Vapour lock

Vapour Lock

Vapour lock is an engine failure mechanism in carburetor aircraft engines with AVGAS and especially MOGAS: fuel evaporates in the supply system into a vapour bubble that interrupts or reduces fuel flow to the carburetor.

Mechanism

Fuel has a low evaporation temperature (especially summer-grade gasoline / MOGAS):

• AVGAS 100LL: Reid Vapour Pressure (RVP) about 38–49 kPa at 38 °C.
• MOGAS: significantly higher (RVP 45–80 kPa) → higher vapour-lock risk.

When fuel in the line between tank and carburetor is:

• Heated by hot engine components (exhaust manifold, engine bay), or
• Subjected to low pressure in the suction line (high altitude, pump actively sucking),

the fuel boils in the line. The resulting gas bubble interrupts the liquid flow to the carburetor.

When does vapour lock occur?

High risk:

• Hot summer day (OAT > 30 °C),
• High-altitude flight (low pressure at tank outlet),
• Right after long ground hold with hot engine (hot start),
• Suction-pump configuration (high-wing with gravity feed is less prone than low-wing with electric pump),
• MOGAS instead of AVGAS in types with MOGAS STC.

Symptoms

• Sudden engine power loss with no other apparent cause.
• Engine stumbles or quits entirely.
• Difficult restart on hot start.

Prevention and recovery

Design measures

• Insulated fuel lines between tank and carburetor.
• Boost pump (electric auxiliary) activated from cockpit — raises pressure in the line.
• Fuel return line — excess fuel returns to tank, cooling the line.
• Better fuel choice — AVGAS has lower VL risk than MOGAS.

Pilot actions

• Activate boost pump before take-off (especially in hot weather / hot start).
• Boost pump during flight on: in hot conditions, high altitude or per AFM.
• Hot start: mixture rich, throttle slightly open, boost pump 5–10 s on, then attempt start.
• In flight on vapour lock: immediately boost pump on, possibly descend (cooler air, higher pressure at outlet), mixture rich.

Practical note

In most Cessna/Piper training types with AVGAS and gravity-fed tanks (high-wing) vapour lock is rare. In low-wing types with MOGAS STC (e.g. PA-28 with MOGAS approval) it is a realistic risk, especially in summer.