If an autobrake setting is chosen prior to touchdown, but after touchdown and nosewheel compression (i.e. after the autobrake has engaged) a different setting is required, is it safe to change the setting during the roll-out? What happens when the setting is changed in this manner? I know application of manual brakes disables the autobrake. Does the autobrake immediately match the new deceleration requested?
This answer was difficult to find as both Flight Operation Manuals and Maintenance Manuals did not say. The only guidance was found in the system description provided by the OEM of the BCSU. I am not mentioning the fleet type because at some point the detailed description might become proprietary.
According to the description I found for a given automatic braking system is that it is armed when the pilot presses one of the three pushbutton switches which set the auto-brake level.
The system will only arm if gear up is not commanded, there is hydraulic pressure as needed, and no existing faults exist that prohibit normal braking, as well as a few other computers available on the associated data bus.
There is nothing that says the system can't be armed after touchdown, however the system will only engage when the spoilers are being activated, or are activated. Or more precisely from the black-box point of view automatic braking is engaged whenever two spoiler signals in three appear once automatic braking has been armed. In other words, once armed, the automatic braking will only engage if spoilers are sending the signal that they are being activated, or are activated.
It specifically says that the pilot can select another pushbutton switch even after arming. The BSCU acknowledges this new command and executes the corresponding task.
Automatic braking is released whenever the braking sequence is interrupted:
- when an arming condition disappears,
- when an engaging condition disappears,
- when one or both brake pedals are held down beyond the defined threshold,
- when the selected pushbutton is pressed again.
The question, therefore is 'After you have already armed at a given setting and the system is actually engaged, because the spoiler actuation has initiated auto-braking, what happens when you press another setting?' As long as the arming conditions still exist, which in most cases would, and as long as the engage conditions exist, as would be the case if the spoilers are activated (at least 2 of three spoiler signals being sent to the BSCU), then yes, you may change the setting after touchdown and the BSCU acknowledges this new command and executes the corresponding task.
This will be quoted from the B737 NG - Systems Summary [Landing Gear] which isn't an official Boeing document, but ...
When a landing autobrake selection is made, the system performs a turn-on-self-test. If the turn-on-self-test is not successful, the AUTO BRAKE DISARM light illuminates and the autobrake system does not arm.
Four levels of deceleration can be selected for landing. However, on dry runways, the maximum autobrake deceleration rate in the landing mode is less than that produced by full pedal braking.
After landing, the autobrake application begins when:
- both forward thrust levers are retarded to IDLE
- the main wheels spin-up
NOTE: Landing autobrake settings may be selected after touchdown prior to decelerating through 60 kts of ground speed. Braking initiates immediately if the above conditions are met.
To maintain the selected landing deceleration rate, autobrake pressure is reduced as other controls, such as thrust reversers and spoilers (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.
So, reading between the lines here, if the autobrake system has not been activated, upon touchdown it can be activated, before slow down to 60kts ground speed, as long as all other stipulations are met. Also, although the sentence as written, is confusing, the last paragraph indicates that the level of braking can be changed during autobraking, by turning the selector AND that will not disarm the system.
I know this is not official documentation and is an older model plane, but thought I'd offer it up.
I have flown the 737NG and now fly 757-200. It is now quite normal for us to set a certain AB setting (e.g. level 3), to match a desired runway turn off. Then once landing distance is assured, at maybe 60-100kt, reduce the setting to 2, then 1, then off (or disarm on 757).
Alternatively you can cancel AB by using manual braking pressure (normal method on 737) or by moving the speed brake lever slightly forward, then back to fully deployed once AB have clicked out (normal method on 757).
All methods work on both aircraft, and the choice of method is more about the best way of avoiding the slight jolt felt as the ABs disengage - it's all about passenger (or pilot!) comfort - oh yes, and also about stopping by the end of the runway!