The Savvy Aviator #42: Don't Worry -- They All Do That

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The Savvy Aviator

This is a story about Cessnas, but please don't hold that against me because the lessons learned are applicable to all aircraft makes and models.

Years ago, I used to travel throughout the country putting on technical seminars for the Cessna Pilots Association (CPA) for owners of various Cessna models, together with John Frank, executive director of CPA. Whenever John and I got to the landing gear segment of the course, we made a point of asking the 20 or 30 assembled Cessna owners attending the seminar for a show of hands:

"How many of you have had a problem with nose-wheel shimmy?" Invariably, virtually every owner in the audience raised their hand.

"OK, how many of you have asked your mechanic about this, only to be told that all Cessnas exhibit nose-wheel shimmy, and that it's simply 'the nature of the beast'?"

Usually, at least half the hands remained up. That was not a very reassuring sign about the competence of the mechanics these owners were using to maintain their Cessnas.

Nose Wheel Shimmy Normal?

Nose wheel shimmy in single-engine Cessnas is not "the nature of the beast," and is readily correctable.

Although nose wheel shimmy is extremely common in single-engine Cessnas, it can and should be fixed. Such shimmy is almost always due to one or more of the following: (1) worn torque-link bushings, (2) an out-of-round or out-of-balance tire, (3) elongated holes in the shimmy dampener linkage, or (4) a defective shimmy dampener.

A mechanic who dismisses a problem like nose-wheel shimmy as "the nature of the beast" and claims that "all Cessnas do that" is just copping out. If a mechanic tells you something like this, you'd be wise to seek a second opinion (and perhaps to change mechanics).

Back in the days when CPA used to operate its own maintenance shop at Santa Maria, Calif., John used to make a standing offer to Cessna owners who brought their airplanes to the shop: If the CPA shop couldn't fix the nose-wheel shimmy, we'd buy the owner a steak dinner at the best steak joint in town. (John never had to pay for anyone's dinner.)

To the best of my knowledge, nobody ever died because of uncorrected nose-wheel shimmy. But from time to time, we hear of a mechanic who dismisses a genuinely serious problem with "don't worry about it, they all do that." And that's scary.

Exhaust Leaks Normal?

An owner who recently acquired a Cessna T310R noticed gritty brown stains developing on top of his left engine nacelle aft of the louvers. He also noticed some cracking and bubbling of the paint. No such symptoms were apparent on his right nacelle.

The owner of this 1977 Cessna T310R became concerned about the gritty brown stains behind the louvers on top of his left engine nacelle, and some bubbling of the paint. His A&P advised him not to worry about it, claiming "almost every twin Cessna has some degree of heat staining." Bad advice. (Click here for a larger version -- 60 Kb.)

Several A&Ps told the owner not to worry about it, because "almost every twin Cessna has some degree of heat staining." But it still worried the owner because he was seeing these stains only on the left nacelle and not on the right.

The owner then did a very smart thing: He sought a second opinion by posting a query on the Cessna 300/400 forum on the CPA website. I saw his post there, and asked him if he would take some digital photos of the brown stains and upload them to the forum so I and others could take a look at them.

The next day, the owner posted some photos of the brown stains on the forum (see photo at right). I replied that I thought that those stains were probably symptomatic of a substantial exhaust leak in the vicinity of the turbocharger, and that I considered it imperative that he have the exhaust system in that area inspected thoroughly and the cause of the leak identified and remedied right away.

Not long afterwards, the owner removed the top cowling from his left engine nacelle and took several more digital photographs, which he posted to the forum. One of those photos (see photo below) showed considerable white powdery deposits on the turbocharger heat shield and firewall. I told the owner that this almost certainly was confirmation that he had a serious exhaust leak at or near the turbocharger. Several other owners and mechanics chimed in and urged that the owner take this situation seriously.

"When it comes to the exhaust system of a turbocharged twin Cessna, you have to take everything seriously and you can't be too careful," I told the owner on the forum. "Too many people have died in these airplanes as the result of in-flight exhaust failures. At one point during the 1990s, we were averaging one fatality per month due to these problems, and the FAA very nearly wound up grounding the whole fleet. Since 1999 we have had zero exhaust accidents, due in part to all the publicity and in part to the new AD 2001-01-16 that we at CPA worked on so actively. That's an eight-year track record of perfect success -- one we're very proud of -- and I'd hate to see it blemished."

The Owner Takes Command

When the top cowling was removed, the owner found white, powdery stains on the turbocharger heat shield and firewall, indicative of a substantial exhaust leak. The owner insisted on a pressure test of the exhaust system, which revealed a gaping leak at the turbocharger-to-tailpipe joint and a loose exhaust V-band clamp. Removing the tailpipe, cleaning up the mating flanges, and retorquing the clamp cured the leak. (Click here for a larger version -- 47 Kb.)

The next day, the owner cleared his calendar and took his airplane back to the shop. "I got some raised eyebrows when I insisted that we pressure-test the system," the owner reported. The owner decided to stick around through the procedure to make sure the exhaust system was checked thoroughly for leaks.

When the mechanic pressurized the exhaust system with shop air and started squirting soapy water on the exhaust plumbing, it was immediately apparent that there was a major leak at the junction of the turbocharger and the tailpipe. "We saw bubbles the size of a man's fist forming between the tailpipe and the turbo," the owner said.

The mechanic discovered that the V-band clamp that secures this joint was extremely loose. The nut on the clamping bolt could be tightened a full half-inch. But even after tightening the clamp, a second pressure test showed little improvement in the leak.

The mechanic then removed the clamp, separated the tailpipe from the turbo, cleaned the mating flanges on both the tailpipe and the turbocharger, and then reinstalled the tailpipe and clamp. A third pressure test showed no leakage whatsoever at the joint.

The owner was very happy about this outcome. He posted the details of his trip to the shop on the CPA forum. "I want to thank everyone here who would not let me accept the word of several A&Ps who told me it was nothing," he said. "It's amazing what two hours of labor can accomplish."

Not So Fast!

But after reading the owner's most recent posting, I still had an uneasy feeling. "When your mechanic tightened the V-band clamp on the turbo-to-tailpipe joint, I hope he used a torque wrench and torqued it to the specified value," I said. "The torque on that clamp is critical, and that particular nut should never be just tightened by hand until 'it feels right'."

Nope, reported the owner, the A&P didn't use a torque wrench.

"After your mechanic cleaned up the flanges on the turbocharger and tailpipe, the flanges should have been inspected with a strong light and magnifier for cracking," I added.

Nope, the mechanic didn't do that, either, the owner said. "Do I need to go get him re-do it, or can it wait until my next scheduled inspection?"

"Redo it," I advised the owner, adding that when the nut is tightened "by feel" it's invariably overtightened, putting excessive stress on the clamp in increasing the likelihood of clamp failure (which could be fatal). I pointed out that the torque is so important that each V-band clamp has a small stainless steel "torque tag" on which the correct torque is stamped.

The owner put his T310R back in the shop to have the clamp retorqued, and resolved that in the future he would take his maintenance business to another shop where the mechanics were more knowledgeable about turbocharged twin Cessnas.

The moral is this: Any time you ask a mechanic about some mechanical discrepancy and get the response "they all do that" or "it's the nature of the beast," consider this a big red flag, and go get an expert second opinion. Doing so might just save your bacon.

See you next month.


Want to read more from Mike Busch? Check out the rest of his Savvy Aviator columns.