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[Chris Scott]

40 nm RADIUS FROM LKP − IS IT ENOUGH?

GreatBear suggests above that the crash might have been outside the 40nm radius from LKP (last-known position), on the basis of back-tracking of body-drift. In response to takata’s comparable “Probable Crash Zone”, posted here on 21st June 2009, FlexibleResponse stated here that “you also need to take into account that the human body does not float in seawater until about 72 hours after death. Then there should be some allowance for the time from establishing positive buoyancy to float from the wreckage at the bottom of the ocean up to the surface. Again some allowance should be made for the average sea current during the ascent time…” (etc.)

Not sure if FlexibleResponse’s assertion has been challenged since but, if not, it put the crash zone back into the circle of 40nm radius. So what about the limited evidence we have of possible flight-paths after LKP, based on evidence of ACARS messages and BEA analysis of aircraft debris?

At risk of re-hashing previous discussions, the implications of the BEA Tests and Research into ACARS messages (Para 1.16 of Interim Reports 1 & 2), as I interpret them, include the following:
02:10:10z (“AP-OFF” message received from ACARS) indicates aircraft at, or close to, planned cruise F350/M0.82, GS<480kt;
02:10:34z (routinely received from ACARS) LKP − FL and GS unconfirmed (?);
02:14:45z (inferred from absence of receipt of an expected ACARS transmission) − flight has ended;
aircraft was intact until impact at sea-level, and impact GS (not stated) unremarkable, so probably below 300kt.

So one may infer that the aeroplane descended 35,000ft in not more than 4min35sec, starting with a GS of, say, 480kt (8nm/min). In addition, it had to reduce GS to something below 5nm/min prior to impact.

This suggests a rate of descent averaging 7600ft/min. That is nearly three times the average VS obtained at normal descent speeds (prior to deceleration) with idle thrust, yet TAS seems to have been considerably lower at impact than at start of descent. Where did the drag come from; to dissipate so much potential energy, and some kinetic energy, so quickly? Spoilers/speed-brakes are unlikely to have been used, and there is no evidence that the L/G was ever extended.

[If the ADR used by the A/THR was grossly under-reading the IAS/IMN, thrust above idle may have been maintained − despite descent − until 02:10:47z (ACARS “A-THR OFF” message received), or even later if the throttles were left open at that point (thrust lock).]

There’s been speculation that the loss of FPV (02:12:10/16) would have been caused by TAS>599kt (10nm/min), although this datum would be from an ADR that had been delivering under-readings of IAS previously. If the relevant pitot had thawed out in 2min, for reasons unclear, excessive TAS remains a possible cause; although the “measured CAS lower than 60kt” may be more consistent with the likely scenario of a blocked pitot tube, coupled with rising static pressure (in the descent) from unfrozen static ports. The third possible cause of the FPV being disallowed, VS>20,000ft/min, is implausible on an intact aeroplane − unless previously frozen static ports were unblocking.

Overspeed - A/C Intact

Although it is possible that the aircraft stalled at high altitude, it seems unlikely with airspeed indications under-reading. Overspeed is more probable, particularly early-on. From the BEA’s comments on condition of the control surfaces recovered, this would not have been supersonic, so that limits average descent TAS to about 600kt (10nm/min). ** In this scenario, with a clean aeroplane, the speed reduction from over 600kt to 300kt − albeit achieved during a shallow descent − would have taken nearer two minutes than one, I suggest. That reduces the average TAS to a figure well below 10nm/min, and an average GS of a similar order. The alternative deceleration scenario of flare into a steep climb, followed by deep stall, would lower the average TAS further, presumably.
(Note: all this is predicated on absence of in-flight structural failure, and an impact time not later than 02:14:45z.)

Ignoring the unpredictable wind components, which are not likely to have amounted to much overall, it therefore seems unlikely that the aircraft would have travelled more than 40nm in the 4:11 after LKP, i.e., averaged a GS of more than 574kt (9.56nm/min). And, even if it could, is it likely to have travelled in one straight line, i.e., continuing north-eastwards along the airway?

Given the above, and inferring from the evidence that flight beyond 02:14:45z was unlikely, the 40nm radius of search from the LKP (last-known position) seems to suffice.

Chris

** If the descent stabilised at, say, M0.95, the IAS/CAS would rise rapidly through VMO; the TAS rising at a much lower rate. Considering the BEA has not reported tell-tale signs of extreme dynamic pressure to the tail fin (V/S) and flying controls retrieved so far, one must assume that the descent TAS at lower altitudes would have been considerably lower than 600kt. No doubt, various computer models will have been run by the BEA, not to mention trials on flight simulators.