Tuesday, May 15, 2018

Goodbye /DME, Hello Equipment Requirements Notes!

(This article is a companion post to episode 178 of the Stuck Mic AvCast)

Big news on the instrument approach charting front! In the near future you will not be seeing any more VOR/DME or LOC/DME or anything /DME procedures in the U.S. In fact, it's already started, but beginning with the 5/24/18 chart cycle, even more changes will take place. These changes could all be grouped under the heading "Equipment Requirements Notes".

Let's take it one thing at a time.

/DME


For as long as I can remember, if you had a VOR/DME or LOC/DME procedure, the /DME in the name signified that you needed to have a DME receiver in order to identify the Final Approach Fix, as in this example.  We may or may not need it elsewhere, but we knew we needed it to identify the FAF, in this case, NUCIK, CNU 12.9 DME.


FAA Order 8260.19H no longer provides for the "/DME" (or /anything else) in the procedure name, so these are being gradually removed. This means that the example VOR/DME RWY 17 above will eventually be simply the VOR RWY 17. But how will we know it requires DME, if it's not in the name? Simply, this removal of /DME from the name is part of a larger move to consolidate all the various equipment requirements notes into one location on the chart, as I'll discuss below.

Other equipment notes


We are also used to notes showing up in either the notes box or in the plan view.





Sometimes this would lead to confusing situations, like this:



Wait, do I need to have ATC radar coverage, or DME, or both?

These notes have always been a bit confusing, as the positioning of the notes affected its meaning. A note such as "DME Required" in the notes box meant that DME was required on the missed approach. If that same note was positioned in the planview, however, it meant that it applied to procedure entry from an IAF. From the AIM 5-4-5a3(b):


This placement criteria is more than a little confusing, however, so the best rule is to always review each approach to determine how you will identify each fix, starting at the appropriate IAF. We're getting out of practice on that with the prevalence of GPS substitution, so it's extra important to take some time to chair-fly the approach when we don't have an IFR GPS on board.

However, that is all changing (gradually).


Equipment Requirements Notes


The FAA Order 8260.19H, which is the FAA document guiding the documentation of procedures, adds a new box to the approach charts - the "Equipment Requirements Notes" box. This box, located near the top of the chart, will spell out what additional equipment is required to the fly the approach, but more importantly, where that additional equipment is needed.

Some examples from the 19H are:

"DME required for procedure entry"

"DME required for LOC only"

"DME or RADAR required to define GIGGS"

Procedures with this new box are showing up starting in the 5/24/18 publication cycle. However, like any change of this nature, it will take years for the updates to make it through all the approach charts in the U.S. inventory.

Here are some actual examples of 5/24/18 charts with those new notes.

First, a VOR/DME that is now going to be a VOR approach with DME required:

Old:


New:


More examples:


On this one, there is no valid fix makeup for DACCA (no DME source, and the angle for the FKL VOR radial would be too obtuse), so RADAR is required. Once you get past DACCA, though, no special equipment or RADAR is required.


Here you need either an ADF to fly the procedure turn based on the VEELS LOM, or a DME receiver to fly the arcs. Once you're established on that 131 course, though, normal ILS, VOR and marker beacon receivers can get you all the way to the missed approach holding fix:



Of course, if you have an IFR-certified GPS receiver, you can usually use it to substitute for these equipment requirements (following the appropriate AIM and AC 90-108 guidance). But putting "or GPS" in every single one of these notes would be a bit redundant...


PBN Equipment Notes


So far I've discussed only conventional, non-RNAV approaches. But don't worry, RNAV approaches get similar changes as well, there just aren't as many different possibilities. However, the changes introduce some terms that will be new to many people.

Very simply, there is a broad concept called "Performance-Based Navigation", or "PBN". PBN systems not only have an accuracy requirement, but among other things also have an alerting requirement for when it calculates it's not able to provide that accuracy. 

Think about a VOR - it has an accuracy requirement (30-day VOR checks), but if the accuracy right at the moment you're using it is degraded for any reason (signal problems, internal equipment errors, who knows), it has no way of determining it and telling you. A "NAV" flag on the VOR will indicate that the VOR transmitter itself is not working, but there's no way for the unit to check itself or the accuracy of the transmitter.

A PBN system, however, has to be able to tell you if the accuracy is degraded. The most popular type of PBN system in GA is, of course, an IFR-approved GPS receiver. But there are other systems - DME/DME/IRU being one of them. The new PBN requirements introduce levels of performance specifications that the aircraft equipment must meet to fly various procedures.

Different types of procedures require different levels of performance. The Required Navigation Performance specification is abbreviated "RNP". Rather than go into too much more discussion here, the AIM paragraph 1-2-2 has the details.

While PBN and RNP concepts can be very confusing, allow me to simplify - most light GA aircraft with a IFR-approach approved GPS receiver meet the "RNP APCH" NavSpec.

The term RNP has actually been around longer than we realize. We're all familiar with the DME/DME RNP-0.3 NA note, it has been on every RNAV (GPS) approach chart for a long time:


Most of us have been glossing over it during an approach briefing because it doesn't mean anything to the majority of light-airplane GA pilots whose airplanes have never had a DME/DME RNAV system. This note is going away. 

What will be replacing it is a note saying "RNP APCH":


This makes effectively no difference to the vast majority of GA pilots because as mentioned above, most popular IFR GPS installations in light GA aircraft meet the requirements for "RNP APCH". But seeing it on the chart could confuse some to think it's talking about RNAV (RNP) approaches, which are still only for airplanes that have a higher level of equipment and for aircrews that have received special training.

These will still be named RNAV (RNP) in the title, but will have the equipment note "RNP AR APCH" (AR for Authorization Required), as in the example below. (Note that this example has the additional requirement that the equipment has to be capable of flying Radius-to-Fix legs (RF required), since all routes into the procedure require such a leg type.) 


Conclusion

Hopefully, these new notes and placement will make the charts a bit easier to understand, although there is always a learning curve with anything like this.

Garmin has published a list of their equipment and RNP levels each one meets:
https://fly.garmin.com/fly-garmin/support/icao-flight-plans
(Near the borrom, titled Garmin Flight Plan Information Excel file)

I could not find such a document from the other GA IFR GPS manufacturers, but will link to it if someone can point it out to me.

All of the above chart samples were obtained at the FAA's website, where you can download the actual charts 19 days before each pub day. Also, you can get advanced notice and draft versions of upcoming approaches, obstacle departures, SIDs and STARs from their IFP Information Gateway.

Actual charts obtained from: 
https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/dtpp/search/

IFP Information Gateway: 
https://www.faa.gov/air_traffic/flight_info/aeronav/procedures/

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!