Friday, March 24, 2006

Weedwhacker Flaps

The WD40 has electrically actuated flaps. A flap-shaped lever to the right of the power quadrant controls movement: up for up and down for down. Full flap extension is 40 degrees. When the flap selector is returned to the neutral position, an electric brake stops the flap motor to freeze the flaps where they are.

An electric flap position indicator above the lever shows the position of the left flap. Both flaps are supposed to move together, and any asymmetry will cause roll. "Split flaps" -- one being extended more than the other -- are vanishingly rare on what I fly now, but the training manual discussion of asymmetric flap deployment leads me to suspect that it is a recurring problem with the Weedwhacker. There is some protection in place, but there are a few holes.

Limit switches will cut off power to the flap motor when the left flap reaches the full up or full down position, or if the left flap fails to move off its stop within one second of power being applied to the motor. One second here equates to 4 to 9 degrees of travel of the right flap. So as I read it, if the left flap moves and the right one doesn't, it's up to the pilot to recognize the asymmetry and shut off the flap motor. Likewise there is no sensor other than the pilot if the flaps are at an intermediate position, and then the pilot attempts to retract them or extend them further. For this reason, pilots are cautioned to disengage the autopilot before operating flaps. (The autopilot is not smart enough to say "that's funny," and would continue to attempt to fly the airplane without reporting its need for an unusual amount of anti-roll input). Other advice to pilots is to move the flaps in one second increments, confirming proper roll control before continuing their travel. If the flap circuit breaker tends to pop, there is likely some problem with the flap mechanism, and the flaps should not be extended beyond fifteen degrees.

Later models have a lever that moves in a track, with the lever position corresponding to the selected flap extension. The flaps will automatically run to the selected position and stop. These ones also sense the position of each flap and compare them throughout their travel, shutting off the motor if the difference exceeds five degrees. It even has a test function: while the flaps are moving you press the test button and it sends a false asymmetry signal to the amplifier, and the flaps should stop, until you release the button. I don't know how the circuitry works for that, but it can't be indistinguishable from the real signal, because if a real asymmetry is sensed, the flap system must be reset by maintenance before it can be operated.


Anonymous said...

Sounds like one of those things you'd have to be on top of. Flap assymetry is a real bugger. Can I guess that you'll be flying a Saab 2000 or 340 of sorts... hmm

Anonymous said...

With the split flap thing and the anti-servo tab, this sounds like a PA31 to me! You also need to remember from what fuel tank the heater draws from and how much you need to allow for it in your flight planning, that sounds like a regular question in ground school.

Sam Weigel said...

Philippe, don't let the wrath of Aviatrix rain down upon you by trying to guess the identity of the Weedwhacker. She's calling it the WD40 for a reason.

Trix, your read into the WD40's early flap system is correct. It's pretty retarded, but you only have assymmetry protection for the left flap, and that only if it's full up or full down. It was subsceptible to split flap because the flexible shafts that connect the flap motor to the flaps were too thin for the aerodynamic loads at extensions beyond 30 degrees. You'll find many early WD40s have an AD on them that limits flap extension to 30 degrees. All in all, it was a poorly designed system, rather uncharacteristic of the WD40. The later "Calco" type system was a huge improvement.