Cessna 206 Flap Control

 

Cessna Flap Indicator Cable

At lunch on Thursday, Marc related a mechanical horror tale. On some small airplanes, the flap controls are simple and straight forward. They'll have a lever, you pull on the lever to raise the flaps, and you push it down to lower the flaps. There is a mechanical linkage and you can observe the direct connection between the motion of the handle and the motion of the flaps.

However, somewhere lost in the mists of time (but likely in the 1970s), engineers were evidently getting bored because they were adding electric motors to everything. It started when they replaced the vacuum motors for windshield wipers and it has grown every year since. Anyway, somebody at Cessna decided they needed to operate the flaps on the 206 electrically. That's not too much of a problem, but they also wanted to be able to set the degree angle of the flaps, not just depend on the pilot eyeballing the flaps. So they devised a kind of a Rube Goldberg system to do this. They ran a push-pull cable from the flaps across the wing, down through the door post, out through the firewall, where it makes a U-turn in the engine compartment and then goes back through the firewall and thence to the flap control lever. 

At this point you could just connect that cable to an indicator dial. The pilot could look at the dial and see what angle the flaps were at, but that's not good enough. I mean we just went to the moon, we should be able to control the flaps automatically.

So what they did is create two parallel levers inside the dash, operating on a common pivot. One lever is connected to the flap control lever, the other is connected to the push-pull indicator cable. Here's the good part (or evil, depending on your viewpoint). The control lever has a protruding prong. The indicator lever has two microswitches. When you push the control lever down, the prong engages the microswitch that in turn activates the motor and lowers the flap. The indicator lever follows the motion of the flap and eventually it catches up with the control lever, the switch is released and the motor stops.

The problem Marc ran into with this old airplane is that the cable was not sliding smoothly, it moved in fits and starts. Solution is to replaced the cable with a new one that is super-non-sticky and guaranteed to be sticky free for the next millennia.

Simple, just replace the cable, except it has to be fished through this convoluted path. Makes trying to drain the AC on the wife's car look like a piece of cake.