Boeing/Airbus Autobrake

[160knots]

Could anybody please explain why autobrakes only operate above a certain speed. For instance on the Boeing Autobrake only operates above 85kt. Why can't it operate at the start of the take-off roll?

Thanks

[NSEU]

I've been told that RTO autobraking can be quite painful (more than Max Autobrakes on Boeings). Your harnesses dig into your shoulders causing pain, so you can imagine the passengers' bodies/heads flailing about under RTO braking.

Presumably you don't want this kind of braking for every rejected takeoff, only for those rejected takeoffs in the higher speed range where remaining runway length/reaction time and the potential for greater injury is an issue.

[Tom!]

Reminds me of the following:
Q: How can you confirm the operation of RTO autobrake?
A: A glareshield shaped dent in one's forehead is usually a good indication :E

[PEI_3721]

NSEU, don’t believe all that you hear. In most autobrake systems, the level of RTO deceleration is slightly higher than the Max landing selection.
RTO autobrake is designed for safety, not comfort; the deceleration will not generate the emotive descriptions or injuries as you imagine.

An RTO system requires some trigger function for arming (e.g. TO power and speed) in order to enable a takeoff and reduce unwarranted operation.
An RTO below 85 kts may not require full braking, but above this value it’s safer to provide the best stopping performance than rely on human assessment and judgement.

[nitpicker330]

Actually NSEU is correct.

I laughed at your enabling a takeoff and no unwarranted operation comment!!! I've never heard of RTO braking activating by mistake on takeoff......

I can tell you that when the RTO function operates on a Boeing the Pilots all complain of sore shoulders later on. It doesn't muck around!!

[deptrai]

In most autobrake systems, the level of RTO deceleration is slightly higher than the Max landing selection.
RTO autobrake is designed for safety, not comfort; the deceleration will not generate the emotive descriptions or injuries as you imagine.

MAX auto will initially - briefly - appply max hydraulic pressure to the brakes, but then try to maintain a constant deceleration (14 f/s^2 above 80 kts and 11 or 12 f/s^2 below, for Boeing. Don't bite me if the numbers are a bit off).

RTO auto will simply apply max hydraulic pressure. Deceleration then depends solely on a/c weight and runway conditions, and can be "violent".

[Capt. Inop]

How can you confirm the operation of RTO autobrake?

Look for bitemarks on the seatbacks in the cabin. :ok:

[BraceBrace]

Heard from a pilot who used to do tests, including RTO test after maintenance on A330/340's. The discussion was about whether to use reverse during RTO, where he simply said: first time he did it his hands got thrown off the throttles by the violent braking action. By the time he had overcome the sensation and found his way back to te reverse thrust, he was pretty close to a standstill.

There is a reason why a not properly working shoulder harness is a no-go...

[PEI_3721]

nitpicker330, et al.
It might be my age, but the modern breed of pilots appear to lack a certain depth in thinking, a need to understand how systems work, and how and when to use them.
Certification standards are designed to enable an acceptable level of safety, which includes passenger well-being, but not necessarily their comfort.
If pilots accept hearsay or have false beliefs about operations there is a risk of these being embedded in the mental processes which surface in demanding situations; e.g. during an RTO you don’t use full braking or reverse thrust, because your subconscious bias remembers some tit-bit of misleading information on Pprune or elsewhere.
Take care; brake for safety not for comfort.

BraceBrace, if the AFM states use reverse for an RTO, then use full reverse; no debate.

[stilton]

I have never understood why Max Autobrakes is not maximum available braking, just like RTO.


There are situations where max autobrakes (if it was max available braking) on landing would be very useful, of course you can override and brake manually but even the best human Pilot will not outperform the rapidity of function and predictable maximum application of Autobrakes.


Probably very rarely used but it could be quite handy.