Common Sense Descents


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Getting 75 tons from cruising 8 miles up at 500 miles per hour down to walking speed at sea level is dependent upon one ever-changing three-point triangle.

That is, the dynamic relationship between altitude, distance and speed.  This relationship is as closely interrelated as a balloon animal: squeeze any one part, and the other two expand.

Descent planning, including mandatory crossing restrictions stipulating specifics in all three parameters would be simple if the triangle of altitude, distance and speed remained fixed. But it seldom does.

Here’s the simple, unrestricted problem: descend from 41,000 feet to sea level. Simple problem, simple math: a comfortable descent rate could be achieved in an idle power, clean (no drag, like spoilers) glide at 290 knots airspeed using the 3:1 rule 3 times the altitude in thousands to lose, or 3 x 41 = 123 miles.

Descent 1

But, here’s the first modification required: the max speed below 10,000 feet is 250. So, you have to allow more miles to decelerate from 280 to 250, plus more miles from 10,000 feet to touchdown because the descent will need to be shallower to keep the speed to 250 knots or less.

Yes, you could add drag in order to maintain the descent rate at the lower speed. But we’re planning the descent efficiently, fuel-wise, and also for passenger comfort: steeper descent angles and rumbly drag devices aren’t as comfortable as a clean descent. Plus, you’ll want to hold drag devices in reserve for when Air Traffic Control (ATC) tosses an unexpected restriction your way.

So anyway, now we have a straight line distance of 133 miles (I added 10 to slow down, remember?) for a clean descent. 290 nautical miles per hour is roughly 4.8 miles per minute. Couple that with a clean, idle descent rate of about 2,500 feet per minute.

The next problem is, however, the straight line. Most of the STARs (Star Terminal ARrivals) multiple lateral segments between a series of points, seldom in a straight line. What happens if you’re issued a revised clearance that shortens the route? That could easily shave off 20% or more of the flight distance, which also shortens the number of miles over which you can attain the descent. So, there’s the balloon animal: shorten the distance and you must increase the descent rate in order to cross the assigned point at the assigned altitude.

UKW STAR

What to do? First and easiest is to increase the speed, which will allow a higher rate of descent. That’s half the reason why I don’t plan descents at speeds over 300 knots–there’s no capacity to add speed if needed to increase the descent rate and accommodate the descent crossing restriction in light of the reduced miles available.

UKW STAR b

The other half is the ride: in the back end of the 737-800, particularly near the tail, all aircraft motion in turbulence, due to the stretched fuselage, are felt more intensely. If you encounter any choppiness at that speed, folks in the back could be tossed about pretty dramatically. Why risk that? Plus, if you plan a descent at 320 or 330–as the on board flight management computers often suggest–and then have to slow because of turbulence, you’re definitely not making your crossing restriction. Now you’ll have to call ATC and ask for relief–that screws up their traffic flow and means an off-course heading and as a result, a delay for you.

So how do you accommodate the shortened distance in real time? First, as soon as you execute the shortened distance in the Flight Management System (FMS), the system will recognize that the 3:1 calculation–the balloon animal of time, distance and altitude–is all out of proportion. The FMS just throws up its hands and switches from “Descent Path” mode to “VNAV Speed,” meaning it’ll hold the speed steady, you figure out how to get back to the descent path.

bug eye cockpit

So I switch the FMS to “level change” mode, meaning I want it to go after the altitude at the max rate with the speed set–then I set a higher speed. That achieves the best rate until, due to the higher descent rate, you re-intersect the normal path. And there’s where you must be on top of the ratios (speed, rate of descent, distance) in order to refuse a descent clearance you know you can’t rationally make.

That seldom happens with a shortcut route clearance, but often will happen if you’re restricted to your cruise altitude past a rational “top of descent” point. Therefore, you have to constantly be aware of the max descent available (with drag and higher speed), sensible (given the chop reports), tailwinds, which rob you of descent mileage, and be ready to refuse an altitude assignment that doesn’t fit those criteria. That only comes from keeping all of the ratios in not only accurately in your head, but also in the jet’s real time performance.

When any parameter changes, as they often do, you have to know how or if you can rationally accept or, even more difficult sometimes, refuse a clearance. I used to fly with a guy who specialized in “creative” refusals: when asked if we could cross a particular waypoint at a certain altitude that was mathematically (and balloon animal-y) unreasonable,  he answer, “We can, but we’ll have to leave the airplane behind.”

Better, I think, to manage the ratios, know what’s practical, plan ahead, and say “no” where required. Anything less, to quote Captain Randy Sohn, a revered name in the pilot world, “Would be considered bad form.” When it comes to balloon animals and jet descents, that just won’t do.

