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So, let us assume you are in a simulator for your favorite jet plane, and the instructor gives you a LOFT (Line-Oriented Flight Training) scenario that goes as follows:

  1. A normal takeoff and climb to cruise altitude, and perhaps thirty minutes of normal cruise flight
  2. The non-recoverable loss of speedbrake (flight spoiler) functionality due to a mechanical jam, or some other malfunction such as the loss of all SECs in a FBW Airbus.
  3. Another 30 minutes of time to deal with the first malfunction: run the checklists, plan and initiate a diversion if needed.
  4. You lose normal cabin pressure for some reason, and have to conduct a rapid descent to get back to 10,000' and complete your diversion.

How do you execute the descent, considering that the rapid descent procedures for most jets rely on using the speedbrakes to achieve a high descent rate without overspeeding the aircraft? To use the 737 QRH checklist for Emergency Descent as an example:

EMERGENCY DESCENT . . . . . . . . . . . . . . . . . . . . . .Announce

The captain will advise the cabin crew, on the PA system, of impending rapid descent. The first officer will advise ATC and obtain the area altimeter setting.

ENGINE START switches . . . . . . . . . . . . . . . . . . . . . . . . CONT

THRUST levers . . . . . . . . . . . . . . . . . . . . . . . . . . . . CLOSE

Reduce thrust to minimum or as needed for anti-ice.

SPEED BRAKE . . . . . . . . . . . . . . . . . . . . . . . . . FLIGHT DETENT

DESCENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Initiate

Target speed . . . . . . . . . . . . . . . . . . . . . . . . . . . Mmo/Vmo

If structural integrity is in doubt, limit speed as much as possible and avoid high maneuvering loads.

Level-off altitude . . . . . . . . . . . . . . . . . . . Lowest safe altitude or 10,000 feet, whichever is higher

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    $\begingroup$ Whenever I see a question title like this, I imagine some pilot somewhere hurriedly typing out his question hoping for an answer before he hits the ground... $\endgroup$
    – Baldrickk
    Apr 25 '18 at 8:58
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The answer is actually quite simple.

FOLLOW THE MANUFACTURES PROCEDURES.

In some aircraft (I don't have a 737 flight manual handy) there is an altitude limitation if the spoilers are inop for just this scenario. When the aircraft was certified, they needed to demonstrate an emergency descent (in the allowable time) without them, and that might not be possible from the certified ceiling. If that's the case in the 737, then descend to the max altitude with spoilers inop (assuming that you started above it).

If there is no altitude limitation and no special spoiler-inoperative emergency descent procedure (I have never seen one of those, but there's nothing to say that a manufacturer couldn't have one) then you simply fly the regular procedure without spoilers. The manufacturer had to demonstrate this during certification and it must have been acceptable at the time.

As rbp says, most of the time you fly Mmo/Vmo at idle power after you don the oxygen mask. IF the flight manual/checklist says that you can put the gear down (some do) then you may, but I wouldn't otherwise. I certainly wouldn't slow to gear speed (unless the manufacturer says to.... Unlikely in a jet like this) because you are decreasing the drag by slowing, and taking time at altitude to do so.

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    $\begingroup$ good answer. cruise in the flight-levels in my aircraft (M20M) is near or below Vlo (140), so I can put the gear down immediately. Vle is 165, so that's the airspeed of an emergency descent, with gear down and the speedbrakes out (which have no operating limitations). gets about 4000fpm descent, which is 4 minutes from the service ceiling of 25,000ft down to 10,000. $\endgroup$
    – rbp
    Oct 12 '14 at 20:02
  • $\begingroup$ @rbp Is that what the manufacturer procedure says to do for your airplane? $\endgroup$
    – Lnafziger
    Oct 17 '14 at 16:10
  • $\begingroup$ yes, and that's what I practiced at the Flight Safety Mooney course in San Antonio. I got a high-altitude endorsement in a 737 level D at Continental $\endgroup$
    – rbp
    Oct 17 '14 at 20:05
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Go to the top right corner of the envelope and stay there (point D in the sample below. Sorry, I don't have the 737 envelope at hand). Engines idle, speed and load factor as high as allowed. flight envelope

If airframe integrity is not critical, lower the gear to increase drag, but expect to lose the gear doors.

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    $\begingroup$ @UnrecognizedFallingObject: Not at all! Yes, power to idle, but then nose down and a steep turn, so the load factor goes up. Nose down as much as needed to stay at maximum speed. $\endgroup$
    – Peter Kämpf
    Oct 11 '14 at 6:58
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    $\begingroup$ @UnrecognizedFallingObject: Not like a split-S at all! Spiralling descent with down pitch at most 15-20° while keeping tightest turn allowed (60-70° bank, 2-2.5G load for typical airliner). $\endgroup$
    – Jan Hudec
    Oct 11 '14 at 16:20
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    $\begingroup$ @UnrecognizedFallingObject More like a steep spiral. A split-S is a roll to inverted and half loop to end up right side up with a 180 deg course reversal. You wont find that in any transport category abnormal procedures. $\endgroup$
    – casey
    Oct 11 '14 at 16:26
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    $\begingroup$ This is the correct answer for a non-transsonic aircraft. In transsonic aircraft, like a 737, the max allowable airspeed is not constrained by Vne, but by Vmo $\endgroup$
    – rbp
    Oct 12 '14 at 15:36
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    $\begingroup$ I would strongly recommend following the manufacturers recommended procedure (which was probably determined using this or a similar method, along with other considerations) rather than creating your own.... $\endgroup$
    – Lnafziger
    Oct 13 '14 at 17:32
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Whenever trying to rapidly descend, you always want to think drag!

Throttle to idle, lots of s-turns, (aileron drag), get down to gear speed, drop the gear, and lower flaps if practical.

A limited crabbing-like manuever (intentional side-slipping) would greatly slow the aircraft as well!

I'm sure that Terry will be here to recant us with a tale soon!

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  • $\begingroup$ Although I am hesitant to say flaps, because they increase lift. $\endgroup$
    – Bassinator
    Oct 11 '14 at 19:36
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    $\begingroup$ +1 for mentioning sideslipping and Terry. I am waiting myself ... $\endgroup$
    – Peter Kämpf
    Oct 11 '14 at 19:44
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    $\begingroup$ @rbp It won't flip, (unless you done goof), but you are correct, a sideslip is more accurate. $\endgroup$
    – Bassinator
    Oct 12 '14 at 16:49
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    $\begingroup$ "unless you done goof"? what does that mean? Next time you're in the air, initiate a 30 degree turn, keep feeding in the rudder on the downward wing side of the aircraft. Make sure you've got a parachute on, and are in an aircraft certified for aerobatics and are at an appropriate altitude. I'll post my unusual attitude training video later today. $\endgroup$
    – rbp
    Oct 12 '14 at 16:58
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    $\begingroup$ here's me in a skidding demo: youtube.com/watch?v=juu7JoSDE1A $\endgroup$
    – rbp
    Oct 12 '14 at 19:56
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There primary consideration in an emergency descent is to descend at the maximum allowable airspeed in the given configuration.

In a jet, this is based on the "barber pole" on the airspeed indicator (which changes based on altitude).

If the speedbrakes fail, you won't get the maximum descent rate, but you won't break the airplane by flying outside the aerodynamic envelope, either.

Here's a jet-type airspeed indicator with a red and white barber pole: http://www.md80.it/cockpit/cockpit/captain/kuvat/asi.jpg

(I have a high-altitude endorsements as per 61.31(g))

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