Originally Posted by
Kagamuga
An area very often overlooked is an accumulation of STC for modifications.
Some years ago a C337 accident in the USA which I recall was an inflight break up; wing separation ..
The aircraft was modified with Vortex generators, and no doubt with an approved AFM supplement and STC. I recall the aircraft had modified wing tips; drooping. The airframe also has a STOL kit fitted with revised aileron and drooping ailerons when flap extended. Also in the mix was a 'speed kit' a series of revised cuffs etc at various fuselage/wing areas and the booms to the tailplane.
Each and every one of the mods was applied in accordance with the various STC's
BUT: no one had certified the COMBINATION of all the mods as one package.
In effect the aircraft was destined for another life. The aircraft was no longer a C337 but a highly modified airframe that resembled at C337.
I can't see how both VG and STOL kit could be added without both having specific approval to do so. How does one come up with performance data unless it's been tested/vetted with both options installed. The speed kit, fair enough just some fairings, account for the weight and add a few knots hopefully, but modifying the wing with lift devices you would have to have the whole thing approved, otherwise the performance data is void. Like PA-31, with VGs you get all the new data for operations with them installed and possible PUS for how many can be missing before performance penalties. If you then added a STOL kit it would have to have performance data and engineering to be used in combination with the VG. No data and no specific approval means they would not be legal to operate in unison.
If I was flying such a modified aircraft I would want to see the performance data state that it incorporates both VG and STOL modifications. That is some statement on the charts that says "STOL mod xxx with VG mod xxx installed" or such. Although most pilot's would probably assume the VGs are part of the STOL kit, that's where being familiar with these additions comes in.
That being said I'm not surprised the FAA might just rubber stamp it after seeing some mods that have been done to transport category aircraft where additional sizable holes have been cut in pressure hulls with no real mention of reduced life or such.