Cold starting conundrums

Aircraft

Ian Fraser looks at the factors involved…

It’s a lovely clear day, no cloud, wind down the runway, and it’s cold. You wrap up warmly and prepare your aircraft for a short flight to blow some of those winter cobwebs away. But it won’t start. While many engine manufacturers recommend engine heaters (or heated hangars) if it’s persistently below -10°C, some of our more traditional carburetted engines (Lycoming or Continental) can be troublesome even in a British chill. Most engines in good repair will self-start in our winters without such cosseting if you use the right approach. But before I describe my winter techniques let us look at some of the basics, as well as potential problems of starting.

To start any aircraft engine requires it to be cranked fast enough to suck fuel/air mixture into the cylinder. It needs a little extra fuel to start – more if it’s cold – and a powerful enough spark at the right time (some 25° later than normal).

Cranking and battery

As long as the battery is strong enough to turn the engine over at two blades a second or more for five revolutions, it should start. Aircraft with ‘retard breaker’ ignition also depend on the battery rather than magneto for the initial spark requiring a strong battery during cranking (more on that later).

Batteries do flatten themselves over the winter despite not being used and don’t perform as well in cold weather. A quick test for battery health is that it should remain above 10 volts during cranking. If it doesn’t, it has a problem. If you can, use a float charger to preserve your battery, it will pay dividends in winter starting and battery longevity.

I covered batteries and starters and their care in more detail in an article in the June 21 issue of Light Aviation. You can find it in the archive at (www.lightaircraftassociation. co.uk/2021/Magazine/June/jun_issue.html.)

Plugs

Sparks are usually generated, timed and distributed by twin magnetos via the plug leads to the plugs. It is important that all the plugs, however new, are clean, free of lead or oil contamination, the gaps are correct and electrode resistance < 5KΩ (Picture 1).

Oiling and contamination normally occur due to slow running on a rich mixture, maybe warming up or taxying. They are normally cleared or ‘burned off’ during cruise, particularly if leaned.

In the summer, that normally balances out, and plugs survive well without much attention (short warm-ups and long flights), but in winter they can deteriorate rapidly. Starting is normally done by just one of the mags, usually the left one. This mag can serve all the bottom plugs or two bottom and two top. The bottom plugs are more prone to contamination but, as they are often connected to the left magneto, we are dependent on them being clean for efficient starting.

Magnetos

These are very simple devices (Picture 2) producing a spark using a coil, capacitor and contact breakers or points. They also contain a simple ‘generator’ which makes them independent of the battery. As the magneto rotates, its ‘generator’ charges the coil and capacitor. When the points open, they discharge creating the spark. For normal running, the spark occurs about 25° before top dead centre. If you tried to start the engine with that setting it would just kick back. To overcome this, most aircraft have one magneto with a starting mode.

There are two types in common use. An ‘impulse coupling’ and a ‘retard breaker’. An impulse coupling is a mechanical device within the magneto, which automatically engages at low revs and temporarily stops its spindle just before the normal spark would occur. Thirty engine degrees later it is released. An internal spring accelerates it rapidly to catch up with the engine. Its increased catch-up speed generates a higher voltage for a better spark, while the delay ‘retards’ the spark to the optimum point to start the engine (about top dead

Left A Bendix magneto on a Continental O-200. This should be the impulse mag.

Below A Continental 0-200 carburettor showing both accelerator pump and manifold primer nozzle. It starts in all temperatures. Many homebuilders and some manufacturers don’t bother with the primer. Lycoming admits in one document that the throttle pump only works as a starting aid at ‘moderate ambient’ temperatures. While they don’t quantify this, my experience suggests they mean above 5° centre, TDC). The impulse coupling disengages at about 400rpm reverting to normal timing. Very simple, and as long as everything is set up correctly, reliable. Such a device can be identified by a click or clunk as you rotate the prop which generally occurs around the compression points. As long as you hear that clunk, it is a good indication that the impulse system is working.

