Originally Posted by
yanrair
What was the most accurate speed readout on the plane indicating, during AF447 and the recent two MAX incidents - that being the GPS?? In AF447 it was reading something like 450 kts at the time of losing IAS. It is not going to change unless you do something like change pitch or power. Which is what happened to AF447 of course. You can fly on GPS speed for a long time until you have sorted out the problem. You can fly an immaculate circuit to land using just GPS. Yet in all the posts so far I have not seen much reference to its use in sorting out conflicting IAS/Stick shaker style events. Climbing out at 15deg pitch, 200 kts IAS Full power. GPS will be reading something similar, depending on wind and altitude. All hell breaks lose ( I am ignoring MCAS here which is a separate matter). Indicated speed all over the place. IAS disagree messages. Stick Shaker going off (one side failure) - what to believe?? Your GPS PITCH AND POWER. They are real, they are going to work and are unaffected by the ADIRU which relies among other things such as AOA and Indicated Airspeed.
There is a THIRD and independant IAS speed tape on the flight deck, right above the gear announciators.
GPS delivers ground speed and is of little value, unless you are close to MSL in calm air.