Seneca II landing gear

My 1976 Seneca II will intermittently fail to complete the retraction cycle after takeoff. Selecting "Gear UP" will begin the cycle by extinguishing the three GREEN down lights, illuminating the red UNSAFE light, electric motor engages and draws power(ammeter reading increases). After 30 seconds, I have recycled to GEAR DOWN, and gotten 3 GREEN lights. Then usually, the second GEAR UP selection will finish normally, and red UNSAFE light goes out. Mechanic unable to duplicate this on jacks and gear swings. All is normal on jacks. Microswitches have been checked, and continuity seems ok. Asking for solutions, comments from experience with this problem, and how to fix it. This problem has plagued me ever 3-4 months, regardless of weather (airplane is hangared) ever since plane was purchased in 2007.


  • edited September 2022
    As the original owner of a 1989 Seneca III, I will be very interested in an 'experienced reply' to this one. Occasionally I experience this same condition, when the gear is very lazy retracting...usually it does lock up, but not without causing some anxiety. Over the years I have overhauled the gear motor, and replaced the gear cylinders. always, the gear performs flawlessly on jacks, when there is no aero pressure. One thing that may be your problem is internal leakage within the gear cylinders> try jacking the plane, retracting the gear, and disconnecting the battery. If the gear falls, likely caused by internal leakage!! Good luck.
    David- Seneca C-FJMM
  • The Seneca gear system is actually pretty simple. There are no uplocks and onlyl hydraulic pressure hold the gear in the up position.

    When the selector is in the up position, the pump runs until the hydraulic pressure switch reaches 1800 psi. during flight, the pressure may bleed off, and the switch closes and pumps things back up.

    That's hint one in the troubleshooting - whether or not the pump is running. Unfortunately, there's no indication it is running, other than what you can surmise from the ammeter.

    The up limit switches simply illuminate the gear unsafe lamp. Any time the gear are not full up or fully down, the unsafe lamp lights.

    While the schematic is complicated, once it is broken down by function, it is a lot more clear how it works.

    If your gear are failing to come up enough to activate the up-limit switches, and the pump is still running, that means that for whatever reason, insufficient pressure is present to achieve full retraction.

    There are a few things to check, of course, obvious stuff like the reservoir actually has enough fluid in it should be first.

    There's also a vent hole, basically a tiny pinhole right next to the filler screw. If this gets blocked, the reservoir won't vent and the pump may not be able to achieve full pressure due to the vacuum in the reservoir. My mechanic told me about a case where he chased a blocked vent for quite a long time that only produced sporadic gear problems. As I recall, someone had simply put an oversized washer on the filler screw and blocked the vent.

    The free fall valve could potentially leak, allowing some of the "up" side pressure to bleed over to the return side.

    Some years of Seneca II have a "thermal fuse", which is just an over pressure valve, to allow for expansion of fluid from ambient heat. This could also conceivably leak hydraulic pressure from the high pressure "up" side to the return side.

    Hope that helps.
  • Sorry to be late in follow-up comment. I replaced the hydraulic powerpack with a certified overhauled unit, and purged all lines before hooking up new unit. Fluid was dark, dirty, and likely had not been changed in 20 years. Also noticed that old powerpack had the vent hole plugged by presence of screw and washer, which is to be removed on installation. Gear has been working fine since performing the above maintenance and repair as described above.
  • Howdy, I'm looking for the part numbers and a source for the hydraulic pump brushes and springs. The service manual describes the inspection procedure, but the parts manual doesn't have the part numbers. Piper tech support is clueless. Anyone have a source?

    '77 Seneca II

  • turbosem81, good reminder. It is required to purge the lines out when swapping pumps (as you might contaminate the new one) but I am sure there are many planes flying with old oil.

    The vent plug is a good tip too. It looks like it should be screwed tight but needs to be open to the air as you have noted. if the vent plug is blocked the pump will struggle to displace the fluid from the tank to the gear legs. This could easily be a reason for slow retraction or two tries.

    Eric Panning
    1981 Seneca III
    Hillsboro, OR (KHIO)

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