Piston Engine Starting Problems

Looking at the various responses to the complexity of the non-automated start procedure for a turbine engine made me think some more about piston engines. These days, or at least a few weeks ago, I have been operating carburettor- equipped horizontally opposed piston engines. That prestart checklist includes ensuring the fuel is turned on in two different places and that the throttle is far enough open for air to enter the carburettor. I engage the starter with a key, just like a car, except that the lighter weight starter motor means that I have to hold the key in the start position for longer before I release it.

Here are some of the abnormal start possibilities for a piston engine.

If you engage the starter and nothing happens, that means there is no electrical power. So you either have a completely dead battery or a broken connection. (Or you forgot to turn the master switch on). You can jumpstart an airplane if it has a dead battery. You can also hand prop it, as you've probably seen in old movies, but if the battery is completely dead, an alternator won't be able to generate any electrical power. A generator will.

If you engage the starter and there is a clunking noise, but the engine doesn't turn over, there's power but it's not energizing the motor. The clunking noise is the solenoid completing the circuit that is supposed to supply energy to the starter motor. If all you hear is a clunk, then the starter motor is inoperative. Get a new starter motor. Hand propping will work in an emergency.

If you engage the starter and there is a whirring, grinding noise, but the propeller doesn't turn around, then the starter motor gear is not engaging properly with the ring gear on the propeller shaft. This can often be remedied by turning off the mags and magnetos then getting out and turning the propeller by hand to a new position before re-attempting the start.

If you engage the starter and the propeller turns around, but not very enthusiastically, and no start is obtained, you likely have a weak battery. It's like trying to start a car on a cold day. Try for 30 seconds or so, but give the starter a rest before trying again. This is best fixed preventatively: minimize electrical use after shutdown and before start. On cold days turn the propeller through by hand a few times to break up congealation (is that a word?) of the oil. Use extra primer, because less of the fuel evaporates in the cold. Use preheating if available. Jumpstarting is an option, and with some battery power remaining, the alternator will function to recharge the battery. Sometimes this is caused by some kind of binding of the crankshaft, which will alos become evident as you turn the propeller by hand.

If the propeller turns, and the engine fires but it coughs and doesn't catch well enough to sustain firing it may not have enough air or fuel for combustion. Or you might have just released the starter too soon. It might just be insufficient primer, or the throttle not open far enough. Or you can mutter "must be vapour lock" and wander around poking randomly at things, hoping it will work next time you try. You may have to beg access to a heated hangar if it's really cold.

If the propeller turns and the engine fires, but weakly, amidst puffs of black smoke, it's probably flooded. You may be able to smell fuel. There is too much fuel present to start the engine. You misdiagnosed the previous problem and overprimed. Pull the mixture to idle, start the engine and then push the mixture knob in after start. If it's really flooded, you may need to wait a while before attempting the start.

If the engine starts fine, then sputters to a halt, I suspect one of two things. The first is carburettor ice, especially if it's a rainy day, with temperatures above freezing but below ten degrees celsius. In such conditions I monitor rpm or manifold pressure as appropriate right after start and am prepared to put the carburettor heat on right after start. If it's not carb ice it's fuel supply, which could mean something missed in the prestart checklist, or an issue for maintenance.

Sometimes the engine starts but the starter doesn't disengage. If you've ever turned the key to "start" a car that was already idling, you know what that sounds like. Ugh.

At engine start it's important to monitor the oil pressure closely. If the oil pressure doesn't come up into the green arc within thirty seconds of starting, you have to shut down the engine and send someone in coveralls to investigate why.

There's also a possibility that while you're attempting to start the engine, it bursts into flames instead of starting. The pilot must surpress the initial impulse to run away and instead keep cranking the engine. If it starts, that will consume the fuel that was feeding the fire, and then after running for a short time, the engine can be shut down and inspected. If it doesn't start, then you keep cranking while you shut off the fuel supply to the engine in multiple places, shut everything down, and evacuate, bringing the fire extinguisher. Now you have the option of running away, or trying to put out the fire, depending on the comparative level of the fire and your bravery/stupidity. If any passengers are on fire, you should expend your firefighting abilities on them, not the aircraft. I've never had nor witnessed a piston engine burst into flames on start up. It must be quite rare, but it was the first emergency procedure I ever learned. Apart from that, the most dramatic way to cook a piston engine is to not notice that the oil pressure has failed to increase. You can write off an engine costing tens of thousnads of dollars by running it without oil for a few minutes. Burning out a starter motor or running a battery flat are minor inconveniences in comparison.