LESSONS ON STARTING A PISTON AIRPLANE ENGINE ....
Acknowledgements: George Scheer (www.thisaviationlife.com)
(Ed.Note: The following wisdom is extracted from George’s article from January 2016, which can be read in full on the website above)
“.... I found myself at the airport on a freezing cold, windy day. A former instructor and I were discussing the state of aviation and our Maintenance Director John joined us in the office to warm himself ... I took advantage of the moment to ask the questions that occur with the onset of freezing temperatures – most notably, “How do I start the airplane when it is so cold?”
John put the question in context:
· “... learning how to be a pilot is about amassing small bits of knowledge which will allow one to make good operational and safety judgments. Take starting the engines of our little airplanes. The first tip is to realise that we are dealing with basic 1940s technology. Those of us of a certain vintage have had the experience of starting cars with manual chokes or throttles or spark advances on the steering column ... Like those cars, the story for our little airplanes is that starting is not a matter of turning a key, but of learning to listen for what the engine is asking of us. Usually, this means what is going on in terms of mixture, because if the mixture is right, the engine will start, no matter how cold the temperature ....”
· “Get the prime right, (and) the engine will start. And getting the prime (initial mixture) right is critical because the capacity of both the batteries and the starters in our fleet is very limited .... For engineering reasons of weight, our batteries and starters are smaller and less robust than what you have in your car .... For this reason, in cold weather we take care to minimally use the battery during pre-flight .... and the more we crank the starter, the lower the voltage from the battery, and the hotter the starter armature .... and batteries cost (at least) $400 to replace ....”
“So, John,” I asked, “what about preheat?”
· “While our little Lycomings will start in very cold weather if we get the prime right, it is best to pre-heat if the temperature is substantially below freezing — perhaps 25F (-4C) or less. (if you have access to) a pre-heater, and if you are going to use it, be sure to get checked out on its use. Allow at least 15 minutes of pre-heater use to do the job”
.... we would all benefit from revisiting the topic of winter operations, at least those related to starting our airplanes successfully, avoiding expensive damage in the attempt, and avoiding engine fires caused by improper starting technique. So, let’s review some of the principles and techniques most critical when engines are cold-soaked ....
· Use preheat when temperatures warrant. Preheating is not so much about getting the engine started as about avoiding wear and tear on the engine when it does start .... not so much about the oil pressure as about clearance for the bearings and the pistons.
· Fuel-injected engines should start readily in cold weather .... normal start technique is all that is necessary. Most problems with fuel-injected starts occur in hot weather, or on restarts of a heat-soaked engine.
· Carburetted engines (e.g. C152 and Warrior) can be more difficult, but with proper technique should start.
· Poor starting technique is both ineffective and expensive .... aircraft batteries are not as robust as auto batteries. If you exhaust a battery by cranking excessively or by leaving the master switch on, it will probably have to be replaced .... batteries are purchased empty and uncharged – someone must install the acid and charge the new battery, clean up corrosion and install the new battery. The cost of flying that airplane just went up about a dollar per hour.
· Starters cost three to four hundred dollars, plus installation .... again, not as robust as those on your car. Aircraft starters, like batteries, are marginal. Starter manufacturers recommend that cranking periods be limited to thirty seconds with a two-minute rest between cranking periods .... I cannot think of a reason to ever crank a starter for thirty continuous seconds, unless perhaps to contain a starting fire, which we are going to avoid.
So, how to avoid exhausting the battery, melting down the starter, or causing a fire? Simple. Achieve the correct level of prime. And avoid cranking for more than a few seconds at a time.
How do we find the correct prime?
· Begin with less than you think you need. I often begin with no prime at all.
· Crank for a few (3 to 5) seconds. If the engine does not start – stop cranking.
Am I dismayed if the engine does not start immediately? Not at all. I am actually pleased. I now know that I need more prime ....
· Prime a little more.
