Common Sense Descents
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.
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.
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.
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.
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.