obstacle clearance surface is at the same height as the obstacle. Now that’s a mouthful, a picture hopefully is a little clearer:


Whatever the glidepath height is at that distance from the runway, well, there’s your DA.

It works out that an obstacle that penetrates the surface within a mile of the runway will usually not cause the LNAV/VNAV DA to be higher than the LNAV MDA. But an obstacle that penetrates more than a mile out will! So when you see this situation occur, you know there’s an obstacle maybe 1-2 miles from the runway. If the obstacle is further away than that, the DA gets really high!

Okay, clear as mud. But what about that visibility value?

Fortunately that’s a little easier to explain.

Visibility values are set so that the pilot has at least a reasonable chance of seeing the runway from the missed approach point. Hopefully sooner, of course, but at least by then. On any vertically-guided approach, this is pretty straightforward – how far is the airplane from the runway at the DA point? Convert that to statute miles, and there’s your visibility.


At approximately 318 feet per nautical mile for a 3 degree glidepath, a Height Above Touchdown (HAT) of 688 ft as in the example above gives a distance, and therefore visibility, of just shy of 2.50 sm. So it’s rounded up to 2 1/2, and published. Approach lighting systems, if installed, get figured into this too, essentially by subtracting the length of the approach lights from the calculated visibility. It’s all in a table that the procedure developers refer to.

For non-vertically guided procedures, however, there is no “DA” point, and most often the MAP is either at the runway end or relatively close to it (sometimes past it on a VOR procedure). For these procedures, the visibility is determined one of two ways. For Cats C and D, the same table as for vertically-guided approaches is used, so the visibility is the same for a given HAT.

For Cats A and B though, a different table is used, and is greatly simplified. A basic visibility of 1 sm is used until HATs start getting over 740 ft for Cat B and 880 for Cat A, at which point it starts increasing. So you will see many, many LNAV (and LOC and VOR) approaches with 1 sm of visibility. Since many Cat A and B aircraft are capable of making a perfectly safe descent at steeper than 3 degree glidepaths, the lower visibility requirement actually gives them a little more flexibility than the faster aircraft.

Like before, approach lights can help here too. There are some other limitations as well.

To briefly recap:
1. The LNAV/VNAV DA may be higher than the LNAV DA if the obstacle is sufficiently far from the runway due to the geometry of the evaluated areas. This is a result of the original design of Baro-VNAV.
2. The visibility values are calculated differently because the approaches are flown differently, and therefore LNAV visibility for Cats A and B will often be less than the LNAV/VNAV Cats A and B.

Simple, huh? I hope this answers some questions about this seemingly strange situation!

Monday, July 13, 2015

My ATP checkride

On Saturday, 7/11/2015 I took and passed my ATP-AMEL checkride! Like many others, I needed to get it done before my grandfathered-in written test expired next summer. Here's a little write-up on how it went.

My examiner was a well-known DPE from Tulsa, OK, Jennifer Wise. The aircraft was a very nice and well-equipped 2011 Beechcraft Baron G58 from Oklahoma Aviation at Wiley Post Airport in Oklahoma City, KPWA.

My ride for the ride!

It's hard to see, but if you look near the right bend of the pilot's yoke, you'll see a little switch labeled "A/C". Yes, it had air conditioning! I never want to take another checkride without it...

Training: 

As mentioned, I trained out of Oklahoma Aviation at PWA. My instructor (Bret Wyatt) and I met on Tuesday and worked in their Redbird AATD for the first 3 days, a couple hours a day. The Redbird decently replicated the power settings and configurations of the Baron, and the G1000 panel was close enough to the real thing to be a good training tool. On Friday, 7/10/15 we went on two flights in the airplane, running through all the required maneuvers for the checkride. By the end of the second flight I felt comfortable and ready for the practical test.

The morning of the checkride Bret and I flew the aircraft to Tulsa/Riverside Airport, KRVS, where we met the examiner in her office (she was the only examiner in OK able to do ATP checkrides in a Baron).


