Maintenance Tip: Aircraft Flaps – A Cold Weather Catch

Maintenance Tip: Aircraft Flaps – A Cold Weather Catch

Just last month, a customer called me with an AOG situation – his aircraft flaps were stuck in the down position after landing. He flies an E90 with the old style flap switches. There are plenty of 90s and 200s still flying today with these switches, which run on ramps instead of the cam type.

This owner-operator is still a little new to the King Air but learning every day. He’s logging a good amount of time in the aircraft, having put almost 160 hours in it between the last two phase inspections.

Acting Up in the Down Position

In this particular situation, it was a normal landing, and up to this point his flaps had been working perfectly. He had checked his circuit breakers and had moved the flap handle up and down to no avail. They would not retract.

Weather-wise, winter had begun and the western United States was in a pretty deep cold snap. The OAT was around 42 degrees Fahrenheit on the ground when he departed that morning, but was below freezing at his final destination of Reno, Nev. Since I know from experience that the older, ramp-style switch arrangement in the flap system is susceptible to freezing, I had a couple questions for him.

First I asked if he encountered rain at any point en route to Reno. He said he hadn’t. My next question, “Did you have the aircraft washed before this trip?” and got a “yes.” We had already discussed the specifics of the OAT on the ground and at altitude, so I suspected the culprit was a frozen up limit switch. When this switch sticks in the actuated position, it prevents the flaps from retracting.

Low-Tech Solution

I told him to go out to the aircraft, turn the battery on and put the flap handle in the “up” position; then I instructed him to stand at the trailing edge of the R/H inboard flap and wiggle it up and down. He did as directed, and lo and behold, the flaps came up!

Needless to say, he was ecstatic. He was able to get his passengers back to home base as originally planned, with the flaps operating normally. They have not acted up (or down) since that time.

I am sure that he had a frozen switch and what I told him to do simply knocked it loose. The ramps for these old-style switches are riveted to the leading edge of the R/H inboard flap. It doesn’t take much moisture for this to happen in freezing temperatures, and a good rattling may be all that is needed to jog it loose.

By the way, the design of the newer cam-type switches makes them much less susceptible to freezing, and they are further protected by a plastic cover.

Flap Motor – A Similar Snafu

I have seen similar behavior in flap motors, where there can be a dead winding in it. If the motor happens to come to rest on that exact spot, the flaps will not come up. But by giving the flaps a good rattling, the actuator moves just enough to jerk the gear box and nudge the motor off the dead winding. The flaps will suddenly spring back to life. Note that this temporary fix will only work if the flaps are stuck in the full down position.

Obviously, with the flap motor, the problem has nothing to do with moisture and freezing temperatures. This can happen in any kind of weather. The thing is, all King Airs have the same flap motor and gearbox. If the flaps are stuck in the down position, this remedy just might get them to go up.

For some reason, rattling the flaps in the up position will not affect the actuator enough to move the flap motor off a dead segment. So this low-tech advice won’t be helpful if the flaps are stuck in the up position. But that is not as much of a problem because you can still take off and do a no-flap landing.

Although it goes without saying, I have to say it anyway: This course of action would only be taken as a last resort, after exhausting all other options as laid out in your POH. And by all means, if you ever have to do this to get your flaps to come up, then your very next stop had better be with a good mechanic that can troubleshoot, identify and remedy the problem(s) in your flaps system.

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