Adjustment suggestion – V I P down-lock limit switches. I would bet that this down-lock switch adjustment procedure is not well understood; a slight misadjusting here will allow the gear to be un-safe, i.e. not “down and locked” over center. I have encountered this misadjusted condition even on new harness installations; it can lead to a “gear collapse”. I wish for all to be aware of this so shout it from the rooftops.
It is of the utmost importance to get this adjustment right!! The limit switches’ adjustment procedure is somewhat vague in the Comanche service manuals; the following will assist and offers a suggestion that may be of some help and clarification in understanding the system. See my further description on page 3.
Proceed as follows [airplane on jacks]. With the gear partially-retracted and the drag link disconnected at the side-brace stud, it is possible to mimic the knee action between the upper and lower drag link assemblies. Make this adjustment so the switching action takes place at the diagrammed value shown; i.e. for the mains 0.125″+0.06″-0.00″ and for the nose 0.187″+0.062-0.00, before the “down and locked” position. Worn bushings here will frustrate this adjustment especially for the mains on the rectangular switch versions. Triple-check this important adjustment for your confidence and you won’t have to go back or wonder about it again. Also please read and understand 6-53d [from SCSM] following. This is again vague without an answer to the question, “How much pressure”? Although I have a problem personally with this 6-53d method, I cannot disagree publicly with Piper’s method.
If you are working on a round-switch version airframe, use the following as a basic setting for installing the main gear limit switches. Adjust the nut position so there is 0.525 inch between the base of the micro switch and its mounting bracket. This works pretty well as a starting point. However, don’t let this suggestion be a set-it-and-forget-it. Triple-check this important adjustment for your confidence and you won’t have to go back or wonder about it again.
Discussion – Limit Switches Adjustment
The dimensional method offered here concerns the round version micro switches only – the rectangular version design does not lend itself to such a discrete method because the mounting bracket and striker plate can be bent; however the theory is the same. Keep in mind manufacturing tolerances when considering my dimensions and measurements; there could be variations but they would be only slight. Use these suggestions with that in mind.
As a basic setting for installing the main gear limit switches adjust the nut position so there is 0.525″ between the base of the micro switch and its mounting bracket. On my test setup this 0.525″ value corresponded with the centerline alignment for the mains; however don’t let this suggestion be a set-it-and-forget-it. Check this important adjustment as previously outlined in this manual.
I have measured switch adjustments in the field and observed them “all over the map”. On one specific airframe [a well-know gear incident] the mains varied from 0.450″ to 0.510″. To determine whether these fell within the safe limits I made a setup using components within the service limits and made the following determination. Note – an engineer would like this to be done geometrically however I don’t feel many will approach this adjustment using that procedure, it’s just too nebulous. The incorrect 0.510” setting allowed the gear-down light to illuminate 0.102” before the centerline alignment position. That could well mean trouble; the 0.450″ setting would be even worse. The switching action should NOT occur before the centerline alignment position. Another consideration is “coast” or momentum; how much occurs? I suspect it is nearly insignificant; Bob Weber of Webco Aircraft fame says “you would be surprised” [I assume he means more]. Premature settings are a gear collapse just waiting to happen.
Understand that in the Comanche system all three down-limit switches must have switched to obtain a gear-down light indication. A too-late adjustment [motor running after down-lock] will tend to “pull” the bearing out of the transmission casting [a Dura problem] and torque-lock the motor, thus opening the 30 amp circuit breaker. This resultant loose bearing is one condition that puts the Dura transmission in the non-serviceable category. Piper addressed this early in production; read SL-315 which can be found in this following link.
