Originally Posted by
Smooth Airperator
I want to know on what planet is a single point of failure (such as MCAS using input from only one AoA source) acceptable? Where has the concept of redundancy gone to in the year 2019? I would love to read the justification for this.
If you take the flying aircraft as a system, there was
not a single point of failure. Apart from AoA there was the flight crew that were expected to understand and cope with a Stab Trim that was repeatedly trimming down. There was an NNC for Trim Runaway also that would have prevented the accident.
It is obvious that the reliance on the crew (with all the other alarms demanding attention) to be able to switch the Stab Trim cut out switches to off was misplaced.
This raises all sorts of other issues. What happens if there is an intermittent fault caused by say a rat that got into the avionics bay and left an intermittent short, that had the same effect of max nose down trim for a few seconds every so many seconds? The Stab Trim switches are there for not only Stab Trim runaway - but I think that as far as most on this board are concerned they are Stab Trim runaway switches not for anything else - which is a real concern and a training issue. What other systems are
only learned in relation to a simple to teach fault response rather than being told this is one of many potential examples?
All automation on the aircraft is built in the expectation that if it all goes to worms the flight crew will sort it out. In Boeing switch everything off (including the Stab Trim) and fly manually; in Airbus drop into alternate and eventually direct law and fly the aircraft. It seems that the limited and highly repetitive sim training and the FOQA studying beancounters are making it difficult for pilots to be that final backstop.