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75/76 Autobrakes/IRU  
User currently offlineSxmarbury33 From United States of America, joined Oct 2000, 447 posts, RR: 0
Posted (14 years 1 week 1 hour ago) and read 1817 times:

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.

5 replies: All unread, jump to last
User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 24
Reply 1, posted (14 years 6 days 21 hours ago) and read 1788 times:

OK.....so what exactly is your question??

User currently offlineHAL From United States of America, joined Jan 2002, 2636 posts, RR: 52
Reply 2, posted (14 years 6 days 20 hours ago) and read 1789 times:

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.


One smooth landing is skill. Two in a row is luck. Three in a row and someone is lying.
User currently offlineFDXmech From United States of America, joined Mar 2000, 3251 posts, RR: 30
Reply 3, posted (14 years 6 days 13 hours ago) and read 1770 times:

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.
User currently offlineFDXmech From United States of America, joined Mar 2000, 3251 posts, RR: 30
Reply 4, posted (14 years 6 days 12 hours ago) and read 1767 times:

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.
User currently offlineSxmarbury33 From United States of America, joined Oct 2000, 447 posts, RR: 0
Reply 5, posted (14 years 5 days 21 hours ago) and read 1720 times:

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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