Aircra& Fuel Systems Chapter 14 FAA Requirements •  The FAA FARs (Federal Avia=on Regula=ons) spell out requirements for fuels systems used on cer=fied aircra& •  The fuel system must ensure adequate fuel flow rate and pressure under every likely opera=on condi=on !
Includes engines and APUs •  EPA regulates some aspects of fuel ven=la=on and dumping !
Goals are to reduce pollu=on hazards FAA Requirements •  Mul=-­‐engine aircra& must have a fuel system configura=on where a failure of a component doesn’t stop more than one engine. •  The pilot must be able to independently shut the fuel off to each engine •  The fuel tank must have at least two vents •  There must be at least ½” of clearance between the fuel tank and the firewall •  The fuel system must be designed to prevent the igni=on of fuel vapor by lightning strikes FAA Requirements •  The unusable fuel supply for each tank must be established It cannot be less than that quan=ty at which the first evidence of malfunc=oning appears !  Also used in weight and balance calcula=ons !
•  The fuel system must be free from vapor lock when using fuel at its cri=cal temperature •  Each tank must be isolated from personnel compartments of the aircra& by a fume-­‐proof and fuel-­‐proof enclosure that is vented and drained to the exterior of the airplane FAA Requirements •  Each fuel tank must have an expansion space of not less than two percent of the tank capacity •  Each tank must have a drainable sump with an effec=ve capacity of 0.25 percent of the tank capacity or 1/16 gallon, whichever is greater !
Place where water and dirt accumula=ons and be drained •  Each fuel tank must allow drainage of any hazardous quan=ty of water from any part of the tank to its sump •  Fuel system components must be bonded and grounded in order to drain off sta=c charge FAA Requirements •  Reciproca=ng engine fuel systems must have a sediment bowl or chamber that is accessible for drainage !
Must be 1 ounce for every 20 gallons of fuel on board •  Fuel filler openings must be marked at or near the filler cover Reciproca=ng engine – “Avgas” and minimum fuel grade !  Turbine engine – “Jet Fuel” !
FAA Requirements •  There must be a fuel strainer for the fuel tank outlet or for the booster pump •  On reciproca=ng engines, one main fuel pump must be engine-­‐driven and there must be at least one for each engine •  Turbine engines require dedicated fuel pumps for each engine •  Fuel systems must be fi_ed with at least one drain to allow safe drainage of the en=re fuel system Fuel Je`soning Systems •  Many aircra& can takeoff with more weight than they can land with Fuel je`soning systems are required on these aircra& !  Federal Avia+on Regula+on Part 23, 25 and CAM 4b !
•  The fuel must discharge clear of any part of the airplane •  The fuel je`son valve must be designed to allow flight personnel to close the valve during any part of the je`soning opera=on Fuel Je`soning Systems •  Each side of the aircra& must have its own independent je`son system !
This enables the pilot to balance the aircra& •  Maintain lateral stability !
Each wing has je`son port with a common manifold connec=ng the tanks •  Boost pumps are used in insure adequate flow •  Je`son systems are designed with limi=ng features, such as dump limit valves or a low-­‐level circuit, to prevent too much fuel loss or fuel dumped at the wrong =me AVGAS •  For reciproca=ng (piston) engines •  100LL (Low Lead) is the only op=on available Blue in color !  Very hard to get outside the USA !
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6 lbs/gallon Usage measured in gallons Low viscosity (watery) High vola=lity •  Easily changes for liquid to vapor •  Remember only vapors burn •  Vapor pressure AVGAS At pressures lower than the vapor pressure, more atoms or molecules of the liquid vaporize and escape from the surface of the liquid. !  Lower vapor pressure than automo+ve gas !
•  Vapor pressure !
AVGAS Lower vapor pressure helps reduce vapor lock •  Vapor lock -­‐ fuel turns into vapors and clogs the fuel system •  A measure of a gasoline's tendency to vapor lock is obtained from the Reid vapor pressure test •  Vapor pressure of 7 psi at 100°F is the maximum allowed for avia+on gasoline. •  Too low a vapor pressure and the fuel will not vaporize to burn •  Pressures at higher al=tudes are lower, thus the need for lower vapor pressure AVGAS •  Detona=on is the rapid, uncontrolled explosion of fuel due to high pressure and temperature in the combus=on chamber Normally the fuel “burns” with a controlled “front” !  Called “knocking” or “pinging” !  Detona=on can destroy an engine in seconds !  The higher the fuel octane or performance number, the more resistant to detona=on !  High compression and turbo charged engines are very suscep=ble to detona=on !
