Originally Posted by
Compton3fox
Looking at the last data points: GS 376kts VS 30976 ft/min = ~305 kts. Decent angle is arctan(305/376) = 39 degrees. <please check my maths!> Based on the video evidence, it suggests that AOD increased markedly in the very last phase of the decent. Maybe up until this point, the A/C was relatively intact but suffered some significant break up close to the ground.
your maths is about right, and the aircraft has a resultant TAS down the flight path of 484 kts, At the lower levels that is an IAS of around 460-475 roughly, which is about 100kts over the Vne of the plane. At very high mach numbers, above the height where a change in the descent occurred to a momentary apparent recovery, the b737 wing reduces lift for a given AOA from the development of strong normal shock on the underside of the wing. a normal shock at high mach is expected, the 737 has a visible shock at around 0.78M, the Classic had one quite observable at at around 0.735. On the top, at speeds over MMo, the shock on the bottom of the wind results in a rapid loss of lift for a given AOA, which results in a decaying flight path, or a pretty hefty increase in AOA needed to maintain level flight.
Adam Air DHI 574, and Silk Air reached speeds higher than this aircraft, but both of those had very odd reasons for the upset. Adam Air was not far removed from Air Asias bad day out for causation. The usual testing limit for Mdive is around 0.06M above the proposed MMo, roughly. The B737 is not much fun at taht but it will do it, but 100kts over Vmo is a tall ask for the aircraft. The increased AOA needed to effect a recovery results in very high buffet and that is one of the items that impacts flutter boundaries.
The simulators that are certified for flight training do not properly represent the aircraft at speeds well in excess of Vmo/MMo, they give a sense of comfort that the MACH/CL, and MACH CM data does not support, the trim change is observable in the MMo-Md dive data. Back in the dark ages this was described as Mach tuck, and the explanation out of the AP3456A was related to the shock foot movement rearwards.... yes, that does occur, but the tuck comes from the development of the shock on the bottom of the wing, and that has a very abrupt loss of CL for the trimmed AOA, and that results in a stunning Cm change. The above just suggests that getting nose low in a jet transport goes bad pretty quickly, and the simulator does not have great fidelity for training in that area. For the occasions that I did tests out beyond MMo to Md the recovery was of enough concern that the flights were planned to have a high thrust level in the descent, so that if the tuck was excessive, a speed reduction from reducing thrust could be made. Some planes are just beautiful, like the Falcon, it is impeccable to very high speeds, with full authority without buffet. The B737 Classic has buffet that was odd given the rule, and the NG has a normal envelope that essentially alters to achieve compliance of the rule, through the little orange line that starts to drop down below the MMo indication, same for a few others, and that gives the compliance to the rule, albeit at a mach a little lower than the nominal MMo. In upsets, where the ATR is engaged, the thrust comes back rapidly to idle. If that hasnt occurred, then more has happened than can be explained by a simple upset. That info resides in the engine EEC memory chips, which have some possibility of having survived. The CVR and DFDR of this aircraft probably survived the average deceleration but any heavy structure that impacts them in the deceleration would have way above the 3400g test levels. The average deceleration would be around 2000-2100g, its about 50/50 the FDR or CVR will have taken higher impact loads.
All in all, would be surprised that the plane in this case was the trigger of the event, and an upset can occur without substantial crew error, or any action by the crew. An upset in severe weather can put the plane into a position that care is needed. The odd thing is that the most significant factor in the recovery of a high speed jet upset is the detection-recognition time of the crew before an effective recovery action is commenced. The only exception that comes to mind on that is the stunning survival of the B747 of CAL on its way to the USA that held together well beyond where it should have failed, it was pretty daggy by teh time it got to California, but it didn't splash down.