Effectiveness of Reverse Thrust?

[Heidhurtin]

Okay, an uninformed SLF question here: Just how much braking effect do modern airliners get from reverse thrust? I'm not asking for retardation figures or anything specific like that, it's just that, whenever I've experienced a landing (all successful so far - thanks for that ;)) I can hear the engines when reverse thrust is used, but can barely feel any effect. When the brakes are applied I can DEFINITELY feel the effect. Difficult answer to put in laymans terms I know, but can anybody have a stab? How much further would the landing run be without reverse thrust?

[Bullethead]

G'day Heidhurtin,

Most, if not all, modern airliners have an autobrake function that automatically applies the brakes after the wheels have spun up on touchdown and will decellerate the aircraft at a set rate. The rate can be preset before landing and there are usually five settings, 1 to 4 and Max.

If, for example, you land with Autobrakes 3 set then the aircraft will decellerate at a preprogrammed rate and if reverse thrust is then applied all that happens is that the brakes release a bit and the constant decelleration is maintained. This is why you will feel no difference when reverse thrust is applied.

Oddly enough Max Autobrakes is not the maximum braking that you can get by standing on the brake pedals as hard as you can.

Regards,
BH.

[Heidhurtin]

Thanks Bullethead, that makes sense. I take it the brief bursts of increased deceleration following reverse thrust (usually after the engine noise dies away) are manual adjustments by the crew after some speed's been lost?

[Bullethead]

G'day Heidhurtin,

The brief increase in decelleration, after reverse thrust is cancelled, is more than likely the autobrakes being disengaged, which is usually done by increasing manual brake pressure until it trips out.

I find that to disengage the autobrakes really smoothly I just increase the brake pressure on one brake pedal and not both.

Each brake pedal controls the brakes on it's respective side.

Regards,
BH.

[perkin]

Are autobrakes a pre-programmed rate of deceleration or a pre-set level of brake pressure? Or does it vary from a/c to a/c? Just curious as the former would require much more monitoring & control by the aircraft systems than the latter...