AVGAS •  Octane ra=ngs and performance numbers measure the resistance to detona=on. •  Octane is a comparison to pure iso-­‐octane Modeled using iso-­‐octane (C8H18) and heptane (C7H16) !  100 octane is equivalent to pure iso-­‐octane !  Anything less than 100 octane is equivalent to iso-­‐
octane with some percentage of heptane !
•  Example: 80 octane is equivalent to 80% iso-­‐octane and 20% heptane AVGAS •  Octane is a comparison to pure iso-­‐octane !
Anything higher than 100 octane are rated in performance numbers •  Achieved by adding aroma=c fuels !
Aroma+c fuels require special hoses and seals, deteriorates normal rubber parts •  Used for military and large transport aircra& •  No longer available AVGAS •  A performance number has two parts (e.g. 80/87, 100/130 or 115/145) The higher the number, the higher the resistance to detona=on !  First number is lean fuel-­‐air mixture ra=ng !  Second number is rich fuel-­‐air mixture ra=ng !
•  The required octane/performance number is label next to the fueling port !
Use the labeled value or the next higher AVGAS •  “Lead” – TEL – Tetraethyl lead – added to AVGAS to raise the octane ra=ng Called a “knock inhibitor” !  Helps lubricate valves !  Fouls plugs !  Poisonous !  EPA is trying to stop the use of TEL !
Jet Fuel •  Used for jets/turbine engines !
Use on some new diesel engines •  e.g Cessna’s 182 NXT and Diamond’s DA42 NG •
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Higher viscosity – thicker Similar to kerosene Colorless or straw color Jet A and A-­‐1 the most common !
A-­‐1 has a lower freezing point •  Jet B is Jet A mixed with gasoline to lower freezing point Jet Fuel •  Usage measured in weight (pounds or kilos) •  6.7 lbs/gallons !
Changes with temperature •  More energy per gallon Jet Fuel •  Retains water !
Jet Fuel Water gets suspended into the fuel •  Water problems !
Freezes in cold temperatures •  Fuel an=-­‐icing products, such as Prist, are used !
Microbes grow in the fuel •  Form a bio-­‐film that can clog filters, corrode tank coa=ngs, and degrade the fuel •  Biocides are o&en added to jet fuel to kill microbes that live on hydrocarbons !
Prist is not a biocide Mogas •  Automo=ve gas •  STC enables use in low compression (octane) engines •  Must not contain ethanol Mogas must contains at least 15% ethanol !  Ethanol is hydroscopic !
•  A_racts water !
Ethanol destroys some types of rubber seals •  Rotax engines allow Mogas with up to 10% ethanol •  As higher vapor pressure than AVGAS Simple High Wing Fuel System Vent equalizes tank
pressure to atmospheric
pressure
Vent system design
to prevent one tank
from draining the
other and limit
pressure difference
between tanks
Simple High Wing Fuel System Vent equalizes tank
pressure to atmospheric
pressure
Vent system design
to prevent one tank
from draining the
other
What is the engine driven pump?
Simple Low Wing Fuel System Simple Fuel Injec=on System X
Simple Twin Fuel System Cross Feed
System
What is the engine driven pump?
Reciproca=ng Fuel System •  Fuel delivery Gravity !  Engine driven pump !  Electric auxiliary/boost pump !
•  Require on low/mid-­‐wing •  Require on fuel injec=on •  Fuel selector/cutoff valve(s) !
Cross feed system for mul=-­‐engine systems •  Allow engine to run from one tank or any tank •  Ven=ng system Equalize to outside atmosphere !  Clogged vent can starve engine or collapse fuel tank !
Reciproca=ng Fuel System •  Filter screen •  Primer Carburetor !  Hand pump !  Electric boost pump !
•  Drain/Sump •  Quan=ty indicators !
