Diesel engine technology is slowly making its way into the general aviation market and with that come some differences in the way aviation technicians must look at line maintenance and troubleshooting. Techniques for diagnostics and tooling for maintenance actions are different in some cases than the traditional gasoline combustion engine. Here are a few maintenance tasks you will encounter differences with when inspecting and maintaining today’s diesel technology aircraft engines.
Differential compression test
Most manufacturers require some form of compression test as part of the engine inspection; in the gasoline aircraft engine, the most common form of compression test is the differential compression test. The differential method requires the input of shop air, at usually 80 psi to be inputted into the cylinder being tested while at top dead center (TDC). The pressure that the cylinder is holding is then measured. This differential test method is often called a “leak down test,” meaning the technician can use the leaking air from the testing procedure to more easily identify the area of leakage. Examples would be compression ring blow-by would vent out the crankcase breather vent, intake valve, and seat defects would leak from the induction system, exhaust valve, and seat defects would leak out the exhaust system and cracks in the cylinder head would simply leak out the crack. Cracks can be located with the old trick of spraying soapy water in the suspect area while pressure is still being applied to the cylinder.
Direct compression test
In the case of the diesel engine, the technician has to use a different form of compression test, called the direct compression test. Since diesel engines have very high compression ratios, shop air will not go to a high enough value to adequately test the cylinder using the differential testing method. This means the technician may have to buy a new tool to add to his/her selection. The typical direct compression tester usually has a pressure range that tops out around 150 psi, which just isn’t high enough for the high compressions of most diesel engines. The technician will need a compression tester that reaches pressures up to 800 psi, although most diesels will range in the 400-500 psi range, some may go higher. As with any other direct compression test, the technician must make sure the engine rotates through at least three compression events and the engine can breathe sufficiently through the induction system.
Coolant system checks
Diesel engines usually are liquid cooled with either a separate cooling system such as an automobile or through some form of oil cooling which is something not too familiar to most aircraft technicians. This brings the requirement to check coolant levels, hose clamp placement, coolant replacement at certain intervals, and most importantly, pressure testing of the coolant system. The coolant system pressurization tool is another tool the technician will have to add to his/her collection. Typically systems are pressurized to double the normal operating pressure, but could have other specified pressure requirements. These pressures are often “held” in the system for a pre-described time frame to check for leaks and seepage within the coolant system.
Gear reduction system checks
Another area technicians must become familiar with is the gear reduction system. As automotive technology blends itself into aviation we are seeing more engines with gear reduction systems. Automotive style diesel engines that are being integrated into aviation applications turn at a much higher rpm than the traditional gasoline aircraft engine. These gearboxes often have filtration systems and chip detectors.
Oil analysis is nothing new to aircraft technicians, but the technician of today must realize that gearbox oil may be a separate system in modern engines thus requiring different oils and a separate oil sample. Maintenance on these gearboxes will also often require the inspection of a magnetic drain plug. The idea is that the magnet in the drain plug catches metal flakes and chips and holds onto them for a more detailed inspection upon plug removal. Analysis of this metal could be as simple as comparing the plug to a picture or emailing a picture of the plug to the manufacturer technical support staff.
Engine static check
The engine static check is also a bit different on the aviation diesel engine. Many aircraft diesels are FADEC (computer controlled) engines; this means that a software package is controlling the engine. The pilot and technician simply position a power level to the desired percent power and the computer does the rest. Traditionally the technician would look up the aircraft static rpm range in the Aircraft Type Certificate Data Sheet and then perform an engine run to full throttle and compare the results to assess engine performance. The technician must account for atmospheric conditions such as pressure and temperature, as they will affect engine and propeller performance.
However, on the FADEC diesel engine, the static check is more like a performance run on a turbine engine. The technician must still account for atmospheric conditions of temperature and pressure, but is not looking at rpm in most cases, but rather percent power. To pass the static check the engine must perform to a prescribed percent power under those conditions or it will not pass.
Diesel technology is slowly making its way into general aviation and the technician of today will need a few more tools in his/her toolbox. They will also need to be familiar with using computers and tablets for diagnostics and troubleshooting. Learning these software systems and diagnostics capabilities can be confusing and complicated. That is why most modern FADEC engine manufacturers are requiring technicians to attend factory training to be qualified with such systems. The laptop or tablet will soon be one of the most used tools in the aircraft technician’s toolbox.
Marshall Tetterton is an assistant professor at Embry-Riddle Aeronautical University Charles Taylor Department of Aviation Maintenance Science. He is a commercial instrument rated pilot with aircraft airframe and powerplant mechanics license. He has an Inspection Authorization with the FAA and more than 20 years of aircraft maintenance experience. He can be reached at [email protected] or (386) 226-6833.
