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Anti-skid issue


tinyclark
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OK, we have an aircraft with a recurring anti-skid issue.

If the aircaft is taxiing, going pretty slow, when they step on the brakes, they can feel the brakes pulsating.

All the tranducer wiring checks out. The control box gets swapped out, and it works for a few flights, maybe...

This last time it was written up by the same crew that had written it up two flights previous. But forget about that, we are assuming it is a real problem.

Anybody seen this before?

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Hows the runway surface? I would think even on dry taxi ways / runways - with deep ruts, or that is poorly maintained - would be able to momentarily trick the system into thinking there is a skid. Pulsating/thumping in the pedals is normal thing when the systems is operating in a anti-skid condition. I know in my truck on some roads even when they are dry - the abs still activates under what I would consider light braking. Obviously heavy gross weights would probably compound the problem. Anti skid system is fairly straight forward - also just because the system tests good from the anti-skid test panel does not mean the anti-skid wheel transducers are any good - one that is failing might be producing a signal less than what the others are producing, or likewise, one might be producing a stronger signal -and as I understand it - the system looks at all 4 sensors and compares there relative strength to determine a skid. Just the thoughts of a young pup.

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Tiny, are they showing any anti skid activity on the FE's anti skid test panel?

If they are going fast enough to activate the anti skid, then if you have a wheel (or more) releasing then the corresponding light on the test panel will flash with the anti skid pulse.

If they systems tests good on preflight, then you get the pulsing with no test lights flashing, probably not an anti skid problem.

How about trapped air in the system, that can give some screwy feelings.

Dan

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On the schematic I'm looking at (E) model there is no pressure reducer valve displayed. However, I remember someone talking about it once, and how they thought it was integral to the brake control valves (not to be confused with the shuttle valves). If so - which is the correct nomenclature - inquiring minds want to know.

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We can't get enough help from the crew to troubleshoot.

"I never looked at the panel", and, they don't want to taxi it for troubleshooting.

They are rewiring the tranducers and replacing them all just because, against my thoughts.

It doesn't seem like an antiskid problem to me either, especially when they say they are below 15 knots when it happens. It just doesn't make sense to me. If it was antiskid, you should be able to feel and hear the valves operating.

Well, when they get done and it comes back, both of us will be on board during a taxi check. Anyone ever see a set of brakes do this if warped?

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Pressure is reduced to 2030 PSI at the brake control valves there behind the rudder pedals.

Dan

Thanks Dan, Knew there was a 2030 somewhere, and that jogs my memory about the notes I had written in my old dash 1.

Shotgun Mx - because if that doesn't fix it.....maybe the next part will. :D

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"the system looks at all 4 sensors and compares there relative strength to determine a skid"

This is true but in context.

Each wheel is separate from each other. The transducer in each wheel sends its own signal and based on the speed or freq of the output the applied voltage will vary. As the freq drops the voltage drops which tells the control box the wheel is slowing down and if the rate of decline (in voltage= speed) the control box detects a skid and modulates the pressure to eliminate the skid. So all four wheels are monitored and modulated individually

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Hi all.

First off I want to say that I usually check this site out about once a week, but never posted on the forums because I have only been out of school a year and don't have much to contribute that you guys don't already know....

That being said,

I am a little familiar with the aircraft in question. I was TDY with it at Kirtland when the problem initially surfaced. My crew was the second to fly it after the brakes were first written up and maintenance had changed the anti-skid control box.

What I can say, is that when we noticed the issue, we were taxiing on the parallel taxiway, after the brakes had already been selected to normal. We were on the before takeoff checklist, and when the pilots would apply brakes, they said they felt a "pulsing feedback" in the pedals. I did check the anti-skid test lights while they experienced the pulsing, and they never illuminated (the anti-skid tested fine during preflight).

The pilots then tried braking the aircraft after slowing to less than 15 knots, and they claimed they still felt the pulsing pedals. We turned off anti-skid, and they said the problem was still there. I do not recall testing the brakes in emergency, as I remember this was all in normal brakes.

After we returned the aircraft, I want to say that transducer wiring was found to be bad and the problem fixed, because the aircraft flew fine after that and returned home on schedule.

Now to clarify, I have not flown the aircraft since then, and I didn't realize the problem was still being reported until I saw the plane roped off on the flight line. I do not know which engineer has been flying it lately, but I would like to find out and ask them about it. I am currently TDY to Kirtland for systems refresher.

-sorry for the long post

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"Hi all.

First off I want to say that I usually check this site out about once a week, but never posted on the forums because I have only been out of school a year and don't have much to contribute that you guys don't already know...."

First congrats on "only being out of school a year" I would trade places in a heartbeat! (wife would not like it but...)

Us older, NOT OLD, guys were once only a year out of school and I would have killed to have a treasure chest full of knowledge as Herky Birds...

Dont hold back... learn and let us know how things change... As the systems change you will become the new EXPERT and we'll just be the OLD guys...

As for the issue.. there is some verbage about pulsing and not a reject item... I will look it up in the AM....can't remember the wording ... TOO MUCH RIESLING.......I mean BEER....

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Well HC-130 FE

Welcome out of the closet and into the light of day:D

From your description, I would say there is a very high likelyhood of the problem having nothing to do with the Anti Skid system and every likelyhood of a mechanical problem with the brake system itself, brake control valve, shuttle valve, fuse, who knows but they really need to be looking at the mechanics of the system.

Dan

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"As for the issue.. there is some verbage about pulsing and not a reject item..."

My bust!!! this is only during a simulated antiskid cycle test... the pulsing or "Bump" is the valve moving...

Think Dan is right... see if you can isolate it to one side P/Cp or one brake left or right, maybe then cap a brake off and see if you can isolate it to just one brake. lets us know what you find....

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Boy after reading this thread I wish I could remember what to suggest. Many times we had Anti-skid issues at Robins with planes coming out of PDM. We had this one plane we taxied I don't know how many times to try and figure out what was causing it. We'd be hanging off the ramp looking underneath the airplane to see which wheel was skidding.

I can remember similar issues as this thread suggests. Brakes pulsing but blow 15 knots. Too many years gone by now.

Maybe Scott Stansfield will see this and post his reply. He'll probably remember it.

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