Ground portion:

We had been told that an FAA inspector would probably be observing this checkride as part of the examiner’s annual requirement, however he was not there yet so we got started with some of the paperwork. When he showed up he briefed that in addition to the usual three possible outcomes of a checkride (pass, fail, or discontinue), there could be a fourth outcome – the examiner herself failed his observation and he would have to take over conducting the checkride. I can only imagine how painful that would have been!

The oral examination was pretty straightforward except for one thing – the weight and balance and resulting performance calculations. You see, with him on board, with the fuel load we had, we were going to be over max gross weight by a decent margin (about 50 pounds). He was insistent that he had to ride along on the checkride, so the examiner and I were trying every which way to figure out how to do it. She asked if we flew for a while by ourselves and burned off some gas, then came back and picked him up to finish, if that would be okay and he eventually agreed.

Still, at just below max gross weight the performance numbers were not too exciting, even with 300 hp per side. It was a pretty warm day which negatively affected takeoff and climb performance. Our main concern was the accelerate-go distance, the distance it would take for an engine to fail right after liftoff and for us to be able to climb to 50’ AGL. At max gross it was about 9100 feet. The runway at Riverside is about 5100 feet long, and there are some takeoff obstacles listed in the departure procedures that are closer than 4000 feet from the runway. Even worse was the situation at Okmulgee (KOKM), where we planned to go for approaches and landings. So it was a reasonable safety call for us to fly most of the checkride without him, and he reluctantly agreed to just observing one takeoff, one approach and one landing.




This is the departure procedure at KOKM. Note the takeoff obstacles listed for RWY 18 - 100 foot trees 1303' from the end of the runway, or just about 6300 feet from the beginning. With a 9100' accelerate-go distance, this was an actual concern.

The rest of the oral examination consisted mostly of questions about the various systems onboard the airplane – describe the fuel system, the landing gear system, what type of anti-ice and de-ice systems does the airplane have, that kind of thing. I was well-prepared for these questions both as a result of reading the POH and a great publication on the G58 produced by the FlightSafety company. She asked a few questions for clarification but there were no surprises. Really, she went right down the list of systems in the PTS. Couldn't have asked for more straightforward!

I will add that the ATP written test and the ATP oral exam are completely different. This was really welcome news. The ATP written was full of arcane questions like “how many flight attendants are required on an airplane with 235 seats if only 150 are occupied” and location of emergency flashlights and such. The oral exam only covered the systems and performance for the airplane being used. Thank goodness!

Flight portion:

(Note: as far as I can tell, we performed all the required maneuvers from the PTS. If I left something out it’s probably just me forgetting about it. Also, virtually all of this checkride is done “under the hood” so I had the foggles on most of the time except for takeoff and landing, and during circle-to-land maneuvers.)

The examiner and I got in the plane and taxied out, leaving the FAA inspector to join us later. Lined up for takeoff on 19R, advanced the throttle, accelerated down the runway, liftoff, gear up, and whoosh - the door came open! (Really, it wasn’t an examiner’s trick.) The airplane, like most small airplanes, flies perfectly fine with the door cracked open, it’s just noisier inside. She had closed it before takeoff and it felt secure to me, but the Beechcraft door locking mechanism is a bit tricky and takes some getting used to (I’ve had it happen myself with a student in a Bonanza). We were already climbing out, so she asked me if it was alright if she called an “audible”, changed our plan (did I really have a choice?), and instead of airwork first, we go do a single-engine ILS RWY 18 approach to landing at KOKM as our first item. Sounded like a good idea to me, so I set up the procedure and she gave me vectors, “failing” the engine somewhere before the FAF and setting zero thrust (the power setting that simulates the reduced drag of a feathered propeller). Although I couldn’t use the autopilot for this approach, the G1000 avionics, flight director and synthetic vision make simple work of staying on course and glidepath. The approach and landing went well, we exited the runway and then got the door solidly closed.



