19

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?

kevin
  • 39,731
  • 17
  • 148
  • 278
  • 11
    Can we get a spoiler alert on this question? –  Aug 02 '16 at 01:40
  • 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? – yshavit Aug 02 '16 at 05:38
  • 2
    @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. – kevin Aug 02 '16 at 09:06
  • Derp, of course. Silly me. :) – yshavit Aug 02 '16 at 12:42
  • 1
    @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 – Jon Story Aug 03 '16 at 12:49
  • 1
    @kevin: "which never correspond to the atmosphere pressure outside." Only if your airliner is pressurised; lots of smaller propliners are unpressurised (they have to stay below 10 kilofeet AMSL as a result, but this isn't really a problem for propeller planes on short hops). – Vikki Dec 12 '19 at 22:07
  • For me, it depends on the plane. On the A320neo, (in MSFS) I do it basically all the time. But on the B747i, only when the plane is already on the ground. Because it's a super heavy aircraft and when the engines are not at full power and the spoilers raised, it will stall. – DaCuteRaccoon Apr 28 '23 at 16:35

1 Answers1

25

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

Jon Story
  • 10,417
  • 1
  • 48
  • 62
  • 2
    Can you please clarify the slower speed - higher descent rate statement you make? It seems to contradict other answers stating the opposite . – Stelios Adamantidis Aug 01 '16 at 14:18
  • 1
    @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 – Jon Story Aug 01 '16 at 14:27
  • There's a hidden assumption here, but I have some problems naming it. One clear counter-example is that in a straight nose-down dive, going faster means descending faster. More in general, going faster at the same AoA means more lift and thus a slower descent/faster ascent. – MSalters Aug 01 '16 at 14:56
  • 2
    @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. – Jon Story Aug 01 '16 at 15:13
  • @MSalters The "hidden" assumption is that he's talking about descending alone (while preserving other flight parameters), not trading height for speed. Such exchange is useless when you want to land. – Agent_L Aug 01 '16 at 15:35
  • 1
    @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) – Jon Story Aug 01 '16 at 15:36
  • @JonStory Neither it was hidden to me. I edited my comment. – Agent_L Aug 01 '16 at 15:37
  • I refer to the section where @casey mentions Often the choice is descend or slow down while you mention if travelling at 160 knots, you'll descend faster than you would at 180 or 200 knots, for example – Stelios Adamantidis Aug 01 '16 at 19:24
  • @SteliosAdamantidis that isn't a contradiction? – Jon Story Aug 01 '16 at 19:47
  • 1
    Yes it is a contradiction, that's what I'm saying. You say lower speed results to greater descent rate, @casey says you will either increase your descent rate or decrease your speed (i.e. you can't do both). Anyway since I believe we abuse the comments, I will post a new question. This out of the scope of How frequent is the use of spoilers during descent? question anyway. – Stelios Adamantidis Aug 01 '16 at 19:56
  • It really isn't: that answer talks about deceleration, mine doesn't? At no point in my answer is an aircraft required to both descend and decelerate – Jon Story Aug 01 '16 at 19:57
  • @MSalters going faster at the same AoA means you fly a turn. Or aerobatics. More lift means more load factor, not a climb. – Peter Kämpf Aug 01 '16 at 21:11
  • @John Story At no point in my answer is an aircraft required to both descend and decelerate But is that not the whole reason for using spoilers/air brakes - to descend faster and decelerate simultaneously? Your optimal glide speed gives you the shallowest descent angle. If you need a steeper angle you have 3 choices: 1) pitch up - your airspeed goes down, but sink rate goes down also. 2) pitch down - sink rate increases, but so does airspeed. 3) spoilers/air brakes - less lift and extra drag, airspeed goes down, sink rate goes up. – TomMcW Aug 01 '16 at 23:45
  • @TomMcW I don't understand what you're asking, is that not covered in the answer? When you want to descend faster, deploy spoilers to both descend faster and avoid acceleration/to decelerate. Spoilers help you lose airspeed and/or altitude, but you can lose either while gaining the other, lose one while the other remains unchanged, or reduce both at the same time... The balance is entirely independent of spoiler deployment, the spoilers just make you more draggy with less lift – Jon Story Aug 02 '16 at 00:36
  • 1
    I too think there is a contradiction here somewhere. Higher speeds equals bigger altitude loss per time, but not necessarily bigger altitude loss per distance. Higher speed is gained either by external power (engines), or by converting altitude to speed. More speed means that more altitude is converted to speed == higher descent rate. – Masse Aug 02 '16 at 07:46
  • @Masse if you're descending at a higher descent rate per minute, at a lower airspeed, you absolutely will lose more altitude per distance - anything else would be impossible. But again, you're discussing aerodynamics, which can vary more significantly, while I'm discussing airliner operation within a fairly defined set of activities. In short, you're over-complicating things – Jon Story Aug 02 '16 at 10:28
  • @Jon Story No, I think these are flight mechanics and you are wrong, by no means slower means more altitude per distance. That's why SIDs (at least those I am aware of) with climb restrictions express it in grade % and not in feet per minute. – Stelios Adamantidis Aug 02 '16 at 10:59
  • @SteliosAdamantidis that's climb, not descent. And I've already clarified that this only applies to Airliners at speeds you would expect an Airliner to be descending at: which is almost without question below their best glide speed, therefore slower = higher descent rate. I include a caveat in the answer to explain that is not always the case, but that it is for the situations described – Jon Story Aug 02 '16 at 11:01
  • 2
    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. – Jon Story Aug 02 '16 at 11:03
  • If thinking through aerodynamics (all planes are affected by it), @JonStory answer is wrong, you get greater vertical speed with higher IAS. This is how gliders manage their speed, by converting altitude to speed (conversely speed to altitude). You want to go faster, push the stick, slower and you pull the stick. Altitude per distance is another matter which takes best glide into consideration, but altitude per time is directly related to speed. The only way to have slower descent and higher speed is by giving the aircraft more power, which can be considered 'airline operations'. – Masse Aug 03 '16 at 07:28
  • @masse again you are talking about an entirely different thing. We're discussing descent on autopilot/autothrottle, which in an airliner typically means idle power for most of the descent. If you need a higher rate of descent than is possible on idle power, you deploy spoilers. That's literally the point of this question. Please, people, unless you have an understanding of the properties of an airliner in descent (in which case I'd be delighted to discuss further or be corrected), can we take this to chat or write your own answer. You're just making the comments section confusing and messy – Jon Story Aug 03 '16 at 12:45
  • @JonStory in that case could I recommend that you modify your answer to accomodate some of this discussion? Clearly there are parts in your answer that doesn't please other aviators. Most likely we just misunderstood your answer or you omitted some parts. – Masse Aug 03 '16 at 13:15
  • @masse If I do that, I'll be taking the discussion out of scope of the question. I may look at the wording later if I have time, to see if we can avoid some of the confusion though – Jon Story Aug 03 '16 at 13:36