Lessons on starting a piston airplane engine

A few years ago, I found myself at the airport on a freezing cold, windy day.   No one was flying.  A former instructor and I were discussing the state of aviation and his future plans – he now flies a regional jet for Air Wisconsin — and our maintenance director, A&P, I&A, instructor, mentor, aviation historian, resident theologian, proud curmudgeon, and fount of knowledge, was working in the hangar, bundled in his winter jumpsuit.  John joined us in the front office to warm himself and I took advantage of the moment to ask the questions that occur to all of us as we cope with the onset of freezing temperatures – most notably, “How do I start the airplane when it is so cold?”

DCP_1670John, as usual, put the question in context.

“As I (John speaking here) often say, learning how to be a pilot is in good part about amassing small bits of knowledge – not the stuff in the POH —  which will allow one to make good operational and safety judgments.

“Take starting the engines of our little airplanes. The first tip is to realize 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.   I drove a friend’s Model A Ford for a while and learned the intricacies of starting a 49 Hudson.

“Like those cars, the story for our little airplanes is that starting is not a matter of turning a key, but learning to listen for what the engine is asking of us.  Usually, this means that we have to cipher what is going on in terms of mixture, because if the mixture is right, the engine will start no matter how cold the temperature.

About now, I am thinking, “So, what shall those of us do who lack that Zen-like oneness with primitive piston engines?”

John answered my unspoken question.  “Get the prime right.  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.  It is easier than most pilots think to exhaust the battery and burn up a starter.  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 we are, of course, very careful not to over-crank the starter.  The more we crank, the lower the voltage from the battery, and the hotter the starter armature.  Melted copper is not far away from those little 400-dollar jewels.  And, yes, the 152 batteries cost $400 to replace. “  (The passage of a few years has increased that price significantly.)

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 or less.  The Club has 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.”

Our conversation continued as I asked more questions.  These are the golden moments – the learning opportunities afforded to us by the wealth of experience in our club.  Our conversation ranged widely but focused on cold-weather issues, including deicing, oleo struts, and such.  Recent events have reminded me that we would all benefit from revisiting the topic of winter operations, at least those particulars related to starting our airplanes successfully, avoiding expensive damage in the attempt, and avoiding engine fires caused by improper starting technique.

Let’s review some of the principles and techniques applicable to all engine starts but most critical when engines are cold-soaked.  We will attempt to document our logic and perhaps dispel a few canards.

1)         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.  You can start an aircraft engine at temperatures so cold that excessive wear will occur in the first few seconds of operation.  And it is not so much about the oil pressure as about clearance for the bearings and the pistons.   For an excellent explanation of the value of preheat read this article by Mike Busch:  The Whys and Hows of Preheating – AVweb Features Article

2)         Fuel injected airplane engines (172 SP and Mooney in our fleet) should start readily in any weather we are likely to have here in North Carolina.  Normal start technique is all that is necessary.  Sometimes it seems that they start even better in the cold.  Most problems with fuel injected starts occur in hot weather — that’s a topic for next spring – or on restarts of a heat-soaked engine.

3)         Carbureted engines (152 and Warrior in our fleet) can be more difficult but, with proper technique, should start.

4)         Poor starting technique is both ineffective and expensive.  We purchase our batteries at wholesale, but they are much more expensive than auto batteries.  Our 24-volt batteries are now more than $300 and our 12-volt batteries are nearly $200.  Installation elsewhere would be expensive, but even when John or James is here to install our batteries for us, there is a cost to the club in lost time and flight opportunities.  Aircraft batteries are purchased empty and uncharged – someone must install the acid and charge the new battery, clean up corrosion and install the new battery.  (They are not always in the most convenient place.  Do you know where the battery resides in a Warrior?  Ever seen it on your preflight?)  Also, remember that aircraft batteries are not as robust as auto batteries.  If I exhaust the battery in my truck, I can throw on a quick charge and be back on the road.  If you exhaust an aircraft battery by cranking excessively or leaving the master switch on, that battery will probably have to be replaced.  We will all pay.  The cost of flying that airplane just went up about a dollar per hour.

5)         Starters cost the club between three and four hundred dollars, plus installation.  Aircraft starters are, again, not as robust as those on your car.  You can crank your car for a minute or more and the starter will probably survive.  Not so the starter on our airplane.  Aircraft starters, like batteries, are marginal.  Why carry around the weight of a heavy-duty starter when you are going to use it for only a few seconds prior to every flight?  The AFM for the Warrior states, “Starter manufacturers recommend that cranking periods be limited to thirty seconds with a two minute rest between cranking periods.  Longer cranking periods will shorten the life of the starter.”  John Hunter interprets “cranking periods” to include any series of cranking events totaling 30 seconds within a 2-minute period.  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.

