Here's more than you ever wanted to know about an MD80
A. Landing Mode Operation
(1) The landing mode operation of the ABS allows the crew three selectable levels of deceleration - MIN, MED or MAX. The MIN decel position produces a decel level of 4 ft/sec2 (1.2 m), MED produces a level of 6.5 ft/sec2 (2.0 m) and MAX corresponds to the maximum deceleration consistent with single system antiskid braking limited by the tire pavement interface. The system
compares actual aircraft deceleration derived from wheel speed with pilot selected deceleration to modulate brake pressure to achieve the fixed deceleration level.
(2) The landing mode operation automatic braking activity is initiated by spoiler deployment with throttles retarded after the crew selects and arms the ABS. Throttle lever position information is used as a brake inhibit function until spoilers are deployed. If throttle levers are advanced beyond 20 degrees, ABS operation is prevented. With throttle levers retarded and spoilers
deployed, automatic braking is implemented after a delay of one second if MAX is selected or 3 seconds with MIN or MED. With another second allowed for hydraulic delays including brake fill, the total delay to the beginning of braking is approximately 2 seconds and 4 seconds respectively. The delays are intended to allow for a normal nose touchdown sink rate while maintaining a predictable stopping distance.
(3) Only one hydraulic manifold is employed for braking in landing mode operation. The landing mode manifold differs from the takeoff mode manifold in that it contains a solenoid valve and servo control valve, while the takeoff manifold contains two solenoid valves. During landing mode operation the first solenoid on the takeoff manifold is activated simultaneously with the landing mode solenoid. This feature provides a check of the takeoff manifold integrity. Pilot takeover of the landing mode ABS is available when spoilers are deployed by either advancing throttle levers (one or both), depressing brake pedals (beyond 25%) or selecting the ABS off by the control panel. Advancing throttle levers produce a DISARM condition in an identical manner to depressing either brake pedal. A DISARM by either brake pedals or throttle levers verifies the integrity of the control box fault monitoring and detection circuits. The MASTER CAUTION indication is inhibited if a DISARM condition is produced by either brake or throttle activation with ABS glareshield lights only coming on to advise the captain that the system has been overridden and is no longer ARMED.
B. Takeoff Mode Operation
(1) The takeoff mode operation of the ABS provides maximum brake pressure on both hydraulic systems. The available deceleration is limited only by antiskid braking consistent with tire pavement interface.
(2) The takeoff mode operation automatic braking activity is initiated by spoiler deployment and throttle retarding. Brakes are applied immediately upon spoiler deployment. The "T.O." position on the control panel, flap angle and ground speed information are employed to differentiate between takeoff and landing mode operation. The flaps must be raised above 22-1/2 degrees
and ground speed must be in excess of approximately 70 knots. If brakes are commanded above 70 kts., full dual system antiskid braking will beapplied to a full stop or until pilot takeover. If commanded below 70 kts. in the takeoff mode the system will apply landing mode braking at the MIN deceleration level.
(3) As in landing mode operation (spoilers deployed), pilot takeover is available by either advancing throttle levers 1 or 2 beyond 20 degrees or depressing brake pedals beyond 25%. Both conditions produce a DISARM which turns on the "ABS" lights on the glareshield without a MASTER CAUTION indication. A MASTER CAUTION indication will occur only if a system failure
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533