Fuel System

["Al Gietzen" <alventures@email.msn.com>]

 

Builders; 

 

Just to add a further comment on fuel system plumbing.  I haven't seen it stated why the certified aircraft put the boost pump in series with the mechanical.  My understanding is that it is because of the tendency of the mechanical (diaphram) pumps; bolted to the hot engine, to suffer vapor-lock.  This is particularly a problem after some time on the ram and taxiways when there is little cooling air flowing, and leads to the potential for vapor-lock during the most critical period - takeoff. 

 

Boost pumps put out higher pressure and will force flow into (through) the mechanical pump insuring flow; therefore we run then during takeoff.  If plumped in parallel, not running the boost pump, or a boost pump failure leaves the possibility reduced/no fuel to the engine. 

 

In working with a friend on his Lancair, we plumbed the pumps in series; and, put an orificed bypass line back to the tank (after the mechanical pump) to allow some continuing flow even during idle conditions to prevent vapor lock. 

 

Al Gietzen RGE