The E90 (and A100, B100) Fuel System

The E90 (and A100, B100) Fuel System

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 PT6A-20-powered C90 was the concurrently produced “small” King Air. The 200-series, 300-series, F90-series, and PT6A-21 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 PT6A-28 engines consumed fuel faster than the PT6A-20 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 300-series.

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 flight-testing program and was certified as the E90. What a nice airplane it has proven to be! It had a 10-year production run – 1972 through 1981. Quite a few of us old-time 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.

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.

Figure 1: Notice that the top of the nacelle is higher than a lot of the wing but is lower than the wing tip.


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 100-series is slightly different from that of the 90-series since the wheel well must accommodate dual main wheels and tires instead of the single-wheel 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 Jet-A wash and the ramp gets wet! Some E90 operators have installed a twisted piece of safety wire running from screw-to-screw 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 gravity-flows 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 is higher than the wing’s filler cap. So why do we even have a filler cap on the wing? Why not just fill the highest spot atop the nacelle?

Here’s the answer: All fuel the engine consumes comes from the nacelle tank only. All other fuel is useless until it gets transferred into the nacelle, so once it gets there the designers don’t want it to be able to easily escape. Thus, a flapper-type of check valve permits fuel to flow to the nacelle but prevents it from easily flowing from the nacelle back into the wing. It would take hours and hours and hours to patiently fill the wing tanks from the nacelle cap.

When the word “Main” is applied to the E90-style fuel tank system, it includes all fuel in the wing tanks outboard of the nacelle as well as the nacelle tank itself. Why is the bladder tank in the wing’s center section – between the nacelle and the fuselage – not included? Because it includes a portion that is too low to gravity-flow into the nacelle. Liquid doesn’t want to flow uphill, right? Hence there must be a system designed to push or pull this fuel uphill to make it all usable to the engine by getting it into the nacelle tank. Hence, the bladder in the wing center section is the “Aux” tank, it has its own filler on top, and the fuel it contains is useless until it gets transferred into the nacelle portion of the Main complex of tanks. For the E90, the Main holds 196 gallons per side of usable fuel and the Aux holds 41 gallons … a total for both sides is 474 usable gallons, 90 more than the C90’s 384.

In the C90, transferring fuel from the wing tanks to the nacelle tank can be done by two methods. First, gravity-flow works well but the last 28 gallons (about 200 pounds) cannot be transferred by gravity flow … it would need to flow uphill. Second, a submerged electric pump – located in the wing’s center section tank, the lowest spot of the wing tanks – cycles on and off as required to keep the nacelle within about 10 gallons of full. So long as the transfer pump is working properly – as it usually is – then no gravity-flow is necessary and none takes place. Although the electric transfer pumps in the C90 have proven to be quite reliable, when and if they fail then the airplane loses 28 gallons of usable fuel on that side.

The transfer system was simplified and made more reliable on the A100 and similar, later designs. A jet transfer pump is now used. The line taking fuel from the boost pump toward the engine-driven, high-pressure fuel pump has a tap-off that sends some of the boost pump’s discharge to the jet pump. What a simple device! It is merely a venturi with no moving parts. Bernoulli’s principal comes into action causing the fuel pressure to be reduced as the fuel’s speed accelerates due to its need to squeeze through the venturi’s throat. This fuel from the boost pump is what causes or motivates the venturi to create suction that can pull the fuel from the aux tank. That explains why the fuel from the boost pump that flows to the jet pump is called “motive flow” since the venturi is not motivated to create suction until flow passes through it. A normally-closed (N.C.) solenoid valve – the Motive Flow Valve – is the only moving part of this transfer system. Whenever fuel is available downstream of the boost pump, the simple action of energizing the Motive Flow Valve to its open position – by moving the Aux Transfer switch from Off to On – begins the fuel transfer action. Simple. Reliable. Nearly fool proof.

In my opinion, there is an even bigger improvement in this newer fuel system than the simplicity and reliability of the transfer system … it is the fact that an engine-driven mechanical boost pump was added to both left and right engines. The drive pad on the aft accessory case of the PT6 to which the boost pump may be installed has always existed. Yet, strangely, not until the A100 – and then the E90 and others – was it ever utilized! What a waste! (Are you reading this, you great King Air modifiers? I still think there would be a market for an STC that would allow an engine-driven boost pump to be installed on A90s, B90s and the C90-series.) Unlike the earlier fuel system, no longer is it SOP (Standard Operating Practice) to operate with the electric boost pump on at all times for the purpose of cavitation avoidance. Instead, the electric pump becomes a Standby Boost Pump, rarely needed at all. How nice!

When I spoke briefly to Mr. Randall on the telephone, he confirmed what I had suspected was his complaint about his E90’s fuel system: Sometimes it vented copious amounts of fuel onto the ramp out of the heated fuel vent under the wing outboard of the nacelle. This only happened following short flight with all tanks topped before departure.

