A nominated weather alternate is crucial for almost all flights operating under IFR (Instrument Flight Rules). Interestingly, only a small fraction of these flights actually divert to their weather alternate. Proper fuel management during this rare phase is vital for a successful outcome.
The primary threat to safety is the delay in deciding to divert. Other risks include being unprepared for the diversion, using an inappropriate flight profile, delays caused by the pilot at the alternate, and failing to declare a fuel emergency when necessary. In the worst-case scenario, any combination of these threats could lead to running out of fuel before landing at the alternate airport.
Minimum Diversion or Reserve Fuel is essentially the total of Alternate Fuel and Final Reserve Fuel. Regulations state that a diversion to the weather alternate should be initiated no later than when the remaining fuel equals the flight plan’s Reserve Fuel. Delaying this decision until fuel drops below Reserve Fuel goes against regulations and could force the aircraft to consume some Final Reserve Fuel before reaching the alternate airport. This situation is classified as an emergency.
Similarly, being unprepared for the diversion, not adhering to the proper flight profile, or causing delays will negatively impact the available fuel. Alternate fuel calculations depend on specific speeds, cost index, routing, and altitude criteria. Not following these guidelines while en route to the alternate can lead to burning more fuel than initially planned.
Reserve or Minimum Diversion Fuel is generally indicated on the operational flight plan. It represents the minimum amount of fuel legally required at the missed approach point for the landing runway at the destination airport. Sometimes, the operations manual allows some of the Reserve fuel to be used at the destination if conditions permit a safe landing. If landing at the destination is uncertain and fuel approaches Reserve levels before the approach, a diversion should be started immediately.
To prevent delays and excessive fuel use during a diversion, pilots can declare a state of “Minimum Fuel” to Air Traffic Control. This declaration is not an emergency but signals that any delay could hinder landing with Final Reserve fuel. If holding near the destination, pilots must exit the hold with enough fuel to complete the approach while still having Reserve fuel left at the missed approach point. If the expected clearance time does not allow for this, a revised clearance time should be requested. If the new clearance time also does not permit approach clearance, a diversion must be initiated.
Pilots should be fully prepared for a potential diversion before starting the approach at the destination. This preparation includes programming the Flight Management System (FMS), reviewing charts, checking alternate weather, and planning the flight profile for the diversion. A briefing about the plan in case of a missed approach should ensure both pilots are aware of the intended actions. If fuel levels are critical and a missed approach is likely, Air Traffic Services should be informed of the diversion intentions, like requesting clearance to the alternate.
During the diversion, pilots should adhere to the operational manual’s criteria for cost index, speed, routing, and altitude. Managing workload during this phase is key; pilots must be ready to conduct the approach at the alternate without delays. If total fuel remaining approaches Final Reserve fuel before landing, a PAN PAN-fuel message should be communicated to ATS. If fuel reaches Final Reserve while still airborne, a MAYDAY-fuel should be declared.
For instance, consider a pilot flying an A320 who commenced an ILS approach to RW27 at Bristol amidst deteriorating weather. Without a diversion plan or informing Air Traffic Services of their intentions, the crew found themselves at minimums in Instrument Meteorological Conditions (IMC) and executed the published missed approach. They held over the aerodrome for eight minutes discussing their next steps, and upon requesting clearance to Luton, their filed alternate, they discovered they were nearly 500kg below the flight plan’s Reserve fuel requirements.
On the way to Luton, the crew was informed of a likely five to ten-minute delay and received a holding clearance. After entering the hold, the captain declared a MAYDAY-fuel emergency. The aircraft was prioritized for landing and safely touched down in Luton, with a total fuel remaining of 860kg at touchdown.
Contributing factors often include pilots hesitating to commit to a diversion until the possibility of landing becomes impossible. This hesitation can delay the diversion decision and potentially compromise Reserve fuel. In severe weather situations, multiple aircraft might be diverting simultaneously, overwhelming the capacity of diversion airports and leading to delays. Thus, effective fuel management and timely declarations of fuel emergencies are critical in such scenarios.
