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 While this has been replaced by the 8260.58, the concepts and calculations are similar.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!

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!

"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!

Flying "teardrop" procedure turns

An interesting situation was brought to my attention a few days ago by a reader (and former instrument student). He (for reasons known only to him) decided to fly the RMN ILS OR LOC RWY 33, in the simulator, using only one VOR and an ADF. Yes, your guess as to why is as good as mine. However, he (correctly) identified that the turn radius depicted by the initial segment starting at HIGAP and arcing to (AFUWY) is way larger than needed in the airplane he was flying (a typical four-place single).

Let’s talk about this type of procedure turn a bit, since you don’t see them very often. Actually, many pilots studying for the instrument written for the last 10 or 20 years have probably seen one at least once, as the Duncan, OK (DUC) LOC RWY 35 used to be an example procedure on the test, and used to have this type of procedure turn. However, it has since been modified and has a (regular) procedure turn. And may not be on the test any longer, though I’ll have to defer to those of you studying for it to let me know about that.

Regardless, it had the same geographic setup as RMN – a VOR a few miles away on final, but offset to one side a couple of miles. How to use it to get turned around and lined up on final? This is a situation where the teardrop procedure turn can be used by the procedure developer. Essentially, from the BRV VORTAC you fly the 122 radial outbound until 10 DME (or intersection with the 279 bearing to EZF), then begin a right turn around to intercept the final approach course on the localizer.

Of course, that turn radius depicted has to cover all speeds of airplanes, right? Accordingly, it is WAY larger than you would need for a light single-engine airplane. If you measure the distance between HIGAP and (AFUWY), it’s about 5.7 nm, meaning a turn radius of half that. But what is the turn radius of, say, a Cessna 172 at 90kts in a standard rate turn?

(Note – way geeky content ahead. CLICK HERE to just skip ahead to the answer and keep reading from there.)

You can easily figure this out if you pull out your copy of “Aerodynamics for Naval Aviators”. What, you say you don’t have one? Of course you do, this is 2015 after all:

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/

This book really has a lot of good stuff in it, which is why it’s still used as a reference, unchanged, since 1965. A quick (?) browse will take you to page 178, which has formulas for turn radius and turn rate. As we are in a standard rate turn, we know our Rate Of Turn (ROT) is 3 degrees per second. What we need to determine is our bank angle from the second equation, then use that in the turn radius formula.

In the second question, solving for phi:

Using V = 90 KTAS and ROT = 3 degrees per second, phi (angle of bank) = 13.9 degrees.

Inserting phi = 13.9 into the first equation gives us a turn radius of……….

2907 ft.

Alternately, you can use Figure 2.29 on the next page if you prefer a chart format. Notice there are two sets of “bank angle”  curves, you use one to solve for radius and the other for rate. Since we know rate, we can work backwards from there. Personally, I prefer the formula method, but that’s me.

Okay, so if you skipped right to here, the radius of a standard rate turn at 90 KTAS is about 2900 feet, or about 1 nm in diameter.

So back to the RMN ILS teardrop – if we entered a standard rate turn at HIGAP, we would be well right of the final approach course after completing the turn. So how to combat that – fly a half-standard-rate turn? Quarter standard rate? No, no need to try to stay on the line – it’s not a DME arc. Instead, begin your turn at HIGAP but then roll out to establish a normal intercept angle to the final approach course. Don’t worry, that whole area between the outbound and inbound legs has been evaluated for obstacles, and as long as you stay at or above 3000 feet until on final, you’ll be safe.

Remember the effect that wind might have on your ground track – a north or northeast wind will tend to push you toward final quicker than normal, and a wind from the west will have the opposite effect, “holding you back” from intercepting the final approach course. The segment between HIGAP and intercepting final is essentially a dead-reckoning course.

Alternately, another solution could be to fly a 10 DME arc from the VORTAC - but obviously this would only work if the facility had DME and the value was published. 

I know what you’re wondering, because I was too. What true airspeed would allow an aircraft to maintain that arc in a standard-rate turn (no wind)? I calculate about 536 KTAS (the method how is left as an exercise for the reader). Not too likely in a Skyhawk, and even more, a “standard-rate”, 3 degrees per second turn at 536 knots requires a bank angle of about 56 degrees. That’s “slightly” past the limit for passenger comfort in commercial air travel, and 536 ktas is “slightly” above the speed limit of 250 KIAS below 10,000 feet anyway (yes, I know, the speed limit is “indicated airspeed” not “true”, but c’mon now).

