The repair of cylinders has been going on for years. If you found a crack in one of your cylinders, no matter what engine, you could have it returned to service with weld repairs or parts replacement. Common repair areas included: fuel injector and spark plug hole, intake and exhaust ports, head cooling fins, barrel thread, valve guide bosses, rocker shaft supports, and so on. You can of course have your cylinders rebored oversize if they are worn internally. This has been the repair mode for years.
The cylinders were repaired by certified shops who knew their business and returned your airworthy cylinder to you or an exchange for a modest price that was significantly lower than a new factory cylinder at that time. If the crack returned, it was not life-threatening and cracked cylinders rarely resulted in injuries or crashed aircraft. As in this AD situation, there never was any evidence of an accident or injury in any of the 30 reported incidents.
Pilot operating conditions
Apparently the FAA did not consider the fact that pilots routinely damage cylinders with operations at high cylinder head temperatures. Common errors in the leaning process can cause serious damage to cylinders and valves. Any temp in the vicinity of 400+ degrees is extremely damaging to cylinders. Detonation can also raise temperatures and severely damage or cause cylinder cracks. The cylinders we are concerned with are on high compression, turbo-charged, powerful engines, with high operating temperatures and they are subject to random internal damage and cracks. This has been routine for years.
In fact, Cessna, the manufacturer of most of the aircraft concerned with the proposed AD has advised pilots in a confusing way in some of its Pilot Operating Handbooks. Here it states that the pilot should lean his engine to 50 degrees lean of peak. According to experts in the field, this is where the pilot should not set his mixture. The cylinder is subject to maximum temperature and internal pressure at this setting, and it will ensure that the cylinder will operate at the hottest and most dangerous level. That is not good practice, but reportedly used by many.
Manufacturing flaws have of course been found from time to time on all manufactured cylinders. The heat-treating process has many times produced faulty cylinders. These problems and others are routinely fixed and fortunately there are usually only a small run of cylinders that are affected.
Some alternate proposals
There are many alternate solutions available to mitigate the threat of cracked cylinders causing further damage. The FAA should consider urging the following procedures, which they themselves recommend.
Although used for years, the traditional differential compression check required during all annual inspections is a poor indicator of cylinder condition and fails to detect leaks many times, but it has been used for years. The best use of this test, as suggested by experts in the field, is to keep a record of cylinder readings over several or more annual inspections, or during the year at routine oil changes, showing the relative trend of readings that are observed. This is not perfect, but along with other tests can be very useful.
Continental (now Avic) has provided very specific instruction on the performance of compression checks on its engines. See Service Bulletin SB03-3, which sets out in detail how to perform the check and more importantly how to make a determination on removing cylinders for cause. However, cylinders need not be removed for low compression readings alone. This instruction should be considered along with the “guidance” contained in AC 43-13, (6-15-98) (Reciprocating Engine Power-Loss Accident Prevention and Trend Monitoring).
In the above Service Bulletin, Continental suggests using a borescope, pricey devices that let you look into the interior of a cylinder and examine the condition of cylinder walls, valves, and other parts of the interior. Borescopes are, however, not yet required to be used by regulation like the compression test but they certainly are a useful practice.
Just the simple careful inspection of each of the cylinders is one of the best procedures to detect any cracks in cylinders. Most, if not all, can be detected visually if you are familiar with where to look. I know many times inspectors find cracks by observation, where the working technician missed them. Of course that’s what inspectors are for.
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