Technical Articles - As published in The Blue

Many of you will realise that engine longevity is strongly-influenced by how the pilot operates his engine and whilst mishandling may not result in immediate engine reliability problems, repeated poor practice can store up problems for the future.

Cross head: Before start-up

Engine start-up is something that we are all familiar with, but what about engine starting in cold weather?  Cold winter flying can produce some of the most beautiful and memorable flights, but when temperatures reach 10 to 20oF (-7 oC to -12 oC) engine pre-heating needs to be considered, even if using multigrade oils.  The primary reason for this is that your engine is constructed of different metals, for example the crankcase is aluminium, the crank is made of steel, and these metals expand at different rates. 

Engines are designed so that the gaps between moving parts are optimal during normal operation, when the internal temperature may be 100oC (212 oF); cool everything down and differential expansion starts to be a major consideration.  Aluminium expands and contracts by a much larger degree than steel, so the aluminium crank case shrinks onto the crank shaft and nips up the bearing clearances. 

If we then consider that the oil is relatively thick when it is cold, then we can start to appreciate that there is a real risk of bearing damage when starting in very cold temperatures.

There are various types of engine heaters available. The first type, the heated dip stick, I would not recommend.  These do little to heat the engine casings and, with no oil circulation, promote overheating of the oil around the dip stick, whilst not heating up the rest of the engine adequately.

The second type uses heater pads under the engine sump and cylinders.  These work well, and have the advantage of being permanently fixed to the aircraft, but pilots should resist the temptation to leave them turned on permanently.  If this type of engine heater is left on for long periods the bottom of the engine heats up, evaporating water out of the oil which then re-condenses on the relatively cool upper engine, causing corrosion.  If the engine has a high-level camshaft, as do most Lycoming engines, then this can lead to the future, premature failure of the cam.

The final type of engine heater uses a hot air blower to circulate hot air around the engine prior to start.  These also work well, but it is worth keeping in mind that the engine needs to be heated for long enough to allow the warmth to penetrate the engine, as it is the temperature of the bearings that is at least as important as that of the oil.

Cross head: Engine warm-up

Once the engine is running, even in summer temperatures, it is sensible to warm the engine on the ground before proceeding to take-off power.  Again, differential expansion is a significant factor here.  Aluminium pistons slide in steel barrels and are a loose fit until the engine is warm.  This is particularly significant in those engines that are capable of high power and are of large cylinder bore. 

Piston rock can result if the piston clearances are too large and high power is applied.  The aluminium end caps that hold the gudgeon (or wrist) pin in place are also common victims of this type of operation – they are forced into the cylinder wall and are the common cause of high aluminium levels in oil analysis results.

The minimum operating temperature for take-off power differs with different engine types and can be oil temperature related (common with many engines, including the Russian radial engines), but may also be cylinder head temperature related (Lycoming recommend a minimum CHT of 150oF (65oC) for flight in many of their engines). 

The Pilot’s Operating Handbook should give the minimum temperatures for your particular aircraft, but many of the lower powered aircraft do not specify any limits.  However, even in these aircraft, I personally like to see the oil temperature off the minimum before I take the engine to high power as I feel that it is kinder in the long term.

Cross head: Avoiding shock cooling

Once flying, the next temperature consideration is that of cylinder head temperature (CHT) management.  Air-cooled engines run relatively high CHTs when compared to liquid-cooled engines, such as those commonly found in automobiles.  This leads to another expansion-related problem, that of cylinder cracking.

In much the same way that a wine glass may shatter when plunged into boiling water, rapid expansion and contraction rates can set up large internal stresses in the metal as one part of the cylinder head cools more rapidly than another.  This can lead to cracking in those areas that are most difficult, and therefore slowest, to cool – typically between the exhaust port and the spark plug hole. 

This phenomenon is particularly associated with higher-powered engines as they produce more heat, and can also fly at higher altitudes where the air is cooler, which allows more rapid cooling when power is reduced.  However good practice can also prevent trouble for lower-powered aircraft and, even if there is not a CHT gauge fitted to the aircraft, flying a sensible profile can help engine longevity.

If you do have reference to a CHT gauge, it is recommended that cylinder head temperature change not exceed 50oF per minute to allow all areas of the cylinder head to cool at a relatively even rate.  This means planning ahead. Reduce power gradually and maintain some power throughout the descent.  Another good tip is to keep the fuel/air mixture at the leaned cruise setting during the descent; lean combustion mixtures burn hotter than when running over rich, so this helps to keep heat in the engine even when power is reduced. 

Of course at low power settings there is no risk of detonation so, as long as the mixture is increased before going to high power, the engine can not be damaged.  If an exhaust gas temperature gauge is installed with a normally aspirated engine, use the mixture setting to keep the EGT close to its peak value. 

This will insure the greatest possible engine heat for the power setting selected; for a turbocharged installation, lean to peak during descent unless otherwise specified in the Pilot’s Operating Handbook, or under conditions where the limiting turbine inlet temperature would be exceeded.

In part two, in the next issue of The Blue, Rob looks at engine shutdown considerations.