Starting an aircraft engine and making it look easy ranks right up there with greasing it on the runway as one of the most frustratingly elusive piloting objectives to accomplish with some consistency. Like landing, engine start is one part science, one part art, one part folklore, and two parts luck. If you don't believe it, you haven't yet tried to start a hot fuel-injected engine.
Perhaps nowhere is the contrast between the airplanes we fly and the cars we drive more black and white than getting the engine started. When was the last time you worried that your car's engine wouldn't start? Probably never, if it's a reasonably contemporary model. No matter the weather or when you last drove it, it's gonna start. At the very most you might need to depress the gas pedal once to activate the automatic choke.
When you turn the key various sensors sniff around to get a feel for conditions and report their findings to the computer under the hood. Based on the field reports, the chip does a few quick calculations to determine what incentives the engine will extort extra gas, an advance on the ignition timing, that sort of thing before reporting to work, which it does almost without fail.
How does that compare with your frame of mind when preparing to start the engine in the airplane you fly? If you're lucky it's a mild spring day, the airplane hasn't been flown in the last couple of hours, and it has a new battery. You review the before-start checklist, prime if necessary based on your feeling of things, set the power controls as directed in the normal procedures section of the flight manual, then silently ask forgiveness for your sins before engaging the starter.
If you're not so lucky, the engine is either ice-cold or stovetop hot. These are the times when pilots resort to folklore and the mystical incantations and complicated machinations of the engine controls that look like they were devised in ancient times by reedy, mustachioed men who spoke in whispers and would not look anyone in the eye, then passed down by generations of pilots.
Starting an airplane engine is difficult, thanks to the traditional low-tech ways of general aviation. The only piston-powered airplane I've flown that was a no-brainer to start every time was a Mooney powered by a Porsche engine modified for aviation use. It was always a cinch to fire up because it had basically the same electronically controlled ignition and fuel injection system as a modern car.
My first airplane, a Cessna 172, was more typical of the tens of thousands of general aviation airplanes flying today. It had two magnetos and a carburetor. Early on an experienced pilot advised me to avoid using the primer and instead pump the throttle three times before engaging the starter. This was a departure from what was recommended in the flight manual, but it seemed to work reasonably well. Pumping the primer injects raw fuel into the intake port of one cylinder, which can load up that cylinder and foul the plugs. Pumping the throttle, on the other hand, forces the accelerator pump in the carburetor to squirt raw fuel through the throat of the carburetor and into the induction system plenum. When you turn the key to engage the starter, the rich fuel-air mix that's sucked into all the cylinders coaxes the engine to life.
The normal function of an accelerator pump is to prevent the engine from bogging when the pilot rapidly advances the throttle, such as when applying full power at the beginning of the takeoff run. But as I learned, it's also an effective crutch to lean on when starting the engine. The trick is to know when and when not to pump the throttle knowledge gained only over time and through experience, and useful only for that specific airplane.
Starting my 172 was one step easier than starting the Cherokee in which I learned to fly. The difference was in the wing. The high-wing Cessna relied on infallible gravity to deliver fuel to the engine. In the low-wing Cherokee, I had to switch on an electric fuel pump to get fuel coursing through the lines to start the engine.
The airplane I fly now has a low-wing design and fuel-injected engines. Fuel injection is a more efficient way to deliver fuel to the cylinders than a carburetor, and it's not subject to carburetor icing, but in certain situations it can be more difficult to start a fuel-injected engine. Typically, the small-diameter metal lines that transport fuel from the flow divider to each cylinder sit above the engine. Within a few minutes after the engine has been shut down, the tremendous heat rising from the cylinders is mostly trapped inside the cowl, superheating the fuel lines to the point where fuel inside the lines boils and vaporizes. This can create a blockage in the lines that persists until the engine and the fuel lines cool and the vapor condenses. The problem comes when you try to restart the engine before it's had a chance to cool sufficiently which may be a considerable length of time in high ambient temperatures.
The flight manual may recommend a complicated procedure for starting when the engine is hot: Open the throttle, switch on the fuel pump, advance the mixture control to full rich to purge the fuel lines of vapor, retard the mixture control to idle cutoff, then engage the starter. When the engine fires or if it fires advance the mixture until it's running smoothly.
Despite the level of detail in the procedures, there's no assurance they'll work. Many a pilot has experimented with throttle, mixture, and fuel pump to blast through a vapor lock and force good fuel to the cylinders.
Here is another complication that you, too, could encounter: The owner's handbook for my airplane doesn't list hot-start procedures only normal and flooded starts. That 's because the airplane was manufactured before the industry agreed in the mid-1970s on a standard for writing a pilot's operating handbook. The new standard calls for normal, cold, hot, flooded, and external power source starting procedures. The manual for the airplane you fly may or may not contain all these procedures, depending on the age of the airplane.
The most reliable hot start procedure I've found is the same as for a flooded start and close the throttle, leave the fuel pump off, position the mixture control at idle cutoff, and simply crank the engine. It starts every time within a few blades, without the worrisome puddling of excess fuel inside the cowling that I used to see when I tried other methods involving full-rich mixture and full throttle. Looking back, I think I learned that procedure from a mysterious guy with a bushy mustache.