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When I first started flying my fuel injected, Lycoming powered Lancair 320, the test pilot (Ivan Checkinov) implored me not to be Russian (or was that rush’n?). He said, “Check lists must require the use of the boost pump during take-offs, low power approaches and landings.” Over time, this procedure was ignored because of numerous departures with the pump left running until long into the flight (I ain’t no ignor-am-us for nuttin). Well, experience and my good angel have conspired to teach me once more.
On the last Friday of Oshkosh 2001, I made another 50-minute flight north and touched down on Wittman Field at 8:30 AM. With the show plane taxiway loaded with departing war birds, the orange shirts directed me to interstitial Limbo and thence, having lost track of any raison d’etre, onto the grass Purgatory alongside runway 9 behind a gaggle of twins. After rattling and rolling along in the heat, I had to sit, idling, for a loooong time awaiting departure. What the heck, it was a beautiful morning to fly back home anyway. During the idling wait, the “idiot” light for low fuel pressure popped on several times and the digital indicator showed the usual 27-psi had dropped to the 12-psi warning level. Energizing the boost pump resolved the fuel problem and got me thinking about something I had read recently.
Engines set up without a post-pump fuel return line system are subject to hard hot starts because the fuel pump becomes heat-soaked from the engine and, when activated by cranking the engine, immediately vaporizes fuel along with resulting pump cavitations, the so called vapor lock condition. Engines, such as Continentals, can benefit from the down-line flow regulator that returns excess fuel to the selected tank while maintaining injector system pressure. Before a hot start, running the boost pump for 1 or 2 minutes with the mixture at idle-cutoff circulates cool fuel thru the engine-driven pump, absorbing a great amount of the heat in the pump and thus reducing or eliminating a major factor leading to vapor lock.
In normal idling operation, a return-line system is an important tool in keeping the fuel cool, as a rule, because most of the fuel passing thru the pump is returned to the selected tank. In non-return-line systems, such as the Lycoming, only the fuel delivered to the engine passes thru the pump and, at idle, very little makes the transit. This allows a hot engine to heat soak the pump that can (and did) lead to vapor lock conditions. Of course, these circumstances could also occur on a hot day during a Vy climb, pattern work or a long slow descent into your typical Midwest 105-degree Heat-index steam bath where cooling air molecules are replaced by lighter and useless water vapor that has not quite yet reached the visible state. It can also happen in the thin air at a hot, high altitude airport with a high density-altitude.
In any event, running the electric boost pump can return enough pressure to the system to force the gases back into solution or, better yet, maintain the required high pressure to reduce the chance of vapor lock from forming during critical operations.
Because U. Ben Slackinoff (a rush’n dingbat pilot) and I now understand the rationale behind Ivan Checkinov’s exhortation to use the boost pump – we will. Especially on takeoffs and landings where the loss of proper fuel pressure, because the engine pump got the “vapors,” could lead to unwanted engine out maneuvers and an overabundance of perspiration on an already hot day.
Scott Krueger
Lancair 320 – N92EX
LML website: http://www.olsusa.com/mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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