Aircraft cowl flaps are movable extensions of the lower engine cowling-hinged, plate-like covers that control the opening through which hot air escapes. They are installed on the bottom aft portion of each engine nacelle, positioned to tap the energy of the slipstream for forced ventilation.
In basic function, cowl flaps open and close to regulate the airflow across the engine compartment; more cooling airflow is admitted when the flaps are extended, lowering both Engine Cylinder Head Temperature (CHT) and oil temperature, as well as Turbine Inlet Temperature in turboprop installations. In the cockpit, a simple cowl flap control allows the pilot to match cooling capacity to flight phase, minimizing the risk of temperature-induced engine damage that can most profoundly affect long-term health and longevity of a high-performance airplane engine.
While open cowl flaps produce some additional cooling drag and slightly degrade performance, the capability this engine-cooling device provides is pivotal: it gives the pilot a direct, immediate means to keep engine temperatures within safe operational limits during high-power climbs, slow flight, or hot-weather operations.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What are cowl flaps in aviation?
Cowl flaps are small doors located at the bottom of the engine cowling. They increase the size of the cowling outlet to increase the flow of cooling air through the cowling. Cowl flaps allow greater cylinder cooling during takeoff and climb and during these phases the cowl flaps are. You can close cowl flaps a bit during cruise, and you refer to the pilot's operating handbook for proper use of cowl flaps. They are fitted to many airplanes that use airflow rather than liquid coolant for engine cooling.
What do cowl flaps do?
Cowl flaps are used to regulate engine temperature by controlling the amount of air that flows through the engine compartment. When opened, cowl flaps increase airflow around and through cylinder cooling fins, which helps dissipate the heat of combustion. This process reduces engine Cylinder Head Temperature (CHT) and Turbine Inlet Temperature, contributing to overall engine fettle and performance.
By creating more suction, cowl flaps pull air through the cowling and across the cylinders, accelerating airflow. This increased airflow cools the engine more effectively, especially in conditions like low airspeed associated with takeoff or high-power operations, where supplementary cooling is required. Cowl flaps are controlled by the pilot or automatically through mechanical or electronic systems, allowing for precise regulation of engine temperature during different phases of flight.
Where are the cowl flaps located on an aircraft?
Cowl flaps are located on the bottom of the engine cowling, typically mounted on the lower section of the nacelle. On single-engine aircraft, they form a ring segment under the radial or reciprocating engine. On multi-engine aircraft, each nacelle carries its own set of cowl flaps, positioned along the lower cowling.
The pilot manages these flaps from the cockpit: cowl flap controls are located on the pilot's left lower side panel, while cowl flap circuit breakers are on the co-pilot's right upper side panel in the bottom two rows labeled Engine & Fuel Systems.
How do cowl flaps work?
When the pilot opens the cowl flaps, a low pressure area forms on the aft side of the engine compartment. This low pressure pulls outside air through the cylinder fins, so the incoming stream draws air that helps dissipate heat of combustion. By controlling the size of the opening, the pilot varies the reduction in air pressure, which in turn regulates how much cooling flow reaches the engine.
When are cowl flaps open?
Cowl flaps are open during takeoff, taxiing and ascent. During taxi, takeoff and initial climb the engine operates at high power while the airplane's forward airspeed is still low, so ram-air cooling is weak. At that stage the cowl flaps are left fully open so the drawn air flows directly across the cylinders and dissipates the extra heat of combustion. If the Cowl Flap Open annunciator is installed, its light normally comes on as soon as the flaps reach roughly the 50-percent-open position, confirming that the extra cooling stream is already being supplied.
When do you close cowl flaps?
Cowl flaps are closed as the aircraft approaches cruise speed in level flight. Seneca V SOPs close them after take-off unless the climb is continued at Vy or Vx, and the QRH repeats the order at the top of climb. During cruise the flaps are kept shut to streamline cooling and recover the extra knots. They remain closed for landing, because the power reduction and higher airspeed already supply ample airflow.
Do cowl flaps affect drag?
Closed cowl flaps decrease the size of the outlet aperture and decrease drag whereas open flaps enlarge the aperture and allow more air to spill, so the airplane accelerates less readily. Increasing the size of the outlet aperture increases cooling airflow, yet every extra pound of air that is pushed through the engine compartment has to be dragged along with the airplane. That airflow increases cooling drag, and cooling drag increases at the cube of the indicated airspeed, so the penalty climbs steeply as the aircraft flies faster. Because the operating engine now faces more resistance, increased drag on the operating engine decreases Vmc, making the minimum-control speed a few knots lower when the flaps are open.
How to open cowl flaps?
In most light aircraft the cowl flaps move by mechanical linkage: a thin, flexible steel wire called a cowl-flap cable runs from the control knob in the cockpit to the hinge line of each flap. When the pilot pulls the knob, the cable draws the flaps inward toward the closed position. Pushing the knob relaxes the cable, and a spring on the hinge then swings the flaps outward to the open position. Because tension in the cable is used in both directions, visual and tactile feedback from the handle tells the crew exactly how far the flaps have travelled. On larger, more modern types the same motion is performed electrically. A small motor, energised by the OPEN/CLOSE toggle on the engine-control panel, rotates a jack-screw or sector gear attached to the flap hinge. Limit switches stop the motor when the commanded position is reached, and a position indicator on the panel repeats the setting to the crew. Whether the motion is cable-and-bellcrank or motor-and-gear, the pilot's task is identical: retard the throttle, set the desired flap position, and confirm positive movement before resuming cruise power.
