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Vapour Lock




A veteran pilot recently passed along one of his many remarkable hangar stories, this particular epistle centering on an experience in which he suffered one complete engine failure while flying a twin, at 10,000 feet over Northern Ontario during a cold winter month. With one remaining engine operational from his lofty perch, he gradually bled off his altitude and timed his descent to perfection, managing still to arrive at his planned destination, minus the power from one unit, gear up to reduce drag in the circuit, then bull’s-eyeing his final approach.


What lead to his power unit’s transformation into dead weight is summarized by the following two words: vapour lock. Here’s an overview of how it can happen.

First, to understand the notion of vapour locking, an equitable understanding of vapour pressure is in order. The latter refers to the amount of vapour that a fuel will produce at a given temperature and altitude. The higher the vapour pressure – a reference which also implies that the substance is volatile – the easier it is for the fuel to vaporize. However, a higher vapour pressure increases the likelihood of a vapour lock since the liquid will boil at a lower temperature.


A fuel with a higher vapour pressure can vaporize prior to reaching the fuel pump or carburetor and can “lock out” any fuel trying to make its way through your fuel lines. Two factors integral to the resultant imbroglio are the effects of the aforementioned temperature and altitude. The higher you fly, the lower the air pressure; the lower the air pressure, the lower the temperature at which a fluid will boil. If, for instance, avgas boils at 140!F at sea level, at 20,000 ft it boils at 100!F.


An aircraft, then, with a high rate of climb, having baked on a field during hot summer months, could reach its cruise altitude quicker than its fuel can cool and thus lay down the prerequisites for a vapour lock to occur. By way of example, if at take-off, a tank of fuel sits at a temperature of 110!F, a quick climb to your cruise altitude may see to it that your uncooled gas slips into a state of ebullience at 20,000 ft if its temperature has not appreciably dropped.


It is fundamental to the properties of avgas that it vaporize easily at the lower temperatures found at altitude, but not so easily that it will cause the onset of vapour lock. Though it is essential that fuel must vaporize in order to burn in your engine, the fuel’s vaporization in the carburetor venturi cannot take place without heat being extracted from the process. Volatile fuels extract more heat than non-volatile fuels but they tend to allow bubbles to form in fuel lines. Such bubbles in the fuel delivery system will interrupt fuel flow, partially or completely causing an engine to fail.


High altitude operations, high temperatures underneath a cowling, and volatile fuel is a recipe for vapour lock formation. Rerouting of fuel lines away from heat sources, fuel coolers, fuel pumps integrally mounted to fuel cells, and heat shields around insulated fuel lines are some standard solutions to prevent vapour locking tendencies. While today’s fuel blends, and modern aircraft designs, significantly limit the threat of a vapour lock silencing your engine in flight, poor fuel system maintenance and hasty engine compartment modifications have contributed to vapour lock incidents.


Our story teller safely brought his aircraft to the security of a runway. Subsequent inspection found a mechanic had neatly ducted air from a nose-mounted heater to a camera operator’s position in the rear of the aircraft. Tidy though it was, the duct stretched along floor boards adjacent to fuel lines which eventually and inevitably vaporized the fuel in his lines.

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