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This answer to a question about emergency descents notes:

[Y]ou 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....

Banking seems like an obvious way to increase drag and accelerate descent when the only objective is to get to a lower altitude (e.g., due to rapid decompression). So does forward slipping. Is either ever indicated in manuals or emergency checklists?

I was wondering if such maneuvers might actually be contraindicated: Isn't MMO determined by the stress limits of the airframe? If so, does MMO (in theory, if not in practice) assume some limit on angle of attack, and would MMO decrease at high angles of attack or slip?

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The problem is that in a deliberately entered spiral dive it is very difficult to keep things in check. In an idle descent, pitch is your only speed control (well... really, in the end, pitch is your speed control all the time; any glider pilot understands that), and in a spiral dive trying to slow down by increasing pitch is trickier because a lot of the pitch input just steepens the spiral and you just pull more G instead of slowing down. Which is the whole reason spiral dives are to be avoided in the first place, especially in the IFR world.

In a jet in an emergency descent you will generally dump flight spoilers with the engines at idle and push over to accelerate to MMO/VMO. An attempt to maintain MMO/VMO in a spiral dive turn would probably end poorly. If you are diving straight at VMO, spoilers out, and want to get down faster, depending on the airplane you might get a faster descent by slowing to gear extension speed and extending the gear, but I don't think most manufacturers recommend that and it's not something I was trained to do (on RJs).

Sideslip doesn't come into it as sidesliping swept wing aircraft deliberately is very dangerous (it doesn't take much skid/slip to make a swept wing airplane want to roll on its back), and nasty dutch roll situations can result. In any jet, once you are off the ground, the yaw damper system takes care of adverse yaw and any random disturbances, and you never touch the rudder pedals unless an engine quits. It's generally feet on the floor after the initial departure profile until final approach.

In any case, in an emergency descent you are way above maneuvering speed so pulling too hard will fold the wings before stalling AOA is reached and trying to get more descent rate by spiraling would be suicidal.

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Isn't MMO determined by the stress limits of the airframe?

No. It is usually determined by the onset of “Mach tuck”, the transsonic loss of lift caused by the flow speed over the wing becoming supersonic in too large region.

It may be also determined by the TAS limit due to onset of aeroelastic flutter, because Mach number follows true airspeed more closely than indicated airspeed.

Both effects are pretty dangerous. The pitch-down moment associated with Mach tuck might put the plane in irrecoverable dive, at least until it gets much lower to warmer (higher speed of sound) denser (higher drag) air, but in the meantime it will likely exceed some other limit and might get damaged.

And aeroelasic flutter will break the plane apart if left to continue for some time.

If so, does MMO (in theory, if not in practice) assume some limit on angle of attack, and would MMO decrease at high angles of attack or slip?

Actually I believe it would. If the wing is forced to generate more lift, the flow speed above it will be higher and it will separate earlier.

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