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Professional Engineer

Single Disk Brakes of B-Model C-130 Aircraft Does Not Hold

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We are experiencing a bit strange problem of single disc (mark-1) brakes not holding B-model aircraft at and above cross-over setting on all four engines. It is happening both in case of parking and pedal pressure braking. It is understood that single disc brakes were designed for T56A7 engines having take-off TIT of 970 whereas subject B-model is fitted with T56A15 engines having take-off TIT of 1085 (thus more torque). Brake assemblies, brake control valves, brake selector valves, brake accumulators, shuttle valves, relief valve have been replaced. Brake pressures have been found normal (1700-1800 psi in case of pedal pressure and 1500-1600 psi in case of parking brake). Thorough bleeding has been carried out to purge any air trapped in the system. Hydraulic plumbing / mechanical connections have been found satisfactory. Though anti-skid system is inoperative in case of parking brake or till aircraft is stationary, however, both anti-skid valves and control box have also been replaced. Anti-skid tester (hytrol tester) results have been found satisfactory. Subject B-model aircraft weight is 172000 excluding 28000 lbs of fuel. Both parking and pedal pressure brakes give abnormal noise and start releasing at all four engines at or above cross over thus aircraft start moving. Ground check out valve rigging has been found satisfactory as hydraulic is not shifting from one system to the other. Moreover, response of warm brakes has been found better than cold brakes. Guidance to fix the problem and sharing of similar experiences would be highly appreciated. 

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A few questions: Are the brakes releasing allowing the wheels/tires to turn?Are all 4 wheels/tires turning or just i,2 or 3? What is the surface the acft. is on like(oily,sandy,slick)?What's your tire pressure?Are these new disks?If so sometimes fresh disks need "burning in" to "cook off" the preservative applied to them before they'll hold.

What is your location and OAT?At that weight that acft. shouldn't even want to creep at T O power!

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1. Yes brakes release momentarily and allow wheels / tyres to turn a bit and then re-engage and it continues. At times brakes totally release and aircraft starts moving. There is no skidding at all. Brakes give abnormal noise while releasing.

2. All four wheels / tyres have been observed turning.

3. Aircraft is on runway made of bitumen, its not oily, greasy or sandy. 

4. Tyre pressure remained in the range of 80+-5 psi.

5. No, these are not new disks.

6. Elevation from sea level is 1668 feet and OAT varies from 28 to 37 degree Charlie.

7. Aircraft weight is 172000 lbs excluding fuel (fuel varied from 28000 to 35000 lbs).

8. shouldn't even want to creep at T O power!??? Did not get it, kindly elaborate. I think creeping at take-off power should be acceptable.

 

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By "shouldn't even want to creep" I mean the acft. shouldn't move forward at all.In the C130 world I only ran A models but at 28000 lbs of fuel and no cargo, at take off power,evan at temps.below freezing the bird didn't move unless the surface didn't provide any grip.  

If the wheels/tires are turning, brake pressure is being relieved or not enough hyd. pressure is being applied.  A lot of components have been replaced and that requires much bleeding.On acft. other than 130s, if a component other than the  brake itself was replaced we would have to push up to 4 or 5 gallons of hyd. fluid through to totally purge the brake system of air. 

You say a noise is heard as brakes release.This makes me think an anti skid valve is cycling  allowing brake pressure to relieve.  

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Is this a new issue?  such as once you put different engines on?  Do you have other aircraft with same engines/brakes with same problem?

I agree that the system should be bleed VERY well.  If you have access to a brake bleed kit that dumps your bled fluid back into the reservoir, you can bleed them very very well and not have to service or replace fluid.

If you have the ability to modify your aircraft, I would look into installing the 1985 and up brake control valve.  They do not reduce the pressure to 2030 PSI, they dump full system pressure (3000) to the brakes and will provide with more holding strength.

If you want to eliminate the anti-skid completely, install a cap on the hydraulic return line from the anti-skid valves and remove the cannon plugs.

bleed the brakes and Do a engine run to check for "creep"

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I was always led to believe GTO of C130 is 155000lbs, so at 172000 you should be grossly

overweight, and I'll assume you mean 72000lbs. -7 and -15 engines are both capable of

exceeding 19600"lb of torque before reaching maximum take off TIT of 972°C and 1077°C

respectively.

 

So from what I'm seeing here is you have an aircraft that is not dragging wheels on a low

friction surface. You have checked and/or replaced all braking system components except

the brake assemblies.

 

Does this problem occur only with brake selector in NORMAL, or in EMERGENCY also?

Did the crew report heavy braking PRIOR to the problem being written up? 

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There are three weight and balance charts for the non-stretch Herks. Actually, I think they are one in the same, but haven't looked at them in a while to be sure. Normal operations max out at 155,000 lbs. It also mentions 165,000 and 175,000 lbs. I've flown once at 165,000. It's a little bit scary. USAF requires waivers for anything above 155,000.

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The more I think about this the more I think you have an electrical glitch that is causing a brake hydraulic problem.

I'm guessing that the 172000 lb. weight excluding fuel is a typo.At least I hope it is!

A very good suggestion from HE HE to cap/plug  the skid valves and disconnect the elec. plugs.

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Well here is my two cents worth. Have you tried pulling the anti-skid circuit breaker and see if the problem still exists? If not, then I'd check all the wiring from the strut to the wheel well overhead terminal strip at WL 165. The terminal strip does get very dirty and possibly corroded terminals. But I would be looking for something loose as in a loose terminal, bad crimp on a terminal end, loose terminal nut. To me, in reading this post it seems vibration is a source. Also look for splices where there should not be any. Check all grounds.

Edited by Ramrod

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