It finally happened to me. A number of times as I have come down to land, the gear hydraulic pump has not come on when the gear switch is put down. A quick opening and closing of the dump valve eliminates the problem. This time the gear wouldn't go up and I could not solve the problem in flight.

Last weekend I flew for 2 hours from Detroit to Newport News, Va and 4 hours back. Going down on 02/28/08 the temp was -14°C at 11,500 ft. Coming back 2 days later the temp was +15°C. My GPS recorded 244 kts on the way down. Back it was 125 kts.

Back on the ground, after the takeoff where the gear wouldn't come up, I took out the passenger seat and looked at the gear pressure gauges. The up gauge read 700 lbs and the down gauge read 1,500 lbs.

The 30°C temperature raise must have taken the down side and raised the pressure to over 1,500 lbs. It looks like enough of this high pressure escaped to the high side to stop the pump from starting up. I loosened a nut on the low side and the pressures went to zero. After tightening the nut, I opened and closed the dump valve and turned the master on and off. The low side recharged, the high side stayed at zero and I left to return home.

In conclusion, I think that from now on, I will open and close the dump valve to remove pressure from the hydraulic system when the plane is parked. As part of my startup checklist, I am adding:

     Master On, gear pump ran to charge the down side.

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Lorn H. 'Feathers' Olsen, MAA, ASMEL, ASES, Comm, Inst
DynaComm, Corp., 248-345-0500, mailto:lorn@dynacomm.us
LNC2, FB90/92, O-320-D1F, 1,300 hrs, N31161, Y47, SE Michigan