Correction: AOPA has corrected a statement regarding Continental’s mandatory service bulletin and FAA airworthiness directive activity. The FAA has confirmed that it has not received any communications from Continental seeking an airworthiness directive. AOPA regrets the error.

At the end of March 2017, Continental Motors issued Mandatory Service Bulletin MSB05-8B that would require tens of thousands of Continental IO-470/520/550 engines to be torn down prematurely to replace the camshaft gear with slightly thicker gear. It required compliance within the next 100 hours of operation, or at no more than 12 years since overhaul, whichever comes first.

The replacement camshaft gear is 0.06″ thicker, and costs$1,200. Removing and reinstalling the engine is typically a $5,000 job and the engine teardown typically costs around $10,000. So this would be a very big deal for affected owners. The number of affected owners is huge. Almost every Bonanza, Baron, Cessna 200/300/400, Cirrus SR22, and Continental-powered Mooney is targeted by this expensive MSB, plus a bunch of other makes and models.

Compliance with manufacturer’s service bulletins is not generally compulsory for Part 91 operators. The FAA would have to issue an Airworthiness Directive (AD) to compel owners to comply.

Continental first introduced this thicker camshaft gear in August of 2005 and started installing it in its factory new and rebuilt engines. But it wasn’t until November 2009 that it started asking field overhaul shops to install the newer-style gear at engine overhaul. Since it did so in a service bulletin (SB97-6B), almost all overhaul shops considered it to be non-compulsory, and therefore almost all field overhauled engines had their older-style camshaft gears reused.

This means that if the FAA were to issue an AD mandating compliance with Continental’s MSB05-8B, a whole lot of low-time (and even no-time) engines would need to be torn apart. Naturally, Continental would not be picking up any of the cost of doing this (as would be the case if this was an automotive recall rather than an aviation recall). Aircraft owners would be hit with the cost, which would be in the tens of millions of dollars.

Where’s the beef?

We talked with the FAA about this, and were told that Continental had provided the FAA data about only three camshaft gear failures in support of its request for an AD. The FAA also indicated that none of these failures were new ones. A search of the FAA’s Service Difficulty Report (SDR) database uncovered a total of 13 SDRs involving the older-style camshaft gears, most of which did not involve actual failures of the gears. That’s an awfully small number considering that tens of thousands of these gears have been in service for more than 40 years.

Even if all 13 of those SDRs represented gear failures (which they did not), my back-of-the-envelope calculations suggests that this would be one failure every 7,000,000 flight hours, which would make the camshaft gear one of the most reliable components of the engine. I’m almost certain that connecting rods and crankshafts fail more often than camshaft gears do.

If these gears have been in service for more than 40 years, and if there’s no new data indicating that they’re starting to fail at an accelerated rate, why would Continental suddenly conclude that a safety issue exists that warrants asking the FAA for an AD that would cost owners tens of millions and put thousands of airplanes on the ground for weeks or months?

By the way, we checked with three major parts distributors (Aviall, Omaha, A.E.R.O.) and learned that the newer-style camshaft gear is presently backordered for two months. We expect the situation to get worse fast as news of MSB05-8B and a possible AD spreads throughout the owner and mechanic communities.

As owners started learning about MSB05-8B through type clubs, online discussion groups and the electronic aviation press, there was “panic in the streets.” Sales of affected aircraft started falling through as prospective buyers walked away from deals. Owners started talking about class-action lawsuits against overhaul shops who reused older-style camshaft gears instead of installing new ones. Some owners with aircraft undergoing annual inspections started asking whether they should be having their engines overhauled. Other owners started worrying whether it was safe to fly their aircraft.

When we contacted numerous well-known engine overhaul shops, we were surprised to find that all of them told us they had been seeing no problem with the older-style camshaft gears, and all had been reusing the gears at overhaul once they passed inspection (which almost all did). The shops were unanimous that they saw no unsafe condition that would warrant an AD.

So what the heck is Continental’s reason for making a federal case of this now? I honestly don’t know, although as you might imagine there’s an awful lot of speculation and a few conspiracy theories floating around.

Late-breaking developments

On April 20, Continental issued a press release indicating their intention to walk back the Draconian compliance requirements of MSB05-8B. In the press release, Continental promised another revision of the MSB within 15 days, and indicated that the revised document would:

  • Eliminate the preemptive camshaft gear replacement in favor of a mandatory repetitive visual inspection procedure allowing on-condition operation until the engine is overhauled or removed for some other reason.
  • Change the 100-hour or 12-year gear replacement requirement to something that owners can live with more easily.
  • Provide an alternative procedure for replacing the camshaft gear that would not require complete engine disassembly (although it would require removal from the aircraft and partial disassembly).

We sincerely hope that Continental’s revised MSB is something that aircraft owners can live with. If not, there’s going to be a huge battle. After all, we’ve been living with these gears for more than four decades, and the overhaul shops are not seeing a problem with them.

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 7,500-plus hour pilot and CFI, an aircraft owner for 45 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike’s book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions.