Fuel systems

An aircraft's fuel system has to store several metric tons of flammable liquid inside the structure, deliver it to the engines at the right pressure and temperature in every attitude and at every altitude, tell the crew exactly how much is left, and manage where the weight sits as it is burned. It must also never become the source of a fire or explosion. These requirements pull in different directions: the best place to store fuel for weight and space is inside the wing, but the wing is also where lightning strikes, where heat from the sun soaks in, and where bending deflects the tanks in flight. This lesson follows the Swift 900 study's fuel system and the much smaller system of the Tern Block 2's turboshaft.

Tanks and their layout

Most aircraft carry fuel in integral tanks: sealed bays of the wing box itself, bounded by spars, ribs and skins, with sealant on every fastener and joint. They use the structure's volume efficiently and add almost no weight of their own. Bladder tanks, flexible cells inside a cavity, are used where the structure cannot easily be sealed, and some business jets add fuselage tanks to extend range. The Swift 900 study has an integral tank in each wing of about 1950 kg each and a center tank in the wing-to-fuselage box of about 900 kg, for a total of about 4800 kg of usable fuel.

Inside each wing tank, ribs with flapper check valves let fuel flow inboard but not outboard, so that when the aircraft banks or accelerates the fuel near the pumps does not slosh away. The innermost bay, the collector or feed tank, holds enough fuel to keep the pumps covered in any normal attitude and is kept full by ejector pumps that use a small flow of high-pressure fuel to draw fuel in from the rest of the tank. Vent lines connect the tank's air space to the outside through a surge tank near the tip, so that the tank does not burst or collapse as the aircraft climbs and descends; the vent outlet is shaped so that ram air slightly pressurizes the tank in flight.

Feeding the engines

Electric boost pumps in each collector bay deliver fuel to the engine's own engine-driven pump at positive pressure. The engine-driven pump, usually a gear pump, raises it to the high pressure the fuel control and nozzles need. The rules require that an engine keeps running if the boost pumps fail, at least up to a specified altitude, so the engine pump must be able to suction-feed from the tank, which becomes harder at altitude as dissolved air comes out of the fuel and vapor forms at the pump inlet. Engineers check this case with the hottest fuel the aircraft might carry and the lowest tank pressure, since hot fuel at low pressure is most prone to vapor.

A crossfeed valve lets either tank feed either engine, so that a pump failure or an engine shutdown does not strand fuel on one side. Crossfeed is also used to correct an imbalance between the wings when one engine has burned more. Fuel imbalance limits appear in the flight manual because a heavy wing needs constant aileron to hold level and reduces the roll control available, especially at low speed.

Fuel also does work as a coolant. Engine lubricating oil, generator oil and sometimes hydraulic fluid are cooled through heat exchangers in the fuel feed line, warming the fuel on its way to the engine. This is free cooling, but it limits how hot the fuel may become, which matters on long ground waits on a hot day. Fuel temperature is therefore monitored and, on some aircraft, warm fuel is returned to the tanks to mix.

Cold fuel and water

At cruise altitudes the air is around −55 °C, and fuel in thin wing tanks cools steadily during long flights. Jet A has a maximum freezing point of −40 °C and Jet A-1 of −47 °C; in practice the fuel's actual freezing point is measured and the crew monitors fuel temperature, keeping a margin above it by descending or speeding up if needed. Long flights over the rural Alaska and the northern plains on cold winter days are where these limits most often matter for Canadian and northern US operators.

Water is the other enemy. Fuel always contains some dissolved water, which comes out as tiny droplets as the fuel cools and can freeze into ice crystals that block filters. Aircraft fuel systems have heated fuel filters or fuel-oil heat exchangers upstream of the filters, and drain valves at the tank low points that are sampled before flight. Some operators also use fuel system icing inhibitor additives. A widely studied accident in 2008, in which ice that had accumulated in a long-haul airliner's fuel feed system was released during the approach and restricted fuel flow to both engines, led to design changes in fuel-oil heat exchangers and new certification tests.

Center of gravity and the burn sequence

As fuel burns the aircraft's mass and center of gravity change, and the sequence in which tanks are used decides how. Fuel in the wings near the aircraft's center of gravity has little effect; fuel in tanks well forward or aft has a large one. Some long-range aircraft carry fuel in the horizontal tail and move it in flight to keep the center of gravity near the aft limit, which reduces trim drag. The Swift 900 study's tanks are all near the wing, so its center of gravity moves only slightly, but the sequence still matters, for a structural reason.

Marcus Webb's structures group wants the wing tanks kept full as long as possible. Fuel in the wing hangs where the lift is, and its weight relieves the bending at the wing root; burning the wing fuel first leaves the lift acting on an empty wing and raises the root bending moment. The system therefore uses the center tank first. Arjun's program checks that this sequence keeps the center of gravity within the approved range at every stage.

