There is a finite amount of fuel which can be loaded on the aircraft based on the requirement for the flight as per policy/regulation/performance or the capacity of the fuel tanks.
It is the Pilot in Command’s responsibility to ensure that the planned final reserve fuel is protected at all times.
The main purpose of defining the fuel requirements and uplifting the minimum required for the flight is to ensure that anytime during the flight, a safe landing can be carried out with the final reserve fuel onboard protected.
Aviation and all its activities are oriented towards commencement and completion of the flight to its destination or if safety is affected, then to an airport where a safe landing can be carried out.
The main intent is protection of final reserve fuel which is intended to ensure a safe landing at any aerodrome when unforeseen occurrences may not permit safe completion of an operation as originally planned. Guidance on flight planning including the circumstances that may require re-analysis, adjustment and/or re-planning of the planned operation before take-off or en-route, is contained in the Flight Planning and Fuel Management Manual (Doc 9976).
There are a number of strategies that can be used to determine the minimum fuel requirement. This could be based on the the type of operation, the area of operation and the prevailing conditions in terms of weather, technical status etc.
The flights can be dispatched with fuel requirements catered for the destination plus one alternate, two alternates, re-dispatch or even without an alternate.
ICAO Annex-6, The pilot-in-command shall continually ensure that the amount of usable fuel remaining on board is not less than the fuel required:
The point to be noted here is that the Annex-6 requirement is that upon landing at an aerodrome (destination or alternate), the minimum usable fuel on board should be the planned final reserve fuel.
The final reserve fuel must be protected at all times.
A flight typically arrives in the vicinity of its destination aerodrome with the following fuel on board regardless of the length of the flight from the point of departure:
At this point in the flight, the PIC must decide in association with operational control personnel, if available, how best to use the remaining and scarce resource.
The PIC has to balance the odds of diverting/committing to the destination where the situational awareness is better in terms of prevailing weather and air traffic conditions as compared to the alternate where the METAR may be available but the air traffic situation may not be accurately judged or known at all. Additionally, if a destination is close to weather minimums or suffering from extended delays, the more information available to increase the PIC’s situational awareness, the better the basis for a sound decision.
The additional circumstances in which “Diverting or Committing” is permitted typically include:
The pilot-in-command shall request delay information from ATC:
ICAO Annex 6 requires: The pilot-in-command shall advise ATC of a minimum fuel state by declaring MINIMUM FUEL when, having committed to land at a specific aerodrome, the pilot calculates that any change to the existing clearance to that aerodrome may result in landing with less than planned final reserve fuel.
After a request for delay information, the MINIMUM FUEL declaration likely represents the second in a series of steps to ensure remaining fuel on board an aeroplane is used as planned and final reserve fuel is ultimately protected. Practically speaking, the PIC should declare “MINIMUM FUEL” when, based on the current ATC clearance, the anticipated amount of fuel remaining upon landing at the aerodrome to which the aeroplane is committed is approaching the planned Final Reserve fuel quantity. This declaration is intended to convey to the applicable air traffic controller that so long as the current clearance is not modified, the flight should be able to proceed as cleared without compromising the PIC’s responsibility to protect final reserve fuel.
The pilot-in-command shall declare a situation of fuel emergency by broadcasting MAYDAY MAYDAY MAYDAY FUEL, when the calculated usable fuel predicted to be available upon landing at the nearest aerodrome where a safe landing can be made is less than the planned final reserve fuel.
This declaration provides the clearest and most urgent expression of an emergency situation brought about by insufficient usable fuel remaining to protect the planned final reserve. It communicates that immediate action must be taken by the PIC and the air traffic control authority to ensure that the aeroplane can land as soon as possible. The “MAYDAY” declaration is used when all opportunities to protect final reserve fuel have been exploited and in the judgment of the PIC, the flight will now land with less than final reserve fuel remaining in the tanks. The word fuel is used as part of the declaration simply to convey the nature of the emergency to ATC.It is also important to note an emergency declaration not only opens all options for pilots (e.g. available closed runways, military fields, etc.) but it also allows ATC added flexibility in handling an aeroplane.
The radio call PAN PAN, used either together or separately, shall be used by an aircraft for the purpose of giving notice that the aircraft has a very urgent message to transmit concerning the safety of a ship, aircraft or vehicle, or of some person on board or within sight. The call simply states that there an urgent message that needs to be transmitted whereas MAYDAY indicates that grave and imminent danger threatens the aircraft and immediate assistance is required.
The fuel requirements are divided into two sections, the flight planning or pre-flight and the in-flight.
The flight planning section primarily deals with the fuel policy and the assumptions on which fuel uplift is calculated. E.g. choice of alternate, no alternate, weather at destination and/or alternates, re-dispatch, technical condition of the aircraft, tankering, NOTAMS etc.
Once the flight commences the bottom line is that at any stage inflight, the aircraft must be able to perform a safe landing at an aerodrome with the useable final reserve fuel remaining.
Throughout the flight fuel checks must be carried out as per procedure to verify and validate the preflight assumptions that of fuel with alternate or otherwise as per the flight plan.
Nearing destination, the PIC has to take a decision , based on the prevailing conditions of weather, traffic, technical condition etc. The PIC can hold at the destination and divert before reaching the minimum diversion fuel, carryout an approach and then divert or commit to land at the destination. If the destination airport meets the requirements fro no alternate with two runways and suitable weather, then the alternate fuel can be used at the destination. At no time the final reserve fuel must be breached.
Therefore various operational strategies and tools are available to the PIC for continuation of the flight to the destination if a safe landing can be achieved.