As pilots, there are many operating requirements and guidelines that we follow regardless of the degree to which we appreciate the engineering limitations that prescribe them. Among the ones with the most serious consequence are those related to the loads placed on the airframe and the thermal conditions the engine is subjected to. Sensitivity to weight and balance, respecting turbulent or icing conditions, and refraining from heavy-handed actions like full deflection of control surfaces are among the wisdom pilots quickly take to heart to avoid exceeding airframe limits. Engine thermal limits, on the other hand, seem a less conspicuous danger and so, perhaps we can be forgiven if we are not as scrupulous in our attention to them. The experience instilled in student pilots during their training further obfuscates the concerns. The repeated, rapid cycling of power to execute countless touch-and-go landings, missed approaches, stall recoveries, and engine-out exercises, inurns us to the enormous stress they place on the precision machine generating that power. Understandably, we want to foster confidence in the machine during the formative days of our piloting tutelage, and the engineering and regulatory rigor we know is applied to our engines, facilitates further dismissal of their vulnerability. Admittedly, the less sophisticated engines encountered in our training may be somewhat more tolerant of harsh operation, but the laws of physics apply, nonetheless. While it might be beneficial in training to inculcate a light hand on the throttle and healthier respect for the mechanical miracle occurring under the cowl, in my experience, I only came to appreciate the following operating nuances later, and primarily through ownership.
Oil is the lifeblood of the engine. We know this even if we fail to appreciate all it does to cool, reduce friction, cushion against shock, protect against corrosion, and remove contaminants. The most accurate reading on oil sump quantity is obtained prior to the first flight of the day and then we rely on stable mid-range temperature as the best indication in-flight of adequate oil quantity. Regular and frequent oil and filter changes may be among the best things one can do to look after the health of the engine, along with ensuring the oil is at the appropriate viscosity to perform its vital functions. Superficial application of preheat may warm the engine enough to permit starting but may be inadequate to de-congeal oil in the sump, lines, cooler, filter, etc. Considerable preheat is required and pilot intuition may not be a reliable guide. The engine may start and appear to run satisfactorily but can be damaged from lack of lubrication. The amount of damage will vary and may not become evident for many hours, or the engine could fail after application of high power. Finally, to help inhibit corrosion when shutdown, I have joined the ranks of those who believe in the value of using CamGuard, after reviewing substantial product testing data.
I sometimes find it hard to remain mindful of just how critically dependent the engine is on proper airflow, to both avoid shock cooling, but even more importantly, remove adequate heat to avoid self-destruction. The often ignored, and much abused, rubber baffles under the cowl are a vital engine safety component and deserving of greater respect. Together with the cowling they precisely shape the airflow, and for this reason, the engine should never be run up under high power with the cowling removed. As pilots, we need to do our part, as well, to actively manage engine cooling. First, by minimizing ground run-ups, as the damage they can cause is seldom great enough to show up right at that moment of abuse. Instead, it goes by undetected, but accumulates with more abuse and finally shows up “down the road” in the form of broken piston rings, scored cylinders, or premature overhauls. When high power run-ups are necessary, ensure the engine is idled (~600-800 rpm) for a few minutes prior to shutdown to remove the excess heat developed. In flight, cowl flaps (and mixture if necessary) should be used to maintain cylinder head temperatures at ~2/3 range of the green arc. Although it is desirable to elevate CHT sufficiently (typically >300°) to burn off volatiles that can accelerate corrosion, the upper limit is an even more critical materials thermal limit not worth encroaching on. On the flip side, avoid long descents at low manifold pressure which can result in excessive engine cooling. If power must be reduced for long periods, it helps to adjust the propeller to minimum governing RPM and set MP no lower than necessary to obtain desired performance. The goal is to avoid having CHT drop significantly for long periods (e.g. <300° F for >5 minutes). Finally, use high power settings judiciously. In cruise, 65% power provides significant improvement in fuel consumption and endurance while still delivering more than 90% of the speed the aircraft is capable of while reducing wear and tear on the engine. Just as smoothness and coordination are a sign of more accomplished flying, so too, thoughtful and deliberate engine thermal management.