The Savvy Aviator #37: Scope That Jug!

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The Savvy Aviator

In 2002, I did something unfortunate: pulled a perfectly good cylinder off of one of the engines of my Cessna T310R. If I had it to do over again, I wouldn't have touched the cylinder. But at the time, I thought I was doing the right thing. It was the usual story. I had just downed the airplane for its annual inspection, and the first items on my checklist were performing a hot compression check, draining the oil, sending oil samples to the lab, and cutting open the oil filters for inspection. All the cylinders had compressions in the low- to mid-70s. All but one, that is. That one measured about 60/80 with air leaking past the exhaust valve. At the time, the engine manufacturer's guidance on compression tests was TCM service bulletin M84-15, which instructed mechanics that a jug could leak considerably past the rings and still be considered perfectly airworthy. However, any leakage at all past the valves was considered unacceptable, according to TCM, and required the cylinder to come off for repair or replacement. So off it came. Pulling a cylinder is a real pain. I spent two hours removing cooling baffles and the exhaust and induction plumbing. It took me another hour to remove the rocker cover, rocker shafts, rocker arms, pushrods and pushrod housings. Finally, I used a cylinder base wrench and a big breaker bar to coerce the eight cylinder base nuts loose. About four hours into the project, I held the offending jug in my arms and carried it over to my workbench to survey the damage. I inspected the cylinder closely, with special attention on the exhaust valve. Surprisingly, I couldn't see anything wrong. The valve looked normal, as did the rest of the cylinder. Yet it must have been bad, I thought, because it had clearly been leaking air past the exhaust valve. I sent the cylinder out for re-valving and honing, installed new rings on the piston, then spent another four hours reinstalling them on the engine and replacing the exhaust, intake and baffles. Like I said, it was a pain. It cost me more than $500 plus a full day of sweat equity. (Had I not been doing the grunt work myself, the tab would have been at least $1,500.)

TCM Pulls A Switcheroo

That episode turned out to be a classic case of bad timing. Had my annual inspection come a few months later, that cylinder would never have been yanked. That's because not long after my jug came off, TCM radically changed its guidance about cylinder inspection. On March 28, 2003, the wizards in Mobile issued service bulletin SB03-3 titled "Differential Pressure Test and Borescope Inspection Procedures for Cylinders." This 14-page document is arguably the best guidance ever provided to mechanics on the subject of when a cylinder should be pulled. SB03-3 differs from M84-15 (which it supersedes) in two crucial respects. First, it reverses TCM's previous position that even small amounts of leakage past the valves during a compression check is unacceptable and grounds for pulling the cylinder. Many experienced A&Ps considered the "zero leakage past the valves" standard as being unrealistic and after 19 years TCM finally agreed with that assessment.

Figure 1: This Autoscope rigid borescope from Lenox Instrument Company costs about $950. It has first-rate optics and is an excellent choice for inspecting cylinders on piston aircraft engines. This model is specifically recommended by TCM in SB03-3. (Click here for larger version - 35 Kb.)

The other difference may be even more important: For the first time, TCM directed mechanics to perform a borescope inspection of the cylinders at each annual or 100-hour inspection, as well as any other time a compression check is done. There's nothing optional about this requirement; SB03-3 states, "TCM requires a cylinder borescope inspection be accomplished in conjunction with the differential pressure test." No ifs, ands or buts about it. This was huge. At the time, relatively few GA maintenance shops and A&P mechanics owned a borescope, and even fewer had a clue how to use one or what to look for. In SB03-3, TCM specifically recommended a particular make and model of borescope: the Autoscope from Lenox Instrument Company in Trevose, Pa. (See Figure 1.) This is a simple, low-cost, rigid borescope developed in the mid-1980s for use by auto mechanics, and it costs about $950. The Lennox Autoscope has excellent optics and provides a remarkably clear view of what's going on inside a cylinder. Although SB03-3 officially applies only to TCM engines, the guidance it offers makes good sense for Lycomings, too.

What To Look For

Figure 2: These valves have normal appearance. Note the reddish symmetrical deposit pattern on the exhaust valve (smaller valve, left), and the brownish deposits on the intake valve (larger valve, right). (Click here for larger version - 60 Kb.)

The accompanying photos (Figures 25) provide some examples of what to look for when performing a borescope inspection of a cylinder. Figure 2 at right shows what valves normally look like. The smaller valve is the exhaust valve, while the larger one is the intake valve. The reddish deposits on the exhaust valve and the brownish ones on the intake valve are typical. These deposits should appear reasonably symmetrical, indicating that the valves are rotating in service as they should be. Figures 3 and 4 below illustrate a burned exhaust valve. Note the asymmetrical appearance, especially the highlighted region where the deposits are minimal or absent (because that portion of the valve is running too hot). This is the classic visual signature of a burned valve. If the cylinder leaks air past the exhaust valve during the compression check and if the borescope shows this kind of asymmetrical deposit pattern, you can be relatively certain that the valve is burned and that the cylinder has to come off. But if the valve looks normal under the borescope, some leakage during the compression check is not grounds for removing the cylinder. (Now they tell me!)
Figure 3: This exhaust valve exhibits the classic signature of a burned valve. Note the asymmetrical appearance and the region nearly devoid of any deposits (white arrows). The intake valve looks normal. This cylinder also had low compression with air leaking past the exhaust valve. It needs to come off. (Click here for larger version - 56 Kb.) Figure 4: Closeup of the burned exhaust valve, with overheated area circled. (Click here for larger version - 70 Kb.)


Figure 5: Two borescope views of the upper cylinder wall ("ring-step area"). The one on the left looks normal, while the one on the right exhibits abnormal scoring and probably needs to come off. (Click here for larger version - 32 Kb.)

The borescope is also a great way to check the condition of the cylinder barrel. Figure 5 shows two borescope views of the upper cylinder bore -- the so-called "ring-step area." The left view is normal; the right one has abnormal wear and scoring -- possibly due to a broken compression ring -- and probably needs to come off. Next time you put your airplane in the shop, ask your mechanic what kind of borescope he uses. If your A&P doesn't have a borescope or doesn't know how to use one, educate him ... or find another mechanic. Pulling a cylinder without first borescoping it is a lot like performing major surgery without first getting a CT or MRI. Don't let any mechanic do that to your engine. See you next month.

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