FAA AC 25-7C, Flight Test Guide for Certification of Transport Category Airplanes, dated
10/16/2012, describes the three types of anti-skid braking systems
identified in §25.109 as follows:
(aa) The efficiency values specified in §25.109(c)(2) are a function of the type of anti-skid system
installed on the airplane. Three broad system types are identified in the rule: on/off, quasi-modulating,
and fully modulating. These classifications represent evolving levels of technology and
differing performance capabilities on dry and wet runways. The classification of anti-skid system
types and the assigned efficiency values are based on information contained in Society of
Automotive Engineers (SAE) Aerospace Information Report (AIR) 1739, titled “Information on Anti-
Skid Systems.”
(bb) On/off systems are the simplest of the three types of anti-skid systems. For these systems,
full-metered brake pressure (as commanded by the pilot) is applied until wheel locking is sensed.
Brake pressure is then released to allow the wheel to spin back up. When the system senses that
the wheel is accelerating back to synchronous speed (i.e., ground speed), full-metered pressure is
again applied. The cycle of full pressure application/complete pressure release is repeated
throughout the stop (or until the wheel ceases to skid with pressure applied).
(cc) Quasi-modulating systems attempt to continuously regulate brake pressure as a function of
wheel speed. Typically, brake pressure is released when the wheel deceleration rate exceeds a
preselected value. Brake pressure is re-applied at a lower level after a length of time appropriate
to the depth of the skid. Brake pressure is then gradually increased until another incipient skid
condition is sensed. In general, the corrective actions taken by these systems to exit the skid
condition are based on a pre-programmed sequence rather than the wheel speed time history.
(dd) Fully modulating systems are a further refinement of the quasi-modulating systems. The
major difference between these two types of anti-skid systems is in the implementation of the skid
control logic. During a skid, corrective action is based on the sensed wheel speed signal, rather
than a pre-programmed response. Specifically, the amount of pressure reduction or reapplication
is based on the rate at which the wheel is going into or recovering from a skid.