View Full Version : Autobrakes

13th May 2018, 12:40
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 737-200ís 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?

13th May 2018, 13:44
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.

13th May 2018, 13:50
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."

14th May 2018, 01:15
Thanks for the informative reply

15th May 2018, 00:26
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.