Fuel Management, II
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.