Bearhawk Fuel System
Related Topics:
The Bearhawk fuel system is relatively simple and was originally designed to be gravity fed to a carburetor. The consequences of uncoordinated flight and its effects on the fuel system are discussed in detail here.
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 time critical.
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 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. Even if in unbalanced flight (with the fuel selector in BOTH), fuel in one tank should be covering the ports and will flow under gravity.
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 (even with only 25 liters remaining in the other tank), with the fuel pump on or off.
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 from that of other Bearhawks particularly if they have a fuel injection return line, or a higher pressure electric fuel pump.
Fuel System Issues
There have been a number of reported fuel system issues observed by Bearhawk owners over the years. It is possible that many of these issues are the result of uncoordinated flight and it's effect on the fuel system, rather than issues of the fuel system itself. This is discussed in detail below, and in the links at the top of this page. Uncoordinated flight effects all aircraft in a similar manner, however the yaw sensitivity of the Bearhawk may mean that the effects are observed in higher frequency on type.
Uneven fuel burn from each tank
This is normally caused by flying out of balance (uncoordinated - ball not centered) and is discussed below.
Balanced Flight
When in flight, it is important to maintain balanced (coordinated flight i.e. ball centered) - at all times because fuel tends to flow in the direction of the ball. This applies when the fuel selector is in BOTH.
If the ball is centered, fuel will flow evenly from each tank down to the pumps. If, for example, 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 (higher head pressure), and as a result when refueling the left tank will take more fuel to refill.
The yaw sensitivity of the Bearhawk may make it more difficult to maintain balanced (coordinated flight - ball centered), particularly for pilots who are new to type.
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. If the ball is not centered, the sight gauges will 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, or cause the pilot to attempt to correct a perceived fuel imbalance that may not exist, (and potentially make it worse).
Air Exercise
To observe the effect of uncoordinated flight on the sight gauges, with wings level and the tanks approximately half full, push on left rudder pedal and observe the ball move to the right side. Then observe the right sight gauge indicating low fuel and the left sight gauge indicating higher fuel. When the ball is centered again both tanks will indicate equal quantities.
Parking on a slope
The same can 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 unintended fuel transfer. Unlike a carburetored engine, a fuel injected engine will not start in the OFF position because there is insufficient pressure. (This assumes no header tank in the system).
Fuel Selector
Most Bearhawks (and many high wing aircraft) have a L - BOTH - R - OFF selector as recommended by the Bearhawk designer. BOTH is the preferred position. This allows fuel from one tank to flood the tank ports even if the opposite side tank becomes unported.
Some Bearhawk builders have installed a fuel selector without a BOTH position in an attempt to prevent uneven fuel burn or transfer between tanks. This may contribute to an issue of fuel starvation if low on fuel and out of balance - with the ball out to the same side as the tank selected. This is discussed HERE.
Safety Bulletin
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.
It is very important when reading the above advice, to be aware that if the aircraft is being flown out of balance, that selecting the tank which indicates (via the sight gauges) it has more fuel in it, could actually be the lowest tank. This is illustrated in the diagrams below.
Selecting either L or R fuel tank (caution)
It may be tempting to select either the L or R tank on the fuel selector in an attempt to prevent or correct uneven fuel burn. However this may lead to another more significant issue when the fuel state is lower. In this case, if flown out of balance with a lower quantity of fuel in tanks, the selected tank may become unported (it happens). The fuel system is designed to be tolerant of this when in the BOTH position, because fuel on the opposite side tank will now be covering the ports and will feed the engine. If only one tank is selected then this is not possible, and fuel starvation may cause the engine to stop.
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 off centered 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.
Sight gauges now indicating same amount of fuel in each tank.
However there is now quite alot less fuel in the left 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.