Originally Posted by
Skyjob
I was always in the understanding that it was mandatory, to get rid of the forward thrust vector, even at idle.
That's it Skyjob
How many accidents have there been because even idle reverse was not selected, which gets rid of the considerable forward thrust from the engines at idle? I can think of so many.
If you don't select reverse idle, you are in fact braking against engine thrust which is going to increase brake wear and heating, even if performance is not an issue.
And even Reverse idle contributes some additional deceleration.
Crank it up to full reverse especially at high speed say >100 its and it really kicks in. Much more effective at high airspeeds hence the need to select right after touchdown. the 737-200 which could reverse almost the entire jet flux being low by pass, using giant buckets, and could stop on reverse alone in about 1800 metres. Landing in Jersey on a wet runway at 1650 metres it was a godsend.
Many of the later High By Pass engines seem to just be big noise machines but even so you are probably reversing 40% of the max thrust - just an estimate. Review any good video of reverse in operation on a wet runway during testing and you can see it is still a big player.
The 737-200 had 15000 lbs of thrust, most of that was directed forwards but because of the angle of about 45 degrees deflection we used to reckon that about 8-10000 lbs was reversed. Thats a lot.
When we got the 737-400 with the CFM engines at 24000 lb thrust, we reckoned we were reversing about half the jet flow through cascades, but the main fan is not reversed and that is a big component. So we reckoned that the -400 was far inferior to the -200 in stopping power under reverse and so it proved to be.
Of course all of this is taken into account in landing performance so even the RJ with no reversers can still land!! But it is severe limited on icy / slick runways where the brakes do very little and the reversers become the dominant retarding force.
R Guy