Delivering uninterrupted fuel to the engine under all conditions of engine power, altitude, attitude, and during all flight maneuvers requires resilient aviation fuel systems
Pilot Handbook of Aeronautical Knowledge, Gravity Feed System
The gravity-feed system utilizes the force of gravity to transfer the fuel from the tanks to the engine [Figure 1]
For example, on high-wing airplanes, the fuel tanks are installed in the wings, and the fuel is gravity fed through the system and into the carburetor
Fuel-Pump System:
Pilot Handbook of Aeronautical Knowledge, Fuel Pump System
If the design of the aircraft is such that gravity cannot be used to transfer fuel, fuel pumps are installed [Figure 2]
For example, on low-wing airplanes, the fuel tanks in the wings are located below the carburetor requiring a pump
Aircraft with fuel-pump systems have two fuel pumps
The main pump system is engine driven with an electrically driven auxiliary pump provided for use in engine starting and in the event the engine pump fails
The auxiliary pump, also known as a boost pump, provides added reliability to the fuel system and is electrically driven by a switch in the cockpit
Volatility & Flash Point:
Volatility: a fuel's tendency to vaporize
A more highly volatile fuel will vaporize at lower temperatures than fuel with lower volatility rating
An increase in temperature increases volatility, forming more vapors
When enough vapors are formed so fuel will ignite readily, that is the flash-point
Flash Point: the lowest temperature at which a combustible liquid (fuel) gives off vapors in sufficient quantity to ignite with the application of flame
Fuel System Design:
The fuel system consists of component, controls, and indicators
Fuel System Components:
Fuel Tanks:
The fuel tanks, normally located inside the wings of an airplane, vary in construction by aircraft, but generally use aluminum, a "wet-wing," or flexible bladders
Tanks will have a filler opening on top of the wing through which they can be filled
The tanks are vented to the outside to maintain atmospheric pressure inside the tank through the filler cap or through a tube extending through the surface of the wing
Fuel tanks also include an overflow drain that may stand alone or be collocated with the fuel tank vent
This allows fuel to expand with increases in temperature without damage to the tank itself
If the tanks have been filled on a hot day, it is not unusual to see fuel coming from the overflow drain
Fuel/Oil coolers function by exchanging the hot oil against cold fuel for cooler oil against now warmer fuel
Fuel Strainers, Sumps, and Drains:
Amazon, Fuel Sump
After leaving the fuel tank and before it enters the carburetor, the fuel passes through a Fuel Strainer [Amazon] which removes any moisture and other sediments in the system
Since these contaminants are heavier than aviation fuel, they settle in a sump at the bottom of the strainer assembly
A sump is a low point in a fuel system and/or fuel tank
Fuel samples should be drained and checked visually for water and contaminants
Water in the sump is hazardous because in cold weather the water can freeze and block fuel lines
In warm weather, it can flow into the carburetor and stop the engine
If water is present in the sump, more water in the fuel tanks is probable and they should be drained until there is no evidence of water
Never take off until all water and contaminants have been removed from the engine fuel system
Because of the variation in fuel systems, become thoroughly familiar with the systems that apply to the aircraft being flown
Consult the AFM/POH for specific operating procedures
Fuel System Controls:
Fuel Primer:
Both gravity-feed and fuel-pump systems may incorporate a fuel primer into the system
The fuel primer is used to draw fuel from the tanks to vaporize fuel directly into the cylinders prior to starting the engine
During cold weather, when engines are difficult to start, the fuel primer helps because there is not enough heat available to vaporize the fuel in the carburetor
It is important to lock the primer in place when it is not in use
If the knob is free to move, it may vibrate out during flight and can cause an excessively rich mixture
To avoid over-priming, read the priming instructions for the aircraft
Failure to prime correctly can result in engine fires
Fuel Selectors:
Pilot Handbook of Aeronautical Knowledge, Fuel Selector Valve
The fuel selector valve allows selection of fuel from various tanks [Figure 3]
A common type of selector valve contains four positions:
LEFT
RIGHT
BOTH
OFF
Selecting the LEFT or RIGHT position allows fuel to feed only from that tank, while selecting the BOTH position feeds fuel from both tanks
Varying the position may be used to balance the amount of fuel remaining in each wing tank
Fuel placards show limitations on fuel tank usage, such as "level flight only" and/or "both" for landings and takeoffs
It is important pilots remember to switch tanks if not on "BOTH"
While language mentions "Each fuel quantity indicator must be calibrated to read "zero" during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply," their accuracy (not calibration) is required to be accurate at any level so as to meet FAR 91.205