C. Marvel designed the pump to prevent larger engines ‘lean cutting’ when you advance the throttle, not to start them.

Starting is a convenient by-product in warmer climes. The real cold starting answer is a panel primer pump.

Below left Internals of a retard breaker mag from a Lycoming O-320. Points can wear, alter the magneto timing, and thus the spark energy. Adjusting the magneto to engine timing won’t solve that, you need to fix the points.

The retard breaker magneto is quite different and depends on the aircraft battery to start. Retard magnetos can be identified by having two connections in addition to the plug leads, rather than one and ‘no clunk’. In start mode, the main contact breaker is bypassed and a second contact breaker, set to open later or ‘retarded’ to TDC, is connected to a ‘Shower of Sparks’ (SOS) device. The SOS device replaces the magneto’s internal ‘generator’ for starting. It is a very simple ‘buzzer’ that produces rapid 12v pulses to trigger the coil and produce sparks. Despite its simplicity, it seems more prone to problems associated with cold, damp and poor batteries, but checking one is also simple. Disconnect the wire that engages the starter solenoid and activate the starter switch. If the SOS is working, you hear a constant buzz at about 500Hz. If it is not working or intermittent there will be no starting spark and it needs attention.

Magnetos are very sensitive to internal settings (Picture 3). If those settings are not correct, the consequence is weak sparks, out of spec magneto drops and poor starting. Fortunately, there is a simple health check. The magneto to engine timing, should have been matched to a good magneto. If it has moved from that original setting it is an indication that one of the magneto’s internal settings has changed. That cannot be resolved by simply twisting the magneto to correct the timing. The magneto itself must be checked to identify and rectify what caused the shift.

Fuel

As with any petrol engine, an aircraft engine requires a richer mixture to start. On a car this is achieved by the choke, but on older aircraft engines, by priming. Priming is squirting fuel directly into the inlet port with a primer pump, or pumping the throttle which injects extra fuel into the carb (Picture 4). This latter system is very sensitive to low temperatures. With a throttle pump, fuel is squirted toward the bottom of the throttle vane of the carb rather than into the inlet manifold (Picture 5). If it is too cold, the extra fuel hits and condenses on the throttle vane and most of it drips back out. While the engine might cough a bit, the mixture is too weak to start it. This is the most common cause of winter starting problems in otherwise healthy engines.

It is important to use the best fuel and oil for the winter season – 100LL deposits lead all over your plugs, mogas deteriorates rapidly and starting is difficult. If it’s permitted for your engine, use UL 91, long lasting and no lead. Straight oils are like treacle at low temperatures and will certainly make initial cranking sluggish as well as clogging the oilways and delaying oil pressure, risking damage. Use a modern multigrade.

My tailored maintenance schedule includes most of these checks as a matter of routine. In winter I pay particular attention to those sensitive items such as plugs and batteries and I can be confident that the engine will start for a winter flight.

This is my winter starting process which has worked reliably for my 35 years of flying in a variety of carburetted aircraft. My RV-6 has a retard breaker mag, throttle pump and no primer, the worst winter combination.

I have never had much success with Lycoming’s pump and crank method for that set-up, so this is how I do it. I prime the engine with throttle (four pumps holding in for a second or so after each stroke), then with throttle ¼in open (and with mags off and aircraft chocked), pull the prop through two rotations (I don’t use the starter as I don’t want it to cough and use up the fuel yet).

I get in and reprime (same process), then open the throttle just a little (Marvel, the carb manufacturer say, 3/32in for the optimum rich slow running spot), crank the engine and it will normally start. In the cold, it is really sensitive to that throttle setting and the pumping action. I don’t crank it for long, respecting the battery and starter maxima. If it doesn’t start in the first five revolutions I wait, reprime and try again. If it won’t fire after a couple of attempts, not unusual if it’s freezing, I use a hairdryer to warm up the carb (pic 6). Five minutes is normally enough. This method has worked on most of the aircraft I have flown. But be careful, live mags bite and dripped fuel can burn. Be safe please. ■