Page 23 - Volume 13 Number 12
P. 23

Ask the Expert
The E90 (and A100, B100)
Fuel System
by Tom Clements
  Reader Ron Randall recently sent me an email requesting that I review the E90’s fuel system. Ron has experience operating a King Air 100, a 200 and two E90s. He wrote that he experienced problems with the E90’s fuel system and requested my review/ input. I am happy to oblige.
Step back in time with me to early 1972. The A100 model had just replaced the 100 (“Straight” 100) as the “big” King Air and the PT6A­20­powered C90 was the concurrently produced “small” King Air. The 200­series, 300­series, F90­series, and PT6A­21 powered and ­135A powered C90 versions were all yet to come.
One negative associated with the straight 100 was its fuel capacity ... 374 usable gallons. That was 10 gallons less than the C90 yet its PT6A­28 engines consumed fuel faster than the PT6A­20 engines of the C90! With this in mind, the engineers designed a new fuel system for the A100 that brought its capacity up to 470 gallons, 96 more than its predecessor. Nice! Equally important – at least in my opinion – was that the A100 featured a highly improved fuel system. In fact, with minor modifications and variations, it is the fuel system that exists today in the 200­ and 300­series.
A C90 was taken from the assembly line in early 1972, provided with the same engines and props that had been used on the straight 100, and fitted with a fuel system very similar to that of the A100. It went through a thorough flight­testing program and was certified as the E90. What a nice airplane it has proven to be! It had a 10­year production run – 1972 through 1981. Quite a few of us old­time King Air users wish that the small King Air produced today, in late 2019, were an advanced version of the E90 instead of the advanced C90 version that is available. An E90GTx ... what a cool machine that would be! (Not that the C90GTx isn’t excellent, also. I’ll just have to keep dreaming of an updated E90.)
Let’s return to the fuel system. The additional fuel was gained by adding two additional tanks both of which reside in the wing outboard of where the C90’s fuel tanks ended. One of these tanks – the one that sits in the wing’s leading edge, forward of the main spar – is a bladder tank, similar to all the other tanks. The other new tank is created by sealing the wing skin between the forward and rear spars in the outboard portion of the wing ... an integral, sealed tank.
DECEMBER 2019
Figure 1: Notice that the top of the nacelle is higher than a lot of the wing but is lower than the wing tip.
Before these two new tanks were added, the highest location in the entire fuel system was at the top of the nacelle fuel tank. A filler cap was located there to allow the nacelle tank to be topped with fuel. In the E90 system, however, the additional two tanks – because of the wingspan and dihedral – move the highest location to the wing tip (see Figure 1). The nacelle tank could now be topped merely by topping the cap at the tip and allowing gravity flow to fill all of the other lower tanks, including the nacelle tank.
Recognizing this fact, the designers of the A100 system (remember, that’s the forerunner of the E90 system) eliminated the filler cap atop the nacelle. It was redundant and unneeded. The nacelle structure of the 100­series is slightly different from that of the 90­series since the wheel well must accommodate dual main wheels and tires instead of the single­wheel design of the 90. For the E90, however, the nacelle fuel cap was retained to save the extra manufacturing cost entailed in having separate C90 and E90 nacelle structures. Woe to be the pilot or fueler who fails to read the warning placard and removes the nacelle filler cap when the main tank is full ... the nacelle gets a Jet­A wash and the ramp gets wet! Some E90 operators have installed a twisted piece of safety wire running from screw­to­screw across the nacelle cap. That’s a clever, simple and useful idea.
The C90 fuel system avoids the use of “Main” and “Aux” nomenclatures. Instead, the terms are “Nacelle” and “Wing.” On the other hand, the E90 – and all of the other King Air models that have the wing’s filler cap near the tip – use “Main” and “Aux.”
When the filler cap midway out on the wing is topped on the C90, fuel gravity­flows downhill into all tanks – the three outboard of the nacelle, the nacelle tank itself and the one inboard of the nacelle. The only problem with doing this – as has been implied earlier – is that the nacelle tank does not quite get filled, since its top
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