Flying the Bearhawk - Fuel System and Fuel Management

Bearhawk Fuel System

The Bearhawk fuel system is relatively simple and was originally designed to be gravity fed to a carburetor.

My aircraft has fuel injection which requires an engine driven pump to pressurize the system. This in turn requires a backup electrical fuel pump that is also used for priming. The backup electrical pump is turned on for takeoff and landing, or any time that a failure of the engine driven pump could be catastrophic, or would not leave enough time to find the electric pump switch easily and turn it on. I leave it on after takeoff until at least 1000ft and check the fuel pressure wherever I turn it off.

My engine driven pump typically provides 22psi and the electrical pump provides approximately 28-32psi depending on power output.

Some fuel injection systems return fuel to the left or right tank.

The tanks feed under gravity through the fuel selector and through the gascolator to the fuel pump. This means that the inlet of the fuel pump(s) are flooded in normal flight attitudes with a positive G loading and even if in unbalanced flight temporarily, fuel in one tank should be covering the ports and will flow under gravity.

Fuel Selector

My aircraft and many Bearhawks have a fuel system with a L-BOTH-R-OFF selector. BOTH is the preferred position. This allows fuel from one tank to flood the tank ports even when the opposite side tank becomes unported. However even when operating in the BOTH position it is important to maintain balanced flight for a number of reasons. If the aircraft is flown even slightly out of balance, it will burn more fuel from the side that provides the higher head pressure. This can result in a flow between tanks via the fuel selector.

A Bearhawk Safety Bulletin states:

The Bearhawk Fuel System as shown in the Bearhawk Book is designed for use without a fuel pump. 

If a fuel pump is used, extra care in flying is required so that neither main tanks become unported, as a fuel pump would rather suck air than fuel. 

If one tank is very low and the other is not very low, set the fuel selector on the fullest tank and fly the plan as not to unport that tank. 

My Fuel System Testing

I've tested the fuel system of my own Bearhawk thoroughly, and even with one tank completely empty it still runs well on BOTH when in balanced flight, with the fuel pump on or off. I've also tested it in unbalanced flight with 25 liters remaining in one tank only, and in a full power climb.

However it is unrealistic to think the engine will continue to run in a very low fuel situation when out of balance. For that reason I maintain balanced flight, and I plan to land with 40 liters remaining (20 each side).

My fuel system may differ between other Bearhawk's particularly if they have a fuel injection return line, or a higher pressure electric fuel pump that may empty the gascolator faster, and if out of balance when feeding from only one tank the gascolator may not fill up until returned to balanced flight. For reference, my Electric Fuel Pump provides a maximum fuel system pressure of 32 psi.

Sight Gauges

When checking fuel levels on the ground or inflight via the sight gauges it is important the the wings are level and the ball centered. This applies to all high wing aircraft with sight gauges.

If the ball is not centered, the sight gauges are unreliable and may indicate incorrectly or even the opposite to the actual fuel levels in the tanks. In this manner it may be possible to run a tank completely dry without realizing it until it is nearly dry.

Balanced Flight

When inflight, (as with all all aircraft capable of feeding fuel from BOTH tanks simultaneously) it is very important to maintain balanced (coordinated flight) i.e. ball centered - at all times because fuel tends to flow in the direction of the ball. If the ball is centered, fuel will flow down to the pumps. For example if the ball is out to the right then fuel will have a tendency over time to flow from the left tank to the right tank. The engine will normally run fine, but more fuel will be used from the left tank, and when refueling the left tank will take more fuel to refill. The same will happen if you park on a slope the ball will be towards the downhill side and fuel will flow through the fuel selector to the downhill wing. For this reason I turn the fuel selector out of the BOTH position every time I shutdown to prevent fuel transfer. Unlike a carburetored engine, a fuel injected engine will not start in the OFF position because there is insufficient pressure. This of course assumes no header tank in the system.

Unbalanced Flight and it's effect on fuel tank quantities

To understand the importance of flying in balance and what can happen if the ball remains uncentered for a period of time, follow the diagrams below and look at the sight gauges for each condition.

A normal balanced condition:

• Ball centered.
• Fuel tanks both half full.
• Sight gauges indicate correctly.
• Engine draws equally from both tanks.

Early in the flight if out of balance:

• Ball out to the right.
• Fuel tanks both half full, same quantity each side.
• Left sight gauge indicates more fuel than right tank.
• Engine now tends to draw more fuel from the left tank.
• Fuel may also transfer from left tank to right tank.
• Tendency to select Left tank at this stage to try and correct a perceived fuel imbalance.
• This will accelerate the fuel burn from the left tank, and accelerate the impending fuel imbalance.

Later in the flight:

• Ball still out to the right.
• However there is now quite alot less fuel in the left tank.
• Sight gauges now indicating same amount of fuel in each tank.
• Engine still drawing more fuel from the left tank.
• Tendency to return the fuel selector to BOTH.

Much later in the flight:

• Ball still out to the right.
• SIGNIFICANTLY less fuel in the left tank.
• Sight gauges now showing less fuel in the left tank.
• Engine still drawing more fuel from the left tank.
• Tendency to now select Right tank in an effort to correct the imbalance.

Before landing:

• Ball still out to the right
• Fuel Selector returned to BOTH
• Engine drawing fuel from Left tank until empty
• After landing, when out of balance forces are removed, fuel will flow back through the fuel selector into the empty Left tank and it will again show fuel in it.

After landing when parked and shutdown:

• Ball now centered.
• Fuel selector in BOTH.
• Fuel flowing back through the fuel selector into the Left tank to rebalance the amounts in each tank under force of gravity.
• If wings are level, fuel amount in each tank will eventually be the same.