Originally Posted by
Bleve
In the aftermath of the AF447 accident, Boeing (and Airbus) substantially changed the Unreliable Airspeed non-normal checklist (NNC). The message that has since been repeatably reinforced is that if there are ANY abnormalities related to airspeed, pilots should NOT make any assumptions as to what indication is right and what is wrong. They should simply run the Unreliable Airpeed NNC methodically until the correct indications are determined. So a stick shaker activating after takeoff (due to a faulty AOA signal) will be treated (and reported to ATC) as 'Unreliable Airspeed'.
The accident scenario that we are discussing might run something like this:
- At rotation stick shaker activates (due to a faulty AOA signal).
- You see normal speed indications.
- Stick shaker at normal indicated airspeeds is NOT logically consistent. Post AF447, that is an Unreliable Airspeed event. (You are NOT jumping to any conclusions at this stage that the IAS is correct and the Stick Shaker is wrong).
- Climb to a safe height and begin the Unreliable Airspeed NNC.
- Report to ATC that you have Unreliable Airspeed (because that is the NNC that you are now working through).
- Manually set a known power and attitude for level flight (the NNC requires the autopilot to be disconnected).
- Unbeknownst to you, MCAS has now been activated because of the faulty AOA signal.
- Your efforts to trim the aircraft for straight and level flight are proving difficult (because of the hidden MCAS intervention). Confusion increases because you can't even establish straight and level flight to continue the NNC.
- You are soon overwhelmed.
bleve
my understanding was that the STAB running fully nose down wasn’t “hidden”_ the pilot tried repeatedly to trim the other way for ten minutes- isn’t that correct? It’s just that every time he trimmed nose up the STAB trimmed nose down and was winning the tug of war. Apart from that your analysis seems entirely plausible. Yes- we revert to pitch power for safe flight once normal speed indications become unreliable. 15 deg pitch in initial climb. 6 deg pitch straight and level.
When all indications fail you pitch power are always valid. And of course GPS which is largely ignored in these discussions but is a life saver
cheers
Y