Just Because You Can, Doesn’t Mean You Should Start a Descent to a Lower Assigned Altitude

At the completion of my instrument instructor checkride back in ancient times, I recall that I asked the experienced FAA Inspector an important question as he was typing up the completion paperwork: “What’s the most important thing I should teach my instrument students?”

“Situational awareness,” (SA) was his answer. I have concluded that he nailed it! If a pilot always knew exactly where he was and could pinpoint himself on any chart or instrument approach plate, then the rest of the instrument flying skills would come together more quickly, safely and correctly. Good old “SA” is the key!

That answer was given to me in the late 1960s. Most instrument training airplanes at that time had two Navcoms (one with a glideslope) and a transponder … and the transponder only provided Mode A. It was quite rare to see DME. An autopilot? You’ve got to be kidding! An RMI? What’s that? With such rudimentary equipment, the challenge to be situationally aware was a mammoth undertaking for all but the most gifted students.

When I started as a ground and flight instructor with Beech Aircraft Corporation in 1972, I was exposed to RMIs (Radio Magnetic Indicators). “What a wonderful aid to situational awareness!” I marveled. Now, without spinning any OBS knob, I could see exactly what radial I was on … and usually I could see the radial from both VOR 1 and VOR 2. Wow! How cool is that?! Plus, the Beechcrafts in which I was instructing all had DMEs. With DME, even one radial was plenty to exactly position oneself with distance information.

And yet, many of the pilots I instructed still struggled to remain correct in their situational awareness. It is a depressing realization that about one in five students – most of whom were professional pilots getting paid to fly King Airs – would place themselves on the wrong side of the VOR station when I asked them to point to their location on the approach plate. They would confuse radials – which always go from the station, remember? – with bearings that usually go to the station.

If I gave this little sermon once, I gave it hundreds of times. It goes like this: “You can search all you want, but you will never find a little symbol of your airplane on that Jepp plate. But you will find VORs and NDBs, so start from those known locations and then follow the proper radial outward. Do the same with another VOR or NDB; where the two radials cross is where you are. Easy! Emphasize the tail of the RMI pointer, not the head.” This sermon seemed to turn on the lightbulb of understanding for a lot of my students.

But then – hallelujah! – moving maps made their appearance. What a wonderful addition! Now the pilot did indeed see his own airplane symbol on the map. I still love RMI needles – bearing pointers – since I am so familiar with them, but their usefulness is a tiny fraction of what they were in the pre-moving map days. To fly IFR without at least one moving map … does anyone do that now?! Why, even a smartphone can do a rather good job of giving a moving map display. And Foreflight on an iPad? Amazing!

So far in this article, all I have addressed is two-dimensional awareness. The title, you may recall, had the word “descent” in it. Before you give yourself a pat on the back and think your SA is a done deal due to your excellent knowledge and skill – aided a little by the moving map(s) you are using – ask yourself how situationally aware you are in the height department. Do you always know when you are likely to get a “slam dunk” arrival? Do you always start down to the newly assigned altitude ATC just gave you, even though the clearance was a “PD” (Pilot’s Discretion) one? Do you fly most legs of an RNAV/GPS approach outside of the Final Approach Fix (FAF) at the published minimum altitudes? My hope is that when you finish reading this article, you will have increased understanding about SA in the vertical, not just horizontal, plane. Done correctly, this addition to your SA will provide increased safety and lower fuel consumption, as well as a better ride and, sometimes, less icing worries.

Increased safety? That’s a no-brainer. We cannot suffer a CFIT (Controlled Flight Into Terrain) accident without hitting the earth, so altitude is our friend here. The requirement to have TAWS (Terrain Awareness Warning System) in King Airs has and will have a positive effect on decreasing these horrible, almost always fatal, CFIT cases. Here is a classic, well-known CFIT accident that probably would not have happened had TAWS existed at the time.

On New Year’s Day in 1985, Eastern Airlines Flight 980, a Boeing 727 on a flight from Asuncion, Paraguay, to Miami, Florida, slammed into a peak in the Andes as it descended in the clouds for an IFR approach at La Paz, Bolivia, an intermediate stop. The wreckage was not found for a long time, being in a horribly unforgiving location above 19,000 feet. Amazingly enough, some adventurers recently made their way to the site and actually found what was thought to be the cockpit voice recorder. However, no usable data has yet been retrieved. Officially, no cause has been found for the premature descent, so what I am about to write has no basis except hearsay from a King Air pilot I trained in the late ’80s.