Flow meters Jet Transport Fuel System •  Mul=-­‐tank Tanks in wing and fuselage !  Must have system to isolate bad tank !  Any engine can be feed by any tank !  All tanks can be used simultaneously !  Outside tank -­‐ Fill last and drain first !
•  Prevents aircra& from =pping over •  Usually has fuel hea=ng system !
Heat comes from oil system or bleed air Jet Transport Fuel System •  Pressure fueling system !
Single point to fuel whole aircra& •  No climbing on top of the wing •  Reduces chances of contamina=on •  Can be done by one person !
Fueling truck pumps fuel into aircra& under pressure •  Set correct pressure for aircra& !
Pressure indicators are used to indicate a tank is full •  Time to switch tanks !
Instruc=onal procedures are normally placarded on the fuel control panel access door •  Types: Fuel Tanks Rigid removable !  Bladder !  Integral !
•  Rigid removable fuel tank Separate units that can be completely removed from the aircra& !  Usually welded or riveted !
•  Older ones are soldered Fuel Tanks •  Bladder fuel tank Flexible fuel “bags” !  Can be patched !  Must be installed flat and without wrinkles !  Must be kept wet !
•  If fuel is drained, coat the inside with engine oil Fuel Tanks •  Integral fuel tank Parts of the structure of the wings or fuselage are sealed with a fuel resistant sealant to form a fuel tank !  “Wet wing” !  Lightest way to store fuel !
Integral Tank Integral Tank Sealing Flapper Valve Acts like a check valve
and allows fuel to flow
only one way
Flapper Valve Used to prevent fuel
from flowing away from
the boost pumps
Fuel Tank Baffling Prevents the fuel sloshing or surging
Fuel Pump •  Boost pumps provides a posi=ve fuel flow from tanks to engine or to engine driven fuel pump Keeps fuel lines full, reduces chances of air bubbles !  Reduce changes of vapor lock by pressurizing fuel system !  Some are used only intermi_ently !
•  Priming/star=ng engine •  Takeoff/landings •  High al=tude or rough air Centrifugal Boost Pump •  Video •  Able to separate the fuel and air/vapors •  Variable displacement !
The amount of fuel delivered is not constant and depends on pressure •  The design limits pressure so regulators are not required !
Doesn’t require a pressure relief valve •  O&en located in the tank Vane-­‐Type Pump •
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Vane-­‐Type Pump Video Video Engine driven fuel pump or boost pump Constant displacement !
Requires a pressure relief valve/pressure regulator •  Establishes output pressure !
Requires a bypass valve •  May contain a pressure compensa=ng diaphragm Vane-­‐Type Pump Vane-­‐Type Pump Vane-­‐Type Pump Fuel Filters/Strainers •  Prevent contaminants/dirt from ge`ng to the engine •  Strainer is a coarse wire mesh that traps large object •  Filter captures very fine par=cles •  Gasolator combines a filter/strainer and a sediment bowl •  The main fuel filter/strainer is located a the lowest point of a fuel system to trap water •  Must have a relief valve to maintain fuel in the event of a filter blockage Gascolator Sump Drain Changeable Filter Element “Washable” Wafer Screen/Filter Carburetor Ice Carb(uretur) Heat Turbine Icing •  Turbine/Jet fuel can trap water At high al=tudes, the temperature lowers and ice forms !  The ice can block the fuel !
•  In addi=on to an=-­‐icing addi=ves, the fuel is heated to melt and prevent ice •  Compressor/bleed air heaters !
Air to fuel heat exchangers •  Lubrica=on oil “coolers” Heat fuel & cools lubrica=on oil !  Fluid to fluid heat exchanger !
Bleed Air Fuel Heater •  Types: Fuel Quan=ty Indicators Sight glass !  Mechanical !
•  A float connected to some indicator Electrical !  Electronic !
•  The indicator should read “zero” when the fuel system reaches unusable fuel level Mechanical Indicator Fuel Quan=ty Indicators •  Electrical and electronic types Transmi_er is located in the fuel tank(s) and sends informa=on through wires !  No mechanical links to indicator !  Indicator can be located almost anywhere !  Can share an indicator !
Fuel Quan=ty Indicators •  Electrical type – as a float moves up and down, it adjusts a variable resistor The float and variable resistor are referred to as the transmi_er !  The change in the variable resister is measured !
