The Anatomy of a Hot Start

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.  All Lycoming powered high performance and turbo charged engines 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.

(Pilots of other airplanes, keep reading.  I have sections below for PA46s, Columbias, and Bonanzas as well )

• 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:  Idle
• Low Boost Pump:  Run for 30-60 seconds (see note below)  (15 seconds in the Turbo Cirrus)
• Mixture:  Full Rich
• Throttle:  Open about 1/4 travel (not 1/4″, that won’t be enough)
• Boost Pump:  Off, but have your finger on Low Boost
• 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, increase the throttle to ensure the engine catches,  then 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, a hot start will be required.  If oil temp is close to 200 degrees, run low boost for 60 seconds in a non-turbo.  If oil temp is 175, run low boost for 45 seconds in a non-turbo.  If oil temp is 150, run low boost for 30 seconds in a non-turbo.  Use 15 seconds for all temps above 150 in a Cirrus Turbo.
• If the oil temperature is between 125-150, 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 and slightly increase throttle to make sure the engine catches
• If the oil temperature is between 100-125, 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, perform a normal prime and start

Piper Malibu (PA46-310P with Continental Engine)

• 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:  Idle
• Low Boost Pump:  Run for 15-30 seconds depending on oil temperature…
  • Greater than 150 degrees: 30 seconds
  • Less than 125-150 degrees:  15 seconds
  • Less than 100-125 degrees, no need to run the Low Boost
  • Perform normal cold start below 100 degrees
• Mixture:  Full Rich
• Throttle:  Open about 1/4 travel (not 1/4″, that won’t be enough)
• Boost Pump:  Off, but have your finger on the Primer Button
• Magnetos:  On
• Starter:  Crank (engine will turn over a few more times before firing, this is normal)
• Engine should fire with the Low Boost Pump off, but…
  • If the engine starts to die, simultaneously increase the throttle a little bit and hit the primer button.
  • Only tap the primer button, don’t hold it as you’ll flood the engine if you hold it At the first indication of start, turn the Low Boost on, increase the throttle to ensure the engine catches,  then adjust the throttle for 1,000 RPM

Piper Mirage & Matrix (PA46-350P and PA46-350T with Lycoming Engine)

• Leave throttle and mixture idle
  • Before you turn the battery on, ensure the mixture is idle cutoff
  • If the mixture is forward and the battery is on, the low boost pump in the fuel tank will start pumping fuel to the engine and quickly flood it
• Ensure Magneto switches are on
• Open throttle 1/4 travel (not 1/4″ as this won’t be enough)
• Crank
• As soon as the engine begins coughing and wheezing (and this is what it will sound like), push the mixture 3/4 of the way forward
• Once the engine has a good solid fire, smoothly and swiftly push the mixture all the way forward
• Reduce throttle
• Note:  The Emergency Boost Pump can be used as part of the hot start technique, but I usually leave it off.
  • Pro to the Emergency Boost Pump is it can help suck more fuel in and purge vapor during the start
  • Con is that if the engine doesn’t start on the first try, you are flooding your motor

Columbia 400

• Throttle and Mixture Idle Cutoff
• Vapor Suppression:  Run for 30-60 seconds.  Reference above temperatures on the non-turbo Cirrus for run times
• Mixture full forward
• Throttle 1″ in
• Prime for 3 seconds, then off
• Crank
• Once engine fires, primer might need to be pushed momentarily to purge excess vapor
  • On Columbia 350s, the throttle should be twisted (or pushed depending on if it’s an Avidyne or Garmin Columbia) in while cranking
  • Be prepared to reduce power once engine fires

Beechcraft 36 Bonanza

• Throttle and Mixture Idle Cutoff
• Low Boost:  Run for 30-60 seconds.  Reference above temperatures on the non-turbo Cirrus for run times
• Mixture full forward
• Throttle open 1″
• High Boost until fuel flow peaks, then off
• Crank
• Once engine fires, Low Boost might need to be engaged momentarily to purge excess vapor

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.