Originally Posted by
yanrair
Gums says 'cutting accident crew some slack". So does everyone I think. If they made an error and we have no idea if they did, it would not be their fault but poor training. I have never met or heard of a pilot who sets out to do a bad job or kill himself. Other than the Pyrenees incident of co
course.
There seems to be a bit of a muddle about AoA sensors and Airpspeed disagree. They are not necessarily connected in any way.
AoA detects a stall by measuring the angle of attack with two sensors, one each side. Measuring the angle of airflow.
Speed is measured by sensing air pressure in a tube facing forwards (pitot)and converting that into Indicated Airpspeed. Not the same as real speed.
GPS measures real speed and is almost never wrong.
Say you have blocked pitot tubes as per aF 447, the AoA still works and stall warning still works.
Say you have frozen AoA vanes so that they are not able to detect stall, or incorrectly detect a stall (as is suggested in these Max incidents) then the Airspeed should still work.
Another scenario is that one airspeed is faulty, but the other two work - there are three airspeed indicators. That one is easy since you go for the two that agree. Backed up with GPS.
And in all these cases GPS will still work.
So the changes of losing AoA information AND Airspeed information at the same time are pretty remote.
This is just for information since there seems to be a misunderstanding about the functions of the various systems here.
There is one last way of determining speed. Pitch and power. 6 degrees of pitch and 60% N1 power will fly straight and level and at a safe speed NO MATTER WHAT ANY OF THESE OTHER SYSTEMS ARE SAYING. [True for lower levels]
The point is I suppose that this is a very complicated area but we can also overcomplicate it.
Yanrair
Actually, before you go any further,
Lion Air [and Ethiopian Air, and indeed Southwest as late as last November 2019], do not have AOA disagree indicator installed in their planes. So, they can absolutely see something was wrong but
NOT TO EXTENT OF AOA DISAGREEMENT/MALFUNCTION. However, they can absolutely see the indirect/direct effect of AOA disagreement: false stall warning and, unfortunately at the time, the still mysterious, ghostly, undocumented and deadly MCAS which thought to be the trim done by "
STS going the wrong direction due to speed difference", as the pilot had written it on their report about the incident in the previous sector, started their discrete 2.5-degree-every-10-second and absolutely unlimited trimming due to a faulty logic loop.
The main reason why the previous pilot failed to write the stick shaker and cutting off the stab trim,
probably because:
1. They thought the nose down trim was simply STS malfunctioning and trimming the other way due to air speed difference [i.e. IAS disagreement].
2. They are convinced that all the plane's problems came from whatever causing the IAS disagreement and unreliable ALT.
Little that they know had the stick shaker and stab trim cut-off incident/action were also being written, we'd end up having a different conversation right now [AND perhaps the MCAS thingy would've been remained hidden...].
In other words, we can just throw off all the discussion about malfunctioned AOA vanes or AOA disagreement stuff when we are trying to imagine about the "why and the what" of the Lion Air's pilot action, because they didn't have any means to consider that possibility.