Hello all,
 
Sometime ago, I watched with surprise 2 separate videos each showing Lancair 4 main landing gear collapse during Taxiing,
Well, I tested mine and with a force of about 50 Lbs pushing backwards on a down locked gear, I was able to dislodge it and unlock the gear. Due to the design angle of the main gear leg, it is difficult to exert a backward force, since the weight of the airplane exerts a force to extend the gear further, With the exception of a rock or small pothole on the runway, the only other situation I can imagine which exerts a backward force on the main gear is Breaking while taxiing.
As all of us probably aware of the lock down pin in the hydraulic cylinder which is pushed down by a spring to engage it in a groove inorder to lock the gear in the down position.
the spring is a series washers type stacked against each other in alternating up and down position (6 pairs = total of 12 washers).
they are sold at McMaster part # 9712k58 with a working load of 110 lbs and a flat load of 175 lbs.
I contacted Lancair and they said they changed the number of washers to improve the spring to a total of 14 ( 7 pair) and they emailed me the drawing which I am attaching in a PDF, however Lancair did not test it against significant loads.
I have 2 questions, first, how far down the groove should the lock down pin travel to secure a down lock position and maybe a mark on the pin above the piston retainer should be placed and checked every annual or so.
The second, and probably will be subject of more discussion is, what is the optimum number of washers in the spring? what is the minimum load it should exert? there is also an offline comments regarding replacing it with a spiral type spring, well these are questions for the techs and engineers in the group!
 
Kamal
IVP 1104T