During an engine runup today, I noticed a problem with one of my PMag units (rough, all 4 EGTs rising, engine would not run for more than about 3 seconds). The engine died when I brought the engine to a slower idle. I then ran the batteries down trying to restart the engine (combination of the cold, new replacement batteries still in the hangar, and not fully charged to start with).
While towing the aircraft back to the hangar (with the master switches off), much to my suprise (and shock), one of the main gear started to fold in. We ran to the wing, and I (not the best idea) pulled the overcenter link down while the other person was lifting the wing. I stepped back and the gear started to fold again. I pulled the link down again while the other person lifted (and applied a side load to help lock it?). As the the overcenter link went overcenter, the nose gear promptly started to retract, with the cowling coming to rest with some weight bearing on the towbar and some slight damage to the tip of the prop.
I reached in the cockpit and turned the masters on (dual buss system). The gear pump did not turn on so I quickly flipped the crossfeed tie and the 2nd buss battery must have provided sufficient current to run the pump. The problem main gear promptly straightened up and locked, as did the nose. Obviously the gear selector was still in the down position, (and I have an airspeed based squat switch.)
I have 3 questions:
1. Any ideas on why the gear could have come up? I have not disassembled anything yet or put the plane on jacks. However, the overcenter links appear fine and recently lubricated, and the springs appear fine (I have the newer style circular springs,not the old screen door springs). The emergency gear dump was tested several flights (albeit a month ago) and had normal pressure in the nose gas strut.
I replaced all the retract and door cylinders and rebuilt the gear pump last year. Since then, it has operated very well, and would hold pressure for hours in flight without the pump needing to "burp" to repressurize the up side. I have never had problems with the down links. Also, the plane had just been running, so there should have been 500-700psi on the down side (I have gauges and have verified it in the past).
It almost seems like somehow the system got pressure on the up circuit. Is there anyway the pump could have operated in reverse or gotten pressure on the up side, maybe while I was cranking the engine on low voltage?
2. Am I the only one who has been having problems with the PMags, It seems like an elegant design, but I have experience one hardware failure (on the ground), one firmware glitch in the air, which significantly advanced the timing on one ignition and caused the engine to run 60degrees hotter than normal, and this most resent failure. I understand there was another firmware glitch (solved in the latest firmware), which causes the timing to get corrupted during starting in rare instances. While factory support has been great, 3 failures in a critical system is getting me a little concerned.
3. Does anyone have any experience with the new MT simitar prop? Since my prop needs to go back to MT to repair the tip damage (was about due for an overhaul anyway), I was wondering if it was worth the upgrade to the new simitar blades (MT was closed today so I did not speak to them). Maybe I can at least get something positive from my experience today.
Regards,
Clark Baker
LNC2, IO360, 800hrs TT
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