Each year a number of unnecessary aircraft accidents are related to fuel starvation, exhaustion or contamination. Most of them were pilot errors and could have been avoided by proper training.
There were numerous reasons for these avoidable accidents: ranging from inadequate fuel systems knowledge by the crew, preflight planning issues, takeoff and landing checks and failing to monitor consumption during flight.
Or even failing to refuel the correct quantity before the flight due to differences in systems used for indicating the amount of fuel, which has caused some aircraft to make an unexpected glide in approach and landing.
During basic flight training or conversion to a new type of aircraft for an aircraft rating the pilot must become intimately familiar with the fuel system, amount and content of the tanks, valves and unusable fuel and consumption of the engine(s) during different flight regimes.
On these pages we will discuss basic procedures to follow when gaining insight in the fuel system and properly managing it during the flight, thereby assuring a safe outcome of the flight.
Fuel Systems Knowledge
There is a difference between exhaustion and starvation: exhaustion means no more fuel onboard the aircraft and starvation means fuel onboard but not reaching the engine for some reason. This could be a closed valve, blocked line or even a leak causing an apparent high consumption.
Some aircraft (C-150, C-152) have two wing tanks but one on/off valve. Fuel should be drawn from both tanks but due to turbulence, flying out of balance, the level will not remain equal in both the tanks.
Contamination is just that: dirty fuel and this could clog filters causing starvation and an engine unwilling to run. Draining fuel during preflight should catch that before it becomes an issue.
Unfamiliarity with the fuel system can (and has) lead to accidents. It is very important that the pilot receives thorough training on the fuel systems of the aircraft they fly.
Some countries conduct an aircraft type rating issue which is then added to the license of the pilot showing the types of aircraft they are licensed to fly. During training for the type rating the aircraft manual must be studied with close attention to:
Fuel grade, system capacity and the usable and unusable quantities
The number of fuel tank/system drain points and quantity measuring procedures
Dipstick, if any, calibrated and marked for the aircraft
Fuel selector operation and markings, cross-feeding procedures
Fuel pump operation, normal pressure and flow indications
Heat exchanger, if any, to combat fuel ice in cold circumstances
Leaning procedures and flow at different manifold/RPM combinations and altitudes
Manifold air pressure (MAP) and RPM for maximum range and endurance
Consumption of any Janitrol cabin heaters
Engine emergency checklists and drills
Procedures for flying with various configurations and fuel loads
Make sure that the person giving this instruction is qualified and has the experience on the type of aircraft, preferably with a current type rating.
If you are not current in a particular type of aircraft that you fly regularly reread the aircraft flight manual and re-familiarize yourself with special attention to the fuel system.
Keep in mind that although an aircraft can carry lots of fuel, you need to make sure that the maximum take-off weight is not exceeded. For example: the DynAero MCR-01, some models can carry up to 150 liters with long range tanks, doing that will result in not being able to take a passenger along for the flight. But then you could fly for more than six hours or so. Alone.
Each year a number of aircraft accidents are related to fuel starvation, exhaustion or contamination. There were numerous reasons for these avoidable accidents: ranging from inadequate fuel systems knowledge by the crew, preflight planning issues, takeoff and landing checks and failing to monitor fuel consumption during flight.
During the preflight phase the pilot must calculate the amount of fuel necessary and comply with legal requirements as minimum reserve during day or night and the weight of the fuel with regard to the aircraft.
Planning For Fuel
Calculating fuel requirements for a flight includes a number of variables, often beyond the control of the pilot (think ATC delays or more wind than anticipated). Taking off with just the bare minimum legal fuel is foolish. It is much better to either remove some weight and add fuel or to plan for a fuel stop along the flight. The amount of fuel onboard should never become a concern.
Warm-up, Taxi and Climb fuel
After calculating the required fuel for the trip an allowance should be made for fuel required for warm-up and taxi. In cold weather warm-up can be much longer than expected as can the taxi times at large and busy airports. Add 15 minutes of cruise fuel for this. For climb out fuel, you will have to work out how long the climb will be to the planned altitude and multiply that with the fuel consumption found in the manual, averaging between maximum power and cruise power to obtain climb out fuel consumption.
