Many pilots have reported problems trying to start The R44 Raven II when the engine is hot. For example, attempting a start after a refueling stop on a cross-country flight. In flight there is enough airflow through the engine compartment to dissipate much of the heat the engine generates, but after shutdown the engine compartment can become “heat soaked” which causes the fuel in the lines to vaporize or change from a liquid to a gas. High outside air temperatures and higher altitudes will also contribute to fuel vaporization. The engine-driven fuel pump is unable to pump this vapor through the fuel lines, causing “vapor lock”, thus preventing the fuel from reaching the cylinders. Of course, without fuel the engine will not start. The pilot can either wait for the fuel to cool, perhaps helping by opening the two small cowl doors on each side of the engine or try this hot engine starting technique.
To understand this technique for starting a vapor locked Raven II engine, let’s review the fuel system in the Raven II. Fuel is pumped from the main fuel tank to the fuel control unit (servo) on the engine which will take whatever fuel is needed to provide the amount of power the pilot is requesting. Excess fuel is then pumped through a fuel return line back into the fuel tanks. There are two fuel pumps in the Raven II, an engine-driven main fuel pump and an electrically driven auxiliary pump. The aux pump, which is activated when the clutch switch is in the “engaged” position or the ignition switch is in the “prime” position, has two purposes; it primes the engine for starting and provides fuel pump redundancy. It’s the aux fuel pump and the return line we will use to eliminate the vapor lock.
For easier starting try the following procedure:
Pull the mixture control knob full out to the idle cut-off position. In this position the fuel control unit will not ask for any fuel.
With the master switch on, activate the auxiliary fuel pump by turning the ignition switch to “prime” and hold for 20-30 seconds after the AUX FUEL PUMP light extinguishes. The aux pump is now bringing cooler fuel from the main tank through the fuel lines and, since the fuel control unit is not asking for any fuel, all the fuel is being pumped through the return line into the fuel tanks, eliminating the vapor lock.
Proceed with the normal starting procedure using a normal prime (2-3 seconds) with the mixture full rich.
You may have to adjust the timing aspects of the procedure to fit your location and individual engine. I’ve used this procedure all over the world: summer in the deserts of the US and Western Australia to the hot, humid jungles of Panama and it works great.
Tim Tucker
November 2018