Flight planning - route, alternate and contingency planning
Flight planning - route, alternate and contingency planning tests route and alternate choice under weather, terrain, airspace and diversion-time constraints. It includes EDTO/ETOPS, equal-time points and contingency fuel.
Rules
- A destination alternate is normally required; if destination weather at ETA is below minima, two alternates may be required.
- En-route alternates must lie within the prescribed diversion time; EDTO threshold and diversion speed determine the radius.
- Equal-time points balance diversion time to two aerodromes.
- An alternate must have a suitable runway, approach, weather and fuel.
Worked example
EDTO diversion time is 120 minutes and diversion ground speed is 420 kt. Radius = 2 h × 420 = 840 NM. An en-route alternate 800 NM from the route is inside the radius; one at 900 NM is outside. If wind changes diversion ground speed to 380 kt, radius = 2 × 380 = 760 NM, so the 800 NM alternate is no longer usable.
Traps
Using TAS instead of ground speed overstates the EDTO radius in a headwind. Still-air distance is not the same as forecast distance. Do not forget the two-alternate requirement when destination weather is marginal. Check alternate runway performance, not just weather.