Mike,
thank you for your discussion-input. As we see, the mechanical fuel pump
helps only in an electrical total failure (which will be on an 914-driven
aircraft an "EMERGENCY") and can bring you in this situation - and only in
that - safely to the ground.
To see the fuel pressure of the mechanical pump more exactly, we have to
consider that the pressure would be higher as laid down in the manual,
because there is no open return line back to the tank. This means, the
available power-situation would be better in an emergency.
Regards,
Gottfried
-----Ursprngliche Nachricht-----
Von: owner-europa-list-server@matronics.com
[mailto:owner-europa-list-server@matronics.com] Im Auftrag von MICHAEL
PARKIN
Gesendet: Donnerstag, 23. Februar 2006 23:57
An: europa-list@matronics.com
Betreff: Re: Europa-List: 914 Fuel Pumps.
--> <mikenjulie.parkin@btopenworld.com>
> At Take Off the engine produce an manifold pressure of 1350 hPa = 39,9
> in.Hg = 19,6 psi
>
> That means, you need a max. Fuel pressure at the carburetor of 24,7
> psi!!!!!!
> The mechanical pump delivers 0,15 bar up to 0,4 bar(5,8 psi). Do you
> see that difference?!
>
Gottfried,
But that 24,7 psi includes the local atmospheric pressure. Consider
pressure at sea level on an ISA day. If the atmospheric pressure was 1013
hpa, then the fuel pump pressure required would be (1350+250)-1013 = 587 hpa
(8.5psi) this as you correctly say is beyond the capability of the
mechanical fuel pump. (Ref - Maintenance Manual p34). However, that is a
max power calculation.
If one considers the max continuous condition as quoted in the operating
manual - 5000 rpm, 31 in/hg, the situation with just the mechanical pump
would be considerably better and certainly sufficient to continue to a
diversion without a problem - all be it at a fairly low altitude.
Example:
Condition - 75% power, 5000 rpm, 31 in/hg. Air box pressure = 1049 hpa.
therefore fuel pump pressure required at 1000 ft (ISA day) - (ambient
pressure = 976 hpa)
(1049+250)-976 = 324 hpa which is 4.7 psi.
Using the minimum pressure required of 0,15 bar a calculated fuel pressure
required from the mechanical pump would be 3.2 psi.
Plainly, pressure altitude is the driving factor here.
kind regards,
Mike (G-JULZ)
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