A Flightdeck Mechanic responsible for coordinating Operational Checks during their Maintenance Checks; reports about damage to a B777-200 from the RAT Motor blades during a Center hydraulic system servicing procedure.
Synopsis
A Flightdeck Mechanic responsible for coordinating Operational Checks during their Maintenance Checks; reports about damage to a B777-200 from the RAT Motor blades during a Center hydraulic system servicing procedure.
Narrative
I am the Flightdeck Mechanic and as so my observations are from the flightdeck only. After performing the RAT system motoring test; the Center reservoir was depleted due to using the Hydraulic Mule to back drive the RAT. We were attempting to service the Center reservoir back up by closing the return on the Mule and operating a Flight Control; but the only Flight Control available at this stage of the visit was the left Elevator. It was going to take hours to fill the reservoir via the Mule through the single Elevator; so we decided to drop the RAT again and back drive it which would fill the reservoir in a timely manner. We have performed this process many times and it works quite well. I obtained clearance and deployed the RAT. The Ground Crew already had Center hydraulic on; via the hydraulic Mule. As they began back driving the RAT; I heard the RAT blades make an unusual sound and the RAT motoring was immediately terminated. I ask the Ground Crew what happened and they informed me that the RAT was not locked down and as they began to back drive it; the Rat drove forward enough for the blades to contact the fillet panel which caused damage to the panel and the RAT blades. All hydraulic power was immediately shut down to assess the damage.
Second reporter narrative
Myself and other Mechanics had just finished accomplishing the RAT Motoring Job Card. We had used a hydraulic Bench to back drive the RAT; so the aircraft Center reservoir was now low. We decided to use the hydraulic Bench to service the aircraft by motoring the RAT and closing the return line[to the Bench]. When we started the RAT motor; I noticed the RAT moving forward and up. I notified the Mechanic on the [RAT] Motor handle to stop; which he did. The RAT damaged a panel/fairing and the blade tips were also damaged.
NASA callback
Reporter stated the B777-200 RAT system takes about 40 gallons of hydraulic fluid per minute to run. While he was in the Cockpit coordinating and operating numerous systems; other Mechanics on the ground were hooking up the Hydraulic Mule; also know as a Hydraulic Bench; to the B777-200's Center Hydraulic System reservoir. Center hydraulic fluid flows into the Hydraulic Bench and is used to power; or back-drive and motor the RAT. That procedure takes the place of airflow over the RAT blades in flight. During that procedure the hydraulic fluid does not flow back to the Center reservoir; but returns to the Hydraulic Bench.Reporter stated that after their RAT Test procedure; the RAT was stowed per their Job Card and the Center reservoir needed to be reserviced. But the wing Flight Controls were not available due to other Maintenance going on and the one Elevator actuator movement would take too long to accomplish the Center reservoir servicing. So they decided to drop the RAT again; hook-up the Hydraulic Bench (Mule) and operate (back-drive) the RAT; using the Hydraulic Bench fluid; but directing the hydraulic fluid into the Center reservoir instead of back to the Hydraulic Bench.Reporter stated the RAT blades turn so fast they actually caused the RAT assembly to rotate forward and up; as if trying to climb back into the fuselage fairing area where the RAT is stowed. They did not use the Job Card to check their procedure because they were out of sequence with the normal RAT check on the Job Card. Each of them had failed to tell each other to check the RAT downlock to prevent the RAT from rotating up and damaging the RAT blades and the Body fuselage fairing. In flight; the air loads of approximately 300-400 miles per hour against the RAT blades holds the RAT down in the deployed position. The downlock is crucial for safe ground operation of the RAT. Reporter stated since then; he and his Supervisor have added additional steps to their Job Card that includes procedures to follow if the aircraft's Center hydraulic reservoir is serviced with their Hydraulic Bench.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.