· Crank a few more seconds. (5 seconds is actually a long time to crank an engine. Time it on your watch and you’ll see.) If the engine doesn’t start, quit cranking.
· Repeat until the engine starts or we exceed the time limit on the starter.
.... the primer simply sprays some fuel outside the intake valve of one or more cylinders. It is statistically likely that this prime is being squirted onto a closed intake valve... some of the fuel you inject is almost certain never to reach the inside of a cylinder. If you pause after priming, this fuel will drain back into the carb heat box where it REPRESENTS A FIRE HAZARD. If you prime a carburetted engine, say, five times it is almost certain that you have created an EXCESSIVE fire hazard. Every one of (our club’s) Warriors shows evidence of fire in the induction system from improper starting technique. In many of these events, the pilot was probably unaware of the damage done!
(.... from the winter edition of the FAA Safety Briefing magazine: “Regardless of engine temperatures, the most important way to prevent carb fires is to avoid over-priming. Start with the smallest number of primer strokes, and increase up to the limit if the engine does not start”)
· Do not prime the engine until you are ready to start the engine .... If you prime the engine, then yell “clear,” then look around, then consult the checklist, then fasten your belts – the prime will no longer be in a useful place .... It (may have drained) into the carb heat box, where it will not ignite the engine – but in case of a backfire might ignite the airplane .... push in the primer, turn the starter, start the airplane .... (then) go back to the checklist and proceed.
· Do not use the throttle to prime the airplane .... pushing the throttle in rapidly will expel considerable fuel from the carburettor – but it doesn’t go to the same place as the primer .... an accelerator pump squirts fuel up into the intake tubes or manifold, which will immediately drain back into the carb heat box if the engine is not turning .... priming the engine with the accelerator pump (throttle) is a splendid way to build a fire in your intake system .... do you remember what to do if a fire occurs in the starting process? ....
From Lycoming: “.... Pilots should be advised that excessive throttle priming can cause flooding of the carburettor and air box, and result in a fire in the induction system, or on the outside where the fuel drains overboard. If the operator floods the engine by pumping the throttle and has a fire, it is possible to handle such a fire in the early stages by continuing to turn the engine with the starter, thereby sucking the fire back into the engine. Furthermore, if there is any fire on the outside of the engine, if the engine starts there is a good chance it will blow out the external fire….
Most Lycoming fuel-injected engines are simply primed by turning the fuel boost pump on, opening the mixture briefly to full rich, and cracking the throttle. Any pumping of the throttle is ineffective until the engine begins to fire.”
(So, with priming,) in all cases less is best. The engine may not start on the first attempt, but you will know that you are not yet flooded and know that more prime is required .... begin with less, crank for very brief moments, increase the prime for another iteration, and work your way toward the best mixture whereupon the airplane will start easily.
Starting a fuel-injected airplane when the engine and fuel system are heat-soaked can be a greater challenge, but the same principles apply:
· Prime less rather than more,
· Your cranking period on a hot fuel-injected engine will be a bit longer than for a cold start, but we are talking perhaps an additional five seconds, five-to-ten total.
· Make your first attempt to start with no prime at all and work your way toward the proper prime and you will be successful.
These principles are intended to avoid completely the need for a flooded start .... (which) raises the possibility of an engine fire and requires a more extended cranking period, neither of which is beneficial.
Do not turn the propeller by hand prior to starting .... if you hang around pilots long enough, someone will tell you that it “loosens up the cylinders” ..... there was a time in aviation before modern multi-grade oils and with radial engines when there might have been some merit to the technique. Not here. Not now. Not these engines.
Understand that there are rare mechanical and electrical problems that can make an aircraft engine difficult or impossible to start ... if it happens and the engine will not start despite good technique, accept the reality. there is no shame in being unable to start the airplane – but there is no excuse for draining a battery or melting down a starter. Take a break, call it a day, or get some help.
If nothing else, remember this: No one can guarantee that they can start an airplane. Everyone can guarantee that they will not damage it in the attempt”.