Originally Posted by
FIRESYSOK
Crews may be aware now, but remember- the Lion Air accident were dealing with unreliable flight instruments along with the MCAS trimming. Merely shutting off the electric stab trim does not eliminate the bad air data indications on the affected side. In itself, a bad set of PFD indications can be a handful wether or not the stab trim is cut off by the crew as recommended.
Clearly, there is no final report from the Indonesians, but unreliable airspeed, etc. was one of the problems as far as I’m aware.
Originally Posted by
Sailvi767
What is there to fix. There are numerous failures that can cause a trim runaway. They have been happening since the first electric trim systems were installed in aircraft. The 737 more than any aircraft currently in service provides a very positive indication of a trim issue via the trim wheels. Solution, disconnect the automatic trim system and continue the fight using manual trim.
Originally Posted by
canyonblue737
it is not dissimilar from a trim runway though, MCAS can be disabled in seconds using the standard trim runway procedure that has you flip two switches on the center pedestal.
Yes, sounds easy if you're 100% confident that MCAS is wrong. But, MCAS is reacting to sensor data- the same data that the instruments in front of your eyes are relaying to you. If your AOA indicator goes haywire, or your ASI is showing lower than stall speed, then you'll have a difficult situation. What MCAS does... it controls flight surfaces to prevent a stall that is likely to occur given the sensor inputs that it's receiving. If you're flying you'll usually trust the instruments- the same instruments as MCAS. Maybe other MCAS interventions haven't been noticed or reported because the rest of the time it's actually functioned the exact same way as the pilots have operated anyway.
If you've just taken off on most other aircraft, if you get a low reading from ASI, or even stick shaker, your first priority is to try to control the aircraft, stabilize flight, level off if at a safe altitude, call a pan and go through the checklists... whereby you eventually should discover that the ASI sensor is bad, Air speed is fine and switch to the alternate input and go on your merry way. Why MCAS is scary is that it 100% relies on sensor data- you're now in the exact same situation as described above, with the exception being that your aircraft has just trimmed nose down by itself, any breathing space you had to troubleshoot has been eroded by MCAS nose down, and any time you could have used to figure out the problem is now spent trying to pull back on the yoke as hard as you can.
It's very easy to say "just pull a couple of CBs, disconnect trim switch, everything will be fine", but in those circumstances, you're on climb out of airport XYZ, suddenly the stick-shaker goes, the ASI in front of your eyes IS showing a reading that's abnormally 'low', the aircraft just trimmed itself nose down to prevent stall, before you have any chance to diagnose the problem you're fighting against the MCAS nose down attitude, your right hand man has 200 hrs all-in and is panicking.
Again, like I said earlier, absolutely not jumping to conclusions as to what caused this tragedy. Again, just re-examining the MCAS issue that has been brought up. Again, stating that MCAS in and of itself should not be a problem- providing the inputs are 100% reliable the software goes unnoticed and may even act as a safety net in the manner it was designed to be. Again, the problem that I see that I haven't seen highlighted previously is the hardware! MCAS relies on data, sensors are the source of the data, why are the sensors so fallible?? Why can't they spend more money on the hardware- ensuring sensors are foolproof with adequate redundancies that can always be relied upon?
Again, Southwest knew the problem and had a quick fix for their own 738 MAX orders, providing an additional AOA indicator for crew to easily cross-check if needed. Note that their statement on 738 MAX listed above is qualified- they're happy with 'their fleet' Possibly their quick & simple solution might be sought by other 738 MAX operators, or even demanded of Boeing to retrofit.