Volatility is a feature until it is not.
A fuel's eagerness to evaporate is quoted as its Reid vapour pressure — the pressure its vapour exerts at 37.8 °C, measured in a standard apparatus. High volatility is what lets an engine start on a freezing morning, so winter motor fuel — mogas, the stuff from the pump — is blended with far more of the light, eager fractions than summer fuel. The number is not printed on the pump, it changes with the season and the country, and nobody at the filling station has ever been asked about it.
Aviation gasoline is the boring cousin, and that is the entire point of it: its volatility is held inside a narrow specified band all year, so the fuel in your tank behaves the same in February and in July. Everything in this article is a consequence of motor fuel not having that property.
Vapour lock is a race between two pressures.
Fuel boils when its vapour pressure exceeds the pressure of its surroundings. In a fuel line there are two ways to lose that race, and a climb runs both at once. Vapour pressure rises with temperature, and it rises exponentially — a line routed past a cylinder or sitting in a cowl after a hot idle can be twenty degrees above the outside air. Ambient pressure falls with altitude: at 1500 metres you have lost about a sixth of it, at 3000 nearly a third. A suction-fed pump makes it worse still, because upstream of the pump the fuel is below ambient by design.
When the fuel does boil, the line fills with vapour. A pump that was moving liquid starts moving gas, its output collapses, and the engine leans out, surges and stops — typically in the climb, or on the restart after a hot turnaround, which are the two moments when you have least height and least patience.
Combien de marge reste-t-il sur la liaison ?
The default is a real scenario, not a contrived one: winter fuel left in the tank, a line at forty degrees after a hot taxi, a climb through fifteen hundred metres. The margin is already negative. Switch the fuel to summer mogas and it turns positive without anything else changing; switch it to avgas and the problem disappears entirely. Then take the altitude back to zero and the margin still does not close: at forty degrees this fuel is already boiling on the apron. Drop the temperature ten degrees and it opens properly. A warm line alone can do it. A climb simply removes the last of the argument.
Ethanol brings friends.
Blending ethanol into petrol nudges the vapour pressure up, which is the opposite of what you want in the air, and at a few kilopascals that is the smallest of its problems. Ethanol is hygroscopic: it pulls water out of humid air, and past a certain water content the blend phase-separates into a petrol layer and a water-and-alcohol layer that settles to the bottom of the tank — which is exactly where the fuel pickup is. It also attacks some sealants, some hoses and some fibreglass tank resins that were specified decades before anyone put alcohol in petrol.
The jam-jar test settles the fuel question in five minutes: put a measured depth of water in a jar and mark the line, top up with fuel, shake it, let it stand — if the water line has risen, the ethanol has left the petrol and joined the water. It says nothing about the fuel system, which is a maintenance question and belongs to whoever signs for the aircraft.
Approvals are three separate documents.
Running motor fuel legally means three permissions lining up, and they are held by different people. The engine manufacturer approves fuels for the engine. The airframe manufacturer, or whoever holds the approval for your aircraft, approves them for the fuel system — the tanks, hoses, pumps and routing. And the fuel itself has to meet a specification, usually with a minimum octane and, very often, a limit on ethanol content.
It is worth knowing which of those three is the one that vapour-locks: the airframe. The engine does not care much what happened upstream, and a fuel that is perfectly approved for your engine can still boil in a badly routed line on a hot day. That is why the question to ask is never just "can this engine run on mogas" but "is this aircraft approved for it, and how is the fuel line routed".
Dans votre cockpit.
Fuel and temperature conspire at both ends of the thermometer. This article is the hot end; the cold end is carburettor icing, where the same volatility that boils fuel in the line is busy refrigerating the throttle body. How much of it you have and how long it lasts is l'article sur la gestion du carburant. And the instrument that turns all of this from folklore into evidence is a recorded fuel pressure trace — engine monitoring explains why a logged trend finds the problem that a glance at a gauge never will.
Fuel pressure and fuel flow on the same recorded log as the cylinder heads, so the first bubble is a data point rather than a surprise.
The engine page beside the horizon, for the climb that turns the cowl into a kettle.
Sensors, displays and engine monitoring for Rotax and Lycoming panels.