This fellow was a rather senior pilot at Eastern while it was still in operation. He had personally flown this route often. Notice the date of the accident – New Year’s Day. He told me that the senior crews had bid that time off because of the holidays. According to him, the crew on this fateful day were new to South American flying. It was standard practice, he said, in this non-radar environment, to be cleared to 16,000 feet by the La Paz controllers when radio contact was first established. Yes, there was no conflicting traffic and ATC was indeed permitting you to descend to this lower altitude. But – and it is a huge “but!” – you cannot go down to that altitude now! The en route IFR chart clearly showed that you were still on a segment with an MEA of 21,000 feet! It was only after that segment had been passed that the MEA did permit continued descent to the assigned 16,000 feet MSL.

Just because you can, doesn’t mean you should! Here is a case where ATC gave permission for the descent yet there was another, overriding, constraint that would correctly force the descent to be delayed, to be in compliance with the MEAs. According to the fellow who told me his theory as to the accident’s cause, receiving clearance to this unsafe-at-the-time altitude was almost universal practice by La Paz ATC.

Take a moment to examine the Instrument Approach Procedure in Figure 1, the RNAV (GPS) Z Rwy 17 at KMTJ, Montrose Regional Airport in Colorado. Imagine that you are approaching from the east, proceeding direct to the GEJYU IAF, 60 miles out, currently maintaining FL220, in radar contact. Denver Center: “King Air XXXX, descend at pilot’s discretion so as to cross GEJYU at or above 10,000 feet, cleared for the RNAV (GPS) Zulu Runway 17 approach to Montrose.”

At what distance from GEJYU would you begin your descent?

If your King Air has an autopilot system that supports VNAV, many pilots would dial in 10,000 feet for the altitude at the fix, use the default three-degree descent angle, and watch the magic happen.

Without a VNAV computer, it might go something like this: “OK, I have 12,000 feet to lose from 22,000 down to 10,000, so I will need about 40 miles [(3 X 12) + 10%]. Since I am 60 miles out now, I’ll start down in about 20 miles.”

For this to work, the rate of descent must be about five times the ground speed (GS). In a C90A, for example, if our GS will be about 240 knots in the descent, I would initially target for a 1,200-fpm descent rate. (Most people prefer to take half of the GS and add an extra zero on the end … which is the same as multiplying by five.)

But now consider another method, and a better one.

WIDRU, the FAF, is where our ducks need to be in their neat little row: 8,100 feet, Approach Flaps, Gear Down and 120 KIAS. Notice the GP angle of 3.00 degrees lurking on the right side of the profile view. Let’s extend that three-degree angle out to our IAF: 6.1 nm from WIDRU to BRUUK and another 10 nm from BRUUK to GEJYU, so about 16 miles total. I can comfortably lose 4,000 or even 5,000 feet in that distance: [(5 X 3) + 10%] = 16.5 nm. Therefore, I could be at 12,000 or even 13,000 feet at the IAF and make a very comfortable, normal descent so as to be all set at the FAF. If you want to get down to 8,100 feet a few miles outside of the FAF to have time to slow and configure, fine. One of the beautiful things about King Airs, however, is their ability to slow while descending on a three-degree angle. If you’d rather use that technique, join the crowd that includes me.

Using this last method, targeting the IAF at 12,000 feet instead of the allowable 10,000, I could delay my TOD (Top Of Descent) for six more miles. Fuel savings? A little. Staying high longer and thereby avoiding the lower altitude turbulence a bit longer? Probably. Going though icy clouds at a higher airspeed while descending with a low angle-of-attack rather than slogging along lower in level flight … yes, I’d prefer that.

Take a look now at Figure 2, an approach to Runway 22 at Cody, Wyoming. You are coming in from the south in your G1000NXi-equipped 350, direct to NICQE, at FL310. You have told Center that you will be asking for this approach with the transition route from NICQE to HOZZR and Center has said to expect that.

