Originally Posted by
NAIA91
In the B737-800 sim we practiced with a runaway engine,has any one of you ever heard of one of those incidents and how this could actually happen? I thought it was a pretty remote chance of ever happening in the real world.
Thanks for any thoughts on this.
Greets Bas
It's known as "UHT" - Uncontrolled (or Uncontrollable) High Thrust. Rare, but it happens (worldwide, ~ 1 per 10,000,000 flight cycles. Lots of potential causes, but the most likely/common is the FADEC losing control of the Fuel Metering Valve and the valve going wide open. If you're "up and away" it's generally not a big deal and you can shutdown the affected engine, but during final approach and on the ground it can get pretty exciting.
I'm aware of only one hull-loss accident due to UHT - a 737-200 roughly 25 years ago (memory is fuzzy if it happened in Egypt or it was an Egyptian operator) - during TO power set, the N2 shaft speed input into the (hydromechanical) fuel control stripped - the control took that as an N2 underspeed and opened the fuel metering valve wide open. Crew elected to abort when they couldn't maintain directional control due to the asymmetric thrust - but due to the failure the engine didn't respond when the thrust levers were retarded and they went off the side of the runway. Subsequent fire destroyed the aircraft but everyone got off without serious injury.
Since this is a single failure, the regulators decided that UHT violated the 'no single failure shall result in an unsafe condition' provision of 25.901(c) - since every aircraft out there has single failures that can result in UHT it made the worldwide fleet non-compliant
Once it became apparent that it was not something that could be addressed in the existing fleet, the regulators granted a 'partial exemption' to the 25.901(c) requirement for already certified aircraft, but all new certs need to address the issue - either by providing compliance, or demonstrating why it's impractical to provide full compliance and hence get the existing exemption extended to the new cert.
New designs need to address UHT - Boeing came up with something called "TCMA" - Thrust Control Malfunction Accommodation. In simple terms, if (while on-ground) TCMA detects an engine either going to high thrust uncommanded, or staying at high thrust after idle has been selected, TCMA will shutdown the engine. TCMA is only active on-ground, since a nuisance TCMA trip could potentially shutdown all the engines which would be obviously really bad in-flight.
The 787, 747-8, and 737 MAX all have some version on TCMA. Airbus uses something different to achieve the same goal, but I don't know the details.