Flight Planning and Monitoring: fuel policy and in-flight replanning
Fuel policy
Commercial planning splits fuel into: taxi fuel, trip fuel, contingency (typically 5% of trip, or 3% with an en-route alternate), alternate fuel, final reserve, and additional/extra fuel. The final reserve is not usable in normal planning - it may only be tapped in a genuine emergency, after which a MAYDAY or PAN is due. Know minimum fuel versus fuel emergency.
En-route planning
Track, true and magnetic headings, drift, ground speed and ETA; MORA/MSA for terrain; equal time point (ETP) for the diversion decision; critical point when the aircraft must be able to reach either of two alternates. Replanning in flight uses actual wind, temperature and fuel flow.
Traps
- Applying contingency to the total rather than to trip fuel only.
- Computing alternate fuel at the alternate's planned mass instead of the mass at the diversion point.
- Treating taxi fuel as part of contingency.
- Forgetting that the final reserve for a turbine aeroplane in commercial air transport is 30 minutes holding at 1 500 ft above the aerodrome in standard conditions.
Worked example
Trip 4 500 kg; taxi 200 kg; contingency 5% of trip = 225 kg; alternate 1 200 kg; final reserve 30 min at 2 400 kg/h = 1 200 kg; extra 500 kg. Ramp fuel = 4 500 + 200 + 225 + 1 200 + 1 200 + 500 = 7 825 kg. Check against MTOW, MLW and tank capacity; if limited, reduce extra fuel, never the reserve.
Rule of thumb
Compare fuel remaining with plan at every waypoint. A persistent shortfall of 5% or more demands a decision, not optimism.