By Jim Taylor, Editor MACS ACtion Magazine
The details concern an aircraft but the lessons stand for any shop. A recent incident investigation report, released by the UK’s Aviation Accident Investigation Board (AAIB) details a chain of human-error events that almost resulted in disaster.
We’ll have to seriously abbreviate the 41-page report, but the story goes like this. A de Havilland Dash 8-100, a twin engine, propeller driven commuter aircraft was being repositioned in a short hop from one airfield to another when the crew noticed fluctuating oil pressure on the starboard engine. After several checks, they shut the engine down and planned a return to the departure field. When oil pressure on the port engine began to fluctuate (and they could see oil running along the housing), they declared emergency, diverted to the nearest airfield and landed safely.
A walk-around revealed oil running down the landing gear struts, coating the underside of the engine nacelles, and running along the fuselage. With access panels opened, leakage could be seen at or near the oil cooler for each engine. A check of the sumps revealed the left engine had lost 3.5 liters and the right 5.5L of oil – a normal supply for flight is 17 liters.
After disassembly and inspection, two oil cooler o-rings on one side were found damaged. The smaller one was split and the larger one cut. Additionally, the inner ring was an incorrect size. Seals on the other oil cooler were also found cut and one was missing a large section of its outer edge. When all the oil cooler o-rings were replaced, the aircraft was tested and returned to service with no further problems.
The AAIB classed this as a Serious Incident and began its investigation. First they found that a month prior to the incident, the aircraft had undergone a rigorous inspection after being out of service for many months. During that inspection, worn pivot bushings were noted for the landing gear doors on both sides.
A trouble-ticket was issued to replace the bushings, and the real trouble began. Two technicians were assigned to the job; they researched the procedure from the maintenance manual as required. The landing gear is very close to the engines on this aircraft, and getting at the bushings is a challenge. The manual’s procedure did not state if removing the oil coolers was necessary for access, but the supervisor chose to remove them to make the job easier.
The job took longer than planned, and the two techs ended their shift with the job incomplete. A third technician (from the next shift) was assigned to finish the job. But it was soon determined by others that the gear door bushings could be replaced without the coolers being pulled, so the tech was instructed to reassemble the coolers and lines as others worked on the gear doors.
Under questioning, that technician stated that “installing the pipes was not an easy task” and he needed to hold the pipe with both hands and hold the flashlight in his mouth. He said he replaced the o-rings on each oil line and that he installed the proper inner O-ring at the left inlet. He also said he did not recall seeing a smaller seal. He was, he said, under time pressure to finish the installation before the end of his shift.
Although the full oil cooler re-installation procedure requires a leak check with the engines operating, the technician forgot and the supervisor didn’t catch it either. But the larger, return-to-service inspection requires a 40 minute high-power run-up anyway, during which somebody noticed oil leaks down the left main gear. A substantial amount of oil was found in and around the oil cooler area. Another supervisor remembered that both oil coolers had just been reinstalled so the attaching nuts and flange fittings were retightened—one by a full turn, another by only a fraction of a turn.
How many errors have you seen so far?
A day or two later, the airplane was flown to a service point for a re-paint. Maintenance crews noticed some oil spots under the engine and investigated but full access was prevented until the paint had been stripped. A day before the in-flight incident, a supervisor had finally opened the access panel and found oil around the port engine oil cooler, but others convinced him is was simply residual from the previous repairs. No oil was seen in the starboard engine.
On the day of the incident, Maintenance asked the flight crew to perform a low power run-up for about two minutes with propellers set to low torque. No oil leaks were seen on the ramp and the oil quantity didn’t change, so they were OK’d for departure. Once airborne, it was almost too late.
The AAIB found direct fault with several technicians and supervisors, as well as the aircraft owner’s training programs. Remember that the whole thing started with a normal wear squawk on the gear door pivots and it all went wrong from there.
It’s not much of a stretch to translate this o-ring saga to any seal, anywhere, or any part adjacent to today’s problem.
Communication, procedure, technique, supervision, quality control — what’s in your shop?
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