We’ve talked about flying at high density altitude, about maneuvers, true airspeed and increased turn radius. We all know that there is less power available, less engine and oil cooling. There is another insidious issue with flying at high DA: the accumulation of heat in the engine compartment and its effect on fuel.
Water boils at 100 degrees centigrade at sea level. As you go up in altitude the boiling point drops. When you boil an egg in Denver, you boil it a lot longer than you do at sea level because the water is much cooler. The same thing happens to your fuel.
For the first flight of the day at high DA, the engine compartment is cool. Your start might require a different mixture or priming, but otherwise will be normal. Now you go out and fly a media flight or two. The heat is building under the cowling and the fuel in the fuel lines is heating up as well. Depending on the conditions that day and the number of flights, the fuel in your fuel lines can actually create a vapor lock.
If this occurs, you may have trouble starting or you may start only to have the engine quit while idling. The problem may persist as long as the conditions remain or until you get cool fuel from the tank all the way to the cylinders. For an air show event, waiting for the engine to cool is usually unrealistic. Once you have resolved that there are no mechanical problems with the aircraft, you may consider the following technique.
Depending on your engine type and manufacturer, one technique to overcome the problem is to utilize your electric fuel boost pump (Assuming you have one). Pressurizing the fuel should be enough to clear the vapor lock and allow a normal idle.
Once cool fuel has filled the entire fuel system, the engine should run normally without the boost pump; however, if you’ve used this technique to start and idle at high density altitude, it is a good idea to leave that boost pump on for the duration of the flight.