FoD Happens

Foreign Object Damage or FoD happens in the jet world.  On one of the pre-flights for our development mule Grey I noticed some fan blade damage.  It turned out there were a handful of blades that had been damaged by what, as best we can guess, was a bolt with some safety wire on it that went through the motor.  No work had been done on the front of the aircraft so it is likely the FoD was picked up by the front wheel and tossed in the inlet.  The inlets on the L39 are high, the plane is meant to be used on unimproved runways and thus FoD is rare but it does happen.

Being relatively new to the Garrett TFE731 series of motors I was pleasantly surprised to learn of the blade damage tolerance limits.  The Light Maintenance Manual (LMM) goes into great detail describing just how much damage and in what locations can simply be dressed off the blades without further action apart from a vibration survey.  In this instance, there were a handful of blades that required dressing and the resultant vibration survey was so good as to require no further balancing.  We can chalk this up to Dirk's skill and experience in repositioning blades to maintain balance.  It was a bit unnerving to watch him go at those beautiful titanium fan blades with a pair of tin snips but this is indeed to procedure as grinding off damage would heat the blade and disturb the state of heat treat.  There is a picture of one of the blades below along with the damage tolerance sheet from the TFE731 LMM.  The total bill was around $2K.

It was good to see that FoD on 731s can easily be addressed in the field and, in most cases, without even pulling the motor.  Individual blades can be swapped out provided you closely match the weight of the blade you are replacing.  Enough motors have come on the market that we have a complete fan set of blades now just in case we see the problem in the future.