737 Autobrake PI distances

[theflyingbus]

I believe the autobrake system applies the brakes appropriate to achieve a set rate of deceleration and all but Autobrake 1 applies continuous braking.

Applying reverse thrust decelerates the aircraft also (I am aware at different rates at different speeds), allowing the autobrakes to brake a little less to maintain the same rate of deceleration for the set autobrake setting.

If on landing you forget to select idle reverse, I would therefore assume that the brakes would have to work a little harder to maintain the deceleration rate.

The Performance Inflight landing distances (except Max Auto) have an addition to be made for not selecting idle reverse, increasing upto over 1000ft for Autobrake 1. (-800)

Thus far no one I have asked knows why, and our manuals on the autobrake system give no answers. So any insight would be appretiated

Many thanks in advance

[Ka8 Flyer]

Possibly because the deceleration rate of idle reverse is higher than that of AB-1 (at high speed of course!) so during that time you will decelerate faster (with the autobrake not applying pressure) than you would with AB-1.
To check this theory we'd have to know what the deceleration rate is with reversers only vs. the set deceleration rate of the AB system (levels 1 thru 4)

[Ka8 Flyer]

I'm surprised to see that no one else has an opinion on this:confused:

Com'on, this is PPRuNe :ok:

[Nightrider]

There is obviously no further reply as it is mandatory to have reversers in at least idle after touch down. An increase in the stopping distance is required if there is no reverser selection possible as this would be a non-normal condition.
The standard configuration after touch down, means reversers at least in idle, was used during the certification process to determine the stopping distance required.
Using more than idle reverse will substantially reduce the required braking efforts to maintain the same stopping distance. This will, however, involve another stress on the engines and reduce their life.
A 737 with a Vref of about 140kn (~54,000 kgs LW, flaps 30) will be able to vacate a runway after about 2500 meters in taxi speed even with no brakes, just reversers in idle.
Reverse thrust is anyway only very efficient above 100 kn, pilots with a feeling for the aircraft will notice the increasing force of the brakes when slowing down through about 80 kn.

[Slick]

The autobrake system on most aircraft does not start working untill after landing when system logic conditions are met, one being reverse selected.

The increase in landing distances is for the time delay with the autobrake system logic ref the above.

In english - immedately selecting reverse after landing is sending a logic signal that you want to stop. One reason not to delay deployment with (or without) an autobrake system.

Max autobrake logic I suppose may not require reverse selected. Or at least the time delay is very much reduced. (thats a guess)


Rgds

[theflyingbus]

Makes sense slick thanks

[WhiteKnight]

sorry I`m afraid but this doesn`t make any sense:

Auto brake activates on a 737NG if:

wheel speed signal more than 60kts
Air/gnd relay in gnd mode
both thrust levers in idle
speed brake lever armed
auto brake system armed


nothing about reverse thrust!!!!!!!!

[JetmousePro]

I don't have the answer, but what I do know is that the PI data assume application of detent 2 reverse thrust (so not idle!) 2 seconds after touchdown and back to detent 1 (idle) reverse thrust at 60kts.

Grtz.

[tulswa]

Reverse thrust will affect how much braking is applied.

For example, if AB-2 (5ft/sec decell rate) is selected on a light airplane and reverse thrust is applied to detent 2, the reverse thrust alone may be sufficient to achieve the programmed decelleration rate.

[JetmousePro]

True, but you have to see it from the other side: on a dry runway wheel and aerodynamic braking is sufficient to achieve the decelleration rate. i.o.w. You don't need the reverse thrust to get it.
This might not be the case on a slippery runway where T/R is needed to get the required deceleration rate.

So the fact that on a dry runway an adjustment has to be made for not using reverse thrust is hard to explain (theflyingbus' problem). On a slippery runway this is obvious.