Originally Posted by
Loose rivets
I'm swinging between both extremes of scenario. However, that balance weight flopping near the end of ET's flight is hard to reconcile, even with Vic's logic.
That slight change of AoA at 05:41:22 - ish, is also troubling me. Why there? Why so consistent before and then after the change? i.e., if it can change at all, why would it remain steady either side of the change?
The correction applied for the difference between AoA vane and AoA body is dependant on Mach number, Pitch rate, Side slip, Flaps, Gear Position and Ground Effect. It is difficult to tell from the 6 figures shown in Fig 9 what form these computations take. Are they look up tables, computed functions, accumulated correction, or serial processed, or what? I’m guessing that they represent different correction lines for different values of the 6 parameters. In which case the single headed arrow line across the Mach number lines could show correction lines to correct AoA Vane to AoA Body from Mach 0 to 0.99, the double headed arrow on Sideslip could indicate + and – sideslip, but there is a single headed arrow for Pitch Rate. Did they not allow for a larger negative Pitch Rate? Could this have led to a negative index in a table? Could some of the computations overflow because of rogue parameters? It could be that one of more of the 6 correction factors used garbage function/table constants to calculate an AoA value of, say 434.5deg, which then ended up as a plausible, but unlikely, 74.5deg on the ARINC 429 bus.
So, the AoA values at the end of ET’s flight (and the 10deg blip at 05:41:22) could be due to the change in the Pitch rate that occurred then, which gave the computations a kick, resulting in different garbage values being picked up. If any of the tables/lines make use of the last computed value of AoA, this can lead to lock in to a particularly high (or low) value. The correction algorithm is far more complex than I first thought, once it goes wrong, any resulting behaviour for a value of AoA is possible!
Whatever damping there may be in the AoA sensor it has to be fairly light otherwise there would be a significant delay on the AoA signal and obviously the pilot does not want to be told he is approaching a stall condition after he has got there! More seriously, if the counterweight without a vane can really get from 35.7 deg to 74.5 deg in 0.75 of a second, then there is very little damping. I do not think it is feasible that the counterweight can then stop, nearly instantaneously, onto the 74.5 deg position, without any overshoot at all, or bouncing off of any end stop that is set at 74.5 deg. Naah, you need software to do that sort of screw up.
Originally Posted by
GarageYears
Well, how does that align with ALL the information?
At 05:38:44, shortly after liftoff, the left and right recorded AOA values deviated. Left AOA decreased to 11.1° then increased to 35.7° while value of right AOA indicated 14.94°. Then after, the left AOA value reached 74.5° in ¾ seconds while the right AOA reached a maximum value of 15.3°
The First Officer called out Master Caution Anti-Ice on CVR. Four seconds later, the recorded Left AOA Heat parameter changed state.
There is very little in terms of cause and effect that would satisfy the heater parameter change other than losing the vane itself. (Unless you want to believe this is entirely spurious).
- GY
Firstly, the AoA values are all driven by the correction algorithm, as I have theorised above, mainly the one using Pitch Rate as a driver.
Secondly, good point, I’ve no idea why the L AoA Heat changed state. I’m puzzled as to why the state was On at the beginning, when the outside temperature was 16deg C. However, I’m more inclined to believe that it was spurious than I am to believe there was anything other than a single underlying failure which affected all 3 of the flights.
Vic