Originally Posted by
syseng68k
Forgive me if I’m missing something here, but if the original driver for the TCMA solution was a mechanical problem, ie: possible fretting corrosion on a splined shaft, would not a simpler solution have been to address that alone, by either redesigning the spline coupling, and / or by testing, then limiting lifetime hours for that component, well within safe limits ?. Instead, it seems that we have a complex software solution, depending on several sensor pathways, that can only increase the possibility of failure, due to the increase in complexity ?.
I suspect that the move to FADECs probably eliminated that specific cause - the N2 sensor is going to be a much lower mechanical load of just a magnet going past a pickup. It's probably possible to also measure the rotation speed of the FADEC alternator by measuring the frequency it produces, though that might not actually be done.
However, the spline shaft is a known single point of failure, and just happens to be the one that failed in 1997. Others exist - Cathay 780 had fuel contamination that caused fuel valves to jam, for example. And tdracer has implied that there are numerous other possible causes.
Wouldn’t the same type of thrust lever management* together with a single WoW-sensor failure be able to command the shut down?
(I know that use of thrust levers is not very probable, but anyway).
I believe this is saying "if another crew threw the thrust levers into/out of reverse at the exact moment a WoW sensor failed", the engines would behave the same.
- Hopefully, the ANA incident we're discussing caused all engine vendors (presumably the same logic exists in the LEAP and GTF, for example) to re-examine their TCMA logic especially around reverse thrust and snap throttle movements
- Few crews are going to be using reverse at or after Vrotate.
It seems to me that the specifications are tighter than necessary if the requirement is only to match a human noticing the high thrust and activating a switch, but if it can be done reliably, there's no reason it can't be better than that.