So it was U hey? Hmm interesting. All of the MV's were on a system of maintenance, I believe to try and extend the life of the donk, last I heard M was past 6k on its donk. Thought with the exception of O, U had the lowest time engine.
Something to do with the cool starts and the short-circuiting of the over-voltage protection (32 volt battery cart if memory serves). Just don't touch the avionics masters with the cart plugged in, lest the magic blue smoke start to pour out of the radion stack, or so I've heard.