I am certain that many of you have read previous articles I have written about this problem, especially as it applies to the F90-, 200-, and 300-series, the ones in which fuel transfer is automated. For these models, I suggest that SOP be to operate with the left and right Aux Transfer circuit breakers pulled, only pushing them in if the aux tanks contain fuel and only when the mains tanks are at least 200 pounds less than full. For the E90, A100, and B100 the solution to the venting problem is very similar: Simply do not turn on the Aux Transfer switches, left and right, until the main tanks are down a bit … like at Top of Climb, after reaching cruise speed.

You see, the transfer system sends fuel into the Main tank faster than the engine is burning fuel from the Main tank. The net increase in Main tank fuel quantity causes a pressure buildup in the Main tank. In theory, the pressure should be prevented from reaching an excessive level by sending some of the fuel through a pressure relief valve and into the vent line from the Main tank back into the Aux tank. Almost always, however, some of that fuel, instead of returning to the Aux, vents overboard. How much? Not much. No one knows with certainty but my educated guess is that less than a gallon or so per side would be vented during the time it takes for the Auxes to empty. Unless someone were flying in close formation with you, the minor amount of fuel streaming out of the heated vents will likely go unnoticed. But in the rather rare case in which you land before the Aux tanks are empty? Now that dripping fuel will be very obvious, perhaps causing the hazmat folks to pay you a nasty visit.

A common response is to open the filler cap at the wing tip to relieve the tank’s pressure. Yes, that is effective … but comes with the nasty downside of gushing fuel out of the filler onto the wing, onto yourself or the lineperson, and onto the ramp! A better way? Leave the transfer switches off while running the engine for 5 to 10 minutes. That will take fuel from the main tank without it being replenished, thus eliminating the
over-pressurized condition and stopping the venting.

However, the very best idea is to avoid pressurizing the Main tank in the first place! Simply don’t transfer until there is some space available in the Main tank. Leave the left and right Aux Transfer switches off until the Main quantity is 1,100 pounds or less. (Full is about 1,300 pounds per side.)

You are correct that the factory procedure is to fill the Aux tanks last and to use them first. This is to avoid excessive wing bending stress under the worst possible combination of wing loading and fuel/payload distribution. Friends, if you ever experience spine-crushing positive G-forces while at the same time with every cabin seat full, lots of heavy baggage, full Aux tanks and almost empty Mains, I’d fret a bit. But waiting to transfer until the Mains are down about 15%? I suggest losing no sleep over that one!

Another problem that sometimes crops up with the E90-style fuel system is the migration of fuel from the Main tank into the Aux tank. Suppose that you had the line person “fill the tips” but ignore the inboards … a very common fuel order when your payload does not allow for full fuel. The next morning when you show up and start checking the plane you find that one side – say, the left – is showing 1,300 pounds in its Main and 0 pounds in its Aux … just as you anticipated. However, the right side is showing 1,100 in the Main and 200 in the Aux. How did that happen?!

There is a vent line connecting the top of the nacelle tank to the center-section (the Aux) tank (Figure 2 shows the line). This line has three parallel paths that connect it to the nacelle: (1) A float-operated valve that should open only when the nacelle’s fuel level starts dropping; (2) A vacuum relief valve that will be sucked open to allow air to enter the tank in the event that the float-operated valve sticks in the closed position; and (3) A pressure relief valve to relieve the pressure buildup caused by thermal expansion or by fuel transfer. If any of these three connections is defective such that it allows fuel to leak past it at all times, then fuel in the nacelle portion of the Main tank will find its way downhill into the Aux. It’s time for maintenance to remove the access cover on top of the nacelle and check the three valves underneath.

Figure 2: The E90 Fuel Schematic from the POM. The Main tank includes the nacelle tank and all five tanks in the outboard wing. The Aux tank is the single inboard tank.

It is common that the fuel migration will cease – because the level in the Main complex of interconnected tanks has reached the level of the nacelle portion’s top – before the Aux tank overflows and starts venting onto the ramp or hangar floor. On the other hand, if the Aux was already filled when this leakage began … expect the angry call from the FBO about the mess your airplane is causing!

There is one other path by which fuel can migrate from Main to Aux: Via the jet pump, through a defective check valve in the line going from the Aux tank to the jet pump. This situation is worse than the leaky valve(s) at the top of the nacelle because more fuel can migrate than the Aux tank can hold! Both the jet pump and the check valve are quite easily accessible in the main wheel well. (The previous sentence is not true for the 200- and 300-series since their jet pumps are located inside of their Aux tanks.) I am happy to report that the problem of fuel migration from Main into Aux is not a common occurrence.

In conclusion, the addition of the engine-driven boost pump, the simplified and more reliable fuel transfer system, the extra fuel capacity … the E90-style fuel system, in my mind, is a real winner that is highly improved over its C90-style predecessor. Its only downside is the tendency to over-pressurize the main tanks and vent a little fuel overboard when one begins Aux fuel transfer while the Mains are already full. Avoid that and you, too, will find this system nearly perfect!

About the Author