Legal Fuel Reserves
The fuel reserves as required by law are basically as follows (they may differ per country somewhat). VFR operations: 30 minutes by day and 45 minutes by night for fixed wing and 20 minutes for rotary wing. For IFR operations: sufficient fuel to divert to a suitable airport and hold for 45 minutes at 1500 feet for non turbine aircraft and 30 minutes for fixed and rotary wing turbine powered aircraft.
Remember that a 30 minute VFR reserve fuel isn't that much at all. It would be wise to have at least an hour of fuel onboard when you land at your destination. Just in case.
To allow for the unexpected things like stronger headwinds, not too efficient leaning method, diversion due to weather, climbing more than necessary and ATC delays, do have some extra fuel onboard. Normally 10 to 15% (of the trip fuel required) should be enough. Take a good look at the weather forecast and note the winds aloft at different altitudes so that a more favorable altitude can be chosen (to your alternate airport too, make sure you do have fuel to go there).
The rate of fuel consumed can vary greatly between different aircraft of the same type due to their age, maintenance condition, hours on the engine and such. With fixed pitch propellers the increase of 100 RPM could increase fuel flow by 10% or more. Not using the mixture knob (even at low altitudes) relates to higher than necessary fuel burn too.
It is therefore wise to determine the fuel flow rate for different altitude/RPM/manifold settings in the preflight planning phase. Round off to the next higher number and make an allowance for the age of the engine (fuel flow tends to increase with engine age).
Weight & Balance
You will often find that the amount of fuel in the aircraft is limited by weight and/or balance. Flying solo in a C-150, for example, usually means that you can carry full fuel. But having a passenger with you equates to leaving fuel out and planning for an extra fuel stop. Other aircraft manuals dictate that fuel difference between the left and right wing tank may have limits, primarily due to the location of the tank in the wing.
Make sure to calculate fuel burn needed for your trip so that you know where the C of G is at the landing weight.
Some aircraft also have a maximum zero fuel weight (MZFW), all weight above that figure must be fuel, or else you will risk damaging the wing structure. See the table for more weight definitions.
Aircraft Standard Empty Weight
The weight of the aircraft including unusable fuel and full operating fluids.
Maximum Zero Fuel Weight
The maximum weight (structural) exclusive of usable fuel. Any weight above this must be fuel.
Maximum Ramp Weight
Maximum weight approved for ground operations (includes fuel for runup and taxi).
Maximum Takeoff Weight
Maximum weight approved for takeoff.
Maximum Landing Weight
Maximum weight approved for landing.
Not normally done with our homebuilt experimental aircraft but during an early return to the airport by the big airliners it is likely that the aircraft weight at that moment in time is more than the MLW (maximum landing weight, due to structural limitations) allows. And as dumping passengers or cargo is not an option, fuel must be either be dumped or burned off before landing. If the aircraft does land overweight it must be inspected before returning to service.
Quantity And Dipping Tanks
Preflight Fuel Management
Before starting any flight you will need to determine the amount of fuel currently in the tanks. Assuming you have calculated the total amount needed for the flight it is just a matter of filling the tanks with the amount required and you're good to go. Almost. It would be wise to check the available fuel by at least two methods: use an empty tank and fill this with known quantities or you will need to use a calibrated dipstick for this type of aircraft.
Due to the construction of the tank and the location where the fuel inlet port and drain port are located, a certain amount of the fuel is unusable and some part of that is even undrainable. Unusable fuel (which includes undrainable) is the amount thatcannot be used in level flight. This amount can vary from model to model and in the aircraft flight manual you will find the specifics for your aircraft.
The experimental aircraft builder/owner must determine for himself what the exact amount of unusable fuel is for each tank in his aircraft and make a note of that in the flight manual. Usable fuel is the only amount which can safely be used in a flight.
Great care must be taken when converting between liters, US and Imperial gallons. Some stations refuel in liters whereas the aircraft fuel gauges show in US gallons and sometimes (to confuse matters even more) on the aircraft wing, near the fuel cap, the amount of that tank is shown in Imperial gallons.
