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Anti Skid Issue


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Here\'s what\'s happening. Aircraft with hyd pressure applied by aux pump.

Parking brake released, normal brakes selected, anti skid on and foot pedals

depressed to hold brakes, The left forward anti skid valve will occasionally

release brake pressure on the left forward brake dropping pressure to zero

and then restores it back to 1800 psi. The other brakes drop just slightly.

The control box, left anti skid control valve and the left forward

transducer have all been replaced, having no affect. Wiring has all been

shook down and visually checked. Aircraft had been flying fine days prior,

came back with a brakes dragging write up, hyd shop bled brakes, ran follow

on maint per tech data, signed it off. Went to fly aircraft, while on spot

running to depart jumped forward. All checks above have been done with aux

pump no engines running. The left forward brake releases, However if the

parking brake is set, anti skid turned off or emergency brakes selected,,,

it does not happen. I have printed out the Lockheed Service News for the

Anti skid system, which has been no help.

Looking for any input...

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Does one of the four anti skid lights on the overhead come on when the brake drops pressure? If it does then there is something somewhere in the system that is telling the anti skid to cycle that brake. If you dont see a light on the overhead .... I would sacrifice a goat on the flight deck to drive out the bad demons.

The slight pressure drop on the other three can be explained by the left forward brake repressurizing and taking some of the system pressure to do so. In all actuality it really sounds like the anti skid system is cycling that brake, but a normal cycle would be in the millisecond range, not a sustained release.

Has the brake been changed? That seems to be the only part not changed yet.

Below 15 knots the anti skid should be deactivated by the control box; there is only one place that provides a release signal to the anti skid below 15 knots and that is at the test panel, has anybody shot the wire to the test panel or checked the switch to make sure its not shorting?

Good luck on this one, let us know what you find.


(I still say try the goat)


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\"In an unprecedented advance in the field of aircraft maintenance, Nepal Airlines fixed an unspecified technical problem that had grounded one of its two Boeing 757 jets by sacrificing two goats to appease the Hindu sky god, Akash Bhairab.

It apparently worked.

The goats were killed in front of the malfunctioning plane on the tarmac at Kathmandu\'s Tribhuvan International Airport in accordance with Hindu traditions, said Raju K.C., a spokesman for the national carrier.

\"The snag in the plane has now been fixed,\" he said, \"and the aircraft has resumed its flights.\"

I tried the sacrificing routine a couple times with no joy...but we used co-pilots instead of goats. Maybe the Gods like goats better than co-pilots??? :S

Now back to the problem at hand. As long as the aircraft is at a stand still the only way for a anti-skid can get a signal to release a brake is through the test box on the flight deck. And it should cycle the green light for that brake.

If everything checks out that way...and the test box does not cycle the light...I would start looking at the brake system. How about the brake shuttle valve porting fluid to the emergency side...you stated that it happening with the aux pump running. The system is pretty simple. Do you have the Mark II or III system?

I am digging in the books for you...and having the wife corral a goat just in case!!!;)

good Luck

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We had a problem almost identical to this at Kirtland not long ago. After we checked pressure and everything we had A/R check the rigging on the brake control valve and one of the was completly out of rig and we had to change them both due to the manufacture issue. After that we had no problem at all. Just something to look at.


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Just in case any of you wanted to know, and I dug this up after assisting with an anti-skid problem.

The self test switch sends 26VAC 400Hz to two of the 4 identical circuit cards in the control box, either FWD or AFT. The 400Hz voltage is knocked down and sent thru the transducer coils back to the appropriate circuit card, basically simulating wheel rotation. Releasing the switch causes the loss of the 400Hz rotation signal, causing the system to think the brakes locked up. This send a pulse to the valve circuit and test light circuit, making the light flash.

I share this as nothing in any of the C130 books we have state that the transducer coils are part of the test.

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  • 1 month later...

Anti-skid test logic provides a test of the skid control circuits and an in flight test of the locked wheel circuits. The parking brake must be off (released).A 26 VAC 400 Hz signal is used as the test signal. During test, the 400 Hz signal is supplied to the control box as wheel speed. This test signal is sent to the forward or aft control section of the control box through the test switch to simulate two wheels spinning. At switch release, the test signal is removed. This appears to the control section as a skid, which results in an output to the valve and the test light. Anti-skid test logic produces test light indications on the anti-skid test panel which vary with the aircraft location and landing situation as follows:

Position the ANTI-SKID control switch to ON, BRAKE SELECT switch to NORMAL, and release the parking brakes. The ANTI-SKID INOPERATIVE light should go out and the four test lights should be out.

Apply the brakes. Momentarily actuate ANTI-SKID TEST switch to the FWD position. Upon release of the switch (center position), the two FWD lights should momentarily come on and pedal kickback should be noted if the parking brake solenoid is functioning properly.

Actuate the ANTI-SKID TEST switch to the AFT position. Upon release of the switch, the two AFT lights should momentarily come on and pedal kickback should once again be noted.

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