Starting a hot engine
How to handle temperature extremes
By Marc E. Cook
A dry wind blows across the FBO's ramp at Phoenix Sky Harbor International. And although it hasn't reached the 100 degrees Fahrenheit forecast for the afternoon, it's still plenty hot. Passengers from incoming aircraft quickly deplane and head for the air-conditioned lobby and a cold drink. Yep, summer's here.
Parked at the end of a long row of airplanes is a Piper Malibu Mirage, loaded and getting ready to go. The airstair door swings closed and is latched. The Mirage pilot moves forward to the cockpit. In a few moments, as the lineman standing in front of the airplane looks on, the propeller begins cranking. And cranking, and cranking. The engine doesn't so much as belch. After a short pause, the pilot tries again, his efforts culminating in plumes of dirty black smoke from the Malibu's exhaust pipes and an engine running raggedly. The effluvia moves off over the airplane's right wing, indicating a stiff breeze from the left doing nothing for engine cooling. After a few moments of running on half its cylinders, the big Lycoming falters and stalls.
Soon after, the pilot tries again. This time the engine lights in a couple of blades but quickly accelerates to runup speed or better. The lineman takes a few steps back, no doubt watching the Malibu hunker down on its gear, wing tips shaking. This time, though, it appears that the engine will continue to run, and in a minute the pilot pulls it back to idle speed. It's easy to imagine his relief at coming through the dreaded hot-start sequence. Now he can flip on the air conditioning in what must be a sweltering cabin and cast a wan smile to the passengers, who surely have eyebrows raised and hands poised on the seat belt buckles to begin the escape.
A pilot seated inside the cool FBO lobby who has just purchased a pressurized twin with fuel-injected, turbocharged engines, wonders aloud: "Is that what I get to look forward to? Or is that a Piper/Lycoming problem?" Not necessarily, and no, in that order. Difficulty in starting hot engines cuts across the spectrum, afflicting all brands of engines and all makes of airframes to one degree or another. And while it's true that certain types of engines — fuel injected, particularly — have gained reputations as difficult hot-starters, the right technique can mean the difference between a running engine and a dead battery.
What makes a hot start hot? For air-cooled engines it's generally the period between 15 minutes and two hours after shutdown. In other words, the time it takes to get everything under the cowl nice and hot but before the heat-soaked engine has had time to cool, which would allow for normal starting procedures to be successful.
Several factors combine to make starting airplane engines — hot or cold — difficult. For one, aircraft fuel systems are intentionally quite simple. They lack chokes for cold starting and are generally not capable of metering fuel precisely at below-idle speeds. Aviation fuel is also culpable. The very qualities that prevent vapor lock at high altitudes and temperatures make the fuel unwilling to vaporize easily at startup. Weak low-rpm spark from magnetos, poor mixture distribution, and updraft carburetion all are part of hard starting.
You can do much to alleviate the difficulties of hot starts through ongoing maintenance and operational tricks. Naturally, you'll want to make sure that your magnetos are strong, spark plugs are clean and properly gapped, and ignition harness is in good repair. Also ensure that the carburetor or fuel-injection system's idle-mixture adjustments are correct — you should get a 50-rpm rise as you shut down with the mixture control. Make certain that your primer system works properly and, in the case of carbureted airplanes, doesn't leak raw fuel into the inlet manifolds.
These are the obvious maintenance items, sure, so here are the equally obvious tricks. Always park into the wind. Our Malibu example was guided into a parking space that faced the terminal building but was 90 degrees to the wind. Insist on being parked into the breeze, even it if means a longer walk to the airplane. Open the cowl flaps and, if you don't mind the somewhat unkempt appearance this creates, pop open the oil filler door, as well. By your doing so, hot air may escape from the top of the cowling, helping to draw cool air up from the outlet area. For airplanes with flip-open cowlings, by all means open them up. Just make sure that it's not so windy that the stays might give way and allow the cowling to flail against the airframe. In short, do everything you can to get some airflow through the cowling after shutdown.
