Are you one of those pilots who hates fuel stops simply for the fact that the engine doesn’t have time to cool off? You pump gas into the plane, hit the bathroom, get in the plane and the oil temperature is still up at 160-170 degrees, leading you to have to figure out how to get the plane going again with a hot engine. For those without proper training, this usually means a lot of jockeying around with the throttle, mixture, and fuel pump to try and get the thing started without flooding it. After coughing and wheezing several times, the engine finally comes to life, leaving you to only guess what worked and without the knowledge of how to duplicate it.
Let’s take a step back for a minute to see what is actually happening with a hot engine. Once the engine is shut off, the fuel in the lines leading from the tank to the engine is vaporized, meaning there is more air in the fuel lines than liquid fuel. In the engine block itself, there is still liquid fuel in the injectors, but only enough for the engine to cough then quit if started.
When a normal priming and starting procedure is performed, too much fuel is forced into the cylinders and the engine becomes flooded. A flooded engine just means that the stoichiometric ratio is way too rich, meaning there is too much fuel and not enough air. The prime does the trick of getting the fuel vapor out of the fuel lines, but it shoves too much fuel into the cylinders. Once the engine is flooded, it’s a waiting game to allow air into the engine to get the mixture right. Lycoming powered 200HP engines in Piper Arrows are notoriously easy to flood when hot.
So, what’s the solution? By taking a step back to see what is actually happening, you can attack the problem from the source, which is the fuel lines. You need to get the vapor out of the fuel lines and get some liquid fuel in there. The procedure for this varies based on the make and model of engine, but I’m going to use the example of the Continental IO-550-N that is in a Cirrus SR22. I found this procedure in the Continental Engine Manual and it works every time.
(Bonanza owners with the 550-B, the procedure differs slightly, as does the procedure if you are flying a Turbo Cirrus. I’ll make notes of the differences. The procedure for the Cirrus SR20 with the Continental IO-360-ES is the same as the SR22 )
- Mixture: Full Lean (this allows fuel in the fuel lines, but prevents it from going past the mixture control into the engine, sending all fuel back to the fuel tank; a small amount of fuel leaks past the mixture control providing prime for the engine)
- Throttle: Full (throttle stays idle in a Bonanza)
- Low Boost Pump: Run for 30-60 seconds (see note below) (15 seconds in the Turbo Cirrus, High Boost for 30 seconds in the Bonanza)
- Mixture: Full Rich
- Throttle: Cracked (In the Bonanza, throttle to full and prime until the Fuel Flow hits 14 GPH, then throttle cracked)
- Boost Pump: Off, but have your finger on Low Boost (Fuel pump stays off for the Bonanza for starting)
- Starter: Crank (engine will turn over a few more times before firing, this is normal)
- At the first indication of start, turn the Low Boost on and adjust the throttle for 1,000 RPM
- The engine will fire right about the time you start thinking it isn’t going to work
A few notes regarding engine temperatures:
- If the oil temperature is above 150 degrees (170 degrees in the Turbo), a hot start will be required. If oil temp is close to 200 degrees, run low boost for 60 seconds. If oil temp is 175, run low boost for 45 seconds. If oil temp is 150, run low boost for 30 seconds.
- If the oil temperature is between 125-150 (150-170 in the Turbo), skip the hot start procedure, don’t prime the engine, leave the boost pump off, and crank the engine, then boost pump on when it starts
- If the oil temperature is between 100-125 (120-150 in the Turbo), skip the hot start, don’t prime the engine, and perform a normal start with the boost pump on
- If the oil temperature is below 100 (120 in the Turbo), perform a normal prime and start
If you’re a Lycoming operator, completely ignore everything just stated. With Lycoming’s the best thing to do is leave everything lean and idle, turn on the fuel pump, crank it and hope that it starts. Don’t give it any fuel at all until the engine fires. The turbo’s especially, you don’t want to add any mixture when trying to hot start them.
There are other “procedures” for hot starting out there, but most of them involve starting with full throttle, which can lead to the airplane shooting ahead on a ramp or taxi way if the brakes aren’t properly set. This can lead to high repair costs, so always be cautious. Figuring out what is happening when the engine is hot will give you a better chance of getting it started right away.