6)         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 and am therefore confident of my next action.  Nothing useful will be achieved by continued cranking and much damage can result.  (See above.)  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.

Remember that the primer simply sprays some fuel outside the intake valve of one or more cylinders.  It is statistically likely, depending on how many primer lines are installed on the engine and which cylinders they approach, that this prime is being squirted onto a closed intake valve of one or more of the cylinders.  This is akin to watering your garden by spraying the hose on the garden wall – from outside the garden.  Some of the fuel you inject with the primer 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 carbureted engine say, five times, it is almost certain that by the time you have filled the plunger a fifth time fuel from the first few pumps will have drained down to the carb heat box. Prime excessively and you have created an excessive fire hazard.

Every one of the WCFC Warriors shows evidence of fire in the induction system from improper starting technique.  In many if not all of these events, the pilot was probably unaware of the damage done.

The dilemma occurs when we fail to start the engine and do not know if the problem is too little prime or too much, i.e., have we flooded the engine.  Gradually work your way toward the correct prime from a known point.  The only time continued cranking should be necessary is in a flooded situation, which our method is designed specifically to avoid, or in case of a starting fire in the intake manifold, which our method should also avoid, since it is caused by excessive prime.

Note this on the subject 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.”   The November / December edition of the FAA Safety Briefing is devoted to cold weather operations.

6)         Do not prime the engine until you are ready to start the engine.  No more than a second or two should pass between the closing of the primer plunger and the turning of 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, will be ineffective, and may be a fire hazard.  As soon as you finish priming, the fuel that has been sprayed on the back side of the intake valve will begin to drain down the intake manifold (the carburetor, as we know, is beneath the engine) into the carb heat box, where it will not ignite the engine – but in case of a backfire might ignite the airplane.  Make all of the preparations, push in the primer, turn the starter.  Start the airplane.  Check that you are not moving.  Check the oil pressure.  Relax.  Go back to the checklist and proceed.

7)         Do not use the throttle to prime the airplane.  Yes, our carbureted airplanes, even our 152s, now have accelerator pumps installed on the carburetors.  Yes, pushing the throttle in rapidly will expel considerable fuel from the carburetor – but it doesn’t go to the same place as the primer.  An accelerator pump is not a priming device; it is intended to provide an extra measure of fuel when the throttle is opened suddenly in flight and the engine inhales a big gulp of air – think of a go around or missed approach.  The accelerator pump is useful when the engine is turning.  It 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.  From the engine’s standpoint, this is akin to trying to drink from a water fountain with your mouth closed.  Use the primer to prime the airplane.  Never use the accelerator pump unless the engine is actively turning so that the intake vacuum will suck the expelled fuel into the cylinders where it belongs. Preparing to crank by priming the engine with the accelerator pump (throttle) is a splendid way to build a fire in your intake system, which has happened to more than one club airplane.  (Do you remember what to do if a fire occurs in the starting process?)

This from an AOPA Air Safety Foundation publication The Engine is the Beating Heart of the Airplane“Do not pump the throttle, as this will simply force raw fuel (which doesn’t vaporize as easily in cold weather) into the intake system, possibly causing an engine fire.”


“From the Lycoming Flyer, a newsletter published by Lycoming, the manufacturer of most of our engines:  Priming can be best accomplished with an engine priming system, as opposed to use of the throttle. The primer pumps extra fuel directly into the cylinder intake porter induction system. Some float-type and pressure carburetors also provide a supplemental source of priming. Lycoming engines of more than 118 HP have a throttle pump which can be used for priming under moderate ambient temperature conditions while turning the engine with the starter.

Pilots should, however, be advised that excessive throttle priming can cause flooding of the carburetor 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.”


Incidentally, opening the throttle farther than recommended while cranking is seldom helpful.  You are introducing more air, not more fuel, which seldom improves the starting mixture.


Again, from the Lycoming Flyer:  The fuel part of the fuel/air mixture may be the part we have the most control over during the engine start, but keep in mind that the amount of throttle opening does have an effect on the air that is pumped through the engine. Just as we compensate for cold/dense air by adding more fuel for start, it may also be appropriate to reduce the air part of the mixture when the temperature is very cold. For example, if the throttle is normally set open one half inch for warm weather starting, it may be helpful to reduce this to one quarter inch in cold weather. Again, it will require some experimentation to determine what is needed to achieve the correct fuel/air mixture for any particular aircraft at any temperature range. When an engine does not start easily, it can be frustrating. Of course this can occur at any time of the year and it is very tempting to just keep grinding away with the starter in an attempt to get it going. Should this happen to you, RELAX. Take care of that starter or it may fail. The general rule for starters is that they should only be operated for short periods, and then allowed to cool. If engine start has not occurred after three, 10-second periods of operation with a pause between each, a five minute cooling off period is required. Without this time limit for operation and an adequate cooling off period, the starter will overheat and is likely to be damaged or to fail completely.