As if having a flight director didn't make it easy enough to fly an ILS, keeping the flight path marker (green circle) right on the runway makes for a perfect approach anyway! (This was taken prior to the checkride and using the autopilot to get the picture, but hand-flying was almost as easy.)
We started our takeoff roll and she “failed” an engine again with the mixture control while on the runway. I brought both throttles back and braked to a stop, maintaining centerline and runway heading reasonably well.

She gave me back the engine and we took off again. After departure she provided me with vectors for the ILS RWY 18 again, but with both engines this time. During the ATP checkride, if the airplane has an autopilot, you are expected to use it for some of the approaches under the idea of “automation management”. Each approach I would ask “can I use the autopilot” to make sure I wasn’t making it harder than necessary! Fortunately this airplane had the fully-G1000-integrated GFC700 autopilot, which is a fantastic device. I basically just watched it do its thing all the way down final. Upon reaching DA she told me to “go visual and land.” At about 50 feet AGL she tried to make up some reason for me to go around, and it came out as “elephants on the runway”, which made us laugh – good as a tension reliever anyway! So I went around and climbed back up, putting the foggles back on, back into the fake clouds.

This was followed by the missed approach into the established holding pattern at the OKM VOR. After entering the hold, she gave me vectors and a climb out to the west for airwork.

The next items were in about this order:

- Steep turns. These were 180 degrees of turn to the left at a 45 degree bank angle, followed immediately by 180 degrees to the right. These were no problem due to the power settings I had figured out in practice – 18”/2300 rpm gave about 140 kias at the entry. When rolling into the turn, bringing power up to about 21” and adding back pressure held it right on airspeed and altitude. But the best part was the flight path marker displayed as part of the G1000 synthetic vision system. Keep the flight path marker on the horizon line, and the airplane will easily stay within 20 feet of altitude.

- Stalls. A series of three stalls is required – clean, landing, and takeoff configuration. One of them was while in a turn. These were conventional and not much different than those on the Private Pilot checkride, except the recovery was to take place at the “first indication” of a stall.

- Unusual attitudes. We did two unusual attitudes, one in a nose-high turn and one in a nose-low turn. She had me tilt my head down and close my eyes while she set up for these. The first one was recovering using my primary instruments (the G1000), the second one was using the standby instruments (standard attitude/altitude/airspeed indicators, but way over on the far right side of the panel).

- Engine shutdown and restart. She “failed” an engine on me and had me go through the actions required to completely feather, shut down, and secure the engine, then start it back up again. I paid careful attention to heading and altitude control since those are what she’s really paying attention to.

- Emergency descent. I think she just told me “let’s see an emergency descent”, so I brought the power to idle, gear and flaps out (at appropriate speeds), and rolled it over into a 45 degree bank, maintaining airspeed near the top of the white arc, just like I teach my students in an engine fire scenario, for example. This resulted in a pretty rapid descent, so we made maybe only a full turn and she had me roll out.


That was about it for the airwork, and we had one more approach and landing to make before picking up the FAA inspector. She had me call Approach Control to get vectors for the KRVS RNAV (GPS) RWY 01L, circle to land. This was flown with a simulated failure of the Primary Flight Display, which was simulated by her covering it up. I went to reversionary mode on the G1000 and used the Multi-Function Display to fly the procedure. I think I probably used the autopilot on this one as well, but maybe not. We were instructed to circle to the east of the runway. In actual instrument conditions this is prohibited by the approach procedure, and for good reason – once I got down to the Circling MDA and went visual, there I was staring at the CityPlex towers near Oral Roberts University (anyone familiar with Tulsa will know what I mean) sticking 648 feet up from the ground about 1.3 nm east of the runway. She told me to just fly my downwind inside that tower which is a local procedure.

That tower rises far above anything within the immediate vicinity and sure looked close once I took off the foggles!
We landed and taxied back in to pick up the FAA inspector. Since she hadn’t told me I had failed, I knew I was passing up until this point. Just a few more minutes to go, but with double the sets of eyes watching me! Since this airplane has rear passenger doors behind the wing (and therefore well clear of the engines), we had coordinated that he would just come on out and climb on board with the engines running. Of course I verified his seatbelt was fastened the best I could, and knew that his visibility would be limited since he was sitting in the rear-facing middle row.

