It’s winter time in Switzerland, as in half of world, and we go through the first very cold days of the season, with temperature below freezing 24 hours a day. This has many consequences for pilots, like frozen fingers when towing planes, an absolute necessity for light aircraft flying IFR to stay out of clouds as any ice won’t melt before landing… the usual stuff.

But for plastic pilots (a.k.a flying with diesel / Jet-A1 engines), low temperatures have special impact. Freezing point of Jet-A1 is higher than AVGAS, and if it get too viscous, the engine could suffer power losses, and even get damaged !

This is why diesel engines have additional gauges showing fuel temperature in both tanks. So, what to do when you get such an alarm ?

Usually this occurs on ground, before the first flight of the day. If the fuel temperature is extremely low (below -30°C / -22°F) it’s even forbidden by AFM to try starting the engine. However the low temperature alert occurs at “higher” temperatures…

When you have to warm the fuel before take off this is easily done because by design some warm unburned fuel comes back from the diesel engine to the tanks (thus no need to start a fire below the wings…). An important point is to know the fuel system design, because even planes with the same engine model have different fuel distribution system.

On the DA40-TDI, the fuel is pumped from left wing tank to the engine, and the warmed fuel goes back to the right wing tank. A transfer pump commanded by the pilot is used to re-balance the tanks. So pilot must use it to warm fuel in both tanks before take-off.

The Cessna 172 TDI has the same engine, but a different design. The fuel is pumped and returned in the same wing tank, so to get both of them warm, it’s necessary to switch the fuel selector once the first tank is warm enough to get the second one up to the required temperature. For all those who are used to AVGAS C172, there is no “BOTH” position on diesel retrofitted C172…

If the low fuel temperature alarm occurs in flight, the AFM recommends to decrease airspeed and increase power, so as to send more warm fuel back in the tanks.

I experienced a low fuel temperature alarm only once, on a very cold and windy day, and it took just a couple of minutes before the fuel got warm enough to allow for a take-off. After the initial climb, and in cruise, it was no longer a problem.