Originally Posted by
Lonewolf_50
A "malfunction that fixes itself." Red Flag.
I'm not sure. In effect the aircraft was repeatedly trimming against them, now I've read some of the archived discussions on Speed Trim/ STS both here and elsewhere and there are plenty of comments saying (paraphrased) "it keeps trimming against me", and plenty of replies saying "that's what it is designed to do". So this could easily be misidentified as STS simply being more enthusiastic than usual.
We may know
now that this was MCAS, but that is knowledge the pilots didn't have. What they did have (on previous flight) was UAS, which might reasonably be expected to cause odd behaviour from the
speed trim system, plus stick shaker. So UAS is what got written up and "fixed".
If the pilots had the AOA disagree (an optional item they apparently didn't have) then maybe that would get written up, if they had known about MCAS maybe they would have suggested that as a cause and AOA would have been identified from that.
What besides (maybe) an AoA probe 'broke' in this case? Still unclear to me.
We don't know if the probe broke or was mis-fitted or why the previous probe was replaced. All we know is the the recorded value was 20deg out. I think these are analog signals so could even be a wiring fault?
Diagnosing malfunctions is, when a problem has persisted for flight after flight, something for a post maintenance check flight. (IMO) not a revenue flight. (Hence my question about MEL?)
But not an actual repair, it seems. So what is going on here: throwing parts in a problem, and hoping that fixes it? I'd hope the trouble shooting trees in the maintenance manuals would be a bit more expansive than that.
Maybe. depends on what actually broke.
Agree on check flight - but easy with benefit of hindsight. MELs and AMMs possibly unlikely to be helpful when the system showing the symptoms is undocumented and the sensor that is (maybe) broken gives no disagree error because the values are never shown to the pilots.