Originally Posted by
tdracer
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All that being said, I'd be hard pressed to come up a scenario for how doing that could cause a nuisance TCMA trip...
There were actually two of these events on the 787/Trent - one in 2016 and one in 2019. I don't know the exact details of the 2019 event, but I assume they were very similar to the 2016 event because the conditions that could trigger the event were very limited and specific.
The 787 reverser "on ground" logic can be satisfied by two low range radio altimeters reading 5 feet or less plus a slight time delay (300 milliseconds), allowing the reverser to be commanded to deploy and reverse power to be set above idle. In the 2016 event, the crew floated the landing below the 5 foot LRRA threshold. They deployed the reversers prior to touchdown and set maximum reverse thrust. As the thrust began coming up they quickly moved the levers back to idle reverse (I suspect because they either dropped hard or anticipated dropping hard - can't remember). Once the main gear weight on wheels switches indicated on ground, the TCMA logic in the EEC was activated. Unfortunately the way it was implemented the original RR TCMA logic did not track engine commands or behavior prior to TCMA activation with weight on wheels. When it activated it saw the thrust resolver angle at idle reverse and N1 above a threshold because the engine was still in the process of decelerating following the prior accel toward max reverse, so the TCMA logic triggered a shutdown because, by the programmed logic, it detected uncommanded high thrust. Since both engines saw nearly identical conditions, they both shut down.
This condition can only occur on the ground. As far as I know at this point all of the engines have had their EEC software updated and this vulnerability was eliminated.