Do you have the same issue if you follow the OP’s steps to reproduce it?
Yes
Provide extra information to complete the original description of the issue:
I have performed a MTOW rejected takeoff at a runway far longer than the FAA minimum for the 747-8. The results: Full manual braking was applied as soon as thrust levers were closed (right before V1). Full reverse thrust was also applied once available. In this case “full manual braking” means the foot pedals were pressed all the way to the floor. The aircraft did not stop by the end of the runway. In fact, it did not stop until falling into Jamaica Bay.
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Another test. Out of fairness, I performed a similar MTOW test starting from the furthest possible distance back on JFK 31L. This test also used only RTO autobrakes and reverse thrust to stop. I have attached a chart of the airport so you can see where we started and stopped. I estimate it took 13000’ to complete the test. I seriously doubt the real aircraft would have such a long stopping distance in this case. https://youtu.be/YZv65frWYiw
Exactly, it shouldn’t take 13000 ft in a situation like that, especially because many of the runways the 747 operates on are in the 9-10K foot range, and some even shorter.
We did adjust the stopping distances based off of Boeing FCOM charts. The minimum dry-runway field length for an MLW of 688,000lbs with no wind, max autobrake, full reverse thrust, and speed brakes deployed is roughly 6600ft. The minimum dry-runway field length for stopping at MTOW with the same max autobrake, reverse thrust, and speed brake is around 10600ft. So, I could see how it might run a bit long with just RTO autobrakes and some reverse thrust, as they do stop pretty long. There is a calculation that shows at MTOW it would take about 40ft to stop with full manual brakes while only going about 20mph(17knots). However, perhaps we underpowered the auto brakes a little too much and need to make some adjustments.
Thanks for the reply. Actually, my issue is not with the autobrakes as much as it is with the toe brakes, because it seems to take longer than 40 ft to stop from 20 mph at MTOW.
This time I was at 685,000 pounds (just below MLW) at KBUF05 (8829’ of useable runway), clear skies setting.
I used full manual braking (feet to the floor all the way until the plane stopped) as soon as I reject the takeoff just before V1. Additionally, full reverse thrust and the speed brakes were armed (not correct for Boeing) but they would have deployed anyway when reverse thrust was set.
I estimate we overshot the 6600’ target by 1500 feet.
Are you saying you did or did not use reverse thrust and speed brakes? The 6600ft distance is based on using max auto, reverse thrust, and speed brakes.
The brakes were tuned in AAU2 erroneously taking thrust reverser credit for manufacturer provided stopping distances. They now accurately match the manufacturer provided stopping distance tables without reversers in SU13.
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