Going Down: What’s Up With Airliner Descents?
Feeling less than comfortable, even downright disturbed with all of the sensations of descending in an airliner? Wonder what all the noise, pitch, turning and weirdness is about? Step inside my head for an inside look at what goes on and why.
Here’s where it all starts:
No, seriously–you’re in my head, remember? That’s the “Jethead” thing. And this is where “descent” starts for me: running the half-mile in high school. Stay with me.
I’m a high school sophomore, hiding in the locker room with the other half-milers who like me, have already finished our heats.
“Cougar Manno, report to the starting line,” blares the loudspeaker in the locker room. Damn.
No, my name’s not “Cougar.” It’s just that after nearly two years on the team, Coach Smith–who also sees me every day in geometry which he teaches–still doesn’t even know my first name. We’re the “Del Campo Cougars–” that’s good enough, make him run the two-mile.
Here’s Coach’s mug shot:
Okay, this is Pythagoras (take that, Coach Smith) but this is where we get back to flying and what my sophomoric high school years have to do with it. And Pythagoras is key.
Say we’re approaching our destination in an airliner. We’re at cruise altitude. Our destination is ahead by a certain number of miles, we’re a certain number of miles up. Picture starting to form yet?
You can see that we’re going to have to slant our flight path downward and cover the distance to the destination at an angle, right? Two important points on that.
First, because Coach Smith taught that our flight path from altitude to destination is the hypotenuse of this triangle–it is longer than both other distances, right? This, if you care to consider it, is why “air miles” are different from “ground miles:” you fly a hypotenuse up and down from altitude, making the straight line distance between two points longer by two hypotenuses–hypoteni?–whatever, you get it, right?
Second, clearly the nose-down angle is going to need to be steeper if we’re closer to our destination than if we’re farther back, where we can make a very shallow descent. And here’s just a little more math to figure the “where” and “when” of a descent:
You have a certain amount of altitude to lose–for example, ten thousand feet–for a restriction. You have a certain amount of time between now and the crossing point. Index the two–a certain number of feet in a certain number of minutes–and you have the required descent rate in feet per minute. If I have ten thousand feet to lose in five minutes, I need a rate of 2,000 feet per minute.
If only it were that simple.
Okay, sometimes it is, and that’s what you perceive as a smooth descent. But other times, Air Traffic Control has specific requirements regarding how soon you’re allowed to descend. They add restrictions, too: cross a certain point at a particular altitude and speed. Then often some other contingency pops up to screw all the angles and numbers you’ve planned:
There goes your formula as well as your smooth, flat descent angle, and here come the speedbrakes:
They disrupt the clean airflow over the wing, and you can see why–they’re like a board pushed out into the slipstream over the normally smooth wing. So there’s a good deal of rumbly vibration, right? Here’s where they are in the cockpit:
The noise and bumpiness are no big deal–the aircraft is designed for this, and most of the noise and turbulence is from the wind. It’s like when you’re driving down the freeway and open a window–lots of wind noise, which is what a slipstream is: disturbed air. Loud, annoying even, but harmless.
What the speedbrakes are doing, however, is important: they’re catching you up on the formula above when some factor alters one of the numbers in any of the three key variables: time, distance, and altitude.
It’s a three cornered relationship–another triangle, right Coach?–whose sides are constantly in flux due to conditions. I’m always visualizing the three variables and how they are fitting and changing due to circumstances like altered restrictions, winds, and speed changes. The alternative to speedbrakes for increasing a descent, which I recall wistfully from my other flying life, you definitely won’t like, or certainly not the 4-G level-off at the bottom:
Sorry, just another quick flashback. Anyway, starting a descent farther out allows for a shallow, smooth descent–think of the triangle. Delaying the descent necessitates a steeper rate: the combination of feet per mile and thus feet per second. There’s the big angle that feels like a plunge when circumstances dictate a higher than usual descent rate.
In my Toronto example above, other traffic below kept us from starting our descent as far out as I’d have liked. Yes, ATC could have vectored that other aircraft out of our way, or even vectored us off to one side. But they didn’t. Suddenly, the time-distance-altitude triangle is changed.
As pilots, we’re always watching that geometric relationship develop in our heads–thanks Coach Smith–and I’m always planning a strategy accounting for the variables like crossing traffic and one I haven’t mentioned yet: tailwind.
Top left corner, “GS 526” means “Groundspeed 526,” even though our true airspeed is in the 400s. That’s because of the “276/107,” which is right above the arrow, which is showing the wind angle. Means that whatever speed we’re showing on our airpeed indicator, add the wind to that, because we’re in the airmass which is itself moving at 107 knots.
It’s this deal:
No matter what speed they’re paddling, the raft’s in the swift-moving roaring torrent of fluid.
Air is a fluid, and 107 knots is a torrent. Which eats up our hypotenuse quickly and in my triangular mental image–I realize we need drag to descend by the restriction. And probably a steeper deck angle, plus drag like the speedbrakes and if we really need all the drag possible, the landing gear too.
Again, more noise, but the gear hanging is like a drag chute slowing us down–we can really lower the nose and keep the speed under control nonetheless, dropping our jet in the technical terms I’ve perhaps used more than once, “like a turd off a tall moose.”
But why, you might ask, don’t we start all descents way out from the destination so as to ensure a shallow, comfortable descent?
Well, for a couple of important reasons. First, it makes good business sense to stay at a higher altitude to take advantage of the lower fuel consumption and the favorable tailwinds. But as a pilot, I’m naturally a fuel miser: I want every pound of fuel in reserve for any contingency we might encounter–weather, mechanical, a runway closure, whatever. Because–another flashback on my part–we can’t just air refuel like back in that other flying life.
Plus, the airspace is crowded more today than ever. If you plan to get into a major airport, you have to do your part to assure the traffic sequencing: increase that descent when Air Traffic Control needs it, and be mindful of the restrictions ahead. Because you folks in back have connections to make and schedules to keep, right?
Which means, of course, skillfully flying the hypotenuse, adjusting the triangle relationship of speed, distance and altitude. Squeeze in the feet per minute required to fit into the traffic mix.
And when you as a passenger on descent hear the noise of the landing gear or speedbrakes, feel the rumble, and notice the deck angle steepening, you can turn to your seatmates with a knowing nod and reassure them by dropping a few phrases since now you know about the “what” and the “why” of the fluid time-altitude-distance triangle.
Just smile and say, “Yup, boards are coming up,” or “guess they needed to catch up on the descent and a speed restriction” or if they still don’t seem reassured, flash a smug grin, then casually turn back to your newspaper with a bored yet oh-so-knowing, “like a turd off a tall moose.” Tell ’em you learned that from Coach “Cougar” Smith back at Del Campo High School.
Now, are you still worried about approaches and landings? Stay tuned, stay subscribed: we’ll take the mystery out of both very soon.