The idea of writing this occurred to me some time ago when I noticed a pilot (whom I generally had a good amount of respect for as an aviator) exhibit a completely "strange" idea of the fuel booster pump's function. The checklist on the Douglas B(A)-26 we were flying calls for these pumps to be placed in HIGH on the downwind leg of the traffic pattern, just prior to lowering the landing gear. When I read the item, he placed them in the LOW position and left them there. I made a mental note of this, determining in my mind that this was another item I wanted to ask about on
the ground, so as to not divert his attention during the checkride.
Later, after shutdown, I asked if he might share with me why he wanted them in LOW. His answer was that he thought the engines had been running richer than normal and he didn't want to "flood" them in the air. Which, of course, left me with a puzzled look (to say the least) on my face!
Let's briefly take a quick look at how the fuel system and pressure-injected carburetor works on these reciprocating engined airplanes in an effort to remove this old wivesí tale before it does any more damage!
Fuel boost pumps have three purposes for being included in the fuel system: (1) provide pressure for starting, (2) condition the fuel and, (3) substitute for the engine driven pump if it fails. These pumps are generally located
Another use that we probably should touch on here is the use of the pump in the low speed position to "condition" the fuel at those higher cruise altitudes where you might see a fuel pressure fluctuation. This fluctuation very likely is the result of fuel vapors/bubbles as altitude begins to affect the fuel in the aircraft's tanks.
And, in conclusion, let's not forget the seemingly elementary - but often forgotten - flight manual requirement that they first be placed in NORMAL, then in EMERGENCY. There's probably some amount of dissension among the troops about this. I can still remember the first time I encountered it, about 1955 or so. Along about that time, the USAF's B-25 fleet was being converted by Hayes Aircraft of Birmingham to the Bendix-Stromberg pressure injected carburetor. This carburetor
required a much higher fuel pressure - on the order of xx PSI - to operate properly. The older Holley carb only needed around 6 to 8 psi, and accordingly, had a simple two-position OFF-ON fuel booster pump switch. In order to avoid a creating a sudden "surge" of high fuel pressure to the Bendix-Stromberg carburetor's diaphragms, a new fuel booster pump switch was part of this conversion. The new procedure called for this three-position switch to first be positioned to NORMAL and the fuel pressure stabilized, then the switch could be moved to EMERGENCY for starting. Whatís that? Well sure, I can hear your question already - so - before going any further, letís talk about it! Yes, the diaphragms can "take it" as concerns the fuel pressure, but the concern here (on some people's part) is the "surge" of fuel pressure. As I said before, competent people, with an intimate knowledge of this carburetor's inner workings, may disagree on the need for this procedure. Some will insist that there is no need to worry about the "surge" hurting the diaphragms.
I just do it, that's the way we were taught, it's in the B-25 flight manual, and it's the way we subsequently trained thousands of cadets in this procedure. This habit pattern became so thoroughly ingrained in pilots that they are likely to habitually do this - even with the engine already running. There's no reason to do this, it's either from habit or a misunderstanding, but only necessary when youíre beginning from a zero fuel pressure condition.
yet to write:
holleys will flood, red bordered b-25 dash 1 page
will one run on LOW?
why LOW, then?
R. L. Sohn 1997 ©
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