2015 737 accident. Idle reverse on wet runway no help

[Centaurus]

Summary
On 5 June 2015, a Boeing 737-600 landed long on a wet runway at Montréal and the crew then misjudged their intentionally-delayed deceleration because of an instruction to clear the relatively long runway at its far end and were then unable to avoid an overrun. The Investigation concluded that use of available deceleration devices had been inappropriate and that deceleration as quickly as possible to normal taxi speed before maintaining this to the intended runway exit was a universally preferable strategy. It was concluded that viscous hydroplaning had probably reduced the effectiveness of maximum braking as the runway end approached.


https://www.skybrary.aero/index.php/...f82c-276530305

Final report

https://www.skybrary.aero/bookshelf/books/4035.pdf

[73qanda]

Thanks Centaurus.

However, it was noted that after an analysis of landing performance in a number of runway overrun events in the USA, the FAA had concluded that the widely accepted assumption that a wet runway will allow good braking action may not be completely valid.

Worth considering when planning your runway performance for arrival.

[Judd]

with just under 700 metres of runway remaining, this was increased to maximum brake pressure and maximum reverse thrust was selected

There is a real trap waiting where the pilot elects to use only idle reverse on touch-down for whatever reason. Idle reverse on the CFM 56 engine is around 22% N1 (same N1 as forward idle). In the case above, the speed was around 80 knots when the pilot realised he had a problem of his own making, including the astonishing act of cancelling spoilers while still at speed.
He then belatedly hauls in max reverse far too late to have any appreciable deceleration effect. Not only is the speed is by now too low for reverse to be effective, but the lengthy period of time it takes to spool up in reverse from 22% N1 to max reverse N1 (around 10-12 seconds) exacerbates the lack of effectiveness of reverse. All the time his max manual braking from 80 knots down would further cause rapid slowing up where the max reverse is practically useless. So the oft used rationale that you can always pull in full reverse as a last resort if floating or landing long on a wet runway, is a myth. Because of spool up times from idle it takes far too long to be effective.

Operators requiring their crews to use idle reverse as policy need to be aware that if an event happens (wet surface for example) where normal braking efficiency is suddenly reduced during the landing roll and too late the crew need to re-introduce full reverse from ground idle, it will probably be a waste of effort as the aircraft will be close to stopping whether on the hard surface or beyond it.

[Checkboard]

The other problem is that the last 700m of the runway is the first 700m of the opposite runway - the bit covered in slippery rubber. That's not the bit you want to be applying max braking on.