seat pan cracks

[wadeg]

Wondering if anyone else has had this problem.
I have had trouble with the fiberglass seat pan cracking. First I had a crack in the fiberglass next to the tube at the right front. Then had one at the left front. Was doing steep turns (only 1.6g) and felt seat sag. Found crack at the top left next to the tube. The points of cracking seem to be at a stress point. The lower ones were fixed and reinforced with multiple layers of glass. I will do the same for the new one.
It was not a problem fortunately due to reinforcing straps under the pan. Anyone else having this problem?
Oh, and I am 165lbs so can't see me as the part needing modification!

[jtpitkin06]

Wade,

Thanks for the PIREP on seat cracks. There are a few isolated cases where the seat actually sagged enough to rub the flight controls, but none with serious consequences.

As you mentioned, owners should look for stress risers and inspect for cracks. My seat pan had several lumps of resin under the thigh hook. It prevented the hook from fully seating on the fuselage. An air powered die grinder with a carbide acorn burr did a great job of smoothing lumps and making a uniform groove. I also filled in a few valleys with Hysol.

Because the seat pan is held only at the back rail and thigh rail it is essentially a sling. Should a hook fail, either top or bottom, the seat could slip down. With that in mind it might be a simple precaution to add a safety strap under each seat to prevent seat sag. A simple 2 inch strap or webbing could do the job with minimum weight.

John Pitkin
Greenville, TX

[Dorsal]

I also found a few cracks due to not properly relieving the seat to match the welds on the frame tubing. The cracks did not propogate far and were easely rectified. I used a "Butt Block" (patten pending) in case the seat ever broke.

[Mnflyer]

Hi I found a few cracks in my seat assy also over the years, repaired them with fiberglass overlays for now its holding. I think that the original fiberglass work was less than 100% at a couple of the cracks there had been ply seperations like the glass had not been completely saturated (my opinion). Mine is a Model III.

[Av8r3400]

My seat pan also cracked and was rubbing on the aileron push-pull rod. Not serious in-flight emergency but the plane became very heavy in the roll control.

I used a variation of the "butt-block" as to avoid paying the heavy royalties to Dorsal.

The reinforcing "rib" on the underside of the seat pan de-laminated from the main pan. After doing a fiberglass overlay of the damaged area, I used a piece of 1-1/4" treated decking material under each side, tie-wrapped to the tubes below the seat.

[HighWing]

I suspect over the years the vendor that supplied the seat pans either changed at times or suffered sever quality control issues. My first Model IV - 1993 - seat pan was pretty much flawless and served for 900 hours over nine years - attached to the tubing with Nylon wire ties. My current Model IV - 1992 - seat pan will need a complete overhaul before final assembly as it is just about as flexible in critical areas as the tip of a fly rod.

Regarding the slings. This first became an issue with the Series V when the console and upswept floor boards were introduced. The IV and earlier models had a definite curl to what John calls the Thigh hook. To accommodate the upturned floor board that curl was cut back probably a half inch or so. The first incident I heard of with this problem was a guy in a V that had his seat slip off the "thigh hook" and land on the control rods on short final in gusty conditions. Slings and such became standard add ons on the later Kitfoxes because of his report and some builders put them on the earlier models as well. I don't think I will worry about a sling after my remake of the current seat as there is too much history with the older models. I know of some flying them that don't even secure the pan to the bulkhead tubing - simply relying on the snap in effect and tension tending to tighten the grip.

Lowell

[SilverFox5]

Hi, This an old subject but thought I would share how I handled this problem. See attached picture. Started with a 3" x 4" x 28 inch long pc of balsa wood. Glued a 1/4" × 3" × 28" pc of plywood to the bottom and a 1/8" × 4" × 28 pc on each side. Made a template from the bottom of the seat pan and started cutting and fitting. Epoxied in at 4 points. Seat pan  sits directly on these support blocks. I weigh 250 lbs and do not trust anything that is not solid to the frame. After all the trimming the total weight is 4 lbs, 2 lbs each. To me well worth the safety security. Note that the co-pilot side had more trimming done after this picture was taken.

Joe Leeman
K5
Rotax 912uls
Freedom, WI

[Meyer]

Is there any risk of bending tubes? Or is the theory that spreading the load across 4 tubes lessens or takes away that risk?

[hansedj]

cracked also had to repair I am 168.

[SilverFox5]

The theory is I feel it much more important to keep the seat off the flight controls. I weigh 250 lbs and a good number of my co-pilots that might fly with me are over 200 lbs. I do not trust the flimsy fiberglass seat w/o something solid to the frame underneath. The only time that bending the tubes would be an issue is in a very hard landing. If this means bending the tubes under the seat i will just need to learn to make a softer landing. Or fix the tubes if I do bend them. More than likely I would survive the hard landing. If the seat breaks in flight and jams the controls I think the outcome is much worse.
But yes I did consider the possibility of bending the tubes.
I'm surprised that kitfox still supplies this seat design in new kits knowing that they could and have broke and caused an accident.

Joe Leeman