Only modifications that carry a Supplemental Type Certificate may be legally installed on an aircraft. The Supplemental Type Certificate guarantees that the FAA has thoroughly tested and reviewed the modification. And it’s the Supplemental Type CertificCessna Floatplane(Photo by TailspinT)ate that insures that the modification is safe and compatible with the particular model aircraft on which it’s being installed. Right?

Maybe not. Owners really shouldn’t place too much stock in an STC. Or so says one former NTSB accident investigator. The investigator, now retired, explained to me that most owners might be surprised by how little work the FAA does before issuing an STC. Sure, the STC process is a huge paperwork shuffle for the modification’s manufacturer. But it’s little more than that. The process seldom entails any real independent engineering cross-check on the FAA’s part.

"Give me an example", I asked. "OK,’ he said. "Let’s talk tip tanks."

A popular modification for many models of Cessna single-engine aircraft are wingtip extensions that

hold extra fuel. The modification for the Cessna 206, for example, adds about three feet to the aircraft’s wingspan and holds an additional 30 gallons of avgas.  According to some, the added wingspan improves the aircraft’s STOL capabilites somewhat.  In addition, the modification allows the pilot to operate the aircraft at a maximum weight 200 pounds higher than the stock configuration.  All that sounds almost too good to be true. It’s no surprise that the tip tanks are a very popular mod.  But beware.

“The FAA did not adequately examine the aerodynamics before issuing the STC,” the investigator offered. "You really have to wonder whether the mod should ever be installed on a Cessna."

The investigator says he examined a Cessna 206 that crashed off the end of a short strip in Alaska. Ultimately, the NTSB determined the cause was pilot error. But the investigator found a couple of things along the way that piqued his interest. First, he learned that the pilot heard the stall warning sounding throughout almost the entire takeoff roll, something you wouldn’t experience in a stock 206. In checking with other pilots of similarly modified 206’s, the investigator found that the "always on" stall warning is typical of aircraft that have had the tip tanks installed.

As it turns out, the tip extensions changed the wing’s average angle of attack. But the Cessna’s original stall warning sensor wasn’t modified to reflect the new configuration. That’s why the warning sounded even when the modified aircraft was flying well above the stall speed.Center of Gravity Envelope

So what’s the problem with that?

“A stall warning that sounds all the time, every time, is no stall warning at all” the investigator told me. "Why the FAA let this modification pass is beyond me. Not even a note in the documentation. I’m sure the FAA wasn’t even aware of the problem, because they didn’t do any testing. And I know – I went through the FAA’s files.”

And that’s not all.

The modification allows the aircraft to operate at higher gross weights. In the case of the Cessna 206, 3800 pounds instead of 3600. That’s a great benefit for the operator. But when the investigator examined the technical data supporting the STC’s issuance, he found none to support the gross weight increase. Without that documentation — in particular, a new weight and balance form for the pilot to use to make sure the plane was properly loaded — operation of the aircraft at the new, higher weights was illegal and, of course, potentially unsafe.

When the investigator pointed out the oversight to the FAA Certification Branch, the office was concerned. Commercial operators scattered far and wide were operating the modified aircraft outside the aircraft’s approved envelopes. "The FAA quickly generated new weight and balance charts. Impressed, I asked them how they did it so fast.. According to the FAA tech, ‘we just used a pen and a straight edge.’ That’s right, no engineering, not testing. Just extend the lines on the stock weight and balance charts and hope for the best."

What does this all mean to the aircraft owner who has purchased an aircraft mod and the STC to go with it? Before having the mod installed, the investigator suggests asking for the engineering data that was supplied to the FAA Certification Branch, including any flight test results, the flutter tests, and the structural tests (both static and dynamic). "If that data is lacking, the owner/pilot is more or less a test pilot."