Tank content is displayed on gauges and these should be reasonable correct when the tanks are empty. Compare the indications of the gauges when dipping the tanks so accuracy can be checked. Remember that during some maneuvers, slipping skidding and in turbulence, tank indications can and will vary accordingly. If the aircraft is equipped with a flow indicator its indications should be checked against the level of fuel in the tank after the flight and the expected calculated consumption.
Fuel Tank Dipping
In some countries it is common to dip the tanks to verify the amount of fuel. It is the most accurate way. But as each aircraft (and tank in that aircraft) are somewhat different it is important to use the correct dipstick for your aircraft.
Each dipstick is calibrated to the fuel tanks of that aircraft. It should have the aircraft registration number marked on it too.
When dipping fuel tanks you will need to follow some simple guidelines:
The aircraft should be parked on a level surface, make sure the tanks are not cross feeding. This will happen when the fuel selector is set to BOTH. Fuel will flow from the tank with the greatest amount to the other tank. This could be a problem if you need both tanks to be full. Just set the selector to either RIGHT or LEFT.
The dipstick must be held perpendicular to the wing unless stated otherwise in the aircraft manual. Usually caused by wing construction, the spar location is the culprit here.
Always dip the tanks after refueling. Even when refueling with a known quantity.
Do the refueling yourself. You are the pilot in command and you are responsible, do not rely on someone else.
If there is no dipstick for your aircraft, you can start the flight with full tanks and keep a good log during the flight. But as dipsticks can be bought for a reasonable price, buy one and get it calibrated (and labeled too) for your aircraft.
As you normally refill the aircraft after the end of the day to keep the change of moisture as low as possible, it would be wise to dip and drain the tanks in the morning. Water and dirt in the fuel will then have sunk to the lowest part to be drained.
This has also occurred a lot in the past, make sure to dip the tanks before the flight as to prevent any surprise. Note that if the aircraft was parked on an uneven surface fuel could flow from the higher tank to the lower tank and siphon out through the vents. Which would leave you with one tank empty.
Having an engine failure just after takeoff could be the worst possible time. A number of these situations were related due to fuel problems and the majority was selecting an almost empty tank (or even switching fuel off due to incorrectly marked selectors), and in aircraft with more than one tank to select. Contaminated fuel has also caused problems in the past.
A lot of these situations can be avoided by using the correct preflight and pre-takeoff checks.
The list below is not complete but these checks are commonly done when verifying fuel supply and switching tanks.
Select the least-full tank before engine start, this will make sure that this tank can feed the engine. Also listen for the fuel pump when switched on, when the engine is running you will not hear the fuel pump. Check for a rise in and a stable pressure. With a gravity fed system there will always be some low pressure due to the design.
Change to the fullest tank before run-up. This will stabilize fuel flow from this tank. Visually check when moving the fuel selector. Selection of the wrong tank (especially when flying different aircraft with similar fuel systems) can be deadly. If you notice, after takeoff, that the wrong tank was selected, wait until you are at a safe height, then switch the fuel pump on and switch tanks. When switching the fuel pump off, keep you hand near the switch. The engine pump could have failed.
If using a timer or watch now is the time to start it or note the time as to be sure not to forget to switch tanks in time.
Keep monitoring the quantity gauges, pressures and flow for normal indications. Scan the pressure during takeoff roll and keep the fuel pump on until a safe height is reached and the aircraft is cleaned up.
When flying low or over inhospitable terrain, switch on the fuel pump.
The reason for having the the fuel pump on when switching tanks is to remove any air that may have collected in the fuel lines which could cause temporarily starvation and probably an anxious moment with the pilot. Not to mention what would happen to the state of mind of your passengers.
It is important to be totally familiar with the fuel system of the aircraft you fly. Be sure to know the number of tanks and quantities, normal fuel flow, number of fuel pumps (the Rotax 914 uses two electrical in series configured pumps) and check the emergency procedures for fuel related engines failures.
Keep the POH or aircraft manual nearby should any confusion arise about the amount of fuel during the flight. Herein you should find the calibration table for the fuel system which relates indicated with actual fuel onboard.
