737NG Autobrake, idle reverse and landing distance

[My names Turkish]

To maintain the selected landing deceleration rate, autobrake pressure is reduced as other controls, such as thrust reversers and spoilers, contribute to total deceleration. The deceleration level can be changed (without disarming the system) by rotating the selector. The autobrake system brings the airplane to a complete stop unless the braking is terminated by the pilot.

From the FCOM. I understand that the rate of deceleration is what the autobrake is looking for rather then each setting applying a fixed pressure. I understand that as thrust reversers are added the brake pressure applied decreases as the system takes into account the effect of the reversers on the deceleration rate and reduces pressure accordingly.

What I am having trouble to understand is the effect they have on landing distance with idle reverse thrust. Our dispatch performance figures are factored as per JAR OPS and they do not take any credit for reverse thrust. My understanding is that the QRH figures which we use once airborne are, for "dry" and "good" figures unfactored. For Flaps 30 Autobrake 3, the no reverse column shows 0 meters additive for no reverse thrust where as Autobrake 2 shows an additive of 60m and Autobrake 1 290m.

If the Autobrake looks for a set deceleration rate then why does the use of reverse thrust vary the landing distance with each autobrake setting on a dry runway when the braking action is not a factor?

[safetypee]

‘Turkish’, I can only give a view based on the broad principles.
As you state, reverse thrust adds to the total decelerating force and thus the brake effort is reduced to maintain the selected autobrake deceleration.
You logically infer (assume) that the ground roll distance is directly proportional to the selected deceleration; normally this would be so. However, during the initial part of the ground roll, the total stopping force from the additive effect of reverse thrust, aerodynamic drag, and any initial brake activity, may be greater than the required for the autobrake deceleration level.
This is shown in Fig 3 (http://flightsafety.org/files/alar_bn8-4-braking.pdf) where in the example the total stopping force exceeds the autobrake requirement when above 80kts (N.B. intermixing force and deceleration).

Without thrust reverse, the total stopping force may be reduced, which might be at or below the level required for autobrake deceleration. Again, from Fig 3, imagine the total force less reverse, but with the brake contribution ramping up quickly after touchdown to alleviate this shortfall, attempting to meet the autobrake demand.
From your examples without reverse, no additive might indicate either a relatively quick acting autobrake (A/B 3) where the brakes combined with aerodynamic (spoiler) force still exceed the required level (brakes etc replace rev thrust), or a slower acting autobrake (A/B 1) where the combined force is initially less than required resulting in a longer distance.

A quick check of an 737-300 QRH (unamended, EU 1.15 factored ?), shows additional distance for all of the conditions that you considered (30 flap, no rev, dry) A/B 3 +60, A/B 2 +60, A/B 1 +510.
I assume that this reflects small differences in the aircraft type and/or system operation from those which you considered.
Also of interest, for good (wet) conditions, the 737-300 distances are A/B 3 +240, A/B 2 +80, A/B 1 +670. Again, I can only surmise that these differences (from dry conditions) are due to the use of different performance parameters in the calculations, e.g reduced friction on a wet runway.

[Avenger]

Autobrake 2 and 1 will clearly command a slower rate of decel than autobrake 3 and therefore a longer landing roll to full stop. The TRs are more effective after a period of deployment and the net effect of no TR is compounded wth the longer landing roll, hence the increase stopping margins. If you consider the RTO scenario, the TRs are the last item requested, after braking and speedbrakes as the short time of initial deployment is not considered a factor in the stop margins.

[misd-agin]

Right after landing, with full reverser and spoilers deployed, deacceleration rate is approx. 2.75(HUD display).

Autobrakes 3 would always exceed 2.75 so using, or not using, thrust reversers would have no impact on total landing roll. You will be deaccelerating at 3 per the autobrakes.

Since autobrakes 2 is less than the 2.75 achieved with reversers and deployed spoilers using autobrakes 2 without reversers would increase total landing roll. With reverser you'd be at 2.75 but without reverser you're at 2 per the autobrakes.

Autobrakes 1 would have a larger penalty than autobrakes 2 due to the reduced deacceleration rate.

The initial 2.75 deacceleration rate decreases as airspeed decreases and thrust reverser and spoiler effectiveness decreases as the a/c slows. So at some point during the landing roll the autobrakes (less than autobrakes 3 selected) will automatically apply to maintain the selected deacceleration rate. Autobrakes 2 will apply earlier than autobrakes 1, which is the distance penalty your manual stated. Since autobrakes 1 will occur last it has the greatest penalty.