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Hot Stuff

At last, I was on my own in a retractable-gear airplane. What’s more, this Cessna’s numerical descriptor started with a two, and not just a one! However, I can still remember my apprehension that I’d 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. I’d watched my flight instructor demonstrate the “hot start” procedure for the Centurion’s fuel-injected engine (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.At last, I was on my own in a retractable-gear airplane. What's more, this Cessna's numerical descriptor started with a two, and not just a one! However, I can still remember my apprehension that I'd 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. I'd watched my flight instructor demonstrate the 'hot start' procedure for the Centurion's fuel-injected engine (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.

The trip down was 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 in fact, I saw that the oil temperature was already in the green. (Oh, just great…) As my friend Dave Green looked on while I started cranking the propeller, let's just say that the situation rapidly began receding from any likelihood that I'd ever be attributed with the same category of sang froid and savior-faire as the bon vivant who'd landed that 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 (and possibly something else less ecclesiastical, along with it). Well, enough with the euphemistic banter: 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.

Now to explain it all, eh? First of all, airplanes aren't the only things that can be plagued by hot start problems. Automobiles 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 Fahrenheit degrees, 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 the term 'vapor lock'. In the Centurion (at least the 210B I was flying), the fuel left in the lines above the engine simply got 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 due to the lack of air flow, heat conduction (as well as radiative heating from other parts that are cooling). This heat soaking is worst shortly after shutdown (say from 30 minutes to one hour afterwards). Once the liquid fuel becomes a gas, all bets are off, as far as flow and pressure continuity is concerned. It's like this: 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 'head' or surface height. If a large slug of air gets in the tube though, your 'pump' (simply the atmospheric pressure acting upon the body of liquid from which you are drawing) gets 'un-primed', the liquid in your siphon tube drops back to its prior level, and you have to 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 hot start checklist goes pretty much like this:

1. Fuel Selector = fullest tank
2. Throttle = Closed
3. Propeller = High RPM
4. Mixture = IDLE Cut-Off
5. Propeller area = 'CLEAR'
6. Rotating beacon = ON
7. Auxiliary Fuel Pump Switch = ON 'HI'
8. Master switch = ON
9. CONFIRM Gear light = GREEN
10. Throttle - FULL AFT while fuel lines are purged When fuel line is purged
11. Throttle - OPEN approximately 1 inch
12. Ignition Switch = BOTH
13. Push switch to Start
14. Mixture - Advance slowly to FULL as engine starts
15. Ignition Switch - RELEASE on engine start
16. Adjust mixture for Density Altitude / Ground Operations
17. Auxiliary Fuel Pump Switch = on 'LO'
18. Throttle - 800 to 1,200 RPM (smooth running)
19. Oil Pressure - in green
20. 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 (like the fuel selector, the beacon, the master switch, shouting 'CLEAR!', turning the ignition, checking for proper oil pressure…) were:

a. Before engine start, setting the mixture on 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.) You'll probably hear the pump doing its thing at this point. (You don't want to leave the fuel pump on too long, lest you go from a hot start, to a flooded one.)

b. Then opening the throttle about an inch, starting the engine, and slowly adjusting the mixture from cut-off to full, 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 simple something is, generally the more reliable it is, and aircraft fuel systems are fairly unpretentious. Fuel injected systems tend not to meter fuel very well at low sub-idle speeds (although carburetors do a better job). 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 will be, as well. But the most glaring attribute of course is that the fuel delivery lines were placed above the engine, right above the cylinder fins. (In some cases, the throttle and metering valves are, as well.)

Generally speaking however, there are a few things you can do to lessen your exposure (and that of your airplane) to hot starting problems.

'When you taxi to your temporary tie-down spot, face the airplane into the wind, to improve air flowing into the cowling.
'If there's any shade to be had of course, use it.

'Leave the cowl flaps open (and there's no danger of wind damage or propeller blast from other airplanes, the cowling itself). If that's too risky, opening the oil filler door is the next best thing, particularly if it is at the top of the cowling.

'When in doubt, avoid the danger of engine flooding and use the hot start procedure first.

Incidentally, I departed Chapel Hill fairly uneventfully (or so I hoped it appeared) with a friendly wave and fond farewell, only to wind up having to dodge scattered thunderstorms on my way back to Maryland. But that's another story.

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