Originally Posted by
syseng68k
SomeoneSomewhere:
Thanks, so what you are saying is that TCMA is designed to address an broader range of problems than just mechanical failure. Older systems might have used a mechanical governor and spill valve flow control, but even FADEC control still has a variable flow rate mechanical valve at some point in the chain. A potential single point of failure, so some sort of second order solution does seem necessary.
It's not just the single failure of the N2 splined shaft - fixing that shaft still leaves single failures. Every fuel control has a
single fuel metering valve - and if that fuel metering valve fails and goes full open, you have UHT. Whenever there is a single valve controlling fuel flow, there are single failures that can cause that valve to do other than what's wanted/expected. Going to FADEC changes the nature of those single failures, but the single failures are still there.
I investigated several FADEC UHT events while I was still working - one was pretty much a worst case scenario: 747-400 during takeoff (PW4000 engines). On thrust lever advance, the fuel metering valve on an outboard engine went full open and the engine greatly exceeded the EPR power set. Crew RTO's at about 60 knots, engine stayed at very high power, the shutdown the engine, and the aircraft never got more than about 5 ft. from runway centerline. In short, the crew reacted perfectly. But the regulators have ruled that we can't depend on the flight crews reacting perfectly - hence the need for TCMA.
BTW, the operator had the even Fuel Metering Unit overhauled before Pratt and Boeing were even notified of the event - destroying any possible evidence of what went wrong - so we were never able to determine the root cause.