The Trouble With Tip Tanks

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."

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Comments (7) Read through and enter the discussion with the form at the end
Dennis Hamblin - March 23, 2010 11:41 AM

The aircraft that you cite in the article was properly certified by the FAA on the basis of extensive engineering analysis and test data. Innuendo, hangar talk, and hearsay were not acceptable as substantiative data for showing compliance to the Civil Air Regulations then, nor to the current Airworthiness Standards now.


Dennis Hamblin
President, Flint Aero, Inc.

Mike Danko - March 23, 2010 12:21 PM

Dennis:

Thanks for your comment.

When you say the "aircraft" was properly certified, I assume you mean that the FAA issued a supplemental type certificate covering the Flint modification. The Cessna aircraft itself was certified long before Flint developed the modication, and Flint had nothing to do with that. Cessna never "recertified" the aircraft with the Flint tip tanks installed. The tip tanks were never a Cessna option, they were never installed at the factory, and Cessna has never approved their use or installation, as far as I know.

It would be interesting to learn what engineering analyses and test data support the Flint STC for the Cessna 206. For example, the STC allows the modified Cessna 206 to be operated at a maximum takeoff weight of 3800 pounds, 200 pounds more than allowed by the engineers who designed the aircraft originally.

How does the Flint modification strengthen the Cessna's wing support structure to accommodate the higher weight? What engineering data did Flint submit to the FAA?

Flint's recently revised Supplemental Flight Manual for the STC (accessible from your website) states "Forward and aft limits of CG and moment diagrams are extended with straight lines to 3800 lb."

What flight test data was submitted to support the new CG envelope? What flutter testing was done?

What data did Flint submit to show that the aircraft's stall warning system would remain accurate and safe after the tip tanks were installed?

Thanks for any information you can provide.

Mike Danko

Phil - April 27, 2011 3:35 PM

Mike,
Your article is interesting, but you are making many assumptions based on one man's opinion, which his coworkers apparently did not side with.

Cessna inevitably certified the 206 to 3600 gross which, for arguements sake, is probably 50% lower than what would reasonably cause structural damage. This 5% weight increase is negligible considering the additional wing area. I would bet that there are numerous 206 operators flying 200 pounds over gross on a regular basis with no additional wing area and no problems.

It must be considered that this 206 operator was likely trying to take off in typical soft/short field fashion with the nose almost off the ground at low speed, which can certainly cause the stall warning to go off... and it should! It's a stall warning, not a stall-already-happened. This 206's wings were probably partially or completely stalled during the botched takeoff, hence the crash at the end. It sounds like it did what it was supposed to do, but the pilot didn't take the hint.

Furthermore, you're asking a small operation to conduct Boeing/Bombardier/Cessna levels of structural testing, which is counterproductive to the entire aviation accessory and modification industry. This type of approach is why a modification that used to cost $100, now costs $5000 or more. Whether we like it or not, if all modifications and accesories were held to the same standard as the aircraft factories, we wouldn't have modifications; we'd only have what the factorys' lawyers allowed us to have. Rather than criticize every way I think modifications could be done better, it's important to recognize that our bigger engines, STOL kits, gross increases, and reliability modifications have improved the safety of our aircraft as a whole.

Phil

Ben Franklin - July 4, 2014 8:10 AM

Flutter testing? Data to show everything is 100% safe after a modification?

Most of us are not paid to ask rhetorical questions and none of us benefit from the answers.

Jerry - September 12, 2014 12:36 PM

Id like to see more stats on accidents before nay saying mods. If you feel Mods cannot prove 100% security just stay away from them. Of course a mod will change many thins. Say Xstol Wingtip mod on a 182. Better climb, cruise and takeoff but could put more stress on components during turbulence compared to a stock 182, so what do you do? fly a little more cautiously.

Ive never heard any such things from any of the operators I know that use said mods. now if Faa isn't doing their job, well then that's a different story.

Jerry

John Ferrell - December 14, 2016 5:57 PM

I like the quote about the FAA not adequately examining aerodynamics before issuing the STC. I think that if I was flying in any sort of aircraft, I would want it to be smooth in the air. I think that aerodynamics might also help someone to feel safer.

Shawn A Roberts - October 13, 2017 10:52 AM

In my experience, every single 208 that left Bethel to fly out to the villages were nearly sitting on their tails, and ALL of them had stall horns blaring well before lift off.
This was so common, no one thought anything of it. Yes, we were nearly always full of passengers and baggage.
This was in the 80's. I know back then, no one asked your weight, nor the weight of your baggage. We were not aware of frequently crashing 208's due to stall after lift-off. I sincerely doubt the Flint tip tanks had anything to do with the crash mentioned.

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