Service Instruction 1479 and You
Service Instruction 1479 and You
by Bill Marvel and Bill Scott
First off, don't try to find us in your roster of C.P.A. members because we're not there! Both of us cut our teeth on the Grumman American aircraft line and still own and fly them. We have been writing an on again, off again series of articles for our American Yankee Association members regarding Lycoming parallel valve engines and some very interesting observations we have made concerning one aspect of them. Those observations have now reached the point where we believe that many others ought to know about them.
Our effort for the past two years has focused on our discovery of an inverse correlation between exhaust valve guide wear and the volume of oil flowing to the rocker boxes of these engines. In other words, the greater the oil volume, the less the wear. In addition, we have discovered that the oil volume fed to the rocker boxes of all the cylinders is surprisingly low in general and most often lowest on the co-pilot side. Coincidentally, it is on the copilot side that most of the reports of valve problems occur. In taking a very close look at the oil flow paths within the engine in general and within the mushroom style hydraulic tappet assembly in particular, we have discovered the reasons for both of these characteristics. We will be telling more about that story in the near future, but are still doing flight test work on one possible solution to this problem. However, in the process of our research, we learned of a development at Lycoming that is of great importance to most operators of Lycoming engines. Fortunately, it fits quite nicely into the area of investigation that we have been doing. This development is the publication of Lycoming'sService Instruction 1479 (S.I. 1479), and the balance of this article relates to it.
For those of you not familiar with S.I. 1479, here it is in brief. It was published on January 11, 1996 and is titled, "TIO-540-AF1A Oil Cooled Exhaust Valve Guide Kit." The TIO engine mentioned is in the Mooney TLS. If you do not own a Mooney TLS and thus think this information does not apply to you, keep reading! You will soon learn that it very much does apply. The kit referred to provides new cylinders machined to accept external cylinder head oil fittings and a different exhaust valve guide. Pressurized oil from the engine's main oil gallery is sent to each cylinder head via external flexible lines. At the cylinder heads, this oil circulates around the new guides and flows into the rocker box. From there it drains back into the engine case via the existing oil return lines. You can see a similar system in the accompanying photograph, which is the setup in a Beech Duke. Although it has a much different engine from the TLS, the Duke's engine incorporates similar oil cooled exhaust guides. Some of you might look at the photo and conclude these are fuel injection lines. Rest assured they are not. In the Duke installation, the fuel injection is integral with the intake manifold.
The reason this kit was developed is that many Mooney TLS owners experienced excessive valve guide wear and corresponding compression loss with 100 to 400 hours on their engines. According to Mooney, these operators maintained cylinder head temperature (CHT) in cruise between 380 and 420 Fahrenheit, which is in the normal operating range. Despite that, valves and guides overheated and failed. Different guides were tried. They failed also. Eventually the oil cooled exhaust valve guide concept was tested in the engine and worked well, according to Mooney. Test flying took place for about a year. S.I. 1479 was the result.
Because Lycoming has now revealed that the valves and guides in this engine run too hot and thus need additional cooling by oil, they have admitted something much more significant. Consider the following sequence of events:
1. Mooney designed the cooling system to maintain Lycoming-designated air pressure differentials, which should provide adequate air flow to meet engine cooling needs.
2. As a result, the TLS runs with cruise cylinder head temperatures ranging from 380 to 420, which is within Lycoming-established acceptable limits.
3. With in-the-green cylinder head temperatures, the engine experiences premature exhaust valve failure problems.
4. Lycoming has designed a solution for this problem that does two things:
a. cools the valve guide with direct oil flow from the oil gallery to each cylinder head, which reduces valve guide wear and allows the valve to have greater heat dissipation through the guide.
b. releases this oil into the rocker box for additional cooling and lubrication
5. If you are an owner of a TLS for which you have paid several hundred thousand dollars and have a guide or valve problem, despite operating with all temperatures in a normal range, Lycoming's response is:
a. We now admit that you fly with CHT levels in an acceptable range, because we have studied the failure mechanism and have found that the problem is with the design of our engine. Air alone won't adequately cool the exhaust valve and guide. Additional oil cooling is required.
b. Since many of these engines failed under warranty, we have a service kit for you that will SOLVE THE PROBLEM.
