Auto Brakes and RTO Operation

[shon7]

During takeoff when autobrakes are on an RTO setting -- does the RTO engage if the thrust suddeny decreases or is there any other condition that signals the RTO to kick in?

[Intruder]

Depending on the airplane, could be a combination of airspeed, thrust lever position (idle), and/or reverse lever position (up).

[Old Smokey]

Well, here it is for one aircraft type, straight from the Boeing 777 Manual to you -

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Rejected Takeoff
Selecting RTO (rejected takeoff) prior to takeoff arms the autobrake system. The RTO mode can be selected only on the ground. The RTO autobrake setting commands maximum braking pressure if:

(1) the airplane is on the ground

(2) groundspeed is above 85 knots, and

(3) both thrust levers are retarded to idle.

Maximum braking is obtained in this mode. If an RTO is initiated below 85 knots, the RTO autobrake function does not operate.
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That applies to the B777. Other aircraft may vary considerably.

Regards,

Old Smokey

[gas path]

Same for the 747 classic and 400 variants.
Above 85 knots and T/L's selected to idle.
The RTO setting applies max hyd. press. (3000psi) and the rate of deceleration is only dependant on a/c weight and the runway conditions. (as opposed to MAX auto which although still applies 3000 psi initially the rate of deceleration is maintained at 11 ft/sec/sec)

[tired]

A340 slightly different conditions (of course!) but same general idea:-

-Max autobrake requires ground spoiler extension command and compression of nose landing gear.
-Ground spoilers extend if speed is greater than 72kts and either all thrust levers are at idle or reverse is selected on at least 2 symmetrical engines, with the remaining engines at idle

So Max autobrake kicks in above 72 knots on an RTO