Unplanned descent speeds
There is a common misconception on this and other threads relating to max achievable rates of descent.
An excellent post by ‘ fdr’ explained in some detail about the aerodynamic effects of subsonic airliners exceeding their certified Mach No.
I think that post or a previous post also explained that flight simulators are not programmed to accurately simulate these situations outside the normal certified flight envelope.
If you inadvertently roll more than 90 degrees of bank in the cruise, and stay in trimmed flight there is no lift up, it’s sideways. If you manage to roll near to inverted, then all the lift of the wing will be towards the ground.
As the speed builds up rapidly, the lift also increases rapidly, accelerating you down.
If you happen to notice the unusual attitude you unload the wings and roll to the nearest horizon, and begin a smooth pullout.
If you’re still distracted, planes will normally enter a spiral descent, but if fully inverted (180 degrees of bank ) you basically start flying the second half of a loop. Without any pilot input, just from the trimmed state of the aircraft.
In these situations the engine thrust is almost irrelevant, full thrust is nowhere near the weight of the aircraft. In the cruise at altitude full thrust is maybe just above 1/20 of the aircraft weight. Airliners have a lift drag ratio of maybe 1/20. Maybe a bit better with more modern aircraft.
But you get the idea, the weight of the aircraft is significantly more than thrust from the engines.
As the plane approaches high transonic speeds , approaching Mach 1.0 you get a dramatic drag build up, Critical Mach drag rise, or whatever you wish to call it.
Airliners are bluff objects designed to fly subsonically, so this extreme drag makes going supersonic very difficult.
So generally the max rate of descent will be about Mach 1.0
As an aside, using the FlightRadar24 altitude readouts, the rate of descent corresponds to about a 1,000 ft a second. 5,000 ft lost in 5 seconds if I read the numbers correctly. Or 60,000 ft a min or 685 mph. Local speed of sound is 707 mph assuming a standard atmosphere and a temperature of -25 C.
So vertical speed M 0.97 , this is only the vertical speed using the data. As stated by previous posts the data from FR24 can be smoothed or assumed. When you allow for the horizontal ground speed to get the true airspeed and Mach No. the numbers are higher.
With previous Jet upsets when the data is pulled and all the circumstances are allowed for, the max speeds always seem to be subsonic, or mildly supersonic.
As ‘ fdr ‘ stated as you descend into thicker air it is also warmer, which means the local speed of sound increases too. So for a given speed, the aeroplanes Mach No will decrease. As you descend into warmer air you should get more control.
There are two big issues with these high speed descents.
1:you can rapidly exceed the aircrafts design max speed which can lead to structural failure.
2: you need a lot of sky to pull out of a high speed descent.
Lastly, I have read that most ‘ Jet upsets ‘ that occur in visual conditions are survived, but those in cloud or at night with a limited visual reference are not.
From my aged memory the CAL 747 that had a ‘ jet upset ‘ was only flown correctly when the crew got a visual horizon at about 20,000 ft. I note that for this unfortunate flight, the cloud base was much lower.
P.S. My credentials; I can’t spell but have flown commercially for far too long.