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Commentary on the Duke Split Flap Event

Bill Hale


Reprinted by permission for purposes of pilot education


Joe Lemanski followed up on the Duke s/n P-105 take off accident in Delaware:


"It looks like the flaps were split for takeoff.  In a Duke the opposite [right] flap is nearly impossible to see from the pilot position.  Joe’s suggestion: Don’t run them up and down after the pre-flight walk around; it’s mostly useless anyway and you can trap the problem at least on the initial takeoff that way."


Tom Turner analyzed the accident [in the January 3, 2008 FLYING LESSONS and Weekly Accident Update]. Tom points out that in sim training, a split flap is frequently mis-identified as an engine out. Shutting down an engine would make the situation much worse.  It’s not a scenario that we practice much. Tom suggests checking the flaps visually after cycling them.  Not likely in the Duke. Then it occurs to me that 300/400 Cessnas are also candidates for this problem as captains can’t see the flaps at all!  Do they ever have this failure?


In reflecting on whether a normal mortal could properly deal with this problem, it occurs to me that rigorous use of “dead foot dead engine”, especially the “verify” part would yield the correct answer.  The method of feathering the  prop in the direction of the yaw, so deftly demo’d by Lemanski in the DAB simulator last year would give the incorrect answer.  I have serious doubts that  any pilot could actually figure this out in the heat of the moment.


So…perhaps we should emphasize inspection of the flaps before takeoff as we can see them in most of the models we fly. 



Follow-up (Thomas P. Turner)

Just so you know where I was coming from, while I was a simulator and flight instructor in the early 1990s the company I worked for added a step to check all position of the flaps to the Bonanza, Baron and Duke checklists we developed.  This was, if I remember correctly, after several reports of flap "bounce" due to mis-rigged approach position limit switches. 


If the switch rollers freeze up the actuating arms can bend, creating a condition where the switches are engaged too close to one another in the flap extension range.  This causes the flaps to drift up and down instead of stopping in the approach position, and subsequent overheat of the flap motor. This may have been a situation unique to a relatively small serial number band that was coming out while I was employed in that position.


In practice I now teach conducting a flap position switch check monthly as part of an owner's "30 day inspection," about which I've written in ABS Magazine and elsewhere.  Otherwise, except for the first Before Takeoff checklist with a new client to demonstrate the operation of the flap system, I don't do a limit switch check before every takeoff.