Helicopter Piston Engine Maintenance

April 1, 2000

Helicopter Piston Engine Maintenance

By Greg Napert April 2000

For the most part, piston-powered helicopters are a relatively rare breed. Turbine-powered helicopters are more popular because of their increased utility and also for what many consider to be better reliability.

Yet, there continues to be a strong market for piston-powered helicopters for the definite advantages that they offer and namely low cost, light weight, and high maneuverability.

Although piston-powered helicopters are used primarily for personal transportation, they are also employed in law enforcement (surveillance), power line patrol, sightseeing, electronic news gathering operations, and helicopter flight training schools.

By far, most of these helicopters, to include the Robinson, Enstrom, and Schweizer lines, are powered by Lycoming engines. There are some exceptions, but they are rare.

If there is one difference between maintaining piston-powered helicopters versus fixed-wing, it is that helicopter engines require quite different treatment from installation to installation. Don't ever make the mistake of assuming that because the helicopter contains a theoretically similar engine to its fixed-wing counterpart, it should be maintained the same. You really have to take the airframe, operating environment, and the instructions of the helicopter manufacturer into consideration in your approach to maintaining these engines. Following are two examples of helicopter installations that illustrate the radical differences in the maintenance approach — Robinson and Enstrom.

Patrick Cox, customer support for Robinson, explains that the engines it uses in its newest helicopters are not Lycoming's H-Model engines that are typically used in helicopter installations. Neither is the engine on the Robinson a V-series or an I-series engine. It is, instead, a slightly modified Lycoming 360 (used on the R-22) or 540 (used on the R-44) Lycoming mounted horizontally, as in a fixed-wing application, except that it is pointed backward. Most of the older R-22s use a 320 Lycoming.

The difference in these engines from their fixed-wing counterparts, according to Cox, is that the cylinders are manufactured to be thinner in order to reduce the weight by 1-1/2 lbs. per cylinder.

"As a consequence, we de-rate these engines for our installation. For example, an O-360 will be rated at 145 horsepower on the data plate and even though it is theoretically capable of 180 hp, as on fixed-wing installations," he says.

"Lycoming makes these cylinders up for us to our specifications. It was critical that we reduce the weight of the engine for the application, so Lycoming came up with a lighter engine design. The engines that we have done this to are the O-360 and the O-540-F1B5. Other than the cylinders, there are no other changes to the basic engine and the block is still the same and the fuel control system is the same," says Cox.

Further, even though the engine is rated at 145 hp on the data plate, it's de-rated further in operation by only allowing 131 hp for takeoff. The thinner cylinder walls dictate that the horsepower rating on the engine be reduced. The advantage to de-rating the engine, however, is that there is significantly reduced wear and tear on the remainder of the engine.

According to Cox, the thinner cylinders have no adverse affect on the life of the engine. In fact, Cox says that Robinson is trying to extend the TBO to 2,200 hours from 2,000. This would work well for Robinson because their airframe TBO is 2,200 hours and making it convenient for maintenance purposes.

Cox explains that further contributing to the low wear and tear on its engines is the fact that the engine is not subjected to excessive gyroscopic loading from a fast-spinning propeller. The engine, on this helicopter is isolated from unusual loading through the gearbox.

"Further, we don't have any shock cooling issues with the engine either because there is no ram air cooling. Instead, there is a centrifugal blower that is always blowing air over the engine. If you chop back the throttle, there is not as much air, so the engine is not Ôshock-cooled.' Additionally, our blower provides more than adequate cooling."

Cox adds, "We are very specific in our 100-hour inspection criteria to make sure the cooling baffles aren't cracked and are in place, because we do rely quite a bit on the fan for cooling. On the R-44, the blower cools your muffler, your battery, your hydraulic reservoir, main rotor gearbox, alternator, magnetos, drive belts, etc. There is really no significant cooling effect from the rotor downwash."

In terms of oil changes, Robinson recommends oil changes every 25 hours, which is much more often than on a fixed-wing installation. The reason is that the helicopter is exposed to a considerable amount of dust while in hover. But just as important, the Robinson models don't come standard with an oil filter. Cox says, "You can get the R-44 with a filter or you can adapt an aftermarket filter kit to the R-22. But our recommendation, based on experience is to change the oil every 25 hours and filter or no filter."