8)         These same principles apply to fuel-injected engines, such as we have on the 172 and the Mooney, which use a different method of priming.  Rather than a mechanical plunger they use the auxiliary electric fuel pump to squirt fuel through primer lines to the cylinder heads outside the intake valves.  Cold starts are generally easy with fuel-injection; hot starts can be more difficult.  But 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.  Eventually, you will find the right level of prime and the correct fuel-air ratio and the engine will start.  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.  Cranking continuously when the prime is not yet correct is that shopworn definition of insanity:  trying the same thing over and over again and expecting a different result.

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, do not crank for a prolonged period.  Fuel-injected engines are prone to flooding when hot, so make your first attempt to start with no prime at all and work your way toward the proper prime and you will be successful.  (Note that the Mooney checklist advises hot starts with no initial priming.)  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.  Unless we have flooded the engine there should be no need to crank for more than ten or fifteen seconds at the most.

9)         These principles are intended to avoid completely the need for a flooded start.  A flooded engine raises the possibility of an engine fire and requires a more extended cranking period, neither of which is beneficial.  If a flooded start becomes necessary, simply follow the AFM/POH procedure, which is always some variant of opening the throttle (more air) and closing the mixture (less fuel) and cranking until the proper balance is restored.

9)         Do not turn the propeller by hand prior to starting.  Yes, if you hang around enough pilots long enough someone will tell you that it “loosens up the cylinders” or some such nonsense.  And yes, there was a time in aviation before modern multigrade oils and with radial engines when there might have been some merit to the technique.  Not here.  Not now.  Not these engines.

10)       Understand that there are rare mechanical and electrical problems that can make an aircraft engine difficult or impossible to start.  This is seldom the case, but if it happens and the engine will not start despite good technique, accept the reality.  You may or may not be able to start a given engine on a given day.  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.  That is a choice you make and is inexcusable.  Take a break, call it a day, or get some help.

How to Recognize and Cope with a Starting Fire

We have mentioned that improper starting technique can cause a fire in the engine compartment.  We are all familiar with the standard advice to continue cranking in the event of a fire, but the problem is that this fire may not be immediately apparent.  The loud pop or bang that you will occasionally hear is usually a combustion event in the exhaust system, again caused by excessive priming, and may cause some internal damage to the exhaust system but will seldom if ever cause a fire.  Similarly, burning fuel in the intake tubes of a Lycoming engine will seldom cause significant damage.  The fires that damage our airplanes are almost always caused by the ignition of a pool of fuel sitting in the carb heat box — fuel that found its way there due to excessive priming or priming attempted with the throttle and accelerator pump in the carburetor.  This is fuel that never made it into a cylinder and drained back down through the intake system to puddle in the carb heat box of a carbureted airplane.

The problem is that the ignition of a fire in the carb heat box caused by a backfire igniting that pool of fuel often produces a subtle, low-frequency “whoompf” noise that can go unnoticed.   The event is often recognized foremost by a sudden engine stoppage either while the pilot is still cranking the starter or has just released the starter and the engine is beginning to spin up.  This is precisely the moment when we need to restart the engine and crank it for ten or more seconds.  If it starts, let it run for thirty seconds or so, shut down, and inspect for damage.  And, obviously, if the fire becomes uncontained, exit the airplane to a safe distance and, if you judge it to be safe, use a fire extinguisher to arrest the fire.

To summarize what we know:

  • Preheat when necessary.
  • Follow the checklist procedure, particularly for fuel-injected engines.
  • Prime less at first rather than more
  • Do not prime with the throttle (accelerator pump.)
  • Prime when you are ready – really ready — to start.  Not before.
  • Opening the throttle further seldom helps.
  • If the engine doesn’t start in a few seconds – stop cranking.
  • Repeat as necessary
  • If the starter begins to slow down, you are melting it down.  Stop.  Go have a cup of coffee. Do not come to our maintenance department and say, “The starter overheated.”  You overheated the starter.   Draining a battery or frying a starter is not something that happened to you – it is a choice you made.
  • Get help if you need it.
  • If you have any reason to believe that a fire has started in the intake manifold continue cranking. Restart the engine and let it run for 30 seconds.

To prevent and manage an engine fire:

  • Follow the good practices outlined above.
  • Realize that a fire in the intake or carb heat box may not be immediately apparent
  • If you suspect a fire, continue cranking to draw the fire into the engine. Keep the engine running for a few minutes, shut down, and inspect for damage.
  • If the fire becomes uncontained, shut down the airplane, shut off the fuel, exit the airplane and, if safe and practical, use a designated fire extinguisher to extinguish the fire.


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.



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