Engine cooling

Learning objective

You can explain how an air-cooled piston engine rejects heat (fins, baffles, ram air) and what cowl flaps do conceptually — without invented AFM positions, oil temperatures or RPM.

60 seconds

The cylinder head dumps heat through cooling fins into air. Baffles force ram air through the fins, not around them. Cowl flaps (if fitted): open = more cooling mass flow and more drag; closed = less cooling, less drag. Which position when: AFM. CHT is the fast head; oil temperature lags.

Definition

Most PPL piston engines are air-cooled — no liquid circuit as the main path. Heat is from combustion; without cooling, CHT and oil temperature rise.

Components:

• Cooling fins — surface area
• Baffles — ducting; missing/damaged baffles = local hotspot despite “lots of ram air”
• Oil cooler — oil carries part of the heat (see engine oil)
• Cowl flaps — variable cowling exit/inlet, not on every type

Rule

More air mass through the fins → more heat rejection, usually more drag. Less air → hotter engine, less drag.

Teaching (not AFM): climb / high power / low IAS → high cooling demand → flaps often open. Cruise → often closed. Descent with an empty cowling into cold air: shock cooling is the teaching name for a too-rapid CHT drop — the counter is per AFM (typically not “idle and nose down”), no universal 1500-RPM rule.

Cockpit

• CHT first: load and mixture act quickly
• Oil temperature as a trend
• Cowl-flap lever: OPEN / CLOSED (or intermediates), labels and limits AFM only
• Enriching mixture can lower CHT (fuel cools) — procedure AFM, not “always full rich in the climb”

No Lycoming O-320 260 °C, no 118 °C oil, no “green before run-up at number X”.

Trap

“Cowl flaps always open until 500 ft after take-off, then closed — every POH says so.” That is false as a universal. Only this type’s AFM.

“Air-cooled means the pilot cannot control cooling.” False once cowl flaps, power, IAS and mixture exist.

Exam knowledge

• Air cooling: fins + baffles + ram air.
• Cowl flaps open: more cooling, more drag; closed: the reverse — no invented positions.
• CHT fast, oil temperature slow.
• Shock cooling is a teaching concept; procedure AFM.
• No universal oil/CHT numbers.

Source

• FAA-H-8083-25B §7 Engine cooling (teaching)
• FAA-H-8083-32A Air-cooled engines / baffles / cowl flaps
• Type AFM/POH for flap use and temperature limits

Check yourself

1. Why air cooling on typical light-aircraft piston engines?

• A) Combustion heat leaves via cooling fins into the ram air; baffles force air through the fins
• B) Air cooling fully replaces oil
• C) PPL trainers are liquid-cooled as standard
• D) Fins are only for cabin heat

2. Cowl flaps — concept (when fitted)?

• A) They change propeller pitch
• B) Open = more cooling (climb, high power); closed = less drag (cruise, descent)
• C) A universal 50 % AFM lever for every type
• D) Ground only, never in flight

1. A — oil also cools; it does not replace ram-air fins.
2. B — no invented lever number; AFM of the type.