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Mark M'Words
22nd Oct 2009, 09:23
Morning everyone,
Does anybody know where I can find the decceleration rates associated with the B752 autobrake settings.

Thanks Mark:ok:

Spanner Turner
23rd Oct 2009, 05:35
Does anybody know where I can find the decceleration rates associated with the B752 autobrake settings.

Yes, I know where you can find that info - in the maintenance manual!

I don't have access to a 757-200 manual but can give you figures from some other Boeing products as a guide only.

747-400
1. 4.0ft/sec/sec
2. 5.0ft/sec/sec
3. 6.0ft/sec/sec
4. 7.5ft/sec/sec
MAX. 11.0ft/sec/sec
RTO. Full 3000psi hydraulic pressure to brakes-nil decel rate taken into account, braking limited only by anti-skid system.

767 (steel brakes)
1. 4.0ft/sec/sec
2. 5.0ft/sec/sec
3. 6.0ft/sec/sec
4. 7.5ft/sec/sec
MAX. 11.0ft/sec/sec
RTO. Full 3000psi hydraulic pressure to brakes-nil decel rate taken into account, braking limited only by anti-skid system.

767 (carbon brakes)
1. 4.12ft/sec/sec
2. 5.12ft/sec/sec
3. 6.12ft/sec/sec
4. 7.62ft/sec/sec
MAX. 11.12ft/sec/sec
RTO. Full 3000psi hydraulic pressure to brakes-nil decel rate taken into account, braking limited only by anti-skid system.

747-2/300
(options vary but our birds only had Min,Med & Max settings)
MIN. 4.0ft/sec/sec
MED. 6.0ft/sec/sec
MAX. Medium level(6.0) until NLG touchdown and then full 3000psi hydraulic pressure to brakes-nil decel rate taken into account, braking limited only by anti-skid system.

:ok:
.

A300-FO
23rd Oct 2009, 14:20
thank you for this helpfull post..just one question..when does the autobrake system activate with reversal selected on ?on which speed ....

STBYRUD
23rd Oct 2009, 14:38
Hmm, I'm not sure if I understand your question, but the autobrakes don't care at all whether the reverers are deployed or not, the famous question from the TRI "What will give you the shortest braking distance, autobrakes on MAX with or without reversers?" points to just that fact ;)

A300-FO
23rd Oct 2009, 15:54
thanx STBYRUD for ur reply..as for airbus 300-310 autobrakes initiated by spoilers extension command during rejected takeoff with reversal selected on and speed is above 85kt that means that if you rejected TO below 85 ,manual braking should be applied
i wonder if this also same with boeing?

stilton
24th Oct 2009, 07:31
On a dry, uncontaminated runway that is true Standby Rud.


However, on a wet / contaminated runway even if you select MAX brakes the
autobrake system can only provide the level of deceleration that the Anti-Skid system can provide, if that is short of the preprogrammed deceleration rate before the wheels skid that is what you will get.


In this case reverse certainly will 'help' and may allow you to achieve 'actual'
MAX braking.

STBYRUD
24th Oct 2009, 08:14
True, the brakes can't provide more braking than the conditions allow, though even in that case if its critical one might waste braking distance using autobrakes because the reverers might provide a lot more braking than the deceleration 'lost' due to the antiskid on the brakes. (which will reduce the brake pressure again if the deceleration rate is above MAX rate if in any other mode than RTO)

Ah, A300-FO, I didn't realize you were talking about RTO, well in that case the 733 applies maximum brake pressure when the throttles are retarded to idle above 90 knots and the autobrakes will be disarmed when the speed brake lever is moved to the down position during the rejected takeoff. Interesting to find out how it works on the 300! :ok:

We Fix Them
25th Oct 2009, 08:19
To make things simple from the Boeing 757 AMM For the 1,2,3,4 or MAX AUTO positions, the system uses the average of the speed signal from the main wheels for spin up logic and the airplane IRS speed data for deceleration control. The system compares the airplane speed with the selected level of braking to generate a pressure command to the control servo valve. The valve provides the commanded pressure via the normal antiskid valves to the brakes.


Autobrake deceleration level inputs
The autobrake system automatically applies brake pressure selected by the pilot:

Autobrake Selector Switch / Brake Pressure / Decel Level
(PSI) (ft/sec/sec)
1 1500 / 4.5
2 1750 / 5.0
3 2000 / 6.3
4 2400 / 8.5
Max Auto 3000 / 11.0
RTO 3000 Max system pressure applied