6. If you are an owner of a non-TLS aircraft and have a guide or valve problem, despite operating with all temperatures in a normal range, Lycoming's response is different:
a. Lycoming does not have any problems with premature valve and guide wear in its other engines. (Author's note: The Mooney TLS engine, being turbocharged, is capable of producing rated power to high altitudes. As demonstrated by S.I. 1479, many have had valve problems while still under warranty. In contrast, the normally aspirated Lycoming engines that most of us fly lose their ability to produce 100% power as soon as they climb above sea level. As a result, they generally encounter valve problems at a later date when the engine is outside of Lycoming's warranty period. Lycoming can thus correctly state that aside from the TLS engine, they have not had any indication -- based on warranty claims -- of valve and guide problems in other engines in their product line.)
b. Your CHT levels must be too hot. The only reason for this type of failure is excess heat. Your baffle seals, leaning technique, maintenance, etc. must be faulty. You must be causing the problem by creating too much heat. We are not responsible for it.
c. You should comply with (Service Bulletin 388B) (S.B. 388B - the valve wobble check) every 400 hours and install new parts at your own expense, as often as is necessary.
What you have just read is a brief summary of the events that led to S.I. 1479 and the current status of it. What is missing is the cause of this situation. The reason for the above series of events is of utmost importance. There are two possible explanations for it:
One possibility is that the exhaust valve, exhaust valve guide and cylinder head design is flawed. The system is unable to transfer sufficient heat from the exhaust valve to the valve guide and then to the cylinder head and atmosphere by air cooling alone, in conjunction with the normal amount of oil that flows to the cylinder head.
The second possibility is that the thermal design of the cylinder head is fine, but that the lubrication system does not provide sufficient oil through the normal flow paths to adequately carry away excess heat. In this scenario, the oil system design in general, and the hydraulic lifter design in particular, is flawed. The system is unable to provide adequate oil to the cylinder head for needed oil cooling.
Regardless of which possibility you prefer, the fact is that Lycoming's issuance of S.I. 1479 clearly indicates their solution to the problem is to augment oil flow to the cylinder heads.
Let's now look at the Mooney TLS engine a little more closely. The engine in this aircraft is a 270 horsepower (HP), 6-cylinder TIO-540-AF1A. However, it produces only 45 horsepower per cylinder, the same as does every 180 HP O-360 engine. The first law of thermodynamics states that energy in has to equal energy out. At a given power setting, a certain amount of fuel is required for each HP produced. The combustion of this fuel yields both power and heat. Each TLS and O-360 cylinder supplies 45 HP maximum each; the remaining energy is heat that has to be dissipated. The thermal loading on TLS and O-360 cylinders is thus the same. Since O-360 engines are also known to experience premature exhaust valve and guide wear, one might wonder if these engines also ought to have the benefits of the S.I. 1479 modification. An obvious question is how different are the cylinders and their components in the TLS and O-360 engines?
The cylinder in the TLS is identical to that in the O-360 except for one minor variation. The TLS uses long reach spark plugs and thus has a slightly longer plug boss for helicoil thread engagement. The original TLS used flangeless guides but they were changed to the part number 74230 guide by Service Bulletin 503 dated April 6, 1992. This is the generic guide for all O-320s and parallel valve O-360s and O-540s.
With this exception, the TLS cylinder is the standard O-360 cylinder that is in use in tens of thousands of airplanes of all makes and models. But it does not end with the O-360. The cylinder head of the TLS cylinder is also used on the O-320 series. If an O-320 cylinder is operating at the same CHT that is causing valve and guide distress in the TLS and in the O-360, is there any reason why it would not experience the same problems? Absolutely not. The cylinder head does not know or care what engine it is bolted to. It only receives a heat input, eliminates all of the heat it can by oil and air cooling, and then operates at whatever temperature the residual heat produces. If this temperature is such that guide and valve life are diminished, it simply means that the cylinder will need replacement parts prior to TBO.
Because of the significance of S.I. 1479 for almost all Lycoming engines, we want to present some thoughts to you regarding our interpretation of it. This service instruction is very important because it is a clear admission by Lycoming of a problem that affects more than just the Mooney TLS, as we have pointed out above. Beyond the obvious statement of the S.I., here are some additional thoughts we have drawn from it.
The problem is directly inherent in the design of all O-320, O-360 and O-540 parallel valve cylinder heads. Any engine with these heads could potentially fall victim to this failure mode. Additionally, other cylinder heads in Lycoming engine models and possibly those of other manufacturers should not yet be ruled out.
