The differential compression check has been a mainstay of piston aircraft engine maintenance for the last 70 years, give or take. Like anything else in aviation that's been around for a long time, various Old Wives' Tales (OWTs) have evolved about the procedure, passed on from journeyman mechanic to apprentice, and later taught in A&P schools and documented in various textbooks and advisory circulars. Ask your mechanic why he performs a compression check a certain way or interprets the test results as he does, and if he's honest he'll probably answer, "That's the way I was taught to do it and that's the way I've always done it."
One of the most pervasive OWTs about compression checks goes something like this:
Now, perhaps this had some validity back in the days when radial engines were king. But for modern, horizontally-opposed engines, it's simply wrong -- certainly wrong for Teledyne Continental Motors (TCM) engines, where the manufacturer has set forth very specific procedures for doing compression tests and evaluating the results. According to TCM, a cylinder with a compression reading of 50/80 might well be absolutely airworthy, provided the cylinder meets certain other requirements.
Another widely accepted OWT is that an engine with compressions in the low 60s is a "tired engine" that will not put out its full rated horsepower. This is just plain wrong. As I mentioned earlier, TCM ran some tests with an engine that was intentionally "tricked out" to reduce compressions to 40/80 and detected no measurable horsepower loss. An engine would very likely produce full rated horsepower (or darn close to it) with compressions of 20/80 -- just not for very long, because the extreme blow-by would wind up pressurizing the crankcase and blowing most of the engine oil overboard.
Even TCM's guidance has been a moving target. For years, TCM Service Bulletin M84-15 was the bible that defined how compression checks should be performed on Continental engines. But in March 2003, TCM published Service Bulletin SB03-3, which superseded M84-15 and radically changed the procedures for performing compression tests and significantly liberalized the standards for interpreting the results of those tests. A lot of mechanics simply haven't kept up with the latest guidance, and are condemning cylinders unnecessarily. (We'll look at what SB03-3 says shortly.)
When an owner asks me whether to be concerned about low compression readings, one of my very first questions are "How's the oil consumption?" and "What do the top plugs look like?" If a cylinder isn't burning oil or pressurizing the crankcase or leaking past the valves, it's probably just fine regardless of what the compression test numbers are.
After repeatedly hearing mechanics say, "If the compression is less than 60/80, the cylinder is unairworthy and has to come off," I got curious where this magic number came from. So I did a little research to see what the FAA has to say on the subject.
The first place I looked was FAR Part 43 Appendix D, which is where the FAA defines what must be done during an annual or 100-hour inspection. Appendix D does indeed require that each annual and 100-hour include a compression check, but it doesn't offer much guidance about how the results should be interpreted. It states, "If there is weak cylinder compression," the engine must be inspected "for improper internal condition and improper internal tolerances." In other words, if the compression is weak, the cylinder needs to come off. But it doesn't define what constitutes "weak" compression.
Next, I turned to the A&P's bible, FAA Advisory Circular AC43.13-1B, "Acceptable Methods, Techniques, and Practices -- Aircraft Inspection and Repair." While non-regulatory in nature, this AC provides a wealth of maintenance information "acceptable to the Administrator" that A&Ps can use as guidance (to cover their derrières) in the absence of specific maintenance instructions from the particular aircraft or engine manufacturer. Paragraph 8-14 is devoted to "compression testing of aircraft engine cylinders." Sure enough, the second sentence of paragraph 8-14 says:
If a cylinder has less than a 60/80 reading on the differential test gauges on a hot engine, and procedures in paragraphs 8-14b(5)(i) and (j) fail to raise the compression reading, the cylinder must be removed and inspected.
That's pretty direct and unequivocal, isn't it? It says that if a jug measures less than 60/80 in a hot compression test, and the reading can't be raised by running the engine and "staking" the valves with a mallet and fiber drift, then the cylinder must be removed. So if your mechanic tells you that a jug on your big-bore Continental measured 58/80 during the annual inspection and has to come off, he's right ... Right?
Wrong! Your mechanic needs to go back and read the very first sentence of AC43.13-1B, which states:
This advisory circular (AC) contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft, only when there are no manufacturer repair or maintenance instructions.
In other words, if the engine manufacturer provides specific instructions for performing or interpreting a compression check, then those instructions take precedence over any general-purpose guidance provided by the FAA. Indeed, TCM provides detailed instructions on the subject in Service Bulletin SB03-3 (and prior to 2003, in Service Bulletin M85-14). So at least for Continental engines, the old 60/80 rule should be tossed right out the window. It simply does not apply.
