Gear Retract Braking Pressure

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Gear Retract Braking Pressure

Postby maro12 » 14 Mar 2018, 21:18

Good Day All,

B737-700, LH MLG WW blade seals inside surfaces show signs of friction wear from contact with the tire. Rubber residual show on surrounding area, also LH OUTB Tire shows signs of friction wear.

Looks like Gear Retract Braking pressure does not stop the wheel during retraction.

Does any of You know by any chance what should be the Gear Retract Braking Pressure value measured on brake quick disconnect when LG Lever Up, or could support me with reference.

I can see in AMM TASK 32-41-00-710-802 Gear Retract Braking - Operational Test, but it is only observing wear indicators movement.

Did anyone faced similar issue before?

Thank You in advance for any suggestions.

Best Regards,
Maro12
maro12
 
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Re: Gear Retract Braking Pressure

Postby sjap » 20 Mar 2018, 16:37

You can put brake press measuring indicators on the brake bleed port, select gear up (with pins of course) and then see what the pressure is.
But in general, blade seals wear a lot in common, and lots of times they think it is a problem, while it is normal wear of the seals..
But to check it i would do the procedure with the indicators to make sure.
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Re: Gear Retract Braking Pressure

Postby maro12 » 20 Mar 2018, 19:19

Good Day Sjap,

That is exactly what we done, but we were looking for any reference what value should we read.

We read LH:420psig, RH:450psig;

AMM does not specify value we should read, so I did not know if it is too low or not.

In order to find out the answer, I compared measured values with DFDR data from two other 737NG (massive help to have shop on-site).

As right after the brake metering valves are pressure transmitters which sends Alternate Brake Pressure to DFDR, on the DFDR graph right after gear UP discrete changing state, You can read Alt Brake Press value for RH and LH gear, until First Officer puts LG Lever to OFF during "After Take-Off Checklist".

On DFDR data I could see values in range: 1st compared Aircraft recorded range LH:465-520, RH:465-515, 2nd compared Aircraft range LH:510-535, RH:450-500.

As I still was looking for any ref. in documentation, I looked up Brake Metering Valve CMM. Apparently there is separate test which checks this output pressure called "De-Spin Actuator Operational Test", which You do on the bench. This test specify, that with Gear UP Hyd pressure between 1500-3000psig applied on "De-Spin Actuator", output pressure from the valve must be between 500 to 575 psig.

If this output pressure out of range, there is the way to adjust it by adding or removing shims in the De-Spin Actuator. Probable causes of incorrect pressure could be also damage of some other internal parts (spring, ball bearings or lever, binding between components).

Next step will be check DFDR data for affected aircraft.

But with all of this information, I am considering measured pressure on brakes quick disconnect as acceptable.

Thank You anyway for Your answer, much appreciated.

Best Regards,
Maro12
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Re: Gear Retract Braking Pressure

Postby tweakjr » 11 Jun 2018, 07:56

Here is an illustration of the gear up brake pressure.

Image

Image

If you are swinging the gear, a quick check is to try and spin the wheel while it retracts. The ~500psi begins when gear handle is UP.
A culprit that sometimes causes the wheel to not spin down is a bypassing alternate anti-skid valve.
Alternate anti-skid gets inhibited for 12.5 seconds when AACU loses gear lever NOT UP switch discretes.

Cheers
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