After takeoff, I contacted departure and was cleared direct to the GNP VOR a few miles south of the field for the full VOR RWY 01L procedure with a circle-to-land. Somewhere in here the FAA inspector unbuckled, turned around and took up some kind of kneeling-on-the-seats position so that he could watch. Quickly setting up the approach, I let the autopilot fly the published procedure turn via GPS courses. Established back inbound, I elected to fly the final approach course by hand, for one reason only – I knew I had to switch the CDI from GPS to VOR mode for the final approach segment (and announced that I was doing this), but I didn’t want to accidentally get into some weird autopilot mode depending on my timing of this change. Admittedly, this just wasn’t something I had done in this airplane, with this autopilot and equipment, so was I hesitant to try something new at this exact moment. I knew I could easily fly it by hand, though, so that seemed the safer way out.

Tower instructed us to break off the approach before I was down at MDA, and to circle to the west for RWY 19R. At that point, to comply with passenger seatbelt regulations, I had to tell the FAA inspector that he needed to turn around and put his seat belt back on. My landing went pretty well, we taxied back in, and I was able to finally relax – I had passed!

Debrief was pretty short, which is exactly what you want I suppose. She said I did well (obviously well enough anyway) and we finished up the paperwork!

Total time in the airplane maybe about 1:45, which includes taxiing back to pick up the FAA inspector. The ride went very quickly, especially since the Baron gets between airports and approaches in no time!

My overall impression of the examiner (Jennifer Wise) was that she made me feel very comfortable. Especially given the difficult circumstances with the extra observer, she made me feel relaxed and at ease. She was friendly and the quizzing during the oral and flight portions was conversational in nature. She was able to find out that I knew the material, without having to resort to trick questions or impossible scenarios. Highly recommended!


A few general notes about the checkride and really instrument flying in general. I used the power setting information available from the American Bonanza Society (they handle Barons too). Flying by-the-numbers was critical to being able to free up extra brain cells for other tasks. For instance, on an approach I used 17”/2500 rpm until just prior to the FAF. Then it was flaps to approach and gear down  to descend down the ILS. This resulted in almost exactly 120 kias and a descent rate that kept me right on glideslope. On a non-precision approach, at MDA bring it back up to 22” (since now the gear is down it takes more power to stay level). Reliable 120 kias all the time. I already mentioned the settings for steep turns. It’s the way I teach my instrument students to fly, and it really works well. Figure out the numbers for your airplane and speeds and try it!

Friday, March 27, 2015

"What's it doing now?", or GPS turn anticipation-gone-wild...

I was on a recent flight with an instrument student in a very well-equipped Bonanza that provided a very instructive example of a few things:

1. Know your avionics equipment.
2. Know your autopilot.
3. When flying instruments, slowing down is your friend!

We were headed from Wichita, KS (ICT) to the Stillwater, OK VOR (SWO) in more or less a direct routing as part of the required “long IFR cross country”. The intent was to fly the KSWO VOR RWY 17 with the procedure turn and everything for training purposes. Kansas City Center provided us with “direct SWO VOR” and “maintain 4000 until established”.

Our approximate course:


The procedure for reference:


Now, this is in an area where Center’s radar coverage does not go all the way to the ground – that’s why the clearance was only down to 4000. You may also notice that there is a feeder route from the PER VOR to SWO VOR published at 3000. Though we were close, we weren’t actually on the PER-SWO route, so we had to maintain 4000 as assigned. In addition, I wanted us to start at 4000 - it would set up a great scenario for the “slowing down and going down” dilemma faced by faster, slipperier airplanes – you can descend OR slow down, but it’s hard to do both at the same time. Being at 4000 once we started the outbound procedure turn, then down to 2600, then down to 2100 once inbound could mean a lot of juggling and planning of power settings and configuration changes (as we all know, CFI's love to inflict this kind of torture…).

