What It Looks Like

Aircraft engines have a love-hate relationship with heat. On the one hand, a minimum level of heat is necessary for the engine to run safely and smoothly. When temperatures are freezing outside, piston engines are preheated so the oil will flow on startup to lubricate all the moving parts. Engines operate best when they�re warmed to normal operating temperature. On the other hand, excess heat can cause critical components to fail prematurely.

Almost all aircraft piston engines are cooled by ram air flowing through the engine cowl. The air sweeps through the thin metal fins surrounding each cylinder barrel, collecting the heat produced by the combustion process inside the cylinders. The cooling air then exits through openings on the underside of the cowling.

Heat becomes more of an issue in airplanes with larger displacement engines and more horsepower. These engines generate lots of heat, and to make matters worse, such engines may be tightly cowled to keep drag to a minimum. That tight cowl makes it more difficult to cool the engine with ram air.

One way airframe manufacturers deal with engine heat in high-performance airplanes is to add cowl flaps. Cowl flaps are like the adjustable louvers on the dash of your car that regulate the amount of cool, conditioned air you want blowing in your face on a hot day. When you first get in the overheated car, you open the louvers to allow the full force of the air to flow though the system. As the car cools down, you gradually close the louvers to restrict the flow and regulate the temperature.

Cowl flaps operate on much the same principle. The �flaps� are adjustable panels on the underside of the engine cowl. Using mechanical or electrical controls in the cockpit, you open the cowl flaps to maximize the flow of ram air through the engine cowl and close them to restrict the flow.

Typically, cowl flaps are open in high-power, low-airspeed situations including takeoff, climb, and slow flight. They�re closed when airspeed is high, such as in cruise flight or when power is reduced, such as during a descent.

Exceptions come into play when the ambient temperature is very high or very low. On extremely cold days the cowl flaps may need to remain closed, even on takeoff and in the climb, to keep the engine temperatures up in the green. If it�s extremely hot, the cowl flaps may have to be positioned partially or even fully open during cruise flight to prevent excessively high engine temperatures.

Oil and cylinder-head temperatures are the critical ones to watch for guidance on how to position the cowl flaps. Unfortunately, age and use take their toll on the accuracy of analog, needle-style temperature gauges found in most piston-powered airplanes. That�s why many owners of high-performance aircraft opt to install digital-display engine analyzer instruments. The wealth of precise data they monitor and display enable the pilot to make informed decisions about engine heat and cowl flap position.

Learning to use cowl flaps properly is an important but sometimes neglected part of transitioning to high-performance airplanes. It�s easy to forget to include cowl flaps in your procedures, but the price of forgetfulness can be costly. The best way to make sure you routinely incorporate cowl flaps in your procedures is to use the aircraft checklist for each phase of flight.