At last, I was on my own in a retractable-gear airplane. What’s more, this particular Cessna’s model designation started with the number two, and not just a one! However, I can still remember my apprehension that I would be stranded in Chapel Hill the first time I flew the 210 to North Carolina from the Maryland suburbs to visit a friend one very warm Sunday, a number of years ago.
While transitioning into the airplane, I had watched my flight instructor demonstrate the “hot start” procedure for the Centurion’s fuel-injected engine (and I had done so several times)—but I can well remember the feeling that either mechanical intuition or just plain dumb luck was going to be needed in ample quantity, if ever I took this airplane anywhere that was remote or isolated. Unfortunately, that day, the place looked pretty sleepy to me. When I landed, I didn’t see a single soul stirring.
The trip down had been glorious, and after a wonderful but all-too-brief visit it was time for me to head back home. It was summertime, and it was North Carolina, and…it was hot…and so was the engine. As soon as I turned on the master switch, I saw the oil temperature was already in the green. As my friend looked on and I started cranking the propeller, the situation rapidly began receding from any likelihood that I’d ever be attributed with the same category of near imperturbability and savior-faire as the bon vivant who had landed that very same airplane there, just a few hours earlier.
As my cool was unraveling—along with any pretense of competence—I resolutely continued to follow the hot-start checklist, muttering a silent prayer for redemption. I survived my trial by fire with my self-image and self-esteem more or less intact. But it was nip and tuck there, no two ways about it.
Airplanes are not the only things that can be plagued by hot-start problems. Automobiles also can succumb to this problem, and if it isn’t something real basic like a rotor or a leaky vacuum hose, it’s usually something sophisticated like a relay or sensor. In this case, I wasn’t depending on some engine coolant or crankshaft sensor or a fuel injection relay. The only timing involved had to do with following the somewhat complicated “knobology” from my hot start checklist, inside the cockpit.
Although you certainly won’t find many hot-start checklists in airplanes with carbureted engines, fuel-injected engines have a particularly sweet advantage over carbureted systems, which is the absence of induction icing problems. Carbureted engines can easily lower the temperature of the air by 50 degrees Fahrenheit, and that allows the formation of ice as the air temperature drops below the dew point. To me, the biggest drawback of having a fuel-injected engine is the very one that I was dealing with that Sunday in Chapel Hill: difficulty starting when the engine is hot.
What happens can be summarized most simply by “vapor lock.” In the Centurion—at least the 210B I was flying—the fuel left in the fuel lines above the engine simply gets vaporized by the heat. After any engine is shut down, many of its parts (such as cylinders and oil) cool; however, others actually heat up, because of the lack of airflow, heat conduction, and heat radiated by other nearby hot parts. This heat soaking is worst shortly after shutdown.
Once the liquid fuel becomes a gas, all bets are off, as far as flow and pressure continuity are concerned. Liquids are mostly incompressible. If you start a siphon going, for example, the liquid will flow up and over any existing surface level, as long as there are no large air bubbles in the siphon line, and the liquid in the “down” side extends below the local hydraulic “head” or surface height. If a large slug of air gets in the tube, however, your “pump”—simply the atmospheric pressure acting upon the body of liquid from which you are drawing—becomes “un-primed,” the liquid in your siphon tube drops back to its prior level, and you have to (literally) suck it up and start over.
These days, a great many newer aircraft have engines featuring fuel injection. But in all of them, when the engine compartment gets hot, the gas in your fuel pump and in the lines can boil off, with the resultant difficulty in restoring the needed flow of fuel to the engine. Each airplane has its own particular hot-start procedure, and you should of course follow the checklist in your own POH. In that particular airplane, the Cessna 210B, the engine-related hot-start checklist goes pretty much like this:
- Fuel selector: Fullest tank
- Throttle: Closed
- Propeller: High rpm
- Mixture: Idle cut-off
- Propeller area: “Clear”
- Auxiliary fuel pump switch: On “High”
- Master switch: On
- Throttle: FULL AFT while fuel lines are purged
- Throttle: Open approximately one inch
- Ignition Switch: Both
- Push switch to Start
- Mixture: Advance slowly to Full as engine starts
- Ignition switch: Release on engine start
- Adjust mixture for density altitude/ground operations
- Auxiliary fuel pump switch: On “Low”
- Throttle: 800 to 1,200 rpm (smooth running)
- Oil pressure: In green
- Auxiliary fuel pump switch: Off (when engine running smoothly)
The high points of this procedure, apart from the usual common-sense items common to all engine starts are:
• 1. Before engine start, setting the mixture to idle cut-off, having the throttle fully aft, and having the fuel pump on High. With this airplane I was taught that keeping the fuel pump on for around 15 seconds was about right, in order to purge the fuel lines of air. What this does, of course, is to pressurize the fuel lines, without dousing the intake ports. With the mixture leaned out all the way, theoretically no fuel is going to the injectors. At this point you will probably hear the pump doing its thing. But you don’t want to leave the fuel pump on too long, lest you go from a hot start to a flooded one.
• 2. Then, opening the throttle about an inch, starting the engine, and slowly adjusting the mixture from cut-off to full rich, adjusting for density altitude and switching the fuel pump to low, and then off, once the engine is running smoothly. Now that you’ve given the hot fuel vapor the bum’s rush and replaced it with much cooler liquid fuel, it can do its work at priming the engine, and once the engine is running, you can optimize the fuel/air mixture to your liking. Switching the fuel pump to low helps avoid the danger of flooding the induction system.
The more simply designed something is, generally speaking, the more reliable it is—and aircraft fuel systems are fairly unpretentious. Fuel-injection systems tend not to meter fuel very well at low sub-idle speeds. The fuel itself tends not to vaporize very readily when the engine is first started. If the spark plugs aren’t clean, or the idle mixture setting is off, starting may be a challenge. But the most glaring attribute, of course, is that the fuel delivery lines were routed above the engine, right above the cylinder fins.
Incidentally, I departed Chapel Hill uneventfully with a friendly wave and fond farewell, only to wind up having to dodge scattered thunderstorms on my way back to Maryland. You can’t win them all.
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