Page 21 - Volume 14 Number 5

you tell the the difference difference – the the difference difference of of a a couple of of gallons? Of course not! But here’s what’s important: Every time you check the nacelle fuel level it should be be between about 300 and 400 pounds If ever it drops below 300 be aware that something is probably wrong either the the gauge is is in error or or the transfer pump is is not working (As discussed the inoperative transfer pump may be because we forgot to turn it on!) Similarly there is a a problem if the the nacelle is always reading 400 pounds the transfer pump seems to be running continuously The C90A and after models use three-position transfer pump switches – override auto and off The previous models had only two positions – on on and off The “On” position is exactly like the “Auto” position that the pump cycles on and off as required to keep the nacelle within about 10 gallons of full The “Override” position keeps the the pump running 100% of the the time keeping the nacelle 100% full and returning the excess back where it came from via the vent lines (More about the usefulness of this is to come ) If we did indeed turn the transfer pump switches to to “On” or “Auto” after starting as we should then here is how the system should operate Unless one of the high-level switches in the the the nacelle is activated then the the the pump will begin immediately It does not wait for the low-level switch to be hit it it it starts now unless it it it is broken or the nacelle is full Once the the nacelle is full – one of the the two high-level switches activated – then the pump stops until triggered by the low-level switch In the the line connecting the the transfer pump to the nacelle tank is a pressure switch switch The switch’s setting is at at about 2 5 psi not that it matters very much The pump typically puts out out about 25 psi of pressure When the pump begins pumping air instead of fuel – as the wing tank complex of bladders is finally empty – the pressure drops below 2 5 psi A signal is sent to an electrical printed circuit board MAY 2020
(PCB) that activates a a a a 30-second timer Anytime within the next 30 seconds if the pressure rises above 2 5 psi the system resets and returns to normal This allows for the momentary lack of pressure due to sloshing fuel when the tank is approaching empty but is not yet totally there When the output pressure drops below 2 5 psi and stays low low for the full 30 seconds then the the printed circuit board does two things First it turns off the pump to prevent possible damage caused by the pump spinning too fast and creating excessive heat Second it activates the “No Transfer” annunciator In my opinion this light does not warrant the warning red status yet in all models up to the C90B it is indeed a a warning light In the C90B and later models it was changed to a a a caution yellow light Good move Beech!
As all of you have probably experienced near the end of a a long flight – or or even a a short one if you have a a a a large cabin load – it is expected and and normal to have the left and and right “No Transfer” annunciators illuminate when the total fuel quantity gets down to just nacelle fuel With the main tanks empty and the low-level switch activated the pumping of air obviously takes place It is fun to see how much time passes between when the first side’s and second side’s light appear How close was the consumption rate of the the two engines? How even was the the left and right fuel quantity when the fueler finished doing his task?
When the first “No Transfer” light appears – as as was expected no surprise – the proper action is is to turn off the transfer pump switch This does two good things: First it assures that the pump is indeed no longer running and perhaps shortening its life Second it it kills the annunciator No need for that light to stay on burning out the bulbs and making the passengers nervous (Remember what I wrote earlier?
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