What It Looks Like

Most aircraft piston engines are air-cooled by ram air - the air that enters the cowl through openings in the nose bowl. This cooling air sweeps across the finned cylinders, collecting heat as it goes, and exits through one or more openings in the cowl. Sounds simple enough but, as with most things aeronautic, there's a lot more that goes on behind the scenes.

Achieving adequate engine cooling under the demanding conditions in which aircraft operate - sweltering summer ramps and low-airspeed, hot-day climbs to altitude, for example - is no small feat. The path that the ram air takes through the cowling must allow for sufficient cooling of the engine in every phase of flight without creating performance-robbing, fuel-hungry excess drag.

The idea is to pressurize the ram air in the narrow space between the top of the engine and the engine cowl, then use the lower-pressure area in the bottom of the cowl to help draw the cooling air through the opposing banks of hot cylinder barrels and heads. The pressurized ram air also is used to cool engine oil (through a small oil radiator) and engine accessories such as the fuel and vacuum pumps and magnetos.

Sealing the gap on top of the engine using flexible synthetic rubber baffles pressurizes the ram air. The slipstream flowing across an opening on the belly of the cowl (louvers, a fixed opening, or an adjustable cowl flap depending on the aircraft) creates the low-pressure area in the bottom of the cowl.

Handling all those cooling chores requires that the flexible baffle seals be properly installed - no gaps and a clean fold against the cowl. Not surprisingly given the temperature extremes, vibration, and the force of the ram air, the material used in engine baffling is subject to wear and deterioration. Small cracks, tears, or breaks in baffling can add up to a cumulative four- or five-inch hole in the baffle system, which means that a lot of air leaks past the seal. The result is less-than-optimum cooling performance. Abnormally high cylinder-head and oil temperatures may be an indication that the baffling system is breaking down.

Engine baffling can be difficult to inspect on a normal preflight check. You may be able to see sections of baffling when opening various engine panels, but much of it likely will be inaccessible. Older flexible baffle seals are thin and black in color. Don't be surprised to find them brittle or tattered. More modern seals are made of a thicker, longer-lasting silicone-based synthetic rubber and usually are orange in color.

The best way to see the baffling system is to visit the maintenance shop when the airplane is going through a 100-hour or annual inspection. The cowling will be off, exposing the entire engine and all of its accessories.