How to prop swing

It’s safe enough to start an engine by hand, providing you observe all the precautions, says Nick Bloom

After getting a PPL, most people continue to fly the kind of aeroplane they learned on, in which case it will almost certainly have an electric starter. However, some very desirable homebuilts and classics have to be started by hand. This needn’t put you off buying such an aeroplane (or a share in one), as prop-swinging really is not difficult or dangerous, providing you proceed carefully. There will almost certainly be an instructor or group member to show you what to do. In the meantime, here are a few basics.

My first aeroplane, an Evans VP-1, had a 1,600cc VW engine that you started by hand, and it was a bugger. The trouble must have been the ignition timing, because when it did fire, the propeller tended to kick backwards. I well remember the stinging pain and ‘banana fingers’ this produced. Other aeroplanes I’ve owned that required starting by hand included a Jodel, Stampe, a Pitts Special, a J3 Cub, a Turbulent, a Laser and my current machine, a Currie Super Wot. They all had their idiosyncracies. The Laser was the worst, with a 200hp Lycoming and a low thrust-line. Also, like the Pitts Special, it had fuel injection. That meant starting in lean and dashing round the wing to push in the mixture control the instant the engine fired; get there too late and the priming fuel was exhausted, the prop stopped.

Some of these aeroplanes were based on farm strips, which meant starting them solo. No one has actually banned solo hand swinging, but it is quite rightly frowned upon. There are just too many things to go wrong. Unless you really, really know what you’re doing, two people are required, one in the cockpit and someone else to swing the propeller.

If you must hand-start an aeroplane solo, tie the stick back, tie the tail down to an anchor and chock the wheels. Having said that, I know the C90 in my Currie Super Wot will start at tick-over and the aeroplane won’t move or lift its tail, so I don’t always do all these things. And I know other pilots who are similarly confident. I never forget, though, the risks I am running.

Before starting, radial engines?and certain in-line engines?must have oil drained from the downward-pointing cylinders or there is a risk of a hydraulic lock. Other preparations including turning on the fuel, priming the fuel pumps if a manual prime is fitted (as with Renault engines) priming the carburettor (where required) and tapping the magneto with a light hammer if (as with Gipsy engines) the impulse mechanism is prone to sticking.

Procedures vary between engines and aeroplanes. Some must be swung on one magneto (the starter mag) and the second magneto only switched on later. Let’s take the Tiger Moth. Before getting in, the pilot (in the rear cockpit) sets the switches on (up) for the front cockpit and off (down) for the rear. The starter?who by tradition is in charge?sets chocks in front of the wheels. The pilot holds back the stick and sets the throttle roughly a quarter of an inch forward from fully closed. The starter calls, “Switches off, sucking in”. The pilot shows a ‘thumbs down’, indicating that the rear magnetos are earthed. The starter acknowledges by repeating, “Switches off, sucking in,” and pulls the propeller forwards through four blades. (Always bearing in mind that there may be an electrical fault and the engine might fire.) The starter then sets the propeller, calls, “Set and contact,” and observes the pilot moving up the rear switches on both magnetos (the starter can see the switches, which is why they’re outside). The pilot gives a thumbs up and repeats, “Contact”. The starter swings the propeller and the engine starts. After an interval to allow the engine to warm up, the pilot crosses his arms, which is the signal for the starter to remove the chocks.

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What if the engine doesn’t fire? There’s one truism that applies to all engines in this situation: it’s either too rich or too lean. Continuing to swing the propeller forwards will make things worse if it’s rich. What’s needed is to ‘blow out’?turn it backwards, with the magnetos off and the throttle open, probably through eight, twelve or even sixteen blades. A strong smell of petrol is the cue, but if in doubt, assume the engine is rich. This is particularly so if the engine is warm. Warm and rich needs a lot of blowing out. (The routine on my Wot is two blades forwards, followed by sixteen blowing out, then two forwards, then sixteen blowing out. Usually three or four repeats will start the engine.)