# Swift 900 study: center of gravity as fuel is burned in two sequences, and the fuel quantity
# error budget of the capacitance gauging. Masses and arms are illustrative.
MAC, LEMAC = 2.60, 11.10                     # mean aerodynamic chord and its leading edge station, m
zfw, x_zfw = 9500.0, 11.80                   # zero-fuel mass kg and its CG station m
tanks = {'center': [900.0, 11.20], 'left wing': [1950.0, 11.98], 'right wing': [1950.0, 11.98]}
def cg(fuel):
    m = zfw + sum(fuel.values())
    mx = zfw * x_zfw + sum(fuel[k] * tanks[k][1] for k in fuel)
    return m, 100 * (mx / m - LEMAC) / MAC
def run(order, label):
    fuel = {k: v[0] for k, v in tanks.items()}
    full = sum(fuel.values()); print(label); step = 0
    while sum(fuel.values()) > 300.0:          # keep 300 kg final reserve
        need = min(250.0, sum(fuel.values()) - 300.0)
        for group in order:                     # burn from the first group that still has fuel
            have = sum(fuel[k] for k in group)
            if have > 0:
                take = min(need, have)
                for k in group: fuel[k] -= take * fuel[k] / have
                need -= take
            if need <= 1e-9: break
        step += 1
        if step % 4 == 0 or sum(fuel.values()) <= 300.0:
            m, c = cg(fuel); print(f"   burned {full - sum(fuel.values()):5.0f} kg  mass {m:6.0f} kg  CG {c:5.1f} % MAC")
m, c = cg({k: v[0] for k, v in tanks.items()}); print(f"full fuel: mass {m:.0f} kg, CG {c:.1f} % MAC")
run([['center'], ['left wing', 'right wing']], 'center tank first, then wings together:')
run([['left wing', 'right wing'], ['center']], 'wings first, then center tank:')
print(f"zero fuel: CG {100*(x_zfw-LEMAC)/MAC:.1f} % MAC   (approved range assumed 15 to 33 % MAC)")
# Fuel quantity indication: capacitance probe error budget (illustrative, root-sum-square)
errs = {'probe and wiring': 0.8, 'density compensation': 0.6, 'attitude and wing bending': 1.0, 'unusable fuel model': 0.4}
rss = sum(e * e for e in errs.values()) ** 0.5
print(f"\ngauging error, root-sum-square of {len(errs)} terms: about {rss:.1f} % of indicated quantity")
print(f"on 4800 kg that is about {rss/100*4800:.0f} kg")

full fuel: mass 14300 kg, CG 27.4 % MAC
center tank first, then wings together:
   burned  1000 kg  mass  13300 kg  CG  28.9 % MAC
   burned  2000 kg  mass  12300 kg  CG  28.5 % MAC
   burned  3000 kg  mass  11300 kg  CG  28.0 % MAC
   burned  4000 kg  mass  10300 kg  CG  27.5 % MAC
   burned  4500 kg  mass   9800 kg  CG  27.1 % MAC
wings first, then center tank:
   burned  1000 kg  mass  13300 kg  CG  26.9 % MAC
   burned  2000 kg  mass  12300 kg  CG  26.3 % MAC
   burned  3000 kg  mass  11300 kg  CG  25.6 % MAC
   burned  4000 kg  mass  10300 kg  CG  25.1 % MAC
   burned  4500 kg  mass   9800 kg  CG  26.2 % MAC
zero fuel: CG 26.9 % MAC   (approved range assumed 15 to 33 % MAC)

gauging error, root-sum-square of 4 terms: about 1.5 % of indicated quantity
on 4800 kg that is about 71 kg

With full fuel the center of gravity is at 27.4 percent of the mean aerodynamic chord. Burning the center tank first moves it aft to about 28.9 percent, because the center tank sits slightly forward of the aircraft's center of gravity, then it drifts forward as the wing fuel goes. Burning the wings first moves it forward to about 25 percent. Both sequences stay inside the assumed range of 15 to 33 percent, so the choice can be made on structural grounds, and the center-tank-first sequence wins. The fuel management computer enforces it automatically, and the crew sees the tank quantities and the center of gravity on the systems display.

Measuring the fuel

Most aircraft gauge fuel with capacitance probes: vertical tubes in each tank whose electrical capacitance changes with the fraction immersed in fuel, since fuel and air have different permittivities. A computer combines the readings of many probes with the tank geometry, the aircraft's attitude and the wing's bending, and the fuel's measured density, to compute the mass of fuel, which is what matters for range and center of gravity. Ultrasonic gauging is used on some newer aircraft and avoids electrical circuits inside the tank.

No gauge is perfect, and the flight crew's fuel planning has to allow for its error. Arjun's error budget combines four contributions by root-sum-square, the usual method when the errors are independent: probe and wiring errors, density compensation, attitude and wing bending, and the model of unusable fuel. The total is about 1.5 percent of indicated quantity, about 71 kg on full tanks. This is one reason fuel planning includes reserves well above the gauging error.

Fuel tank safety

An empty-looking fuel tank is not empty: above the liquid is a mixture of air and fuel vapor that can be flammable over a wide range of conditions. A spark, a hot surface or a lightning arc inside a flammable tank can cause an explosion. The 1996 loss of an airliner off New York, attributed to an explosion in a heated center tank, led to far-reaching rules on both preventing ignition sources and reducing flammability. Fuel pumps and probes are now designed so that no single failure can create a spark inside a tank, wiring in tanks is limited and protected, and bonding straps and fasteners are designed so that lightning current cannot arc inside the tank. For transport aircraft with high-flammability tanks, such as heated center tanks, the rules also require flammability reduction, usually by filling the vapor space with nitrogen-enriched air from a small onboard inert gas generating system.

The Tern's turboshaft fuel system is simple by comparison: two wing tanks of about 300 kg each, a collector tank in the fuselage between them, two electric boost pumps and a filter. Its fuel is only for the range extender, which burns about 170 kg per hour at cruise. But the Tern adds a particular risk the Swift 900 does not have: the high-voltage battery and cables. The zonal safety analysis checks that no high-voltage cable runs through a fuel tank or vent, that battery venting cannot reach fuel vents, and that a battery fire could not heat a tank wall. These are the kinds of interactions that the common cause analysis of Lesson 1 exists to find.