We descended to 4000, but hadn’t slowed down yet – we still had a ways to go, after all. Eventually the GTN 750 showed about 10 miles to go to the VOR, and we were doing around 155 kts GS (and airspeed too, it was pretty calm). The GTN's CDI output was in “GPS” mode – appropriate for this phase of the flight, and the autopilot was in GPSS mode, following the GPS course exactly.

Note - the following screen captures are from Garmin's GTN 750 simulator - so they're not from the real flight. However, they're representative of what was going on and pretty accurately depict what was happening.


At about 10 miles out, the pilot told me he’s going to start slowing down. Okay. Shortly thereafter, the GTN then shows us the following course:


Holy turn anticipation, Batman! The GTN plotted a course that would turn before the VOR (as expected) to intercept the procedure turn outbound course. However, due to our ground speed and the angle of turn, it had to lead the turn by several miles. (If you're interested, the turn radius of a standard-rate turn at 155 KTAS is about 5000 feet, so twice that to make essentially a 180-degree turn). This several-mile lead turn would make us roll out on the procedure turn outbound PAST where the GPS had also calculated we should have finished the procedure turn and been back inbound (dashed white line). Notice the "miles to go" in the bottom right corner (7.4nm) is still showing the distance to the VOR. How far until the turn starts is not depicted.

At this point the pilot realized he'd sure better get slowed down. The Bonanza is pretty slippery, of course, and we were only able to drop a few knots by the time the turn started. We elected to leave the autopilot on to "see what it's going to do" now - something I wouldn't have recommended in actual IMC, but a possibly informative moment in training.


The GPS started around the turn as expected, and rolled out on the PT outbound course. The GPS auto-sequenced to now highlight the PT course. Notice that we have not yet started the PT yet, and are at the end of it - we should be pointed the opposite way. Also, our TAS (GS) is still pretty high (105-110 is normal in the Bonanza) because of the previously-discussed need to descend and slow down simultaneously:


Now I was really intrigued - how is the GPS going to get out of this? Keeping in mind that GPS-steering essentially tries to correct left/right deviations from course - and at this point we are well left of the intended course, which is over a mile southeast of us at this time. So it should correct to the right, right?

And it did!


At this point the programming of the GPS apparently decided we must have already completed the procedure turn and therefore should be inbound, as it did two things - one, it highlighted the inbound course as our current leg, and two, it kept us turning around to the right to intercept, the "opposite" way that a PT is normally flown:





Finally, having intercepted the final approach course, the GPS and autopilot did line us up nicely on final:


Back on course, the pilot switched from GPS to VLOC mode and the mean instructor made him turn the autopilot off and hand-fly the rest.

I love educational moments like this! There were several lessons to be learned:

- SLOW DOWN! There's never such a thing as slowing down to approach speed and configuration too early, especially when you have a big turn coming up. Had we been down at 105-110 KTAS before the first turn started, the turn radius would have been much smaller and the outbound course would have been intercepted in plenty of time to perform a "normal" PT.

- PLAN AHEAD! An approach briefing is more than just reading the altitudes and heading off the chart. Know where you are on the chart. How are you going to get into the approach? What altitude? What are you going to have to do to make that altitude? When to slow down? How much turn? Lead it or don't lead it?

- Don't give up CONTROL to the machines! If you don't know what "it" is doing, whether "it" is the GPS or the autopilot, take over and fly it by hand. I had no idea how this was going to turn out, and I wouldn't have wanted to find out in actual IMC.

- As much as you can, KNOW your equipment and how it functions. Sadly, I looked in the GTN750 pilot's guide and couldn't find much about how it calculates turn anticipation, or at what point it starts showing it (note that the first picture above doesn't even show the turn yet).

Lots to learn in this flight, but that's one of the main purposes of the "long IFR XC" in training. I'd say mission accomplished!