Lots of pointless deliberation hereabouts about the configuration/attitude etc of AF447 at impact. I'd suggest that it's fairly obvious that it spun in in a flat spin - and that airbrakes/speedbrakes/spoilers (at least) were deployed out of desperation. It's more productive to backtrack to why/how this accident happened..... particularly as they are unlikely to ever recover the DFDR (or CVR). The effort this month is in reality a nominal and token, yet enforced, one. It has only a very slim chance of any success.
.
from the post by Noelbaba (
The Last Four Minutes Of Air France Flight 447 ). This exposition is a confused mish-mash - and if it has an authoritative sourcing, it is indicative of an attempt to muddy the waters with a welter of disinformation.
.
Three critical aspects mentioned, yet hidden away (and
AFAIK) not yet commented upon indepth here on Pprune, still require logically derived answers:
A. Suddenly the gauge indicating the external temperature rose by several degrees, even though the plane was flying at an altitude of 11 kilometers (36,000 feet) and it hadn't got any warmer outside. The false reading was caused by thick ice crystals forming on the sensor on the outside of the plane. These crystals had the effect of insulating the detector. It now appears that this is when things started going disastrously wrong.
QUES: 1. How does this falsely high reading of TAT affect the ADIRS speed-readouts/data inflows? (if at all) i.e. before considering the Thales pitots' complications?
2. Is it possible that the TAT sensor's heating can be overpowered by an accretion of supercooled ice crystals to the same extent as the Thales pitot heads' pitot heaters?
.
ANSWERS: (probably duff, but that's why I'm asking): -
Answer: 1. If the ADIRS TAT data inflow is falsely high then that (at a constant thrust and IAS) would generate a lower TAS, but that high temp wouldn't affect the IAS (which is after all what predicates the aerodynamic stall speed). "To hold a constant Mach Number (considering that temperature is decreasing and TAS increasing thus MN increasing) you would have to raise the nose (reduce your IAS/CAS) and increase your angle of attack to climb at a constant Mach Number/hence IAS decreases?"
Again, temperature has no effect. At a constant Mach Number climb, both CAS and EAS are reducing, and increasing Angle of Attack is required. The logical extrapolation of this is that if you maintained a climb at constant Mach Number, assuming that you had a lot of excess thrust, you would climb all the way to where the AoA went all the way to the stall. What you have to remember is that the Airspeed Indicator measures Impact Pressure, which is the total sum of Dynamic Pressure (related directly to EAS) and compressibility (related directly to Mach Number). It does not measure Density, which is related to Pressure and Temperature. In fact, all of the manometric instruments on the aircraft, the ASI, Mach-Meter, Altimeter, and VSI only sense Pressure.
You, the pilot, have to apply Temperature to Pressure Height to find Density Height and True Altitude, and to CAS and/or Mach Number to find TAS.
During climb, with decreasing pressure, the aircraft must fly faster, i.e. a higher TAS, to achieve the same Impact Pressure, because you're at a constant CAS. This is so, even above the Tropopause where there is no change in temperature with change of Pressure Height (except in the case under consideration here). Because the aircraft is flying faster, compressibility increases, you are experiencing an increasing Mach Number. Note this - On a very hot day, your TAS will be higher due to the lower density AS WELL AS the lower Pressure, but this temperature factor puts you not one jot closer to the speed of sound, because at the higher temperature, the speed of sound (M1.0) is ALSO higher (but, however, NOT in the false TAT case). Thus, the 'hot day effect' of a higher TAS than that due to pressure change alone, has no effect upon Mach Number whatsoever...... but it WOULD HAVE for AF447.
Consider the formula for CAS calibration -
Vc = SQR ((Y/(Y-1)) * Po/Qc * [(Qc/Po+1) ^ ((Y-1)/Y)-1]) * SQR (2 Qc / Rho0)
Where –
Vc = Calibrated Airspeed in ft/sec,
Qc = Impact Pressure in Lb/ft^2,
Rho0 = Sea Level Air Density = .0023769 slugs/ft^3,
Po = Sea Level Air Pressure = 2116.2 Lb/ft^2,
Y = A constant for air = 1.4, being the ratio of the Specific Heat of Air at constant pressure to that at constant volume (Y = Gamma, I don’t have a Greek key-board)
The Left-most portion of the equation is the ‘f’ factor, the compressibility effect –
F = SQR ((Y/(Y-1)) * Po/Qc * [(Qc/Po+1) ^ ((Y-1)/Y)-1])
At altitude, the Static Pressure, P, is substituted for Po in the left-most radical.
Temperature does not even get a mention (except in establishing standard Sea Level Density for calibration purposes, and then it's a constant). However a falsely high temperature might induce the ADIRS to assume a lower thrust and/or TAS/Mach and so cause the autothrust to incrementally offset the (ADIRS) perceived under-performance. Net effect would be to cause AF447 to fly faster than "indicated" (to the pilots) - and thus, in TAS terms, even closer to the coffin corner envelope. You then only need the Thales pitot clog-up over time to create a coffin corner encounter (see theShadow's post at link
http://www.pprune.org/rumours-news/3...ml#post5537109 for the outcome of that cumulative {over time} additive error). The conclusion is that a freeze-over of the TAT sensor would have put them, courtesy of the ADIRS, at a higher actual TAS. Usually the temperature isn't a factor or a consideration, except when it's a "phony" one.
.
Also, in this regard, the article claims fairly authoritatively that AF447 was heavy (i.e. cargo and pax-laden) so went early direct to best height (FL350) looking for optimum SFC (i.e. to "make" fuel and avoid any enroute drop-short of destination reserves requirements.). That in itself would have quite early put a heavy AF447 closer than normal to the upper RH edge of the operating envelope and that much closer to a Mach Crit encounter. The fact that they never reported "Level FL350" - but only read-back that final cleared height - may indicate that they levelled and soon thereafter accelerated straight into a compressibility encounter..... having already accumulated some of the icing-induced errors on climb.
____________________________________
Answer: 2. Once again depends (as theShadow points out with respect to the pitot heaters) upon the heating capacity of the sensor's protection. Having atmospheric precipitation freeze upon impact to form ice is quite a different thermal proposition in aviation to accumulating an agglomeration of already formed supercooled ice crystals over time during a relatively smooth cruise in Cirrostratus/Cirrocumulus. But in the Thales pitot head, by contrast, the tendency was to "collect" the ice crystals inside a tube and thus gradually (insidiously) clog the pneumatic flows and sensed pressures. An assumption can be made that a false high TAT (and consequent high undetected TAS), due to a thickening sensor ice-coating, was a possibility.
____________________________________
QUES: 3.
From the article: "The crew immediately recognized that the three airspeed indicators all gave different readings. "A situation like that goes well a hundred times and badly once," says Arnoux, who flies an Airbus A320 himself."
I sorta beg to differ here - entirely. I agree with theShadow that the effect was in fact quite insidious and that all three pitot heads were affected simultaneously and thus never "differed" - (thus no alert to pilots and nil breach of triple redundancy disagreement rules as far as the ADIRS was concerned). See ASW article on the A346 double flame-out due to fuel computer programming: "THRICE ALMIGHTY, the Virtues of Triple Redundancy" (at link
Thrice Almighty - The Virtues of Triple Redundancy | Air Safety Week | Find Articles at BNET
________________________________- Over to others who may be better informed/educated.
.