Our Tiger Moth team does it this way: the starter calls, “Off and open”. The pilot opens the throttle all the way and puts the rear magneto switches down. He then gives a thumbs down and repeats, “Off and open”. The starter reverses the propeller for however many revolutions he/she feels is appropriate, calls, “Set and contact,” and the procedure starts again from the beginning.

However, particularly for the first start of the day, the problem may be not enough fuel reaching the cylinders. In the case of the Tiger Moth, continuing to swing the propeller should fix it. With other aircraft, you may need to do more to ‘prime’, either by moving the power lever fully forwards then fully back two or three times (three times for first start of day works perfectly with the C90 in my Currie Super Wot), or by operating the primer, if one is fitted. (If using a primer, don’t forget to lock it.)

How exactly does one go about swinging a propeller? First make sure you have a reasonably non-slip surface to stand on. Second, wear gloves?ideally thick mittens. Third make sure nothing’s dangling, like a scarf, tie, or headset leads. Then ‘set’ the propeller (as in “Set and contact”?although ‘set’ can also mean setting the throttle). In other words, rotate the propeller forwards until you can feel the stiffness which indicates a cylinder coming towards the top of its stroke. To swing the propeller, you then pull it firmly past that stiff point and let go immediately it’s past it. Keep both hands clear as you let go. It’s the following blade that might get you.

While you are setting the propeller, a spring inside the starter magneto is coiling. (Assuming it’s an impulse mag; some have different arrangements such as ‘shower of sparks’, or none at all.) As you pull the propeller forwards, you should hear a click. This is the spring mechanism letting go, sending a jolt of electricity to the spark plug. If the timing is set up correctly, the plug will fire just as the cylinder passes the top of its stroke and is beginning its way down the cylinder. Note that it’s the un-coiling spring that turns the magneto and produces the spark, and it does so much faster than your hand on the propeller, which by itself is relatively slow. So if you have the ‘click’ impulse mechanism (most engines do), you don’t need a lusty pull. However, it’s as well to pull firmly, in case the engine has been mistimed and there is a risk of backfiring.

A hot, rich engine can also fire prematurely, causing the propeller to turn violently the wrong way. A hot engine can even ‘kick’ when being turned backwards, so when blowing out, still treat the propeller as ‘live’.

You can turn some propellers without hooking your fingers over the back of the blade. That reduces the risk of ‘banana fingers’ if you get a backfire, but gives you a less lusty pull. Remember, the more firmly you pull, the less the risk of a backfire.

You can practice prop-swinging before doing it in earnest to your heart’s content by switching off both magnetos, and the fuel and putting the mixture control to lean. I should. Rehearsing in this way will lose you your fear of the propeller, which is understandable. Remember back in the day, everyone started engines this way. As a rule, stand up straight, stand quite close and use both hands for anything over 60hp. Look at the propeller the whole time and don’t allow yourself to be distracted.

By the way, some people, Pilot’s Editor being one of them, stand behind the propeller to swing it, the better to be able to reach throttle and mag switches. Most stand in front since it gives more control. (Philip is tall and has long arms.) When the engine fires it can come as a shock, so try to be prepared. Calmly step back. Don’t rush.

Of course, there are dangers. An individual cylinder firing?even on a 30hp engine?can break your wrist or crack your skull if you have either in the way of the down-going blade. Over the years I have seen a half-dozen injuries through prop-swinging, including several people who had to be rushed to hospital and one man who was permanently disabled. I have also read periodically about runaway aircraft that were prop-swung with no one in the cockpit. Typically the throttle was set too far open, or its locknut wasn’t secured adequately, or the pilot forgot to close the throttle after blowing out. The aeroplane either wasn’t chocked or it jumped its chocks. Not so very long ago, a Stampe at a fly-in actually took off and climbed away with no one it its cockpit, ending up lodged in a tree. A mate of mine set our Pitts Special on its back after a grim whirligig with him hanging desperately to one strut. It was that, or have it plough into millions of pounds’ worth of parked helicopters. As Dixon of Dock Green used to sign off, “Mind how you go”!