ANA 787 Engines shutdown during landing

[flash8]

Originally Posted by Check Airman

Thanks. My question was targetted to the 737. Any practical reason to select reverse at 10ft?

As I recall possible below 20' RA on the classic (possibly 10') but in my opinion not very wise! I'd go as far as to say no practical reason whatsoever beyond get down-itus.

[Dave Therhino]

Originally Posted by Check Airman

What’s the advantage to being able to select reverse before you’re on the ground? Without checking my FCOM, I’m almost positive you can’t do that on the A320.

It allows the reverser deployment to get a head start so you can get reverse thrust right at or very shortly after touchdown.

[Dave Therhino]

Originally Posted by tdracer

...

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.

[tdracer]

Originally Posted by flash8

As I recall possible below 20' RA on the classic (possibly 10') but in my opinion not very wise! I'd go as far as to say no practical reason whatsoever beyond get down-itus.

I have a vague memory that the 737 uses Radio Altimeter to enable the T/R (I thought it was 5 ft., not 10, but as I noted it's a vague memory). This is different from other Boeing installations, which use Weight On Wheels (WOW) -often in combination with the Radio Altimeter - to enable the reversers. Apparently the low wing on the 737 can allow it to float in ground effect enough to prevent WOW from going true.
Dave T - really, really surprised if the 787 enables the reversers only with RA - I'd expect as a minimum that the track lock would have WOW logic. There is a reason why we don't do that on the other aircraft - RA can be fooled in heavy weather...
Then again, the 787 did a lot of 'reinventing the wheel', throwing out decades of experience on why we did some things the way we did.

[Dave Therhino]

I wasn't speculating - I saw a detailed report of the event and analysis of the cause.

[navefenix]

Yo me it looks likely to be an TCMA issue, bulleting included

[tdracer]

Assuming TCMA works as intended, it does eliminate the risk of uncommanded/uncontrolled high thrust (UHT) on the ground. There is still some risk that UHT could happen in the air during final approach - if you're less than ~100 ft. it could be pretty exciting. It's shown to be controllable, but the pilot does need to be paying attention and fly the aircraft or it could end badly. The good news is that the exposure is maybe 30 seconds per flight, and the probability of UHT is something between one in 10 million and one in 100 million flight hours, so the odds of it happening during final are astronomically high. However UHT can be caused by a single failure so we need to show it's controllable (ref 25.901(c) and 25.1309 - you can't use probability arguments for single failures that are catastrophic).