If we add this procedure into our flight plan, we will see that 7,500 feet is already populated in the altitude field associated with the FAF, ELYPS – 7,500 will be in blue or cyan, designating a “true,” necessary, constraint. (“Blue is True; White sheds Light” … thanks to FlyingLikeThePros on YouTube for that memory-jogger.) The VNAV computer, using the default three-degree angle – same as this approach, by the way – will now extend the glidepath all the way out to NICQE. Using our “three times the altitude to lose plus a cushion” rule of thumb, let’s predict what altitudes will be shown on the flight plan for the fixes outside of the FAF: 6.2 nm to DUSCA equates to about 2,000 feet. Thus, I would expect to see a white altitude number close to 9,500 in the flight plan at DUSCA.

Another 7 nm back to HOZZR … again a little over 2,000 feet, so let’s guess 11,700 feet there. Now continuing back to NICQE, 16.1 nm, means about 5,400 feet, so the reference altitude there would be about 17,000 feet.

What will be my flight plan’s TOD position then? Since I am currently at FL310 and want to cross NICQE at 17,000 (31,000 – 17,000 = 14,000 feet; 14 x 3 = 42 + 10% is about 46). So, I reckon the TOD will be not quite 50 nm away from NICQE.

It will be no surprise then to receive this clearance from Salt Lake City Center when about 15 miles or so from TOD: “King Air XXXX, descend at pilot’s discretion so as to cross NICQE at or above 11,000 feet. You are cleared for the RNAV (GPS) Runway 22 approach to the Yellowstone Regional Airport.” Ain’t technology great?! Monitor carefully, adjust power as needed, be configured no later than ELYPS, enjoy the magic.

Suppose you’re in a King Air without the VNAV capability and had not taken the time to “do the math” while reviewing this approach. Receiving that same clearance, the tendency would be to actually cross NICQE at the 11,000 feet altitude the controller stated, rather than taking advantage of the “at or above” clause (31,000 down to 11,000; 20K to lose, 20 X 3 = 60 + 10% = about 70 miles). So, we would have been premature in our ideal descent profile by 20 miles. Dangerous? No. Big mistake? No. But not the most efficient or comfortable method, in my opinion.

Figure 3 is the RNAV (GPS) RWY 36 approach to Polson Airport in Montana. To be ready at 6,400 feet at or before DODSE, the FAF: (A) at what altitude would you like to cross ARLEE on your way to the CABLI  IAF? (B) Would a standard three-degree descent angle keep you at or above the appropriate MEAs for the various legs of the transition?

The answer to (A) should be near 15,000 feet; and (B) is “yes.”

Lastly, take a look at Figure 4, the RNAV (GPS)-B approach to Sandpoint, Idaho. Same questions: (A) At what altitude would you like at the CESIG  IAF? (B) Will your three-degree glidepath to 4,900 feet at FINTA, the FAF, keep you above the various MEAs?

The answer to (A) should be near 12,000 feet; and (B) is “yes.”

Do you grasp the point that I am trying to make? Be situationally aware in three dimensions and optimize your descent profiles when ATC permits. Just because you can descend to a lower altitude when ATC provides the clearance, doesn’t mean that you should start down immediately. Yes, of course, if there is no “at pilot’s discretion” included in the descent clearance, then you must start down without delay. However, I rarely use a three-degree descent angle in this case. If I aim for 1,000 fpm, ATC rarely complains.

All of the approach examples that I have used here came from approaches that we actually flew in the C90A that I manage and fly out of the Phoenix area. The owner organized a six-day trip to various great locations in the Northwest and I was honored to be one of the crew. We were on the road from Oct. 5-10, 2018. Take a look on FlightAware if you wish to see our tracks: N3190S. I realize that those of you based on the East Coast would be rather surprised to receive so many “at pilot’s discretion” descent clearances. Folks, those types of clearances are routine out here. I am finding that these wonderful GPS approaches provide a great deal of flexibility. Ask and you shall receive. Honestly, in every case we requested the approach and the IAF that we wanted and were given permission for those procedures without delay. I personally think ATC is quite happy to wash their hands of us when they turn us lose to conduct the approach procedure totally on our own. I find myself doing less and less ILS approaches when there is an LPV option to the same runway. Why worry again about the magenta needle to green needle transition?