If a fuel pump is installed in the fuel system, a fuel pressure gauge is also included
The normal operating pressure can be found in the AFM/POH or on the gauge by color coding
Fuel Flow Gauges:
Fuel flow gauges indicate the amount of fuel flowing through the system
Aviation Fuel Grades:
Pilot Handbook of Aeronautical Knowledge, Aviation Fuel Color-Coding System
Aviation gasoline (AVGAS) is identified by an octane or performance number (grade), which designates the antiknock value or knock resistance of the fuel mixture in the engine cylinder [Figure 4]
The higher the grade of gasoline, the more pressure the fuel can withstand without detonating
Lower grades of fuel are used in lower-compression engines because these fuels ignite at a lower temperature
Higher grades are used in higher-compression engines, because they ignite at higher temperatures, but not prematurely
The proper fuel grade is stated in the AFM/POH, on placards in the flight deck, and next to the filler caps
If the proper grade of fuel is not available, use the next higher (never the lower!) grade as a substitute
This can cause the cylinder head temperature and engine oil temperature to exceed their normal operating ranges, which may result in detonation
Care must be exercised to ensure that the correct aviation grade is being used for the specific type of engine
Auto gas should NEVER be used in aircraft engines unless the aircraft has been modified with a Supplemental Type Certificate (STC) issued by the Federal Aviation Administration (FAA)
The current method identifies AVGAS for aircraft with reciprocating engines by the octane and performance number, along with the abbreviation AVGAS
Although AVGAS 100LL performs the same as grade 100, the "LL" indicates it has a low lead content
Fuel for aircraft with turbine engines is classified as JET A, JET A-1, and JET B
Jet fuel is basically kerosene and has a distinctive kerosene smell
Since use of the correct fuel is critical, dyes are added to help identify the type and grade of fuel
In addition to the color of the fuel itself, the color-coding system extends to decals and various airport fuel handling equipment
For example, all AVGAS is identified by name, using white letters on a red background
In contrast, turbine fuels are identified by white letters on a black background
Pilot Handbook of Aeronautical Knowledge, Aviation Fuel Color-Coding System
Refueling Procedures:
Fuel Cap Unsecured
Nylon, Dacron, or wool clothing is especially prone to accumulate and discharge static electricity from the person to the funnel or nozzle
To guard against the possibility of static electricity igniting fuel fumes, a ground wire should be attached to the aircraft before the fuel cap is removed from the tank
Because both the aircraft and refueler have different static charges, bonding both components to each other is critical
This means by contacting metal to metal, the static differential charge is equalized
The refueling nozzle should be bonded to the aircraft before refueling begins and should remain bonded throughout the refueling process
When a fuel truck is used, it should be grounded prior to the fuel nozzle contacting the aircraft
If fueling from drums or cans is necessary, proper bonding and grounding connections are important
Drums should be placed near grounding posts and the following sequence of connections observed:
Drum to ground
Ground to aircraft
Bond drum to aircraft or nozzle to aircraft before the fuel cap is removed
When disconnecting, reverse the order
The passage of fuel through a chamois increases the charge of static electricity and the danger of sparks
The aircraft must be properly grounded and the nozzle, chamois filter, and funnel bonded to the aircraft
If a can is used, it should be connected to either the grounding post or the funnel
Under no circumstances should a plastic bucket or similar non-conductive container be used in this operation
Some sort of eye protection should be worn and refueling performed in a well ventilated area to avoid bodily harm
Static electricity is formed by the friction of air passing over the surfaces of an aircraft in flight and by the flow of fuel through the hose and nozzle during refueling
Static electricity discharge poses a safety risk when refueling, resulting in the installation of static wicks and use of grounding wires when refueling
Grounding cable should be attached to an unpainted, non-corroded surface of the airplane
If not specified by the manufacturer, the exhaust stack or tie downs may serve this purpose well
Ground yourself as well by holding the wire with bare hands to avoid being a static course
Even the fuel nozzle can carry static electricity - keep it in contact with the metal inside the fuel port to maintain it's ground as well
Fuel Contamination:
Accidents attributed to powerplant failure from fuel contamination have often been traced to:
Inadequate preflight inspection by the pilot
Servicing aircraft with improperly filtered/mixed fuel from small tanks or drums
