One of the most important factors in the continued health of an aircraft engine is the use of proper operational procedures by the owner. These are based on common sense but, there are some basic maintenance and flying requirements that will determine the in-service life of your engine.
Engine health rarely deteriorates rapidly; it is more often a gradual process. The owner or pilot is in the best position to monitor the symptoms of deterioration but, do not try to diagnose the symptoms yourself: go to a specialist! Engines sometimes produce misleading symptoms that tend to indicate the wrong problem. However, you should be aware of the proper handling techniques and know the major signs of engine deterioration.
Let us start with a zero-hour engine, either new or overhauled. These engines will have been given a production acceptance test; for the Gipsy Major this consists of an initial run-in followed by an endurance and oil consumption test at 2100rpm. The new engine is far from fully run in and the finishing of this process rests with the owner and pilot.
Unlike a car engine, the aircraft engine must not be "run-light" during the run-in period. The main reasons for this are that the aircraft engine is designed to operate at a higher constant cruise power, and the clearances are much greater, unlike the car engine which is designed to run at variable power outputs with relatively short time at high power. The primary object of running-in an aircraft engine is to bed in the piston rings and cylinder surfaces. To understand why this is not carried out at low power settings we need to look at the primary function of the piston ring and to see what happens to the ring and cylinder surface during the run-in period.
To allow for the differences in thermal expansion, cooling, lubrication and, to a lesser extent, manufacturing tolerances between the cylinder and piston, the engine is designed with a clearance between the piston skirt and the cylinder wall. The Gipsy series engines were designed to operate with a radial clearance of up to 0.5mm. This gap must be sealed to prevent the combustion gases escaping downwards past the piston into the crankcase and also to prevent the oil, used for lubricating and cooling the piston, from passing upwards into the combustion chamber. Rings are, therefore, fitted to the piston to seal this gap.
Much development has been carried out into piston ring technology and all Gipsy engines use a three ring arrangement; two plain compression rings to give gas sealing and one stepped scraper ring that combines its oil control function with gas sealing. In a perfect world these rings would have a mirror smooth finish and seat perfectly onto the cylinder wall to give a 100% seal. Unfortunately, this does not happen. It is impractical to produce rings and cylinders with a mirror smooth finish and even if this could be economically achieved, it would be unlikely to result in a 100% seal. Although the standard of the finish given to a face of a piston ring looks smooth, if looked at under a microscope it would appear very rough, it is this surface that needs to be broken-in during the run-in period and to aid the smoothing process the cylinder wall is honed to give a cross hatched surface.
During normal operation of a new engine a film of lubricating oil holds the piston ring away from the cylinder wall. To achieve a proper bedding in, the piston ring must rupture the oil film and make a metal to metal contact with the cylinder wall. During this contact the peaks on both surfaces become white hot and rub off. This process will continue until such time as both the ring faces and cylinder wall have established a smooth, compatible surface between each other.
The one problem with this process is the film of lubricating oil which is present on the cylinder wall to prevent such metal to metal contact during normal engine operation. To achieve a satisfactory run-in this oil film must be ruptured to allow a degree of contact between the ring and cylinder, but enough oil must remain to aid cooling and lubrication of the piston skirt. It is in this area that the cross hatched surface of the cylinder, the use of the correct oil and correct operating techniques, combine to aid run in.
There are two main types of piston engine lubricating oil: compounded ashless dispersant, commonly but erroneously called detergent, and non-compounded commonly called straight, oils. The compounded oils have been developed to give superior lubricating performance with greater film strengths than the straight oils. It is, therefore, necessary to use the straight oil during the run-in period to aid the rupture of the oil film.
The piston ring is forced on to the cylinder wall by combustion pressure. This is the second key to a successful run-in. Combustion gas pressure increases with engine load and to ensure that there is sufficient pressure to force the piston ring onto the cylinder wall it is essential to maintain a high rpm during the run-in period. Using low power will not only lengthen the run-in period but any prolonged use of low power will allow the cylinder surfaces to glaze over before the rings have achieved a satisfactory seating.
During each power stroke the cylinder walls are subjected to high temperatures in the region of 4,000 deg F. Although this period is very short it is long enough to oxidise small quantities of oil that have settled in the valleys of the cross hatching on the cylinder wall. Eventually, this oxidisation of the oil will fill the valleys and produce a hard, smooth, flat surface. This is the ideal situation achieved after a normal run-in when the rings have completely bedded in. If this glazing occurs before the ring bedding-in is completed, the seal between the ring and cylinder will not be complete, and excessive oil consumption will result. The only remedy will be to re-hone the cylinder walls and start the run-in process again. It is for this reason that the cylinders must be honed and cross hatched during top overhaul or at any other time that the piston rings are changed.
Run the engine in for 50 hours or until the oil consumption stabilises.
Use a good quality non-compounded aircraft engine oil, (grade 100). For operations below 0 deg C use grade 80, and above 30 deg C use grade 120.
Use full power during takeoff and maintain at least 75% power for climb out.
Maintain at least 65% power in the cruise and intersperse with maximum power for 30 sec every 30min.
Avoid long glide approaches and maintain the maximum power possible.
Keep ground running, idling and taxying to a minimum.