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No normal brakes with Anti-skid on

Herk Jerk

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Looking for some troubleshooting help with the ant-skid system. During taxi after flight, below 15 knots, with anti-skid sytem on we have no nomal brakes, when anti-skid system switched off normal brakes work fine. We have already changed two control boxes, both control valves, all 4 transducers and normal brake selector valve, no joy. Anti-skid system works as advertised during system checks.

We are currently checking all wiring, but so far all seems good.

Any suggestions would be great.


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Check your touchdown switch and relay operation.

If the touchdown relay did not remove the 28 volt DC from pin “E” upon touchdown of the aircraft, wheel rotation above approximately 15 knots would override the touchdown signal and allow normal brakes with anti-skid protection down to 15 knots.

BEN beat me to it....

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Yes, Ben Legere…. The possibility of intermittent malfunction of the Up Contact of the Landing Gear Lever is there in which electrical power will find its bath to close the Normal brake selector valve leaving the aircraft for some time without any braking system (since the other valve "emergency" is already closed).

BUT ……..the nature of the defect as been described (hitting only with low speeds) is weakening your BET to win (no relation between speed and the valve closing through this circuit.

I am supporting the possibility of electrical power gets its way to Pin "F" as mentioned by NATOPS1 either through internal malfunction within the touchdown relay (B3 / B2) or through any momentarily electrical leakage found its way to Pin "F"

Relation of aircraft signaled to be as in flight(while in ground) and the speed of 15 knots is there; in which the override feature-within he anti skid system-will prevent normal brake to be inoperative till speed dropped to 15 or below.

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