Originally Posted by sensor_validation
Is it it just a coincidence that it all seems to be coincident with the selected Mach drop from M0.82 to M0.80? This seems to have been commanded at 2:09:50, unfortunately the plot of "Navigation Parameters" on English P109 is corrupt while the selected Mach is 0.82 - as mentioned before it appears they are plotting 0 when no update.
Nothing looks corrupted here. Change of Mach mode "Selected", set at Mach 0.80, occured at 0209:58 (not 0209:50), only 7 seconds before UAS. Previously, it was Mach 0.82 and the blue line drop is readable behind the "Mach Number" green line. Before this change, the flight guidance mode was different ("Managed"), hence it is painted (in blue) differently with down lines, but the top of blue block is showing Mach 0.82 until this point.
I think that it is not a coincidence as the turbulence level slightly increased, hence aircraft speed was reduced... while the phenomenon occured at this point. The conditions were changing: feeling of warming, smelling, noise, etc. The very same elements were also described during other flights encountering UAS conditions.
Originally Posted by sensor_validation
The engine N1s start to ramp down from 2:10:00, but seem to have little effect on the indicated speed - but presumably is responsible for the co-ordinated drop in pitch angle? The longitudinal acceleration suggests the plane is slowing (although I don't understand the negative bias on the plot).
Maintained airspeed is primary due to momentum rather than immediate thrust change at 0209:58. In fact, it seems that the first result of this thrust reduction was a slight drop in pitch, down to zero or slightly below, which was followed by a small acceleration, small loss of altitude, slightly negative vertical/speed and also an increase of ground speed (turbulence can play its role also). This looks consistent with airspeed slightly increasing up to 0210:07, when ADR1 became invalid. Hence longitudinal g should have followed the same trend. Once this inertia moment was consummed, pitch increasing, low thrust and climb started altogether to bleed airspeed. But this was not effective before five or six seconds following A/P disconnection (about 0210:11, when gentle descent stopped and climb was resumed).
Originally Posted by sensor_validation
It appears the ISIS calibrated airspeed when recovered after the dip is "stuck false high" in period 2:10:10 to 2:10:13 - during which the engine N1s are locked low - but groundspeed doesn't drop off much (perhaps due to plot scale).
I don't think that ISIS is either "stuck" nor reading "false high" at this point. Last valid ADR1 was 274 kt at 0210:07 and climb was still not to be resumed until 4 seconds later, hence, 270 kt on ISIS between 0210:10 and 0210:13 doesn't seems false reading but actual airspeed.