Friday, July 15, 2005

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.

8 comments:

Aviatrix said...

Might not be any blogging over the weekend, but as usual, I'll probably find a way.

david said...

Nice list -- it's definitely a keeper in any piston pilot's bookmark list. I have a couple of extra comments.

First, when you write that the "starter doesn't engage", I think you're talking about a sticky bendix -- sometimes it's possible to fix that by working the bendix forward with a screwdriver until it's engaged with the starter gear (I've had it demonstrated, but haven't tried it myself); however, the very best solution, if you're going to be stopping somewhere isolated and are worried about the next start, is to reengage the starter immediately after you've shut down the plane and turn the propeller, say, a half turn. The bendix will engage and stay engaged for your next start, since the prop doesn't spin fast enough to through it back.

The second point has to do with carb ice, and is actually more of a question than a comment: is it really possible to get carb ice before the engine has started? I wouldn't have thought that there was enough airflow through the venturi to cause the icing until the engine itself was actually firing. I'd be more likely to suspect ice on the air filter or even surface ice over the air intake, but I fly a plane that's not prone to carb icing (unlike the old Continental-powered Cessnas), so I don't have any practical experience with it.

Anoynmous said...

Carb ice can form amazingly quickly after the engine starts to run in some situations. That's why it's a suspect "...If the engine starts fine, then sputters to a halt."

One of my college roommates had a car that would sometimes exhibit the "whir-grind noise" symptom of failure to engage the starter gear. The culprit was a short area of broken teeth on the ring gear; if the engine happened to stop with that area in the wrong place, it wouldn't start. As cars have no handy propellers, he'd have to pop the hood and apply a significant twisting force to a small pulley in order to move some good teeth into position.

Old Blind Dog said...

I wouldn't recommend hand propping the engine unless you have been taught the proper technique. It is a good way to lose an arm or your life if you don't know how. Given that, I'll also say that most of the engines you will encounter have too high a compression ratio for you to be effective at hand propping. It works well on low compression engines like the radials seen in the movies and the little Continentals on Aeronca Champs.

dibabear said...

I'm with OBD. If it doesn't start and it's not obvious I'll send in the guy in coveralls. Hand-propping isn't something I'm inclined to try.

Aviatrix said...

Responses to some of these comments in the July 19th entry.

david said...

An AME hand-propped my Warrior's 160 hp 0-320 once, so after I bought the plane, when my battery was almost flat. It took surprisingly little movement to start the engine -- I'd say less than a quarter turn. Terrifying stuff. He did it with his left arm, body, and head wrapped around the cowling as close to the windshield as possible, presumably to keep him from slipping.

Paul Tomblin said...

David, are you saying that he hand-propped the engine from BEHIND THE PROP? I thought only float plane pilots did that.