Incidentally, my exhaustive computer search of FAA publications turned up one other place where the magic number 60/80 is mentioned: Advisory Circular AC20-105B, "Reciprocating Engine Power-Loss Accident Prevention and Trend Monitoring." This little-known publication is actually quite well-done and worth reading. It's largely devoted to subjects like fuel exhaustion, starvation and contamination (since that's usually what makes engines quit), but it does get into various maintenance subjects as well, including fouled plugs, stuck valves...and yes, compression checks. Interestingly enough, AC20-105B says that if a cylinder measures less than 60/80, a visual borescope inspection is recommended (as opposed to pulling the jug). Nobody ever accused the FAA of consistency ...
But once again, the FAA makes it clear that if the manufacturer provides instructions on this subject, then those instructions must be followed and contradictory information (from the FAA or whomever) should be disregarded.
In December 1984, TCM issued Service Bulletin M84-15 titled "Cylinder Leakage Check (Compression)" to explain how to perform the test procedure properly and how to interpret the results correctly. They issued this document specifically to discredit the various OWTs about compression checks, and to deal with an epidemic of perfectly airworthy cylinders that were being arbitrarily removed simply because they failed to make the "passing grade" of 60/80 on a single differential compression test.
The biggest effect of M84-15 was to establish that leakage past the rings could be considerably in excess of the old 60/80 standard without rendering the cylinder unairworthy. It required that mechanics calibrate their compression test gauges against a master reference orifice to determine the go/no-go leakage limit. For most gauge sets, this number wound up being somewhere in the high 40s or low 50s.
At the same time, M84-15 stated that no leakage was allowed past the intake or exhaust valves. None whatsoever. Zero. Nada. Zip. Most mechanics considered this to be an unrealistic standard that, if taken literally, would condemn lots of perfectly airworthy cylinders to be pulled for valve work.
Obviously, TCM must have agreed with that assessment, although it took them 19 years to come around. (Nobody ever accused TCM of acting impulsively.) In March 2003, TCM issued SB03-3 to supersede M84-15. The new Service Bulletin greatly liberalized the criteria for valve leakage. It told mechanics not to reject a cylinder based on any single compression test, but to fly the airplane for at least 45 minutes and redo the test. It also instructed mechanics not to remove a cylinder with substandard compression test results until the cylinder undergoes a borescope inspection to confirm that there is indeed a bona-fide problem with the cylinder.
It is clear that TCM's intent in issuing first M84-15 and then SB03-3 was to discourage mechanics from pulling a cylinder unless they can firmly establish that the cylinder is seriously ill.
TCM's current guidance, Service Bulletin SB03-3, starts out by emphasizing that the differential compression test has significant limitations that necessitate its results to be corroborated by other non-invasive methods. It states that TCM requires a cylinder borescope inspection to be accomplished in conjunction with the differential compression test. It also makes clear that SB03-3 takes precedence over the old 60/80 guidance found in FAA AC43.13-1B.
In addition to the differential compression test and borescope inspection, SB03-3 instructs mechanics to consider several other factors -- oil consumption, the appearance and color of engine oil, and evidence of crankcase pressurization (such as excessive oil on the belly of the aircraft) -- when evaluating the airworthiness of a suspect cylinder.
Compression test gauges are notoriously inaccurate, and it's quite common for different gauges to give markedly different readings. For that reason, TCM does not publish any specific value (60/80 or whatever) as the go/no-go threshold for the compression test. Instead, TCM requires that your A&P establish a go/no-go threshold for his particular compression test gauge by using a special calibrated master orifice tool (TCM p/n 646953A) that represents what TCM defines as the maximum allowable leakage for a cylinder. SB03-3 instructs your mechanic to hook up his compression tester to this calibrated master orifice, measure its leakage just as he would a cylinder, and write down the value he gets. That value then becomes the maximum allowable leakage for your cylinders when using his particular compression tester. (For most compression test gauges, this value turns out to be in the low 50s or high 40s.)
Alternatively, your mechanic can purchase a compression tester (like the Eastern Technology Corp. Model E2M) that has the calibrated master orifice built right in. Turn a valve one way and the gauge measures the leakage through the master orifice to establish the go/no-go limit, then turn the valve the other way and it measures the leakage of your cylinder. This simplifies the procedure considerably.
SB03-3 goes on to provide a step-by-step explanation of precisely how to perform the compression test. It makes interesting reading, and differs in several important details from the method I've generally seen practiced by A&Ps in the field.
I've watched a good number of mechanics perform a lot of compression checks over the years, and indeed done quite a few myself. The usual procedure I've observed -- and indeed the one I was taught -- is to rotate the prop to bring the piston in the cylinder being tested to top-dead-center (TDC), hold the prop firmly while turning on the air valve to apply 80 psi through the test gauge, and then rock the prop back and forth a few degrees either side of TDC until the highest stable reading is obtained.
