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Aircraft Systems Topic 25

Anti-skid, and Auto-Braking Systems

This mini-editorial is a minor extract from the ATPL Aerodynamics and Systems course, run by Avfacts. It does not cover the topic in itís entirety.

Anti-skid is incorporated to protect the tyres from the effects of skidding, and to assure maximum braking is achieved especially when the runway is slippery, and there is a chance of hydroplaning. The same system is often fitted to automobiles.

The use of powerful hydraulic braking systems have the adverse side affect of not allowing the pilot to appreciate the wheels may be locking up. The anti-skid system will reduce the braking effort applied to the tyres and allow them to spin-up again. It recognises any potential wheel lock-up through wheel speed sensors.

The wheel speeds are sent to an anti-skid computer, which compares the rate at which the wheel slows to a computer model of brake performance under perfect conditions. If the wheels slow too quickly ABS relieves enough hydraulic pressure to stay within the model deceleration limits. It must be appreciated that aircraft ABS systems, unlike some automobiles, do not "pulse" the brake pedal, or stop and restart wheel rotation. They simply make possible the maximum deceleration that can be achieved in the conditions.

Although normally selected ON for dry runway operations, ABS really comes into itís own if the runway is contaminated with water, ice, or snow. The antiskid on one brake unit being inoperative will mean another 250 metres extra runway required in dry conditions for the B767, which has 8 main wheels. A wet or puddled runway will require a good deal more runway. Total failure of the anti-skid system on the 180, 000 kg B767 will mean a reduction of about 17, 000 kg from the planned takeoff weight. Failure of the anti-skid braking system is thankfully quite rare.

It is important for you to understand that the anti-skid computer will be confused if you release brake pedal pressure during the landing roll. You should hold a constant pedal pressure and let the anti-skid do the work of limiting wheel lockup.

Using Autobrakes (B767)

On approach you have two braking choices

1. To use manual braking through the pedal brakes, which applies brakes via the antiskid modulating valves. Any lockup is detected, and brake pressure (hydraulic pressure applied to the brakes) is reduced to allow wheel spin-up, at which point more braking effort is again applied. This gives maximum deceleration achievable in the conditions. Maintain steady brake pressure and let the anti-skid valves do the job. - do NOT release the brake pedal force, or you will confuse the antiskid computer.

2. To select (arm) Autobrakes on (settings 1, 2, 3, 4, or Max Autobrakes). The pilot does not apply manual braking at all through the pedals in this case.- brakes will come on automatically once the wheels have spun-up after touchdown. In setting the autobrakes you are scheduling the degree of deceleration desired (ie: 1 is light braking, 4 is lots). Typically setting 1 or 2 is sufficient (normal). Autobrakes also work through antiskid modulating valves, to prevent wheel lock-up. The rate of deceleration achieved is sensed by the Inertial Nav System (INS/IRS), and braking effort is trimmed by the autobrake computer to supply that deceleration rate. If you apply significant pressure to the brake pedals during the landing roll with Autobrakes operating, the Autobrakes will disarm, assuming you wish to take over using manual (pedal) braking.

Application (contribution) of reverse thrust during autobrake operation will see the autobrake wheel braking effort reduce, as the autobrake computer (through sensing inputs supplied by the INS) attempts to maintain the desired deceleration rate per second. Cancelling reverse (typically at about 80 kt) will see the opposite happen.

You can increase or decrease the autobrake selection at any time. Eg: if you are using autobrakes setting 2, and you are running out of runway, you could select autobrake 4. If you are targeting a particular exit taxiway to leave the runway, and it is a fair way ahead, then de-schedule the autobrakes setting to reach the taxiway at the appropriate speed without first stressing the brakes, and then having to apply power again to get there.

This editorial is NOT designed to replace company operating policy. Where any conflict exists, always use the policy outlined in the approved AFM.

I hope this editorial is beneficial to your understanding of braking systems. ATPL training course topics can be found at :

http://www.aviationshop.com.au/avfacts/atp/default.htm

This outlines the topics contained within each of the online or book based courses that are designed to meet the Australian ATPL theory training syllabus.

To assist those having difficulty finding suitable Aerodynamics and Systems texts, limited edition texts (Aerodynamics & Systems Parts 1, 2, and 3) are now available at the online pilot shop, and most pilot supply shops around Australia. These are not full courses, but are designed as focussed ATPL references texts, and are extremely popular. Full ATPL course information is available at:

http://www.aviationshop.com.au/avfacts/Courses.htm

and: http://www.aviationshop.com.au/avfacts/avfacts/atp/default.htm

The full range of ATPL study reference texts are available at:

http://www.aviationshop.com.au/avfacts/Study_texts.htm

A B737 web site that you may find interesting, can be found at:

http://www.b737.org.uk

Best regards

Rob Avery

ATPL Lecturer

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Marty says ... "Goodbye to GA".

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