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Mooney Electric Flap Problems
ShopTalk - May 2020
This month’s ShopTalk will cover a common electric flap problem with our aging Mooney fleet. The fleet is not getting any younger so almost all the ShopTalk articles are written about this ongoing issue and how to deal with this issue (preventive maintenance).

In 1969, Mooney stopped producing airplanes with hydraulic flap actuators and adopted an electric actuator system. The hydraulic system was a piston pump in the forward belly (just under the copilot’s seat) attached by mechanical linkage to a handle just right of the manual gear johnson bar on the lower instrument panel. A slave cylinder located in the belly attached to the flap mechanism under the baggage floor on the rear spar.

With the newer system, a simple electric switch, circuit held closed for flaps down or open for up, was installed in the center console just right of the throttle/prop/mixture controls. The flap position indicator was still located next to the trim position indicator in the lower console. When the throttle console disappeared in 1978, this switch was relocated to the top portion of the lower console. The indicator was relocated to the right portion of the pilot’s instrument panel when the M20M first came out but the K and J models were still in the lower console. This switch is pretty much indestructible. So far, we have never had to replace one in 41 years of working on Mooneys.

Of course, there has to be a way to limit the up and down travel of the flaps, and that is the culprit that messes with the flap system. Here are two typical scenarios:

1) Flaps won't retract after landing as you taxi to your parking spot, or better yet, they retract with the switch in the full up position but only after you hit a bump in the ramp (that's my favorite one).

2) One day on a test flight to rig the flight controls, after using full flaps the darn things would not come up. We had to fly back to the airport at 90 knots. That's a real bummer in a M20K! (If I wanted to fly around at 90 knots I would work on Cessna 150s).

In Figure 1, you can see the typical Mooney electric flap system mounted under the baggage floor. The electric motor and its jackscrew are attached to the flap mechanism on the rear spar. This mechanism attaches to your flaps via a short rod end on the inboard side of each flap.
>The two switches located above the jackscrew are the up and down limit switches. These switches are actuated by the rolled doughnut area on the end of the jackscrew cylinder. These are typically plastic switch bodies with metal contacts inside of the bodies and an actuating lever. The switches can be adjusted on the rail that their mounting block is set on.

Due to the location and environment of these switches, they tend to get dirty and need annual cleaning with an electrical contact lubricant type cleaner (not WD40). This is one item not found in the service manual. Try to keep in mind your model’s service manual was written for a new airplane, not a 43-year-old M20J. Even if you clean and lube these switches every annual they will still eventually fail. We suggest replacing these switches when you replace your engine or every 2000 hours. The two switches will set you back about 100 bucks and take an hour to change them. That's cheap preventive maintenance for any airplane.

When you are away from your regular mechanic, consider what an FBO will charge you to fix your flaps, let alone the issue of being stuck miles from your home base. Of course, we are assuming that where you have landed has a mechanic with a Mooney Service and Parts manual and has the correct part (a big assumption in the year 2020).

Figure 2 shows a collection of some failed switches straight out of the aircraft, some physically broken, some internally inoperative.
If you have not yet replaced both gear switches, order them from your favorite Mooney dealer and get your mechanic to replace them at your next annual, especially if your airplane has more then 2000 hours on it. A little preventive maintenance will enhance the reliability of your gently aging Mooney.

If you have questions about this or any other ShopTalk article, please e-mail me at shoptalk@knr-inc.com or call me at my aircraft repair shop, 307-789-6866. All ShopTalk articles can be read here at www.knr‑inc.com. Until next time, enjoy flying your Mooney.