Autobrakes

[stilton]

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]

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]

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]

Thanks for the informative reply

[vapilot2004]

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.