ANTI-SKID AND AUTOBRAKES.
From comments and questions we have received we understand that there are some uncertainty and confusion regarding the anti-skid system and the Automatic Brake System (ABS) in MD-80. This is an attempt to disperse the fog and explain the differences of the two systems.

At first it should be pointed out that it really is two different systems that work independently of each other even if the anti-skid system works in conjunction with the ABS.

Let us start with the anti-skid system as it has a kind of first-born right.

ANTI-SKID SYSTEM.
Each of the four wheel brakes are powered by both left and right hydraulic systems. For normal pilot operated braking the desired brake pressure is modulated with the brake pedals, the left pedals providing pressure to both wheels on the left main gear and the right pedals providing pressure to both wheels on the right main gear, Full pedal deflection will provide full hydraulic system pressure (3 000 psi) to the brakes. Depending on the runway surface conditions the pressure applied to the wheels may be too high causing the wheels to skid and ultimately lock. In both cases braking efficiency is de-graded.

The anti-skid system has one main purpose: to prevent wheel skidding during braking so the available friction coefficient can be utilized to the fullest degree. Secondarily it will also prevent wheel lockup and tire blowouts.

The main components of the anti-skid system are the wheel speed sensors, the anti-skid servo valves (which are supplied with the metered pressure from the pedal controlled brake control valves) and the anti-skid control box which contains one control card for each wheel.

The anti-skid system senses the rotational speed of each individual wheel. if a skid should develop causing the wheel speed to drop below a reference speed (based on past wheel speed) the control card for that wheel will signal the associated anti-skid servo valves (one for each hydraulic brake system) to bleed off pressure, allowing the wheel to regain its speed. Pressure will then be gradually re-applied again, until a new impending skid develops and the process repeats itself. The anti-skid control box will sense the impending skids and effectively modulate the pressure for each individual wheel to a level with a slight skid (which provides optimum braking). If a serious skid should occur, pressure is released and the system starts all over again to find the new modulating pressure. The maximum pressure the anti-skid servo valve can supply to the brake is the metered pressure from the brake control valve, i.e. the pressure derived from the brake pedal deflection. Any variation of the input pressure (changed pedal deflection) will disturb the control system and cause it to seek a new modulating level, i.e. some braking efficiency is lost.

To prevent wheel lockups each wheel is paired with another and the paired wheel speeds compared; if the speed of one of the paired wheels becomes substantially lower than the other, the pressure to the low speed wheel is released to allow it to speed up again.

The anti-skid system will control the brake pressure, as required, down to a ground speed of approximately 10 kt. Below this speed brake control valve pressure is applied directly to the brakes ("manual" braking).
From this description it should be evident (isn't it?) that to achieve maximum "stopping power" the brake pedals should be depressed fully and held. The anti-skid system will then able to modulate the brake pressure for each individual wheel as required up to the full 3000 psi system pressure depending on runway conditions.

AUTOMATIC BRAKE SYSTEM (ABS).
The automatic brake system is a system that relieves the pilot from the task of applying the brakes in landing and at a rejected takeoff. With the system armed it will actuate the brakes automatically when the spoilers are extended an ground. Simply it can be said that the system does what the pilot's feet otherwise would do.

The main components of the ABS are the auto-brake control panel, the auto-brake control unit and the landing and takeoff manifolds. The manifolds correspond to the pedal operated brake control valves in the normal brake system. The ABS always provides symmetrical brake pressure to the left and right brakes.

The system has two main modes; the landing mode and the takeoff mode.

Landing mode.
For landing, three retardation levels may be selected: MIN, MED and MAX.

In the MIN and MED levels the ABS is programmed to brake the aircraft at a predetermined deceleration. This means that if reverse is used to aid in retarding the aircraft, the ABS will reduce the applied brake pressure to maintain the selected deceleration. In the MAX level, full brake hydraulic pressure is applied and any reverse will further aid in deceleration.

In the landing mode, only the right hydraulic brake system is utilized. This means that only four of the eight brake cylinders in each brake are powered. The available braking torque is, however, sufficient for any braking demand.

When landing with the system armed the ABS will be activated at spoiler extension with a slight delay (3 seconds for MIN and MED, 1 second for MAX). The landing manifold will regulate brake pressure to the left and right brakes as commanded by the ABS control box to achieve the selected deceleration level (the deceleration is calculated from wheel speed signals borrowed from the anti-skid system).

The anti-skid system works exactly as for a normal braking, monitoring all wheels for skid; if any wheel should develop a skid, the anti-skid system will reduce the pressure to that wheel via the anti-skid servo valve. The input to the servo valves from the ABS landing manifold will remain unchanged but the output will he governed by the anti-skid system as in a normal "pilot" braking.

If the runway conditions are such that the selected deceleration level cannot be achieved and during MAX auto braking, the anti-skid level will be governing the retardation.

When the speed decreases below the anti-skid capability, the ABS regulated pressure from the landing manifold will be applied directly to the brake.

Takeoff mode.
The takeoff mode has two submodes depending on the ground speed at activation.

· Takeoff rejected below 70 kt:
At spoiler extension the ABS will revert to landing mode MIN level and apply brakes after a 3 second delay at the minimum deceleration level utilizing the land manifold for brake control.
· Takeoff rejected at a speed above 70 kt:
At spoiler extension the ABS will activate both the takeoff manifold, supplied from the left hydraulic system, and the landing manifold and apply maximum brake pressure without delay, i.e. both left and right hydraulic brake systems will be utilized. The two manifolds will provide full brake pressure as long as the system is active (i.e. spoilers extended).

The anti-skid system will monitor the wheel speeds and control the pressure to the individual brakes as for a normal braking with the pedals fully depressed.
 
 

Auto-brake disengagement.
When the pilot takes over braking and the brake pedals are depressed to 25% deflection the auto-brakes will disengage. If the brake pressure applied by the auto-brake system was higher than that corresponding to 25% pedal deflection the immediate sensation will be a feeling of an "acceleration" (actually a reduced retardation) until the pedals are further depressed. Furthermore as the input pressure to the anti-skid servo valves is changed the anti-skid system will react by seeking a new skid level.

Braking efficiency.
There have been quite some discussions about the efficiency of auto-braking vs "manual" (actually pedal) braking with the general opinion that the shortest landing distance is achieved with auto-brakes.

The question has been posed to the manufacturer of the system and the following answer has been presented:

"The primary difference between the automatic braking system and manual anti-skid braking in the landing condition is the single hydraulic system used (auto-brake landing manifold), as opposed to the dual hydraulic system used with the pilot initiated brake system. From the standpoint of brake torque, it may be noted that twice as much torque is available during pilot initiated braking.

Although the performance (on dry runways) is certified as identical between the "MAX" auto-brake selection and full manual (anti-skid) braking, performance on low friction runways appears to be different. As noted by SAS, performance with "MAX" auto-brake seems to be superior to the pilot applied braking. If a difference in low friction runway performance actually exists, brake torque gain effects may be influencing the overall system performance.

During extremely low runway friction runway conditions, the torque levels necessary to control the wheel are correspondingly low and the reduced torque may provide an efficiency advantage".

Another difference that may affect total braking efficiency is the fact that during MAX auto-braking the anti-skid system is not "disturbed" by variations in input pressure as may be the case during pilot braking when any change in pedal deflection will case the anti-skid system to react.