Maintenance matters

Flying Safety, April, 2004

Editor's Note: The following accounts are from actual mishaps. They have been screened to prevent the release of privileged information.

FOD it is the ultimate enemy of our aircraft and engines, and unfortunately we put it there. Here are a few cases where we, the maintainers, created more work for ourselves and damaged aircraft.

F-15 Engine FOD

The mishap sortie was planned, briefed, and flown as an F-15E air-to-ground operational test mission. The mission was uneventful until after landing. The aircraft displayed an automatic thrust departure prevention system (ATDP) caution, indicating a possible problem with the avionics interface unit 1. The maintenance crew chocked the aircraft and began troubleshooting the ATDP caution. To facilitate the maintenance repairs, the left engine was shut down and the right engine rpm increased to 78 percent to allow sufficient avionics cooling for extended single-engine ground operations. Approximately one minute after increasing the rpm on the right engine, the crew heard an abnormal amount of noise and felt abnormal vibrations from the right side. The right engine was shut down and the crew egressed normally.

After engine shutdown, maintenance performed a borescope inspection on the right engine and found damage to four blades, in the 3d, 4th, 12th and 13th stages of the compressor. This damage was square and sharp at the edges and consistent in size, indicating that something metallic had caused the damage. Maintenance impounded the aircraft, pulled the engine and sent it, along with the borescope recordings, to the engine back shop. After opening and dismantling the engine core, engine back shop technicians discovered numerous areas of FOD damage, including blades and stators throughout the compressor section.

Photographs of the damage, as well as three of the damaged blades, were sent to a laboratory to be analyzed. Results of the analysis indicated a high amount of zinc and iron in the damaged areas. This result is consistent with galvanized steel. The size and shape of the damaged portion of the blades were analyzed. The laboratory personnel concluded that a general-purpose cotter pin was the most likely object. Further, because aircraft-grade cotter pins are typically fabricated from stainless steel, the cotter pin most likely came from flightline-authorized ground equipment.

This example shows that we must ensure the ramp, as well as the aircraft, is FOD free. Do you do a ramp FOD walk prior to your aircraft coming back from a sortie?

[ILLUSTRATION OMITTED]

Old C-130 Engine FOD

After a local night vision goggle training mission, maintenance personnel discovered damage to the #3 engine compressor section, and reported it as a FOD incident. The engine was taken to the engine regional repair center for tear-down and analysis. The engine was replaced and the mishap aircraft returned to service.

During teardown, FOD damage to the first stage rotor and stator, the third stage stator, and the fifth stage rotor was discovered. The first stage damage was partially coated with a small amount of blue dye, used after repair to alert other maintenance personnel to the repair and that the damage is within technical order limits. The third and fifth stage damage had the presence of soot trails behind the nicks, which could mean that the damage had been there for a while. There was no evidence of any repair attempt to the damaged third and fifth stage areas. Impact marks on the third and fifth stages are consistent with damage caused by cotter pin ingestion.

Investigation of maintenance and home station check records revealed no documentation of a prior blend job or FOD damage to the engine. This is not unusual since prior to the Oct 02 release of AFI 21-101, blend repair documentation was not required. Somehow, this engine ingested a cotter pin that damaged the engine. Maintenance personnel discovered the damage to the first stage, repaired (blended the blades) and dyed the area to indicate the damage was within specifications. However, personnel did not inspect further sections of the engine for any additional damage, nor did they report the FOD.

During a routine intake inspection maintenance noticed the dent in the first stage, saw the blue dye and assumed the engine damage was within technical order specifications. Over time, most of the blue dye wore off, so maintenance personnel "assumed" the damaged was new and reported the FOD incident.

How good is your FOD program, and when FOD is found, how good is your inspection? Do you look just at the surface or perform a thorough investigation?

MH-53 and A Cotter Pin

While performing the intake inspection for a compressor wash, a maintenance technician noticed some nicked #2 engine compressor blades. Further inspection, including a borescope, found all 14 stages of the compressor damaged beyond repair. During the inspection, technicians found a partially intact cotter key jammed in the root of one of the first stage compressor vanes. The cotter key originated from the support stops located on the #2 engine air particle separator (EAPS).