Wrong, says TCM! The piston should be positioned so that it's just starting to come up on the compression stroke in the normal direction of crankshaft rotation. Then while firmly holding the prop stationary (preferably with the help of an assistant), the cylinder should be pressurized to 20 psi and the prop slowly rotated (against the air pressure) until the piston reaches TDC -- you can tell when you get there by a sudden decrease in force required to turn the crankshaft. At this point, the prop is held stationary, the air pressure is increased to 80 psi, and the leakage reading is noted. The prop may be rocked back and forth a few degrees to obtain the highest possible reading.
With the piston at TDC and the cylinder pressurized at 80 psi, the cylinder must be checked to determine the location of any leakage. This is done by listening carefully for any sound of air leaking through the exhaust port (exhaust pipe) or the intake port (induction inlet) to determine if air is leaking past the exhaust valve or intake valve; if the leakage is past the rings, it can be heard by listening at the oil filler or engine breather.
SB03-3 also contains specific guidance on the procedure to be used for borescope inspections. A borescope is an optical device that can be inserted into a cylinder through the upper spark plug hole, and allows the mechanic to get a good look at the condition of the cylinder's barrel, combustion chamber, and intake and exhaust valve. While borescopes have long been used for routine inspections of turbine engines, their use has been rare in the maintenance of piston engines.
Clearly, TCM intends to change that. SB03-3 makes it mandatory to perform a borescope inspection in conjunction with each compression check. Since many shops didn't even own a borescope prior to the issuance of SB03-3, the service bulletin even includes a recommendation about what kind of borescope to buy (Autoscope 6600K from Lennox Instrument Company, approximately $1,000). If your shop doesn't have a borescope, it cannot legally perform a compression test on a TCM engine.
SB03-3 advises that each cylinder should be inspected twice: once with the piston at bottom-dead-center at the end of the power stroke (so that the exhaust valve is open), and again with the piston at bottom-dead-center at the end of the intake stroke (so that the intake valve is open). Items that should be looked for during borescope inspection include:
SB03-3 includes a number of photographs to help mechanics identify what normal and abnormal cylinders look like through the borescope.
In addition to explaining exactly how to perform the compression test and borescope inspection, SB03-3 provides precise guidance on how mechanics should interpret the test results. Here are some highlights:
TCM's SB03-3 applies only to Continental engines, but its fundamental concepts make a lot of sense for any piston aircraft engine. The most important of these concepts is that cylinders should never be removed on the basis of any single compression reading. If a cylinder shows substandard compression, the airplane should be flown and the test repeated. Absent any other abnormal symptoms, a substantial amount of leakage is acceptable during the compression test (down to 50/80 or lower for most test gauges). On the other hand, excessive oil and carbon deposits in the combustion chamber coupled with excessive oil discharge from the breather may warrant cylinder removal even if the compression reading is okay.
Never allow a cylinder to be pulled on the basis of a single compression test -- unless the gauge reads zero or something close enough to zero that you're pretty sure you have a hole in the piston or a chunk missing from the exhaust valve. If the compression is simply weak (as opposed to non-existent), fly the airplane for another three to five hours and then recheck the compression again. There's at least a fair chance that the cylinder will pass muster on the re-test, and that you'll be spared the expense and hassle of an unnecessary cylinder replacement.
Another tip: If you're worried about a particular cylinder, check its compression first, before the cylinder has much of a chance to cool off. The cooler the engine gets, the looser the fit of the piston in the cylinder, and the larger the compression ring gap. All other things being equal (which they never are), the first cylinder to be checked will have the best compression reading, and the last will have the worst.
If the compression reading for a particular cylinder is lower at this year's annual inspection than it was last year, don't necessarily assume that those two data points define a downward trend that will continue. That cylinder might well come out better at next year's test, while some other cylinder shows a downturn. Just remember that there's a substantial element of random noise in differential compression readings, which is why they're notoriously non-repeatable.
The differential compression check remains one of the oldest and most useful tools for measuring the top-end health of a piston aircraft engine. But it's essential to understand the limitations of the test, and how to properly interpret the results. TCM's Service Bulletin SB03-3 is the most helpful guidance ever published in this regard, so make sure you and your mechanic are familiar with what it says.
Despite what the Friendly Aviation Agency would have you believe, there's simply no meaningful difference in airworthiness between a cylinder that measures 62/80 and one that measures 58/80. There's nothing magic about the number 60 ... unless, of course, you happen to be an airline captain.
See you next month.
Want to read more from Mike Busch? Check out the rest of his Savvy Aviator columns.