krujje wrote,
In aircraft I've seen, TR deploy on take-off is considered a catastrophic event, therefore effort is made through system reliability and redundancy to ensure the probability of it happening is better than 10-9.
True regarding the probability calculation, hence the emphasis on system reliability. However, only catastrophic if the problem is not identified as such, the very remote event of an uncommanded TR deployment.
With hindsight, a Fokker 100 (TAM flight 402) accident on Oct. 31, 1996 could have been prevented if the crew had identified the problem to be an uncommanded #2 TR deployment. ATS disconnecting and the thrust lever retarding to idle (of the affected engine) are indications for such an unlikely event but apparently were not recognized as such by the crew. Whatever the perception of the crew was regarding the situation, human factors and situational awareness become vital elements. In an action to correct the problem, the thrust lever was advanced in order to "restore" thrust on the #2 engine, which consequently made the aircraft uncontrollable.
No alerts were presented to them as would have been anticipated to occur in case of a TR deployment during most phases of flight. But TR alerts are inhibited above 80kt until 1000ft AGL on this aircraft, the philosophy being to prevent high workload during the critical phase of a takeoff. Training and the AFM/FCOM cover this issue as part of the design and certification process.
Investigation later revealed there was a design flaw (a dormant failure) in the electrical control circuit which may have contributed to the uncommanded deployment. The electrical control circuit was later revised which now makes such an event even more remote.
Summarizing, if the symptoms of an ATS disconnect and a retarding thrust lever had been recognized for what this implied, corrective action would have been taken and the aircraft would have remained controllable with a deployed TR using the procedures in place to cater for such a remote event.
As is usually the case, hindsight has the benefit of 20/20 vision.
Regards,
Green-dot