Cox als o says, "Change the air filter at least every 100 hours, and in some applications where low-level, dusty environments are encountered, we recommend increasing the frequency of filter changes."

In terms of general troubleshooting and other maintenance items on the helicopter, Cox says that Robinson tries to generally follow in-step with Lycoming's recommendations. "Our approach to engine installations is to make it simple, light, and economical."

Enstrom installs its own STC turbocharger to enhance the performance of its engines at altitude.

By contrast, Enstrom helicopter piston engines require a quite different approach. Lee Burdue, Technical and Training coordinator for Enstrom, explains that piston installations in their helicopters are somewhat unique in the marketplace.

First, the engines in their "C" and "F" models are turbocharged by Enstrom under an STC in order to produce the rated horsepower to a given altitude. Second, the configuration of the helicopter is such that the engine compartment does operate at elevated temperatures when compared to fixed-wing installations.

"These higher temperatures," Burdue explains, "can do such things as cause the diaphragm in the fuel servo to become stiff and brittle or can cause premature magneto point wear. If the fuel diaphragm becomes stiff, it will affect the fuel flow and the engine take more fuel to do the same job. You can tell when this is a problem by checking the manifold pressures versus fuel flows." He adds, "A typical indication that you have a problem with the diaphragm would be a manifold pressure of, say, 28 inches, with an associated fuel flow of 130 to 140 pounds, instead of the correct 102 to 105 pounds of fuel flow."

The result of these higher than normal temperatures means there is a requirement for more frequent maintenance intervals as well as attention to specific areas of the engine that are affected by the heat. Enstrom does have a non-turbocharged "A" model helicopter that runs a bit cooler but does not offer the advantages of the turbocharged "C" and "F" models.

All engines used on Enstrom's helicopters are HIO-360 Lycoming engines that are designed specifically for helicopters.

Enstrom offers a class to maintenance personnel in which it teaches some of the basics of maintaining its engines.

Burdue says, "We begin by addressing the fuel servos that we use on our engines. Much of the troubleshooting of the fuel servo can be done by knowing what the fuel flow should be depending on specific manifold pressures."

He continues, "For instance, if you're pulling 28 inches of manifold pressure and you're indicating a fuel flow of 105 pounds per hour, they're okay; there is no fuel problem, so you can eliminate the fuel servo and fuel related items. We also feel it's important for the technician to understand fuel flows and how the fuel is distributed throughout the engine."

A critical part of properly maintaining Enstrom's engines is to understand their turbocharging system. Burdue says it's important to understand that with a turbocharger, you always develop deck or manifold pressure whether the turbocharger is working. On a normal non-turbo engine this would normally be around 27.5 inches of manifold pressure, yet with a turbocharger, you develop at least 30 inches of pressure, even if it isn't working. This is important to know for troubleshooting purposes. Just because you have 30 inches of pressure, it doesn't mean the turbocharger is working correctly. Turbocharger TBO is around 2,000 hours and but that depends on how it is used and if it's abused.

"Another problem area we see in terms of troubleshooting is the engine operating at high density altitude due to elevated intake temperatures," says Burdue. "This can be attributed to where the air filter intake assembly mounts to the intake pipe. If it is not seated correctly and there is a gap behind the air inlet the engine air, which is hot inside the compartment, will blow out and leak across the gasket and get sucked back into the intake. This will require that the engine make more power to do the same job. By ensuring this area is sealed, you can gain up two to three more inches of manifold pressure."

Burdue explains, "We use a dual Bendix magneto on our turbocharged engines. Again, the high heat in this engine compartment can contribute to premature wear on the magneto points, and this requires more frequent inspection. We recently have developed a magneto cooling kit to blow outside air into the magneto area to help cool it down. And we strongly recommend this kit in southern climates where outside air temperatures are high to begin with — and especially if you are doing a lot of prolonged hovering, such as in training exercises. This is a fairly inexpensive modification that is well worth it. Other operators who have experienced this problem simply schedule a point change every 100 hours (at the 100-hour inspection). Many helicopters don't need it, however and particularly if you operate the helicopter in a cool climate."

