Insights

Don't fly without it

I continually observe pilots make operational errors that reflect a serious lack of system knowledge. You may not have a mechanical background, but that doesn't mean that you cannot learn the essentials of proper aircraft operation.

Are you required to have an in-depth understanding of every mechanical device that's on the airplane? Fortunately, the answer is no -- the airlines also support that philosophy. It does mean, however, that you must have a general understanding of each aircraft system and an in-depth understanding of every control, switch, gauge indication, and annunciator light that you would operate or observe during aircraft operation.

A carbureted fuel system is just one example. When I ask a pilot to describe that system, he will usually recite the general system description that's illustrated in the pilot's operating handbook for the aircraft. Then, in the airplane, he will make several operational fuel-system errors.

When using the fuel primer before engine start, he will pull out the primer and immediately pump it four or five times without pausing. How much priming occurred? Little, if any. You must pause for a few seconds each time the primer is pulled out to allow fuel to fill the primer's fuel chamber.

In other cases, primer operation is correct, but it is pumped too many times. Excess priming increases engine wear, because the extra fuel washes engine oil off the cylinder walls. And in temperatures that are well below freezing, a pilot may close and lock the primer rather than leaving it out and charged, which permits extra shots of fuel if the engine begins to die after it starts, a common occurrence in low temperatures.

Instead of using the primer, he may prime the engine by pumping the throttle several times. This is acceptable in warmer temperatures, but it should never be done with updraft carburetors -- the ones that are located below the engine -- when the engine is not rotating. The fuel will fall into the air intake duct, not the engine, and create a fire hazard should the engine backfire during starting.

It is permissible to pump the throttle while the starter motor is rotating the engine, because the fuel-air mixture is being sucked into the engine. However, rapid pumping does nothing, nor does leaving the throttle at a position that is not recommended for starting -- usually one-quarter to one-half inch open.

Rapid throttle pumping does not allow the carburetor's accelerator pump to recharge with fuel, and a throttle that's too far open lets too much air in for the small amount of fuel that exists. The throttle should be rapidly opened and then quickly repositioned to the required start position. Pause for a couple seconds so that the accelerator pump will refill and then cycle the throttle again. In mild or warm temperatures, two or three of these cycles will usually provide enough prime for engine start.

After engine start, the pilot will pull the mixture back somewhat in order to lean the mixture and prevent spark plug fouling during taxi, a common occurrence with low-lead fuel. This is incorrect. He should slowly lean the mixture until engine rpm increases slightly and then enrichen the mixture slightly so that the engine will not "stumble" when taxi power is applied.

If the rpm increase is excessive, the idle mixture is too rich; if no rpm increase occurs and the engine starts to die, the idle mixture is too lean. In either case maintenance is required. This leaning procedure is also required for fuel-injected engines.

After engine runup, the throttle must be fully closed in order to check idle rpm. If it is too high or too low, maintenance is required. Some pilots will apply carburetor heat during the idle rpm check, and that is absolutely incorrect.

The idle rpm check is critical for fuel-injected engines, because if the engine will not idle and power is reduced to idle for an appreciable time during a landing approach, fuel vaporization may occur, and power will be nonexistent when the throttle is advanced -- the cause of several landing accidents. The idle fuel situation is not as critical for a carbureted engine, because fuel is located in the carburetor's fuel chamber, and it will immediately enter the engine when the throttle valve is opened.

Study your aircraft's systems, and when the POH leaves you with questions, consult a mechanic. I've never known one who would not address a pilot's concerns.

Ralph Butcher, a retired United Airlines captain, is the chief flight instructor at a California flight school. He has been flying since 1959 and has 25,000 hours in fixed- and rotary-wing aircraft. Visit his Web site.