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For how long can a commercial airliner fly at a 90deg roll angle ? Or can it at all ?

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    $\begingroup$ Depends on what you consider to qualify as flying. Does it have to stay at the same flight level? (spoiler: it won't) Or are you interested in the sink rate in this attitude? $\endgroup$ – AEhere Jan 7 at 12:56
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    $\begingroup$ FWIW, I don't think this is primarily opinion-based. Data may be hard to come by, because few people are likely to be crazy enough to pull a stunt like that (and even fewer are likely to be in a position of being able and willing to allow people to pull the stunt), but that just makes the question difficult to answer, it doesn't mean that answers cannot be based in fact. That said, as AEhere pointed out, an Edit to clarify what constitutes "flying" in this context would likely help answerability. $\endgroup$ – a CVn Jan 7 at 14:00
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    $\begingroup$ Airliners being able to perform 25 sec parabolic flight, could roll 90 degrees at start and end of parabola. Assuming 15deg/sec roll rate, there is 13 seconds left of 90deg bank angle ballistic flight. $\endgroup$ – qq jkztd Jan 7 at 16:22
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What you're talking about is called "knife edge" flight, where all of the lift is coming from the sideways fuselage, making it a lifting body aircraft with big fins (the wings) sticking up and down that aren't doing anything (rolling sideways during a ballistic arc doesn't count). The rudder is doing the job of the elevator, and the elevator is doing the job of the rudder.

To do this is requires a fairly deep fuselage profile, preferably reasonably flat sided so it makes a decent lifting body, with a really powerful rudder to effectively function as the elevator for this lifting body, and it's helpful to have a high power propeller engine on the nose helping with the lift generation.

Just about the only airplanes that meet those characteristics are purpose built aerobatic machines that can sustain knife edge flight indefinitely or some military jets. Certainly no airliner could ever do that.

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A commercial airliner will not support sustained coordinated flight with a 90 degree roll attitude. This is because the vertical lift at 90 degrees is greater than the weight (and therefore lift needed) to support the aircraft.

In general, aircraft with can perform sustained flight in a 90 degree roll attitude will have either some way of creating lift in that direction or will use thrust vectoring or both. Commercial airliners do not have these attributes. Actually very few airplanes do.

So in the case of a commercial airliner, in a 90 degree roll, one can expect that it will be "falling" towards the ground, with increasing velocity as it accelerates. This is because it generates insufficient vertical lift in that flight attitude.

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Boeing considers 90 degree bank to be an out of control event, and this paper discusses methods to recover.

https://www.boeing.com/commercial/aeromagazine/aero_03/textonly/fo01txt.html

HIGH BANK ANGLES. A high bank angle is one beyond that necessary for normal flight. Though the bank angle for an upset has been defined as unintentionally more than 45 degrees, it is possible to experience bank angles greater than 90 degrees.

Any time the airplane is not in "zero-angle-of-bank" flight, lift created by the wings is not being fully applied against gravity, and more than 1 g will be required for level flight (figure 6). At bank angles greater than 67 degrees, level flight cannot be maintained within flight manual limits for a 2.5 g load factor (figure 7). In high bank angle increasing airspeed situations, the primary objective is to maneuver the lift of the airplane to directly oppose the force of gravity by rolling to wings level. Applying nose-up elevator at bank angles above 60 degrees causes no appreciable change in pitch attitude and may exceed normal structure load limits as well as the wing angle of attack for stall. The closer the lift vector is to vertical (wings level), the more effective the applied g is in recovering the airplane.

A smooth application of up to full lateral control should provide enough roll control power to establish a very positive recovery roll rate. If full roll control application is not satisfactory, it may even be necessary to apply some rudder in the direction of the desired roll.

Only a small amount of rudder is needed. Too much rudder applied too quickly or held too long may result in loss of lateral and directional control or structural failure.

NOSE HIGH, HIGH BANK ANGLES. A nose-high, high-angle-of-bank upset requires deliberate flight control inputs. A large bank angle is helpful in reducing excessively high pitch attitudes. The pilot must apply nose-down elevator and adjust the bank angle to achieve the desired rate of pitch reduction while considering energy management. Once the pitch attitude has been reduced to the desired level, it is necessary only to reduce the bank angle, ensure that sufficient airspeed has been achieved, and return the airplane to level flight.

NOSE LOW, HIGH BANK ANGLES. The nose-low, high-angle-of-bank upset requires prompt action by the pilot as potential energy (altitude) is rapidly being exchanged for kinetic energy (airspeed). Even if the airplane is at a high enough altitude that ground impact is not an immediate concern, airspeed can rapidly increase beyond airplane design limits. Simultaneous application of roll and adjustment of thrust may be necessary. It may be necessary to apply nose-down elevator to limit the amount of lift, which will be acting toward the ground if the bank angle exceeds 90 degrees. This will also reduce wing angle of attack to improve roll capability. Full aileron and spoiler input should be used if necessary to smoothly establish a recovery roll rate toward the nearest horizon. It is important to not increase g force or use nose-up elevator or stabilizer until approaching wings level. The pilot should also extend the speed brakes as necessary.

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