In-Flight Replanning and Diversions
When the plan stops working
Replanning begins when an assumption fails: weather below minima, a closed runway, an unserviceable aid, a fuel burn higher than planned, a passenger or technical issue, or a change in the operational requirements. The decision to divert should be made against pre-briefed criteria, not improvised under pressure. Know your diversion triggers before departure.
Doing it in the air
Replanning means recalculating track, heading, groundspeed, time, fuel and ATS coordination from the present position. You must be able to pick an alternate, compute the track and distance to it, estimate fuel required, check performance for the new destination and revise the ATS plan. Consider airspace, terrain, weather and the availability of the alternate at your ETA, not at the time you make the decision.
Traps
Examiners test whether you recheck the alternate's weather for your arrival time, and whether you account for the fuel already burned versus fuel remaining. Another trap is assuming the nearest aerodrome is the best diversion; suitability depends on runway, services, weather and approval. Watch for questions where the diversion changes the ATS unit responsible for you.
Rule of thumb
Use a simple decision gate: if the remaining fuel above final reserve is less than the fuel needed to reach the diversion plus a margin, divert now. Waiting for more information costs fuel, and fuel is the resource that decides the outcome.