Electrical Indicator Fuel Quan=ty Indicators •  Electronic type – also called Capacitance type !
A capacitor is an electronic device that consists of two plates separated by a dielectric (insulator) •  Change the proper=es of the dielectric and the capacitor’s proper=es change •  Changes can be electronically measures !
Fuel and air serve as the dielectric •  As the fuel and air mass changes, the capacitance changes !
FAA says weight instead of volume •  Is not effected by temperature changes !
Colder fuel is more dense and heaver Fuel Quan=ty Indicators •  Electronic type Mul=ple probes can used in a single tank to get a be_er measurement !  No moving parts !  FAA says they incorporate signal amplifiers !
•  Consists of a bridge circuit, an amplifier, an indicator, and a tank unit Electronic Fuel Indicator Drip Fuel Gauge Fuel Flow Indicators •  Measure the flow of fuel •  Types: !
Pressure •  Piston fuel injec=on Vane !  Turbine !
Fuel Injec=on Pressure Gauge Vane Flow Gauge Turbine Flow Gauge Turbine Flow Gauge Fuel Totalizer •  A computer that monitors fuel usage •  Example informa=on: Total fuel used !  Total fuel remaining onboard !  Total range at present airspeed !  Rate of fuel consump=on !
FAA Ques=on •  A digital fuel totalizer automa=cally indicates the amount of fuel used, fuel remaining, current rate of consump=on, and fuel *a. weight, aboard at takeoff. b. remaining at the 45-­‐minute IFR fuel reserve. c. remaining, flight =me at the current power se`ng. FAA Ques=on •  A fuel totalizer is a component which indicates the a. total amount of fuel being consumed by all engines. b. amount of fuel in any given tank. *c. amount of fuel in all tanks. Fuel Temperature Gauge •  Informs the pilot when the fuel is cold enough to start forming ice Fuel Pressure Gauge •  Pressure transducers converts fluid pressure directly through a transmi_er and sends an electric signal to the indicator •  Measures the pressure at the carburetor to indicate the fuel pumps are working •  Differen=al pressure gauges measure the pressure across fuel filters !
If the pressure difference is too high, the filter is clogged •  Pressure sensors measure the flow from a tank !
If the pressure is low, the tank is almost empty and it is =me to switch tanks FAA Ques=on •  As a general rule, which statement is true regarding fuel leaks? a. Stains, seeps, and heavy seeps are not flight hazards. b. All fuel leaks regardless of loca=on or severity are considered a hazard to flight. *c. Stains, seeps, and heavy seeps, (in addi=on to running leaks) are considered flight hazards when located in unvented areas of the aircra&. Troubleshoo=ng •  Following the manufacturer’s recommended procedures !
Use fuel system schema=c •  Look for leaks !
AVGAS leaks leave a blue mark •  Blue stained fuel injector is a sign of a clogged injector Jet fuel leaves an oily film !  Look for dirt s=cking to unusual surfaces !  Look for cracked fuel lines !
Troubleshoo=ng •  Turn the fuel pumps on and pressurize the system Check that there are no leaks !  Check that all the pressures are normal !  Watch if fuel is flowing !
•  Pu`ng the wrong fuel in can require a total system teardown and engine rebuild Repair •  Replace gaskets and seals with the manufacturer’s recommended part Clean/condi=on seat before reinstalling !  Check age !
•  Replace/repair any damage lines !
Look for cracking and wear •  Inspect filter content, may uncover a failing component Tank Repair •  Empty before repairing !
Outside if possible, no fumes inside buildings •  Wet Tank !
Replace rivets and sealant •  Bladder !
Patch or replace Tank Repair •  Welded/Soldered Flush with steam !  Fill with CO2 !  Repair !
•  Clean weld with solu=on of nitric or sulfuric acid !
Inspect repair by pressurizing tank •  Brush a soapy solu=on on the outside and look for leaks FAA Ques=on •  When moving the mixture control on a normally opera=ng engine into the idle cutoff posi=on, engine RPM should *a. slightly increase before the engine starts to die. b. slightly decrease and then drop rapidly. c. remain the same un=l the cutoff is effected, then drop rapidly.