[Denhamblin]

I was recently made aware that some here have questions about the AD issued on Aviation Enterprises regarding extended wing tips and whether Flint Aero will also be affected. There will be an AD forthcoming dealing with our wing tips as well and we are currently working with the FAA and the Small Aircraft Directorate on this matter. The incident in Farmington, New Jersey prompted the FAA and the SAD to look into not only AE's modification but Flint Aero's as well (based on one Service Difficulty Report on a Flint modified T337G that reported some wing skin cracks at WS150 underneath the removable fuel tank access cover). I have had an engineer working on this project for the last few months and the engineering study has been quite intense. The FAA was able to get engineering data directly from Cessna though that information could not be shared with us. We have had to reverse engineer the wing which has taken quite some time.

To deal with this upcoming AD we are looking at two options. One would be a structural reinforcement while the other would involve reworking the limitation for “zero fuel” in the tip tanks. The current limitation is the requirement of maintaining at least 12 gallons of fuel in each tip until the aircraft gross weight reaches 4,330 lbs. This limitation has been in place from the time the STC was first issued in 1991. It is possible that the "fix" will include a combination of the two options.

The structural reinforcement will be relatively non-intrusive and includes reinforcing the stringer splice at WS150 and some externally mounted stainless steel straps on three stringers and the forward spar cap outboard of WS150. At WS150 there will be two bands of SS attached chordwise. One band will be 3.5” x 19.75” and will be riveted to the underside of the removable fuel panel and will attach at the existing screw locations at WS150. A second band (5” x 19.75”) will be riveted to the exterior of the upper skin and will also attach at WS150 via the existing screw locations. The stringer straps will be 1” wide and begin 2" outboard of WS150 and terminate 3" outboard of WS177. The spar doubler (also 1” wide) will originate at WS162 and terminate at WS192. These reinforcements will be required only on the upper wing surface. Of course, this “fix” will first have to be approved by the FAA though so far they have been very happy with the data we have supplied. Please note that the plans for the structural reinforcement is preliminary and may change as we continue to refine our data.

The limitation option is still one we are looking at. We believe the aircraft is fine at 4,330 lbs with a minimum of 12 gallons in each tip. At higher weights we believe that there would need to be more than 12 gallons in each tip. Right now I would error on the side of caution and recommend keeping the tips tanks full until the gross weight drops to 4,330 lbs and then begin transferring fuel from the tips to the mains. We are still working on what the “zero fuel” weight would be but we feel the weight of 3,300 lbs that is listed in the AD which affects Aviation Enterprises is much lower than it needs to be. Right now, though, I would recommend keeping a minimum of 12 gallons in each tip at all times (or at least when the gross weight is above 3,300 lbs) until we finalize the "zero fuel" weight. The limitation option may not be practical in the long term though it would at least be a required temporary limitation until the aircraft is reinforced.

This AD will only affect 337s that do not have the 150.6 gallon capacity factory fuel tanks, what we refer to as the “long wing” aircraft. The wing structure of the “long wing” is stronger than the “short wing” aircraft. I would note that the aircraft involved in the New Jersey incident as well as the one in the recent Avon Park incident were "short wing" aircraft.

I think we are all aware that the pilots involved in the New Jersey incident were asking a lot of the airframe. The FAA quickly issued a SAIB (CE-10-20) though the response to that SAIB was tepid in their view. Flint Aero was included in that SAIB. The latest incident in Avon Park, Florida has the FAA additionally concerned because the wing failure occurred in the same general area (perhaps a bit inboard) though this aircraft did not have wing tip extensions. I do not know if this latest incident will have an effect on the upcoming AD for Flint Aero but they did contact me for some input.

I do appreciate Larry for allowing me to use this forum to get some information out. If anyone has any questions feel free to contact me via phone (619-448-1551), e-mail (Dennis@flintaero.com) or here in this thread.

Dennis Hamblin
President, Flint Aero, Inc.