If there's one main advantage to carburetion over fuel injection — next to simplicity — it's easier hot starting. (In fact, several pilot's operating manuals for carbureted airplanes list no specific hot-start procedures.) Because the carb is usually below the engine, it's not as likely to get cooked. Also, carburetors tend to meter fuel better at low engine speeds than do fuel-injection setups.
When faced with a hot start, first follow the procedures spelled out in the operating manual. Usually they take into account any quirks of the installation and are your best baseline of comparison. Unless it's extremely hot, you'll probably do just fine with the normal-start recommendations — carb heat off, mixture full rich, little or no prime. (Try it first without priming; if the engine doesn't catch right away, use minimum priming.) Don't pump the throttle as a primer unless you are cranking the engine at the same time. Raw fuel will accumulate in the airbox and create a serious fire hazard. (Some models, like the Cessna R182, with its horizontally mounted carburetor, specifically call for pumping of the throttle before start, but it's the exception rather than the rule.) If your airplane is equipped with an electric boost pump, use it for starting — and keep in mind that you might have to use it during taxi to ward off vapor lock.
Fuel injection suffers from two main problems with regard to hot starts. One, most systems don't meter fuel very accurately at low engine speeds, particularly the Bendix injection setup found attached to Lycomings and, rarely, to Continentals. And, two, the fuel-delivery lines reside on top of the engine, directly over the cylinder fins. Heat rippling off the just-shut-down engine boils the fuel out of the lines. Moreover, many of the Continental installations have the throttle body — which also houses a pair of spool valves that meter the fuel to the injector spider — above the engine, too, just asking for vapor formation.
As with the carbureted models, try the factory recommendations first. For an example, here's the hot-starting rundown for a Continental TSIO-360. (While the turbo Continentals have altitude or deck-pressure compensating fuel pumps, the recommendations listed here work for the other models, too.) Throttle, full open; mixture, idle cutoff; boost pump, on for 10 to 15 seconds or until the fuel pumping pulsations stabilize, then off; mixture, full rich (momentary priming with the boost pump or primer may be required); throttle, 1/2-inch open; engage starter. Why these steps in this sequence? First, because of the design, the Continental fuel-injection system — which, like the Bendix setup, is continuous flow — requires both the throttle and mixture controls to be full forward for maximum fuel flow. But because your main concern with hot starts is to minimize vapor in the system, you must pressurize the system with the mixture in idle cutoff. The idea is to purge the vapor in the delivery system without also allowing excessive amounts of fuel into the intake ports; with the mixture in idle cutoff, no fuel actually flows to the injectors.
How do you know that the vapor's been purged? Until you have moved the mixture control from idle cutoff, you don't, really; in some installations you can hear the pump operating. A steady whine usually indicates that the pump is moving fuel; a wavering tone suggests that it's trying unsuccessfully to transfer vapor. Sometimes, however, the distinction is subtle.
The next step calls for actually priming the intake ports with fuel. Most operating handbooks call for priming a hot engine until fuel flow stabilizes. Some instructions say to set the throttle and mixture to maximum, while others dictate moving the throttle from idle with the boost pump running and watching for an increase in fuel flow on the gauge. Either method accomplishes the same thing — moving vapor from the delivery lines, replacing it with cool fuel, and priming the engine itself. A few installations have throttle-skewed boost pump schemes — the greater the throttle opening, the greater the boost pump output — intended to keep you from flooding the engine by switching on high-flow boost at a small throttle opening.