Storing aircraft with partially filled fuel tanks
Lack of proper maintenance
Types of contaminate:
Chemical: usually resulting from inadvertent mixing of petroleum products, this type affects the chemical and physical properties of the fuel and can only be detected by specific laboratory tests
Material (particulate): generally consists of water, microbiological growth, and particulate matter which can usually be detected visually
Microbiological: consists of living organisms that grow at the fuel-water interface
Fuel should be drained from the fuel strainer quick drain and from each fuel tank sump into a transparent container, and then checked for dirt and water
Dirt should be easy to see floating in the sample but water, since it is more dense, will sink to the bottom
Water will be clear against the AVGAS color you are using
The mixing of different fuel types may not be apparent by look or smell, but should be considered
When the fuel strainer is being drained, water in the tank may not appear until all the fuel has been drained from the lines leading to the tank
This indicates that water remains in the tank, and is not forcing the fuel out of the fuel lines leading to the fuel strainer
Therefore, drain enough fuel from the fuel strainer to be certain that fuel is being drained from the tank
The amount will depend on the length of fuel line from the tank to the drain
If water or other contaminants are found in the first sample, drain further samples until no trace appears
Water may also remain in the fuel tanks after the drainage from the fuel strainer has ceased to show any trace of water
This residual water can be removed only by draining the fuel tank sump drains
Water is the principal fuel contaminant
Suspended water droplets in the fuel can be identified by a cloudy appearance of the fuel, or by the clear separation of water from the colored fuel, which occurs after the water has settled to the bottom of the tank
As a safety measure, the fuel sumps should be drained before every flight during the preflight inspection
Fuel tanks should be filled after each flight or after the last flight of the day to prevent moisture condensation within the tank
To prevent fuel contamination, avoid refueling from cans and drums
In remote areas or in emergency situations, there may be no alternative to refueling from sources with inadequate anti-contamination systems
While a chamois skin and funnel may be the only possible means of filtering fuel, using them is hazardous
Remember, the use of a chamois will not always ensure decontaminated fuel
Worn-out chamois will not filter water; neither will a new, clean chamois that is already water-wet or damp
Most imitation chamois skins will not filter water
Color Identification:
Note that water and/or Jet A fuel mixed with the common 100LL will still appear blue, albeit not as blue
Water will separate from fuel however, unlike Jet A which will remain mixed
Smell:
Jet A Smells like kerosene
Touch:
Pour fuel on a white paper towel
If the fuel evaporates quickly, leaves a slight blue tint, and is dry to the touch, those are good signs that it's 100LL
If jet fuel is present, it will leave an oily sheen and it won't evaporate quickly
Gascolators may be used to internally filter water and debris prior to fuel routing to the engine for combustion
Aviation Fuel Anomalies and Malfunctions:
Fuel Imbalance:
Many aircraft are equipped with a fuel selector which allows you to select which tank, or both, from which to draw fuel
Aircraft can at times develop a fuel imbalance from various sources:
Prolonged turns in the same direction
Mechanical reasons
If a fuel imbalance occurs, select the appropriate (fullest tank) to even out the fuel levels
System Failures:
Running a tank completely dry may allow air to enter the fuel system and cause vapor lock, which makes it difficult to restart the engine
Vapor Lock: on fuel-injected engines, the fuel becomes so hot it vaporizes in the fuel line, not allowing fuel to reach the cylinders
Loss of Fuel Pressure:
Loss of fuel pressure can be caused by malfunctioning/failed pumps or cavitation
Fuel Leaks:
Fuel leave severity will dictate the response required
Aside from running out of gas (a common accident casual factor), fuel leaks can lead to inflight fires
Managing Fuel Anomalies/Emergencies:
Fuel Advisories:
Minimum Fuel:
Pilot:
Advise ATC of your minimum fuel status when your fuel supply has reached a state where, upon reaching destination, you cannot accept any undue delay
Be aware this is not an emergency situation, but merely an advisory that indicates an emergency situation is possible should any undue delay occur
On initial contact the term "minimum fuel" should be used after stating call sign
"Salt Lake Approach, United 621, minimum fuel"
Be aware a minimum fuel advisory does not imply a need for traffic priority
If the remaining usable fuel supply suggests the need for traffic priority to ensure a safe landing, you should declare an emergency due to low fuel and report fuel remaining in minutes
