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I fly a plane at 40000 feet. There is a sudden depressurization; what is the maximum vertical speed I can keep to come back at a proper altitude? Is this speed different for different emergency situations? (Engine stopping/fire/general emergency).

Are there any numerical maximum values in different scenarios for, for example, a A320 or a B737?

dalearn
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Gianni Alessandro
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  • Several thousand feet per minute. It is not possible to provide a precise answer to your question with the information provided. – acpilot Oct 06 '16 at 06:43
  • Your edit of the question now makes my answer seem out of place. – SentryRaven Oct 06 '16 at 06:51
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    Get as much drag out as permissible: spoilers, gear, etc. and descend as fast as you can. Mmo / Vmo. Most airplanes will get down in around 4 minutes to 10,000 FT MSL from their maximum operating altitude. – wbeard52 Oct 06 '16 at 06:56
  • @SentryRaven My question is "which is the speed"? So, I want a numerical value in some possible situations. And since I understand it may be too broad, I ask for a specific value of a generic airplane, for example A320 or B737. Why shouldn't be a legitimate question? – Gianni Alessandro Oct 06 '16 at 06:56
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    Your initial question asked " Is this speed different for different emergency situations? ". My answer was based on that. I'll let someone else answer this, you are changing your question every few seconds and I cant keep up with that. – SentryRaven Oct 06 '16 at 06:58
  • @SentryRaven My initial question, as you can also see from the chronology, was "which is the speed". My edits were done because you asked for clarification and then changed the question in order to let it fit with your answer. Let someone else answer this. It's okay. Thank you anyway. – Gianni Alessandro Oct 06 '16 at 07:05
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    Are you asking for the speed as shown on the VSI (Vertical speed indicator) which is a rate of descent indication, or an airspeed/mach number to fly while descending without exceeding structural limits of the aircraft/wing? – KorvinStarmast Oct 06 '16 at 11:42
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    In 747-100/200 aircraft, after you got the gear out, you kept the nose down to whatever angle was required to maintain mach .82 and then 320 kts IAS and accepted whatever descent rate that produced. I never bothered in the sim to monitor what the rate of descent was since that wasn't a controlling factor, but I think it pegged out the meter, which stopped at 6000 fpm. – Terry Oct 06 '16 at 19:54

4 Answers4

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enter image description here
(wikimedia.org) Part of a Boeing 747-400's autopilot control panel.

Maximum rate climbs and descents are achieved by using a speed climb/descent mode, where the pitch (nose up/down) controls a selected speed. The button FLCH (flight level change) activates such mode (shown above). Each manufacturer has its own name for it, but it's the same functionality. On an Airbus it is activated by pulling the altitude selector knob (Airbus calls it open climb/descent).

In level change climb, the thrust levers are engaged at the maximum thrust for a climb, the autopilot then controls the plane's nose to hold the selected speed. For maximum climb rate, a low speed (ideally Vy) achieves that, i.e., all of the climb thrust minus what's needed for the low speed is used for climbing.

For descents, the reverse is true -- fastest forward speed possible and idle thrust -- combined with the speed brakes. The plane will nose-down to achieve that speed. The rate of descent will be very steep initially, then it will ease off to hold the speed. Speed brakes add drag, so to achieve the same speed the aircraft will nose down more, resulting in a faster descent rate.

As the plane descends into the thicker atmosphere, the descent rate will again start to decrease -- as the plane will be producing more lift and drag. So we can't pinpoint an exact value, but 6,000-7,000 feet per minute is not uncommon.

Once a rate of descent is achieved, whether it's 1,000 fpm or 8,000 fpm, the g-force on the passengers will be 1 g, as there will be no acceleration in the vertical.

enter image description here
(YouTube) A Boeing 777 flight crew demonstrating such procedure.

  1. A lower altitude is selected
  2. FLCH is pressed
  3. The thrust levers are idled -- by hand is quicker than the auto flight system (this does not disconnect the auto thrust)
  4. The speed brakes lever is deployed
  5. A faster speed is selected -- the maximum allowable indicated airspeed increases as the plane descends into the warmer air where the local speed of sound is faster (thereby allowing a faster true airspeed, up to a limit).

The above applies for a rapid decompression, other scenarios would likely require a descent, but not that steeply.

