Engine Forensics

Failure Analysis Service Technology (FAST) FOD kit

Forensics. The mention of the word brings to mind television shows about crimes and autopsies. The dictionary says it’s the scientific analysis of physical evidence. But what does the science of forensics have anything to do with aviation? When it comes to foreign object debris (FOD) the answer is, quite a bit, at least if you choose to take advantage of a really great scientific tool. But more on that later.

To hear the old-timers talk (I’m an old-timer but not that old), FOD wasn’t such a big deal in the heyday of the big radial engines. You didn’t want to drop a washer down a carburetor or leave loose hardware inside an engine during assembly, but generally speaking an old removed cotter pin was not going to cause the complete destruction of the engine or millions of dollars of damage. Yes, there were exceptions for you hangar lawyers out there, but they were exceptions rather than the rule.

That changed with the advent of the jet engine with lots of tiny little blades spinning at high speed and vacuuming up every loose object within reach of an invisible vortex. Even the seemingly most innocent and smallest piece of debris could now cause the loss of an aircraft, extreme damage to an engine, or at the least, disruption in aircraft availability.

In the case of the recent Canada Geese vs. aircraft incident, the cause is easy to determine. Once you’ve seen and/or smelled the results of bird ingestion, you never forget the sensory impact and you always remember the smell of whole cooked bird, feathers and all. But what about when you find FOD damage to the engine and the cause isn’t immediately obvious? Do you just blow it off and say, “Oh well, that sort of stuff just happens?” Hopefully we’ll do our best to look deeper because it can be very expensive to do otherwise.

Military applications
Contrary to popular opinion, the U.S. military is interested in being cost effective, but even more so they are concerned about preserving combat assets and their availability. In civilian terms, that means we work hard to have aircraft available and capable of performing the missions our government assigns. It’s just a simple fact that crashed and broken aircraft don’t do anybody any good, except maybe the bad guys out there. FOD is a hazard that’s particularly impactful to single engine fighters like the F-16 and the upcoming F-35. The military has had FOD awareness and prevention programs in place almost since the inception of jets, and most FOD damage is preventable by one means or another. But before you can put a fix in place, you have to know the root cause.

Take for example a recent incident at the 122nd Fighter Wing in Ft. Wayne, IN. The 122nd is a combat veteran Air National Guard unit flying F-16C Fighting Falcon aircraft. The Viper (the real nickname of the F-16) is well known for being a FOD sucker and the low slung intake and the high volume of airflow of the engine create the ideal ramp vac. With the engine running on a humid day you can usually see a vortex that comes out of the intake and curves gracefully down to the ramp and madly spins around like a mini Texas-Twister, sucking up everything in its path. Needless to say, F-16 bases have ramps you could eat off of because they are FOD walked daily and vacuum sweeper trucks are constantly in operation. However, even with the best efforts, sometimes FOD damage happens, and when there are rashes of similar events people start getting really worried.

Analyzing engine damage
Over a period of a few weeks in the summer of 2008, 122nd FW maintenance personnel noted a rising trend in FOD events. Two aircraft had sustained airframe damage to a ventral fin and a strake, both caused by an impact of an unknown source, and in the same time frame two engines also received blendable FOD damage. We are not talking just a little ding here and there. Both engines had fan and IGV blades damaged which required further inspection and blending. These events were found during postflight inspections by the aircraft crew chiefs, which means that the aircraft received the damage after launch and probably flew that way.

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