Another consideration with these magnetos is the critical nature of the magneto timing. Newer engines are timed at 20 degrees, and the old "A" model is timed at 25 degrees. Unfortunately, you're forced to look through a small hole in the fan to align a small dot in the magneto, and this can be difficult to do correctly. If you time the magneto only two degrees out of time, it will have a negative impact on power.

"We stress getting the engine at top dead center (using one of several methods) on compression and then precisely lining up the marks on the magneto and then double checking to achieve proper timing. We also stress how to tell by instrument readings that the engine is out of time. For instance, if it is slightly advanced, you will have the engine run better (lots of power) with the EGTs being on the cool side. This may seem better, but it is actually detrimental to the engine as it may result in carbon fouling or other related problems," says Burdue.

Another item that is unique to Enstrom engines is the "correlated" throttle system. The correlated throttle is a system that connects throttle linkage to collective lever movement. The result is that you set the throttle at one place, and then, as the collective pitch is increased, the throttle automatically increases to provide the power you need to keep the engine rpm constant. "This gives the recip helicopter the feel of a turbine helicopter," says Burdue.

Correlation is accomplished through proper rigging of the throttle cables. It's an all mechanical system that uses cables, pulleys, and cams.

Burdue continues, "Unfortunately, rigging the correlator can be a bit confusing as you are balancing one part of the system against another and you have to keep going back and forth in order to achieve the desired adjustment. We like to impress always starting from step one of the post-flight rigging procedure and then following the procedure until you find what is out of rig — then making the necessary adjustments. Don't start with the pre-flight rigging procedures (preflight rigging is set at the factory and seldom needs adjustment) and don't just take a guess at what needs adjusting as you'll become even more confused and then make it difficult to get it back into rig.

"One of the tools we like to encourage technicians to use on our engines is the instruments in the cockpit and particularly the exhaust gas temperature on all of our helicopters and the graphic engine monitor that comes standard with some of our models and is optional on others," he says.

The Graphic Engine Monitor (GEM) allows you to monitor the cylinder head temperature of each cylinder, plus the exhaust gas temperature. One example of the use of the GEM is if a spark plug goes out, for instance. If a plug is inoperative, you can observe that the temperature of that cylinder will be hotter than the rest with both magnetos running. If you then switch from one magneto to the other, you will see that one cylinder is cooler than the other with one of the magnetos switched off. This will tell you exactly which cylinder the plug is in, and which magneto the problem plug is associated with.

The air filter assembly must be properly sealed to prevent re-introduction of heated air into the intake.

Another example of using the GEM for troubleshooting is through monitoring the EGT on all cylinders and this will tell you, for instance, you may have a bad fuel control. If you have a bad magneto, you can tell by abnormal temperature indications across all cylinders while the engine is switched to one magneto. Or, these abnormal temps may be that you have one of the magnetos incorrectly timed, whether it's retarded or advanced. Advanced timing will actually give you lower EGT readings. For example, a low reading of 1,350 degrees across all cylinders (which is 100 degrees too low) will tell you that the magneto is probably one or two degrees out of time. This can result in detonation or pre-ignition and depending on if the magneto is advanced or retarded.

"In general," says Burdue, "we like to think of all items on a helicopter engine as being critical. For instance, in addition to the timing, we like to make sure that you always have good compression and good plugs on each cylinder. The reason that you need this is that it takes most of the 225 horsepower for the helicopter to run well. So, if you lose one cylinder or even one spark plug on a four-cylinder engine, that represents a large portion of the available horsepower. We also recommend frequent differential compression checks to monitor the condition of the engine and don't recommend any more than 10 percent difference from the highest to the lowest cylinder in terms of compression.

TBOs on the engines used in Enstrom helicopters are 1,500 hours and a bit lower than their fixed-wing counterparts. "We occasionally have received requests for TBO extensions," says Burdue, "but since Lycoming is the manufacturer, they are the ones who you need to go to for the extension. I have not seen them grant many extensions, but when they do, they typically require you to do more frequent inspections, oil changes, etc."