737 a wide

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11 Responses to “Common Sense Descents”

  1. Cedarglen Says:

    Another very informative post, Chris, and thank you. While not a current pilot (so add some salt) I think I’d be less reluctant to tell ATC, ‘no cannot comply,’ if they are stretching your airplane’s comfort level. If/when, for whatever reason, you do have to toss out all of the drag and those sky brakes thus making the back end’s ride a little bumpy, forewarn your FAs and make a very clear PA to inform your PAX that you’re sorry for the expected rough ride and that ATC gave you no choice. Avoidance is always best and that’s why they call you ‘Captain.’ When avoidance fails, blame it on ATC and proceed for a safe, if bumpy approach. I’ve had my share of those “E-Ticket” rides. I don’t really mind, when I know what is coming. -C.

    • Actually, this post was with you in mind: a return to the hands-on flying. Back to aesthetics next.

      But I have to disagree with you on the clearance. Too many pilots don’t say no when really they should, on descent as well as on climb: “be level in two minutes.” The answer is, “unable.” Climb rationally and never flirt with a slow speed stall in order to comply with a clearance. Rather, get a new clearance.

      Descents, same way: fly your jet conservatively with plenty of leeway on all clearances. There are too many wild cards (tailwinds, further restrictions) to do anything else. I’d never tell anyone that ATC didn’t give me any choice, because as pilot-in-command, there’s ALWAYS a choice.

      In fact, if you fail to comply, at your violation hearing, the first question will be, “Why did you accept the clearance?”

      There’s no good answer to that question.

      • Cedarglen Says:

        Thanks, Captain Chris and your points are smack-on, esp. the last two. If you don’t like it, say so and (briefly) explain why. The last one, ‘why did you accept…’ speaks for itself. ATCs are friends and helpers there to **assist** pilots in safe operations. Solo or with a multiple crew, younger folks and/or ‘young’ pilots too often forget that the PIC has to own everything, even substandard ATC direction and clearance.
        And thanks. Progress is far better than raw hours for two reasons: some prior stick and rudder and that small measure of mature thought that comes only with a lot of pretty annual calendars. Do you ever read N631S? Frank is an IFR-rated PP that I admire, not because he gets to fly a lot, but because he does his private flying to professional standards. Thanks, Chris.

      • I just finished Deke Slayton’s autobiography–excellent story, from WWII to the space shuttle.

  2. roberthenryfischat Says:

    Reblogged this on robert's space and commented:
    yeap.triangulation…

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  4. Saurabh raje Says:

    Just a small question, do you use v/s button? Maybe that would be a little simpler than changing the speeds to get the right descent rate?

    • I seldom use the v/s option. In my experience, I get after the descent profile most efficiently and quickly with level change. Within about 40 miles of the airport, both level change and profile can get you down and slow too early.

  5. I have a dumb question,actually a lot of dumb questions. I was listening to a online ATC feed and I heard a Pilot say Speed Alive. My hubby explained to me what it meant. Now my dumb question is this- Do pilots say Speed Alive on every flight? My friend said no,it depends on the airline. Is It a checklist type rule to say speed alive? Do pilots have to say Speed alive at all? Do you say it? Also my other is question is what is your favorite airplane or jet? Mine is the MD-80. But it seems like no one loves the MD-80 anymore except for pilots that flew them. My last dumb question is about the FLY QUIET signs I usually see at KBUR. Hubby explained noise abatement to me but I still get nervous that when I’m on a jet that has to FLY Quiet,that we will not go fast enough for takeoff. Can a jet crash dut to the Fly Quiet thing?

    • I’m not familiar with the “speed alive” call out or what it refers to. We don’t say it at my airline, so I guess I can’t answer that.

      The “fly quiet,” or noise abatement as you correctly termed it, has its safety drawbacks. For instance, at Burbank, that means staying at climb power and takeoff configuration and speeds till 1,500 feet. What we’d rather do, as pilots, is lower the nose and accelerate, gaining precious speed and retracting flaps. Speed is the safety margin and noise abatement profiles–and they’re everywhere–delay acceleration, leaving you vulnerable longer to things like an engine failure. An engine failure is harder to handle slower and “dirty,” meaning flaps and/or gear extended. Noise abatement delays the clean up of flaps and acceleration and keeps the nose high–exactly how you don’t want to encounter an engine failure.

      The MD-80? I flew it for 20+ years but I don’t miss it. Today’s Boeings and Airbuses are much more versatile and capable, from the engines to the aerodynamics to the flight management systems.

      Have safe, worry-free travels–

  6. Your brew of Euclid at several miles per heartbeat moves quickly from beer to brandy. Within the distillate, the knowldege – wisdom – of knowing when to say “unable” stands out.

    Followed the Seven from the outset, read aviation since knee-high. Outstanding, all; for me three up-thumbs were Cooper, Schirra, Slayton. We forget so many so soon – life flies on readily, more quickly than regard.

    I see a book in your future. And, thank you.

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