While the Continental TSIO-360 manual spells out a cracked-throttle start, there's another technique that often works better. If you understand that during a hot start, your job is twofold — get the vapor out of the system, and provide a fuel/air mixture that will sustain combustion — then the following procedure will make sense. After priming to a steady fuel flow — just enough to see the needle peak, no longer — retard the throttle to idle and leave the mixture at full rich. Begin cranking. Steadily increase the throttle from idle; use a rate that would get you to full throttle in five seconds or so. At some point in moving the throttle, the engine should fire — after all, you're only adjusting the amount of air the engine receives to combine with the fuel from the priming and the slight flow available at cranking speeds.
When the engine starts, retard the throttle to a high-idle setting and get a free hand to the boost pump switch. Usually, the engine will start and run for a few seconds on the prime fuel, then it may well falter. Hit the boost pump — the low setting if you have more than one — a second or so at a time. Provide just enough fuel boost to keep the engine running; too much will flood the induction system and put the fire out.
Lycoming fuel-injected installations (the vast majority of which have the Bendix RSA injection) require a different technique. Because this system doesn't meter fuel well at sub-idle speeds, you'll have to help it along even more so by priming. Another consideration with the Bendix installations is that the fuel pressure is regulated at the throttle body; you can't force extra fuel through the system with the boost pump as you can with the Continental scheme.
Here's the recommended procedure for the Aerospatiale Trinidad TC, with a Lycoming TIO-540 — throttle, full; boost pump, on; mixture, move from idle cutoff to full rich for one second, then back to idle cutoff; boost pump, off; engage starter. (Could you just as easily push the mixture to full rich, toggle the boost pump, and then retard the mixture? Sure. You're accomplishing the same thing.) In theory, the engine will catch within a few revolutions and you can push the mixture to full rich and retard the throttle to idle.
Why does this installation require a different technique from that of the Continental system? Due to poor low-speed metering, the Bendix setup calls for the priming to provide the majority of the fuel needed in those first few tentative combustion events. As you continue to crank with the throttle wide open, the mixture becomes progressively leaner — there being less and less fuel remaining in the induction system — eventually reaching the right ratio for combustion. Can you use the Continental technique on a Lycoming? Of course; and according to some pilots, it works better in some installations. The point is this: If the handbook technique fails to work adequately and you've ruled out any mechanical problems, try the other method.
Eventually every pilot of an injected engine will inadvertently overprime the engine and be faced with a flooded start. (This is all the more likely if you leave the boost pump on too long for priming or, in a Lycoming, leave the pump on while you bring the mixture to full rich after the engine starts. Sometimes the flow at that setting will be too great for prevailing conditions and you'll end up with the engine stalling after running for a few seconds.) Your task for the flooded start is pretty straightforward — get the excess fuel out of the intake system. Unless you've really overdone it — the odor of avgas overcoming passengers and a lurid stream of the blue stuff exiting the bottom of the cowling — you can use the textbook flooded-start procedures. (If you have overdone it, go to the coffee shop and have a slice of pie while the induction system drains.)
Like the Bendix/Lycoming recommendations, the flooded-start drill calls for turning off the boost pump, retarding the mixture to idle cutoff, and opening the throttle fully. Begin cranking. It may take a few seconds to get the engine to even consider running — be sure to adhere to the usually specified 30 seconds' max cranking duration. When the first brave cylinders begin to take, gradually reduce the throttle opening. Don't be in a hurry to move the mixture to full rich; there's enough fuel hanging around to support combustion for a short time. When engine speed begins to fall, gently move the mixture to full rich. Once the engine's running, be prepared to toggle the boost pump to ward off vapor formation. (In Continentals, be careful of using any more than the low boost settings or be prepared to lean the mixture to mitigate the fuel flow. It's painfully easy to flood the engine again with hamfisted boost-pump operation.)
Sound like a lot of fuss to get an engine running? (Especially compared to the turn-and-go nature of modern cars?) Well, it can be. But what you ought to remember is that eventually, with the right technique and barring mechanical difficulties, the engine will start. It just takes patience and an understanding of your airplane's fuel-delivery system. All lessons that our Mirage pilot in Phoenix probably has learned by now.