Capt Traub's paper on jet inertia and horizontal vortices
jcjeant,
Don't take lomapaseo's riposte too hard. On the other hand, wasn't it you who were complaining (23rd March) about thread drift, after a discussion on possible use of AoA in recovery?
Have read through (Capt) Ellis Traub's 1966 piece once and, in case it helps anyone, am prepared to give one retired line-pilot's opinion on it. But it doesn't cast any new light on the AF447 inquiry.
Traub explained the inertia characteristics of the large jet transport well, bearing in mind that Eastern had only been flying them about six years; other US carriers not more than eight. Eastern, Wikipedia says, had lost a DC-8 in 1964, due to "degradation of aircraft stability characteristics in turbulence because of abnormal longitudinal trim component positions." This was reckoned to have happened while climbing through about 16000 ft.
For me, his argument weakens from the bottom of page 7. He poses a scenario where a horizontal vortex forms at 28000 ft, about 20 miles in diameter, with a circumferential velocity of about 300 kts. The transiting aircraft flies through one side, experiencing a rapid increase in headwind-component (to 300 kts). IAS is 280 on entry, and the pilot initially tries to maintain IAS or Mach by raising the nose. This is unsuccessful.
Traub argues that the uncontrollable rise in speed leads to the centre of pressure (lift) moving forward, leading to the pilot trimming forwards to counteract. My recollection is that, certainly on the early jets, an over-speed in Mach leads to the centre of pressure moving aft (causing Mach "tuck").
He also argues that the shock wave at the nose leads to unreliable airspeed/Mach indications, because the pitot tubes are mounted aft of the nose (unlike supersonic experimental aircraft).
During the vortex transit (about 2 minutes), he suggests that ground speed could have reduced to zero. Considering the TAS and G/S on entry were about 450 kts (my guess), and because of the aircraft inertia he is describing, I think that is a considerable exaggeration. But, on the other hand, we don't need to have ground speed as low as that to be in trouble...
Reduction in thrust (of engines mounted below the wing) increases the trim problem, in his scenario.
The aircraft emerges from the vortex with (a) massive loss of airspeed; (b) severe nose-down trim. He argues that the ensuing dive would be difficult to recover without eventually over-speeding and/or applying too much normal acceleration (pulling too hard). Fair enough. (The latter was Bernard Ziegler's main argument for removing the pilot option of "pulling" more than 2·5g on fly-by-wire Airbuses, but this does not apply if the FBW system has been sufficiently degraded.)
The paper may be badly flawed, but Traub's basic jet-inertia argument is universally recognised in relation to downbursts/microbursts close to the ground, and often practised in the simulator.
Whether the high-altitude, horizontal vortex he described is now a known phenomenon I don't know. but doubt it would be found in the vicinity of ITF (ITCZ) convective storm cells. If it did, the BEA and others would know.