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    Speaking from the standpoint of a 747 captain that retired in 1999, I find it almost unbelievable that the autopilot would be used in an emergency descent. Obviously times have changed, and autopilots these days are probably much better than those on the 747-100/200 aircraft. I never had to do an emergency descent for real, but the first thing in the procedure in my sim training was to simultaneously disengage the autopilot using the control yoke switch while pulling the power. – Terry Oct 06 '16 at 16:33
  • The A/T circuitry was still there, although disabled in some of the airplanes. In others it was still operable but not maintained, and we didn't use them except to maybe play a little. 741/2s had a significant drag rise at mach .86, so if you adjusted power to that speed, the thrust levers required almost no adjustment. And, of course, the flying pilot would usually simply tell the f.e. to "give me .86". I typically didn't touch the thrust levers after takeoff to the top of descent unless the f.e. was asleep. – Terry Oct 06 '16 at 19:34
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    just nitpicking now, but best climb rate speed (Vy) is not lowest speed. It is 'low'' but still quite a way faster than 'lowest', wether you mean Vmd or Vs – Radu094 Oct 06 '16 at 20:11
  • @Terry I'm thinking in an emergency descent would be the best time to use the autopilot. You already have enough problems already, and with three button twists I can tell the aircraft to descend as quick as possible, while deviating from the airway simultaneously. Not a bad deal. Sorry if it sounds like I'm advocating against manual flying. – Radu094 Oct 06 '16 at 20:15
  • @ymb1 still nitpicking to point out that climbing (in a motorised a/c) involves using extra engine power to gain potential energy. not swaping kinetic for potential (which is what a glider does). Same for descending is losing potential energy to drag, rather than converting it to kinetic which will just speed you up into a destructive dive – Radu094 Oct 06 '16 at 20:29
  • @Radu094 Why fumble with autopilot settings in an emergency? A rapid descent is simple enough to be manually flown: Cut power, deploy speed brakes, push yoke or stick forward until you fly at the maximum allowed speed for that configuration and weather. Control speed with the stick. – Peter Kämpf Oct 06 '16 at 20:54
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    @PeterKämpf well, because I would think this is a time when one can use all the help one can get. Any maneuver is easy enough if you think just maintain speed and bank angle. With AP taking the plane down (and gettig me off route) I can check oxygen mask and flow, establish comm with crew, check everyone is alive back, declare a mayday, check off route mora and start planning an escape route if above 10,000 , there are probably 17 other important things I'm not even thinking about – Radu094 Oct 06 '16 at 20:58
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    @Radu094 I think the best thing to do is to use the procedure that you were trained for and thus most comfortable with. In my case back in the previous century (that really sounds like a long time ago) we were trained to not use the autopilot, and we were comfortable with that. Why they now train to use the autopilot I can't say, although I imagine the superior capabilities of today's autopilots over what I used probably are part of the reason. Our emergency descents were low workload for the flying pilot. Non-flying pilot did the comm, f.e. did the outflow valves, fuel, talking to the purser. – Terry Oct 06 '16 at 21:15
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    @ymb1 I watched the 777 youtube example. Some things I noticed different from what we used to do are: (1) we always started from an explosive decompression or the cockpit-altitude-above-10,000 horn going off, (2) the nose didn't go down very far and didn't get started down quickly (we started with at least a 5 degree immediate nose down, (3) there was no shaking with the speed brakes deployed (the 747 shakes noticeably with full speed brakes), (4) really fancy O2 masks, (5) the cockpit stayed quiet (not the case in an old 747). All in all, just another day at the office. LOL – Terry Oct 06 '16 at 22:20
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    @Terry One reason I could think of for using the autopilot is in case there was a problem with the oxygen system. If the pilots started suffering hypoxia effects or even passed out temporarily the plane would continue descent down to a safe altitude and stop there until they came to. Sudden decompression can damage hearing and possibly introduce a lot of noise that would hamper crew communication. – TomMcW Apr 22 '18 at 18:11
  • According to FAA regulations, the descent must be accomplished in less than 10min. If the situation allows the use of the autopilot the pilots can focus on other tasks during the 5-10min amusement park ride - making it the best use of crew resource management. – jwzumwalt Jun 01 '18 at 07:02
  • @Terry Hi Terry, I tried to access http://TalkingOfFlying.terryliittschwager.com , but it is down. I would love to read your stories. – Cloud Jun 01 '18 at 08:31
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    @Cloud Hmm, it appears I screwed things up when I changed things so that it can be accessed using talking-of-flying.com, which is working, but I will fix the old way of getting it was well. You might also be interested in the 1990s Flying/Email Journal link accessible through the talking-of-flying.com page, especially the links in the cyan color. We should probably delete these comments after you read this as they are not directly related to the post. – Terry Jun 01 '18 at 19:43
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In a rapid depressurization, you'd typically descend at MMO and transition to VMO until reaching your level-off altitude. This is accomplished with the speedbrakes extended. What vertical speed you'd get is unknown; essentially you take whatever rate this procedure gives you. Initially, it might be 6000+ FPM, after transition to VMO, maybe a little less than that.

