By Douglas R. Herlihy
During a takeoff from a short remote strip, the single-engine air taxi Cessna U206 failed to accelerate before running out of runway, impacted in the water and overturned. Miraculously, neither the four sportsmen nor the pilot were killed. The pilot likely misjudged the distance needed for his loaded aircraft, but his reply was most revealing to investigators, when asked whether he recalled the sound of the stall warning horn prior to lift off attempt. His aircraft had been equipped with wing-tip fuel tanks and he said, "with this tip-tank modification, the stall warning is always on, during take off run."
A survey of other such models of Cessnas equipped with these wing-tip fuel tank found that often, a stall warning does sound at lower-than-normal speeds and thus routinely is ignored. The modification, through the increase in the wing span and aspect ratio of the wing, tends to change the location of the aural stall warning device as a function of its location on the effective wing span. (Simply put, it's a new wing that reads speeds with an old indicator.)
A search of certification records through the FAA Certification Office revealed an interesting and not uncommon set of circumstances relating to the approval of this modification. The records showed that the FAA required no engineering test data to be supplied by the third party component manufacturer as a basis for approval of this modification. Undoubtedly the extended wing now provided added lift and greater performance. The FAA went an added step and granted an increase in the aircraft's gross operating weight, again without recorded flight test data. When this weight increase was challenged by another branch of the FAA (Flight Standards), the Certification Office merely created a new Cessna 206 weight & balance envelope with a pencil and a ruler.
Moreover, when Cessna investigators were asked about the performance of this new wing, they admitted that they "had no idea how this wing would perform, it's simply no longer our design." Beside the stall warning change, it certainly raises questions as to other aerodynamic changes this modification has produced in the wing, such as the static and dynamic moments with the fuel added to the wingtips. How does it affect the airplane's spin recovery, for example? Cessna doesn't know. It's never been tested, and that wing is not described on the original type certificate (TC).
Modifications to basic airplanes, approved by the FAA in the form of Supplemental Type Certificates (STC), or even "field approval" may or may not have sufficient supporting engineering data. When a stall/spin accident, or landing impact short of a runway, or takeoff crash occurs, we should be looking at how the many modifications may have affected the performance. Who designed this wing anyway?
Doug Herlihy is a former Investigator-in-Charge with the NTSB Anchorage Field Office, now managing partner of Herlihy & Leonard
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