sxmarbury33
Posts: 405
Joined: Fri Oct 20, 2000 5:41 am

75/76 Autobrakes/IRU

Tue May 21, 2002 9:32 am

I read somewhere that the autobrakes are tied into the IRU to give a constant deceleration rate. I dont quite see the benefit of having this unless the aircraft is heavy coming in and has to do a quick turn. In this senario the pilot can use heavy reverse and less brake so that they dont heat up affecting the rto performance. Other than that i see no reason. Lets say im taking a 757 into KSNA and using autobrake 3. Im going to want all the brakeforce of auto 3 plus any bouns that the reverse can give me. Im just assuming that this system cuts out in the event that max auto or especially RTO is used. Even though the pilot can always elect to maunually brake i just dont understand the logic of the system so if any of you have opinions or info please post.
 
cdfmxtech
Posts: 1319
Joined: Wed Jul 05, 2000 11:37 am

RE: 75/76 Autobrakes/IRU

Tue May 21, 2002 1:49 pm

OK.....so what exactly is your question??
 
HAL
Posts: 1743
Joined: Tue Jan 15, 2002 1:38 am

RE: 75/76 Autobrakes/IRU

Tue May 21, 2002 2:23 pm

The autobrake setting is a TOTAL deceleration force. If you have a short runway and elect to use autobrakes 3, the brakes will modulate off as more reverse is used to keep a steady deceleration force. That way you know about how long your landing roll will be, and not be subject to vagaries like uneven pedal application, or delayed reverser application.

HAL
One smooth landing is skill. Two in a row is luck. Three in a row and someone is lying.
 
FDXmech
Posts: 3219
Joined: Sun Mar 19, 2000 9:48 pm

RE: 75/76 Autobrakes/IRU

Tue May 21, 2002 10:01 pm

A large part of the reason is the autobrake system depends on a properly functioning antiskid system.

Should the antiskid system fail, the autobrake will not arm or will disarm.

One of the parameters the antiskid system requires is a valid IRS input.

Prior to the IRS interfacing with the antiskid system, aircraft speed and deceleration signals were a function solely of wheel speed transducers in each wheel assembly sending its RPM to the A/S computer.

The problem with this is should the wheels stop spinning due to severe hydroplaning or severe runway ice, the computer might think the aircraft stopped moving.
With the IRS coupled to the antiskid and thus autobrake, these systems aren't solely dependant on the spinning wheels for spinup/speed/decelleration signals.

As a sidenote, the A300-600 uses nosewheel speed sensors for its antiskid reference speed, as well as the IRS.
You're only as good as your last departure.
 
FDXmech
Posts: 3219
Joined: Sun Mar 19, 2000 9:48 pm

RE: 75/76 Autobrakes/IRU

Tue May 21, 2002 10:35 pm

Also, when the autobrake system is selected to maximum and or RTO (rejected take-off), full hydraulic pressure is directed to the brake system.

The deceleration rate is at the mercy of the antiskid system which will in essense control the braking enough to avoid locking the brakes.

Generically speaking, if the autobrakes are set to maximum during landing, there is a time delay prior to the brakes applying.
You're only as good as your last departure.
 
sxmarbury33
Posts: 405
Joined: Fri Oct 20, 2000 5:41 am

RE: 75/76 Autobrakes/IRU

Wed May 22, 2002 1:36 pm

About the use of Max auto why is it not as powerful as either full manual or RTO? And if you were in an emergency situation where you needed to stop quick would you even use the autobrake?

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