But, you set an airspeed, rather than a vertical speed.

Edit:

  • MMO = maximum Mach Number (often, .82 or above, depending on aircraft type)
  • VMO = maximum Indicated Airspeed (often around 340 knots)
Ralph J
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  • The vertical speed is your horizontal speed divided by the actual L/D at the given speed and configuration. With spoilers out this should be less than 10. Details depend on the type of aircraft. – Peter Kämpf Oct 06 '16 at 20:49
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I can't answer for to 40,000 feet (FL400—Flight Level 400—in pilot parlance), nor to an A320 or B737. I can answer for a piston engine airplane with retractable landing gear at FL250, though.

As a private pilot, I frequently fly our Mooney 252 (M20K) at FL250. I did an emergency descent drill, running through the procedures to initiate a maximum-rate safe descent, something I would do if I had a fire in the airplane or an oxygen emergency. (The 252 is not pressurized: I have supplemental oxygen on board, delivered to my face using an oxygen mask.)

The procedure: power to idle, speed brakes deployed, landing gear extended ("down"), 160 KIAS (knots indicated airspeed), propeller control full forward (increases drag). The 252's maximum speed with gear down is 165 KIAS, and I want that little bit of extra margin. My vertical descent rate exceeded 5,000 feet per minute.

There are other scenarios, though, for "emergency descents": you might want to go as far along the ground as possible (engine failure, glide to a distant airport for landing), when you want to fly at the best glide speed (compensated for wind and for actual airplane weight, ideally); you might want to stay aloft as long as possible (engine failure right over an airport, and you want the time to attempt a restart or to prepare more thoroughly for the landing), so you'll fly at the best sink speed. In my airplane, best glide is at about 85 KIAS, best sink is about 75 KIAS.

ammPilot
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  • you can probably increase that 5000fpm to 7000 or more if you put a boot full of left rudder and opposite aileron to slip the a/c. The noise will be horible at 160kts though if you survive it will probably be a fun story to tell – Radu094 Oct 06 '16 at 20:23
  • Interesting thought, @Radu0904: add a slip to the maneuver. I don't know that the noise would be so horrible. I have used slips to lose altitude quickly on final when needed (airplane without flaps, or when all the drag on the Mooney is already deployed), and the noise isn't at all horrible—but, that's at about 80 KIAS, not 160 KIAS. Next time I'm up doing some emergency descent drills, I might add this and see what the descent rate is. – ammPilot Oct 07 '16 at 23:20
  • The best configuration and technique depends on the airplane. In a Navion I owned, I could only get about 1200fpm using flaps and gear at Vfe (max flap speed) . I was better off (2000fpm) staying clean and side slipping at top of green Vno (normal operating speed). – jwzumwalt Jun 01 '18 at 07:16
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The Mesa E175 that had an emergency descent about a year ago peaked around 7000 fpm, in total taking 6 minutes to descend from FL340 to 10,000 ft.

enter image description here
(flightaware.com)

In light of the comments above about MMO/VMO and lift/drag limiting descent rates, you can accelerate descents by banking the aircraft into a turn. This allows you to increase angle of attack, thus increasing drag to keep speeds down, while rotating the lift vector away from the vertical so it's not working against you. The combination allows for higher descent rates with the same MMO/VMO limits. Some operators train for this, but my impression is not many, and it tends to scare the passengers.

Mark
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