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Having flown a number of flights as a passenger recently, I observed that:

  1. Roughly 50% of pilots used spoilers above FL180
  2. Almost everybody used spoilers to pass the 10,000 feet / 250 knots mark
  3. Roughly 25%~33% of pilots used spoilers around 7,000 feet ~ 8,000 feet down to glideslope capture

Are modern airliners so "slippery" that an engine-idle descent is not enough to meet operational requirements? (on the other hand, makes them really good gliders in the event of a total engine failure!)

I was told that companies like to use engine-idle descent as much as possible because it is the most fuel-efficient way to get down; spoilers are only "sometimes" used when ATC clears the descent path unexpectedly, which usually means a short-cut and passengers can expect an earlier arrival. That was in the 90s'.

Has it changed in the last decade? How frequent is the use of spoilers during the descent portion in today's flights?

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    $\begingroup$ Can we get a spoiler alert on this question? $\endgroup$ – user14257 Aug 2 '16 at 1:40
  • $\begingroup$ Out of curiosity, how do you observe the altitude when you're a passenger on an airliner? Are you checking an altimeter on your phone? $\endgroup$ – yshavit Aug 2 '16 at 5:38
  • $\begingroup$ @yshavit the new Android-based onboard entertainment system has many new functions, including a PFD-like screen which shows speed, altitude, v/s, heading, winds aloft, pitch & bank. The altimeter on a watch or phone would only measure cabin pressure, which never correspond to the atmosphere pressure outside. $\endgroup$ – kevin Aug 2 '16 at 9:06
  • $\begingroup$ Derp, of course. Silly me. :) $\endgroup$ – yshavit Aug 2 '16 at 12:42
  • $\begingroup$ @yshavit and on a clear day, you can have a decent idea by looking out of the window... the 10,000ft/250 kt point is also typically marked by the cabin seatbelt signs being turned on, too, which gives you a nice point of reference $\endgroup$ – Jon Story Aug 3 '16 at 12:49
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Very frequent.

Modern airliners are certainly fairly slippery, but it's not so much that they're super-gliders that we can't wrestle down onto the ground.

It's more a function of busy airspace than a property of the aircraft. When you remove power from any aircraft, it's going to descend - the only question is the rate. I promise you that if you found yourself a long way from an airport in a powerless B737, you'd find that descent rate to be faster than you'd like!

The main reason for spoiler use isn't that a modern aircraft glides "too well" - but rather than the airspace is busier so aircraft are expected to respond a little quicker than in the past, which can make it more convenient to descend faster as a pilot. It also means that ATC will often ask pilots to maintain a higher-than-ideal airspeed on the descent: if travelling at 160 knots, you'll descend faster than you would at 180 or 200 knots, for example... but more often these days, you're asked to maintain a higher minimum airspeed for longer. That's often convenient for ATC as it gets you where you're going faster, and it's convenient for you too, because you get to go home faster: but it means you're travelling a lot further in the next minute, making it harder to hit the descent rates required, necessitating the use of spoilers in some circumstances.

Clarification: In this answer I've focused on the later stages of the flight: "Slower = descend faster" is not necessarily a universal constant - aircraft glide best at their "best glide speed" which is a function of drag, weight, and airspeed. This answer assumes that, as for most airliners, best glide speed is in the region of 200-250 knots, and that an airliner is unlikely to descend at/faster than their best glide speed in the approach phases where speeds are typically in the 150-200 knot range.

At lower altitudes when descending under ATC, the above ATC-centric discussion applies. At higher altitudes, it tends to revolve around efficiency and company policy: the longer you can stay at cruising speed/altitude, the more efficient your flight will be: therefore it saves the airline money to descend as late as possible, requiring a higher descent rate. Spoilers, of course, help with that.

TL;DR: Some of it stems from the aircraft design being more aerodynamic and therefore better at gliding, but it's more closely related to ATC being busier and therefore keeping aircraft higher/faster for longer, which requires higher descent rates and therefore the use of spoilers, along with the fact it's more efficient to stay high/fast in the cruise for as long as possible then descend later/faster

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    $\begingroup$ Can you please clarify the slower speed - higher descent rate statement you make? It seems to contradict other answers stating the opposite . $\endgroup$ – Stelios Adamantidis Aug 1 '16 at 14:18
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    $\begingroup$ @SteliosAdamantidis I'm not sure where that answer contradicts mine? They discuss entirely different concepts. An aircraft will glide "best" (in terms of altitude lost per unit of distance) at the best glide speed: typically 200-230 knots in most airliners (A330 is ~210 knots, ~220kts for a 767, ~200 kts for a lighter aircraft like an A320/B737). This answer is discussing slower speeds, though and is relating to altitude loss per unit of time (which isn't necessarily synonymous with altitude loss per unit of distance) I think you're possibly confusing different concepts between the two answers $\endgroup$ – Jon Story Aug 1 '16 at 14:27
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    $\begingroup$ @MSalters you're getting into aerodynamics, not airliner operations, and outside the scope of this question. If your A340 is in a nose dive, you have significantly bigger problems than whether your spoilers are deployed. We're talking about deploying spoilers in a constant airspeed descent, or when slowing in the descent, during normal airliner operations. $\endgroup$ – Jon Story Aug 1 '16 at 15:13
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    $\begingroup$ @Agent_L - I see where you're coming from, although I didn't see that as a hidden assumption, but rather one implicit in the question in that it relates to airliners (as opposed to aerobatic or combat aircraft) $\endgroup$ – Jon Story Aug 1 '16 at 15:36
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    $\begingroup$ Guys, if you think I'm wrong please feel free to write your own, better answer and let the voting system sort it out... this is just becoming a convoluted chat in the comments section now and isn't really helping anything or anyone. An airliner descending at idle power at 150 knots will, for every single modern airliner of Q400 size or larger, descend at a higher rate than one at 200 knots, and that's the final input I'll be putting into this part of the discussion. If you disagree, please write a better answer. $\endgroup$ – Jon Story Aug 2 '16 at 11:03

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