Originally Posted by
The Bullwinkle
I’m still really confused as to why the MCAS is being held totally responsible for this crash.
Didn’t this flight and the Lion Air flight both have unreliable airspeed problems immediately after takeoff?
...
I’m honestly confused as to why everybody seems to be jumping on the MCAS bandwagon!
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.