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Foil in the Brake Valve Inlet Filters



Global services provider operating a fleet of aircraft.


When reports of “spongy” parking brakes on cargo planes began, it was not a threat to safety or service. But with growing frequency, maintenance checks revealed what appeared to be pieces of foil in the filters that keep the hydraulic fluid clean. The foil particles trapped in the filter almost blocked the flow of hydraulic fluid, causing the brakes to feel spongy. Early speculation that contract maintenance crews were introducing the foil by not using proper sealing caps during maintenance checks proved to be wrong: the root cause of the clogging remained unknown.


Unscheduled maintenance in response to complaints about the brakes was driving up operating costs. After days of troubleshooting failed to find root cause, the problem was referred to the company’s own Kepner-Tregoe rational process experts, Program Leaders (PLs) who are certified to teach and facilitate Kepner-Tregoe Problem Solving & Decision Making, a structured approach to rapid issue resolution. They quickly developed an effective problem statement and examined the “IS” and “IS NOT” data. It became apparent that they needed to know where else this was happening in the world.

Contacting other aircraft owners, the PLs learned that another company had experienced the same problem in the same model plane. Pieces of laminated aluminum shims, used within the brake manifold, had flaked off into the hydraulic fluid due to age and hydraulic pressure. The problem had been corrected successfully by replacing the shims with one-piece steel shims.


A research engineering firm examined the particles for the Program Leaders and verified that the aging shims were the cause of the problem. The aircraft manufacturer was informed and proceeded to notify other owners of this aircraft about the problem and solution.

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