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As the Wrench Turns

AS THE WRENCH TURNS

David Vital, Director of Maintenance davidvital@wvfc.org

Hot Start

Happy Spring to all of WVFC. As we move into the hotter months of the year, I would like to bring back the hot starts article. This is to remind everyone of the procedures that seem to work the best for Cessna 172SP’s and most fuel injected engines.

Every year, a number of WVFC pilots are stranded somewhere when they are unable to start a Cessna 172S (or other fuel injected plane for that matter) when the engine is hot.  The typical scenario is a trip for lunch or dinner somewhere, say a 1 hour stop, and then trouble starts when attempting to restart the engine.  The problem is usually a combination of factors. 

The first issue is over-priming when the engine is already warm.  Typically for a hot start, use only a little priming (some engines will start with no priming at all when hot).  Cold engines like a decent amount of priming but warm or hot engines don’t need the same amount of raw fuel to get going.  In fact, what often happens is that this over-priming of the warm engine actually floods the engine with fuel and the fuel/air mixture isn’t right and the engine won’t start. 

A common scenario here is the pilot pumps the throttle trying to get the plane to start and if the mixture control is anywhere but idle/cutoff then more fuel will enter the engine and it definitely won’t start.  What’s called for in this case is a so-called “flooded” start procedure.  A POH may have specific flooded start procedures and if it does then follow it.  Many manuals don’t have a procedure so it’s a good idea to follow a generic flooded start procedure.  Though maybe counterintuitive, the throttle should be placed in the full open position and the mixture at idle-cutoff.  Now turn the key to crank the engine. The effect is to flush excess fuel out of the engine.  During the purging process, at some moment, the fuel/air mixture will be correct and the engine will start.  Next, one has to quickly reverse the controls and smoothly move the throttle to idle and smoothly advance the mixture to rich to keep the engine running.

A second issue with hot starts is so-called vapor-lock in fuel injected engines.  After being run, when a hot engine is stopped the fuel in the fuel injection lines might vaporize and create “bubbles”, called vapor lock, that get stuck in the fuel system.  When it comes time to start the engine, the fuel pressure in the fuel system might not be adequate to push the vapor lock through the system and the engine will not start.  Most fuel injection planes have an electrically driven auxiliary fuel pump.   The pump may be used for take-off and landings and possibly other emergency situations.  One additional use is to help overcome vapor in the starting scenario above.   Again, a POH may or may not recommend the use of the auxiliary fuel pump for a hot start (or may not mention it all) but if you suspect vapor lock, it might be a good idea to try the hot start with the auxiliary pump running.

If there is ever any doubt about whether an engine is starved for fuel or flooded with fuel, assume it’s flooded.  Because adding fuel to a flooded engine is a lot worse than starving an engine that is starved.  If the flooded start procedure doesn’t remedy the situation then it’s OK to proceed with a normal start with some priming.

Starting in May we will go back to club BBQ’s on the last Friday of the month. See you there!

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