Fuel Remaining: A phrase used by either pilots or controllers when relating to the fuel remaining on board until actual fuel exhaustion. When transmitting such information in response to either a controller question or pilot initiated cautionary advisory to air traffic control, pilots will state the APPROXIMATE NUMBER OF MINUTES the flight can continue with the fuel remaining. All reserve fuel SHOULD BE INCLUDED in the time stated, as should an allowance for established fuel gauge system error
Controller:
When an aircraft declares a state of minimum fuel, relay this information to the facility to whom control jurisdiction is transferred
Be alert for any occurrence which might delay the aircraft
Emergency Fuel:
Emergency fuel is a declaration of emergency
Emergency fuel is asking the controller for priority, going direct to the nearest airfield to land immediately
Common Training Aircraft Fuel System Characteristics:
Cessna 172:
Two tanks hold 100LL aviation fuel (AVGAS)
Each tank holds 28 gallons for a total of 56 gallons of fuel
Each tank has 1.5 gallons of unusable fuel for 3 gallons total unusable
Total usable fuel is therefore 26.5 gallons per tank or 53 gallons total
Each fuel tank is vented through individual vents
"Fueling to the "tabs" equates to about 17.5 gallons in each tank
Fuel is measured by 2 float type transmitters
Fuel pressure is measured through a transducer
Piper Arrow:
Two tanks hold 100LL aviation fuel (AVGAS)
Each tank holds 38.5 gallons for a total of 77 gallons of fuel
Each tank has 2.5 gallons of unusable fuel for 5 gallons total unusable
Total usable fuel is therefore 36 gallons per tank or 72 gallons total
Each fuel tank is vented through individual vents
Fueling to the "tabs" equates to about 25 gallons in each tank
Tanks are integral
Separate fuel quantity gauges exist for each tank
Fuel selector can be set to OFF, LEFT TANK, or RIGHT TANK
Private Pilot (Airplane) Operation of Aircraft Systems Airman Certification Standards:
Objective: To determine the applicant exhibits satisfactory knowledge, risk management, and skills associated with safe operation of systems on the airplane provided for the flight test.
Private Pilot (Airplane) Operation of Aircraft Systems Risk Management:
The applicant is able to identify, assess, and mitigate risk associated with:
PA.I.G.R1:
Detection of system malfunctions or failures
PA.I.G.R2:
Management of a system failure
PA.I.G.R3:
Monitoring and management of automated systems
Private Pilot (Airplane) Operation of Aircraft Systems Skills:
The applicant exhibits the skill to:
PA.I.G.S1:
Operate at least three of the systems listed in K1a through K1l appropriately
PA.I.G.S2:
Complete the appropriate checklist(s)
Private Pilot (Airplane) Systems and Equipment Malfunctions Airman Certification Standards:
Objective: To determine the applicant exhibits satisfactory knowledge, risk management, and skills associated with system and equipment malfunctions appropriate to the airplane provided for the practical test
Causes and remedies for smoke or fire onboard the aircraft
PA.IX.C.K4:
Any other system specific to the airplane (e.g., supplemental oxygen, deicing)
PA.IX.C.K5:
Inadvertent door or window opening
Private Pilot (Airplane) Systems and Equipment Malfunctions Risk Management:
The applicant is able to identify, assess, and mitigate risk associated with:
PA.IX.C.R1:
Checklist usage for a system or equipment malfunction
PA.IX.C.R2:
Distractions, task prioritization, loss of situational awareness, or disorientation
PA.IX.C.R3:
Undesired aircraft state
PA.IX.C.R4:
Startle response
Private Pilot (Airplane) Systems and Equipment Malfunctions Skills:
The applicant exhibits the skill to:
PA.IX.C.S1:
Describe appropriate action for simulated emergencies specified by the evaluator, from at least three of the elements or sub-elements listed in K1 through K5 above
PA.IX.C.S2:
Complete the appropriate checklist(s)
Aviation Fuel Case Studies:
NTSB Identification: ANC14CA014: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot's failure to adequately remove water-contaminated fuel during the preflight inspection, which resulted in a total loss of engine power
NTSB Identification: CEN13LA354: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot's failure to identify the water contamination of the fuel system during his preflight inspection, which resulted in a total loss of engine power during the airplane's initial climb
NTSB Identification: ATL03FA133: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot's failure to maintain control of the airplane during a VFR pattern for a precautionary landing, which resulted in an uncontrolled descent and subsequent collision with terrain. Also causal was the pilot's inadequate preflight inspection of the aircraft, which resulted in his failure to secure the fuel cap
Safety is always a consideration and the handling of fuel is an especially regulated operation
It is advisable that the pilot remove all passengers from aircraft during fueling operations and witness the refueling to ensure that the correct fuel and quantity is dispensed into the airplane and that any caps and cowls are properly secured after refueling
