Hanging in the Breeze

The most commonly glossed-over subject on complex checkouts could cost you several knots in cruise and a couple hundred hours of useful life from your engine. It’s not the landing gear, or the controllable pitch propeller, or even the mixture control. The cowl flaps can have a profound long-term effect on the health and longevity of a high-performance airplane engine — an engine that already stands a very poor chance of reaching the factory recommended Time Before Overhaul (TBO).

Most lightplane engines depend on air for cooling. A small opening or lip at the rear of the cowling deflects the slipstream enough to create a low pressure area in the bottom of the engine compartment; low air pressure itself cools the engine, and it pulls air through the cowling for more cylinder and oil cooling. All that air flowing through the engine compartment, though, creates a tremendous amount of drag.

In airplanes built for cross-country speed, this cooling drag is minimized by keeping the rear cowling opening just big enough for adequate airflow in cruise. At lower forward airspeeds, like takeoff and climb, the pressure differential may not be enough for adequate cooling. For cooling at these high power/low airspeed phases of flight, cowl flaps (controllable openings that provide a greater airflow deflection and therefore greater pressure differential) are added to many high-performance airplanes.

Why It Matters

  • Forget to open cowl flaps for taxi, takeoff or climb, and the engine may develop hot spots. Translation: you’ll be headed to the overhaul shop a lot sooner.
  • Forget to close them in cruise and the rapid increase in cooling airflow may result in damaged cylinders; the cooling drag may also rob several knots from your final cruise speed.
  • Leave the cowl flaps open for descent and landing, and the engine may suffer shock cooling — cooling unevenly, and too fast.
  • Close the cowl flaps for parking, to help keep birds out of the cowling.

Cowl flap use is critical, but easy and following a few simple steps can save you a lot of unnecessary expense — or worse. Here’s how:

C Checklists: Follow the appropriate one — before, during and after flight.
O Open your cowl flaps for start-up, taxi, run-up, and for taxi-in after your clear the runway.
W Watch cylinder and oil temperatures and close the cowl flaps as the aircraft approaches cruise speed in level flight. But, if the engine is getting abnormally hot, or you’re in a configuration where it’s likely to get hotter (taxi, takeoff or climb), open the cowl flaps.
L Leave the cowl flaps closed for descent and landing — and then some…

BOTTOM LINE: Proper cowl flap use is an exercise in procedure and temperature control. Follow your checklists, but do not be afraid to deviate from them if your engine temperatures move out of the “green.” When it comes to temperature, engines like consistency and they hate surprises. Do yourself a favor and be careful. Make an effort to keep your engine temperatures steady and “in the green.” It’s your engine and your wallet, but more important — it’s your life.