Fuel - Diversion to Weather Alternate

Fuel - Diversion to Weather Alternate


A nominated weather alternate is an integral part of the flight plan for practically all flights conducted under IFR (IFR). In reality, only a very small percentage of these flights actually terminate with a diversion to the weather alternate. Appropriate fuel management during this infrequently encountered phase of flight is critical to an acceptable outcome.


  • The principle threat to safe operations is delaying the diversion decision.
  • Secondary threats include:
    • being unprepared to carry out the diversion,
    • use of an inappropriate flight profile during the diversion,
    • pilot induced delays at alternate, and
    • failure to declare a fuel emergency when appropriate.
  • In the worst possible circumstances, a combination of any of these threats could lead to fuel exhaustion prior to landing at the alternate aerodrome.


Minimum Diversion or Reserve Fuel is the sum of the Alternate Fuel and the Final Reserve Fuel. Except where allowed by specific criteria published in the Company Operations Manual, regulations direct that diversion to weather alternate is to be initiated not later than the point in time at which the fuel remaining is equal to the flight plan Reserve Fuel. Delaying the diversion decision beyond this point to where the fuel remaining is actually less than Reserve Fuel is contrary to regulations and will result in a situation where some of the Final Reserve Fuel must be consumed before the aircraft reaches the nominated alternate aerodrome. By definition, this is an emergency situation.

Similarily, being unprepared for the diversion, not following the appropriate flight profile or any pilot induced delays will all negatively impact on the fuel remaining. Alternate fuel is calculated using specific speeds, cost index, routing and altitude criteria. Failure to adhere to these criteria whilst enroute to the alternate aerodrome is likely to result in burning fuel in excess of the amount anticipated by the flight plan.


Reserve or Minimum Diversion fuel is, in general, clearly indicated on the operational flight plan. By definition, it is the minimum amount of fuel legally required to be available at the missed approach point for the landing runway at the destination aerodrome. The operations manual may allow for some of the Reserve fuel to be consumed at destination when weather, traffic and airport facilities allow the pilot to consider that the landing at destination is assured. If landing at destination is not assured and fuel will be depleated to Reserve fuel prior to the approach, a diversion should be initiated without delay. The following actions can help to mitigate delays, excess fuel consumption and the significant workload associated with a diversion.

  • Under provisions of an ammendment to ICAO SARPS effective November 2012, pilots can declare a state of "Minimum Fuel" to Air Traffic Control. This is not a declaration of a fuel emergency but it is a means of clearly identifying the fact that any delay could result in the inability to land with Final Reserve fuel.
  • If in a hold in the vicinity of the destination aerodrome, pilots must plan to leave the hold with sufficient fuel to proceed by a realistic routing to commence the approach, conduct the approach and still have Reserve fuel remaining at the missed approach point. If the expected further clearance time (EFCT) for leaving the hold does not allow for this profile, a revised EFCT must be requested. If, at the revised further clearance time, approach clearance is not available, a diversion must be initiated.
  • If the potential for a diversion is high, the pilots should be fully prepared for that eventuality prior to commencing the approach at destination. This includes programming the FMS where applicable, reviewing the appropriate charts, obtaining alternate weather and reviewing the flight profile to be flown in the event the diversion becomes necessary. A briefing regarding intentions in the event of a missed approach should be conducted to ensure that both pilots are fully aware of the intended flight profile.
  • If fuel is critical and the potential for a missed approach high, Air Traffic Services should be pre-advised of the pilot's intentions in the event of a missed approach - as an example, "in the event of a missed approach, (callsign) requesting clearance to (alternate) via (routing) at (altitude)".
  • The diversion should be flown using the diversion criteria of cost index, speed, routing and altitude as prescribed by the operations manual or operational flight plan.
  • Workload management during the diversion is critical and pilots should be fully ready to conduct the approach at the alternate aerodrome without inducing any delays.
  • If it becomes apparent that total fuel remaining will reach Final Reserve fuel prior to landing at alterate, PAN PAN-fuel should be broadcast to ATS.
  • If fuel remaining reaches Final Reserve while still airborne, MAYDAY-fuel should be declared.

Typical Scenarios

  • The pilot of an A320 with legal fuel remaining commenced the ILS approach to RW27 at Bristol in rapidly deteriorating weather. A diversion plan had not been discussed nor had Air Traffic Services been advised of the crew's intentions in the event of a missed approach. At minimums, the aircraft was still in Instrument Meteorological Conditions (IMC) and the published missed approach was carried out with a return to the hold over the aerodrome. The crew spent a total eight minutes in the hold discussing their course of action and when they requested a clearance to Luton, their filed alternate, they were almost 500kg below the flight plan Reserve fuel requirements. Enroute to Luton, the crew was advised that there was likely to be a five to ten minute delay inbound to Luton and was given a holding clearance. The hold was accepted but, shortly after entering the hold, the captain elected to declare a MAYDAY-fuel emergency. The aircraft was given priority for landing and touched down in Luton without further incident. Total fuel remaining at touchdown was 860kg.

Contributory Factors

  • Pilots are often reluctant to commit to a diversion until any possibility of landing at the intended destination has been negated. This can lead to delays in the diversion decision and can, potentially, compromise Reserve fuel.
  • In cirumstances of severe weather at destination, it is possible that many aircraft could be diverting at the same time. This can potentially overwhelm the capactity of the diversion aerodrome(s) and result in delays. Fuel management and appropriate declaration of fuel emergency becomes particularly important under these circumstances.

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