Although this table is only valid when the aircraft is flying straight and level (or on level ground), it will give you a good idea of how much is onboard during the flight.
If using a dipstick to check fuel levels, it should be calibrated and marked with the aircraft registration. The reason for marking a dipstick is simple, not all aircraft and their tanks are created or formed equal, or have the same contents.
If you are using a J-Air fuelhawk then you need to created a simple chart which converts the dipstick indication to real gallons. Easy!
Logging Rate Of Consumption
Fuel Log Keeping
Maintaining an accurate log during any flight is mandatory. Monitoring the consumption rate in combination with the fuel gauge readings is an important task of the pilot in command. Making this a part of the cruise checklist is crucial, it will keep you from forgetting to change tanks when needed and thus creating an unbalance in the wings (in case of light aircraft with two or more wing tanks) or even trying to fly on an empty tank.
From the beginning of the flight you will have recorded the engine start time, aircraft takeoff time, selected the correct tank and determine a 'land by time'. After 30 minutes or so in the flight, change tanks (in case of two wing tanks) take note of the time and tank, keep on doing this every hour afterwards, this way fuel imbalance is no more than 30 minutes of flight.
Deduct the fuel used from this tank and note that on the log. This creates a running total (in minutes, liters or gallons) which should be kept for all tanks on board. This will give you a good idea of how much fuel is left in each tank at any time during the flight.
Fuel gauge readings should also be recorded on this log. Any problems with the fuel gauges can be noted as well. If for any reason your cockpit workload is too high (inexperienced pilots are easily overloaded in busy airspace and when weather plays a role), remember the overall fuel quantity and keep an eye on the 'land by time'. Just make sure that you are on the ground before this time passes.
Whenever you are changing tanks in flight, the electric fuel pump should be switched on just before switching tanks and left on for a short period of time afterwards, 30 seconds should be enough. This is done so that any air in the fuel line from the other tank is purged.
Keep your hand near the fuel pump switch when switching it off. If the engine RPM drops for any reason drops you can switch the pump back on immediately. At this point you are quite certain that the engine mechanical pump has failed.
Never run a tank dry as that could introduce air into the fuel lines. Some engines have trouble restarting when air gets into the system. And never (only when you really need to) change a tank when over stretches of water, inhospitable terrain etc. Emergency landings could be really difficult.
An accurate fuel log should be kept by the pilot and checked after the first landing to verify the actual fuel level with the calculated consumption. This is even more important when hiring aircraft, as consumption can and will vary between the same type of aircraft and engines due to pilot technique.
Pilot who regularly fly their own aircraft have a will current history of the actual consumption of their aircraft, those who hire usually have not.
It must be remembered that a fuel log alone should not be relied upon. Variables as a loose fuel cap, leaking drains, higher consumption due to turbulence and not so perfect leaning techniques can result in higher fuel usage.
Checking total consumption
After completing the flight make it a habit of checking the fuel level in the tanks. This will give you a good indication of the actual consumption and will be a reference for the upcoming flights. At the same time you can verify the fuel gauges for accuracy.
Normally the aircraft should be topped off after the last flight of the day to minimize entry of water due to condensation in the fuel system. However, this may create a problem for the next pilot as this could mean a weight & balance problem and fuel needs to be drained from the aircraft. Try to determine if the aircraft needs full tanks for the next flight before topping off.
Leaving a topped off aircraft in the sun creates a temperature rise and a possible fuel spill from the tanks, fire could result.
Not leaning an engine during cruise could mean that your still air range is about 15-20% less with some engine types. Most aircraft engines can be leaned when power is set 75% or below and at any altitude. And not only when cruising higher than 3000 feet, which is teached at some flying schools.
By leaning, the engine runs more fuel efficient and with higher temperatures, the only requirement is that you keep an eye on the EGT and or fuel flow gauges. You do not want to burn out the exhaust valves prematurely. For aircraft with fixed pitch propellers and without EGT indicator engine leaning can be done by RPM only: just pull, gently, the mixture knob until you reach max RPM. If you pull to far the RPM will drop quite suddenly. Just put it back in and you are fine, no worries.