The 500 degree red line cylinder head temperature is meaningless if the exhaust valve and guide can be destroyed at far less than this. What does the 500 degree limit protect, and what is the point of protecting it, if other components are damaged at a lower temperature? Should red line be lower? How much lower? What is the critical component -- the valve, the guide or something else?
The upper limit of the normal operating range for cylinder head temperature (green arc) is in reality considerably lower than published. Since the TLS destroys valves and guides while operating in cruise at CHT levels from 380 to 420, is a practical upper limit of the green arc 375, 350 or 318.6? Cylinder head temperatures well below what we have considered normal are actually excessive from the standpoint of exhaust valve longevity.
S.I. 1479 demonstrates that it is, in fact, possible to operate the cylinder within the normal CHT range and yet have destructive heat loads imposed on the exhaust valve and guide. This is in direct contravention to our own F.A.A. Advisory Circular 65-12A, Airframe and Powerplant Mechanics "Powerplant Handbook," Chapter 10, which states, "...as long as the cylinder head temperature is kept within the prescribed limits, the temperatures of the cylinder dome, exhaust valve, and piston will be within a satisfactory range."
One cannot rely on cylinder head temperature in an effort to avoid damage to the exhaust valve and guide. This leaves even the most well-equipped aircraft totally unable to advise the pilot of potentially damaging temperatures in the engine's upper end.
This revelation is somewhat at odds with the very concept of air-cooled aircraft engines. It says that air cooling alone, in conjunction with the normal oil flow quantity to the cylinder head, is not sufficient to adequately cool all cylinder components in some normal operating conditions. In these conditions, an additional source of cooling oil to the cylinder head must be employed to maintain acceptable temperatures.
Based upon the above paragraph, one can conclude that there are two engine operating regions pertaining to air cooling, at least in the Lycoming parallel valve cylinders. One is a region where air cooling alone, in conjunction with current oil flow quantities to the cylinder head, is sufficient to meet all requirements. Another is a region where an additional source of cooling oil to the cylinder head must be provided or components will be damaged. There is no way for the pilot, or anyone else, to know when the engine passes from one operating region to the other.
Based upon the above paragraph, one can conclude that the mission the aircraft usually flies has a direct impact on the health of its exhaust valves and guides. If it usually flies at very low power settings, such as local sightseeing, air cooling and the current quantity of oil flow to the cylinder head may alone be sufficient to dissipate valve and guide thermal loads. But if the exact same airplane is flown on a different mission, such as long flights with cruise power and leaned mixture set for hours at a time, guide and valve life may be diminished unless additional oil cooling to the cylinder head is provided. This will be the case even if all temperatures are maintained at normal levels. This variation of component longevity with mission undoubtedly accounts, in part, for why some aircraft experience valve and guide distress and others do not.
Service Bulletin 388B, or some variation thereof, is the only way an operator can know for certain if his aircraft is being adversely affected by insufficient oil flow. Compression testing will only reveal the problem late in its development, if at all. No amount of proper maintenance, oil changes, correct leaning technique, good baffle seals, instrumentation, etc. will prevent an engine from encountering valve and guide problems if it operates at temperatures where additional oil cooling is necessary but not provided.
Because of the above paragraph, Service Bulletin 388B should be interpreted in the proper context. It should be considered as evidence as to whether or not the problem Lycoming designed into the cylinder head/oil system is or is not going to manifest symptoms for the type of operations the owner is flying. The owner should realize that a change in the aircraft's mission could well cause the problem to occur even though it has previously been absent.
One of the stated reasons for using a derated 180 HP O-360 in the new Cessna 172 is to achieve greater reliability. However, remember that the O-360 cylinder head is identical to that in the O-320. One has to wonder, given the fact that the O-320 also experiences exhaust valve and guide problems, if this concept will succeed. How will an O-360, derated to produce the same power as an O-320, achieve greater valve and guide longevity when both use the same cylinder head?
If this article does not result in contentious discussions (maybe even heated arguments?) among C.P.A. members the world over, we're not sure what will! Our view is that the above observations are merely logical conclusions drawn from careful thought about the implications of S.I. 1479. If you want real controversy, wait until our next installment, titled, "Hydraulic Lifters 101." It will vividly show that the mushroom style hydraulic tappet used in most Lycoming engines was not originally designed to operate with pushrods, why this lifter supplies far less oil flow to the rocker boxes than does the Continental version of the same design, and how the problem of adequate rocker box oil flow was eventually solved by a different lifter design altogether. It will leave you scratching your head wondering why things were done the way they were.
Precision Engine
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