So what’s the deal with this huge turn radius on the approach?

Above about 180 KTAS, a standard rate turn requires greater than 25 degrees of bank. As a result, faster aircraft use 25 degrees of bank as a maximum, regardless of the “degrees-per-second” that result. (Note I am not a jet pilot, but this comes from AIM 5-3-8j6, which admittedly only references holding patterns. Please correct me if I am wrong.)

At 25 degrees of bank, the speed necessary for that 4.7 nm diameter circle is “only” 273 KTAS. Still pretty quick, but not out of the realm of possibility at the maximum 250 KIAS, depending on atmospheric conditions!

This type of teardrop used to be seen quite often on the military “HI” approach charts, where the idea was to cross the field at a high altitude then have an outbound and inbound leg long enough for the descent – a “high-altitude penetration turn”. But even then, many U.S. military bases do not have them anymore. I’ll have to defer to any military aviators who read this to let me know why.

I have no idea how many of these teardrop procedure turns are around. It doesn’t seem to be very many, and the number is likely getting smaller as the teardrop is replaced with other options. But if you see one, now you know what it’s all about!

"Higher in a minute" vs. "Climb via the SID"

This isn't really a TERPS article like most of the rest of mine, but it's an important point that a friend's recent corporate jet flight out of Teterboro, NJ brought up.

Here's what happened:

He was cleared to depart runway 24 using the TETERBORO NINE departure, then to "climb via the SID, expect FL xxx 10 minutes after departure...". The TEB9 departure requires a couple of intermediate level offs before climbing up to your cleared altitude:

After takeoff, having leveled off at 1500 on heading 280, but prior to reaching 4.5 DME, ATC told him in the initial call, "Off of Teterboro, N12345, radar contact. Higher in a minute." In typical NYC-area fashion, the other nonstop radio communications prevented any immediate clarification.

His question was the same as mine and yours - what exactly does the controller mean by "higher"? That's not standard phraseology. Higher than what in a minute? Typically something like this would be used when a delay is expected to the final altitude, FL xxx in this case. But since in this case there are some intermediate altitudes, there are essentially two possibilities:

1. I can't clear you to FL xxx right now, but can soon. Since you were already cleared to climb via the SID, climbing to 2000 is fine but I'll have your "higher" altitude in a minute.
2. I want you down at 1500 feet for now, don't climb up to 2000 yet. I can get you "higher" than you are currently in a minute.

Who really knows what ATC wanted? The pilot chose (wisely) to assume the worst and stay at 1500. Fortunately, immediately upon crossing 4.5 DME he received a climb to 11,000, so the issue resolved itself without any further difficulties.

The FAA has recently (April 2014) been implementing new "Climb via the SID" terminology, which in large part is designed to reduce this kind of ambiguous situation. However, it actually caused the confusion this time.

NBAA has a great write-up and slideshow briefing on "Climb via" (and its sibling "Descend via" for STARs) at the following link. It's worth a read if you fly anywhere that you're commonly issued SIDs and STARs.

http://www.nbaa.org/ops/cns/pbn/climb-via/

Notice that subsequent altitude assignments effectively cancel the "climb via" authorization. A case could be made that in this example, that's exactly what happened.

But I think the most important thing to take out of this scenario is what we all learn in Private Pilot training - if you don't understand what ATC wants you to do, don't assume, ask! If the frequency is so busy that you can't get in a word, then use good judgment and take what action is necessary. In this case, the PIC and SIC both decided they would stay at 1500 - in my opinion, the absolute right move.

A long time ago (yes, in a galaxy far, far away too), someone told me that whenever you're trying to decide to doing something that you're not sure about, think how you would sound trying to explain your decision to a jury (or the NTSB for example). "Well, the controller told me 'higher in a minute', so I went ahead and climbed from 1500 to 2000 because I thought he meant I could go higher NOW, but even HIGHER in a minute." Doesn't sound very convincing, does it?

It may very well have been what the controller wanted, but we don't know. Good communication is the key!