PPRuNe Forums - Autobrakes

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stilton

13th May 2018 11:40

Autobrakes

Autobrakes provide a deceleration rate selectable in several increments depending on what is required

This rate is sensed through an inertial input however older aircraft such as some B727 and 737-200’s had autobrakes fitted with no inertial system or input available

curious as to how these systems functioned, was a deceleration rate determined in some other manner or did it just provide varying levels of brake pressure depending on selection?

DaveReidUK

13th May 2018 12:44

I suspect deceleration rate is determined by monitoring the rate of rotation of the wheels. The latter is needed anyway by anti-skid systems. In the days of steam-powered aircraft like the 727 that would have been done mechanically. Newer aircraft presumably do it with transducers.

MarkerInbound

13th May 2018 12:50

From a very old 727 manual on the bookshelf -

"Aircraft deceleration is measured from the anti-skid wheel speed signals; then, auto brake pressure is adjusted up or down to maintain the selected rate. The auto brake system will attempt to maintain the selected rate regardless of reverse thrust application."

stilton

14th May 2018 00:15

Thanks for the informative reply

vapilot2004

14th May 2018 23:26

Not only the jurassics. On the classic 737, AB decel control is also done programmatically with no feedback loops (outside of what the anti-skid does).

For each setting of auto brake, there is a uniquely programmed ramp up of brake pressure, done in steps, all of which is based entirely upon pre-calculated deceleration rates. After the ramp-up, about 3 seconds in, the max pressure per AB selection level is then simply held.

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