"Contact," I repeated. Holding the stick in the crook of my right arm, with my right hand on the throttle, I opened the switches with my left hand, and made a thumbs-up.
The swinger placed both hands on the prop and pulled it down towards him. There wasn't so much as a click.
I immediately called out, "Switches off," and closed the switches again, before extending my arm into a thumbs-down where the swinger could see it.
The instructor frowned from the front cockpit. "I think the impulse is jammed."
The swinger followed the procedure for this situation. He opened the engine cowling, and then walked back into the hangar. Returning with a spanner, he banged it twice against the housing of the impulse magneto. He then closed the cowling again.
If you're used to flying modern planes, you've probably never thought about how the engine works. That's a luxury Tiger Moth pilots don't have.
As the prop shaft turns, an electromagnet (or magneto) produces a voltage, which causes the spark plugs to spark, igniting the fuel in the cylinder to push the shaft another turn. The switches on the left side of the cockpits short the magnetos to the fuselage, stopping them sparking, and preventing the engine firing. Thus closing the switch actually turns it on.
That works when the engine is already running, but it needs something to get it going. The swinger's job is to swing the prop by hand, to generate that first spark. To help him generate enough oomph to cause a spark, one of the two magnetos is special. This impulse magneto is sprung, with a mechanism like a watch spring, to build up the energy as the shaft turns, and release it into the coil in one go. The click noise we should have heard is the sound this mechanism makes, when it's working. Occasionally it sticks, and a sharp tap is needed to free it. It's easy to see where to hit it: it's the only part of the engine assembly covered with scratches.
|Picture courtesy Cambridge Flying Group|
With the maintenance delivered, and the cowling closed again, the swinger returned to his position on the left side of the propeller. Again he swung it, and this time we heard the click, but the engine didn't start.
"Switches off." By now I'd got so used to reacting quickly, I was afraid that when the engine did start, I would shut it down before I realised. I needn't have worried. The very next try, the Gipsy Major engine burst into life. After checking that the oil pressure came up to the right level, I set the throttle to 900 rpm to let it warm up.
I had a nice view while it was warming up. Although Cambridge is a fairly small airport, local aerospace company Marshall's performs maintenance on larger aircraft, and occasional training. Today, there was a 737 doing circuits, presumably to practise instrument landings, so I watched that roll across the apron a few times. Sitting in my stationary Tiger Moth, I felt like a child in a parked car pretending to drive it while the real cars go past. Still, I wasn't too embarrassed to wave to the flight crew as they rolled past.
Once the engine had been running for a few minutes, I next had to test that both the magnetos are working correctly. You turn one off, then back on, followed by the other, to check they each work in isolation, and to check that closing the switches correctly stops the magneto. If you do this at 1000 rpm, you should hear a difference while each one is off, but you can't really measure the rpm drop because it's so small. You then throttle up to 1600 rpm and repeat the check, but this time you should be able to see a 100 rpm drop on the tachometer (the rev counter).
This was fine, so I had to check the area behind was clear for the next check. I'd already done this before starting the engine at all, but it's best to check again, as running the engine at full power might end up sand-blasting someone's paintwork, or knocking the hat off a passer-by. At Cambridge, the area where the Tiger Moths start up is away from the other aircraft but right next to the car park, and it's generally considered rude to point your slipstream at the duty pilot's car! In this case, a cyclist passing the airport had stopped to watch us, but well out of the way, so I opened the throttle to full power to check the engine response.
Normally, after finishing all these checks, you'd radio the tower to request taxi, and soon you'd be leaving the ground behind. But on this particular day the weather was too bad to actually fly the old bird, so practising starting the engine was to be my whole lesson for today. But having started it, I'd have to shut it down as well. First I throttled back to 900 rpm again to let it cool down. After a few minutes, I had to perform the magneto check again, and I'll explain why in a moment. The shutdown itself is simply done by turning off the magnetos, and as the propeller slows, smoothly opening the throttle wide to suck air into the engine. Once the engine has stopped completely, you close the throttle again.
Now it should be clear why you need to double-check that magnetos can turn off. If you try to stop the engine, but one magneto is still firing, and you open the throttle at the same time, the airplane will drive forward unexpectedly. This could be a particular problem on an airplane with no wheel brakes, but as you usually shut the engine down after taxiing towards the hangar, it's likely the plane won't be out of control for long before it's halted abruptly by a building or another aircraft.
Swinging the prop is the most dangerous part of a Tiger Moth flight, but it's not dangerous for the chap in the cockpit: only the "swinger" standing out front. This Tiger Moth has had its prop swung quite a few times since it was built in 1944, so I can understand that sometimes it takes a few tries to get going. Some mornings I have the same problem myself, and I'm less than half its age.