Instrument Flight Planning, Navigation, and Weather
Domain 4 tests the planning chain: regulation-driven fuel and alternate decisions, weather diagnosis, and en route navigation, including where the plan is most likely to break.
Legal planning floors
- 91.167 fuel: fly to the first airport of intended landing, then to the alternate if one is required, then for 45 minutes at normal cruising speed.
- 91.169 alternate: required unless, from 1 hour before to 1 hour after the estimated time of arrival, the ceiling is at least 2,000 ft and visibility at least 3 SM. An instrument approach must exist at the destination regardless.
- Alternate minimums are 600 ft and 2 SM for precision approaches and 800 ft and 2 SM for non-precision, unless the chart's alternate minimums section publishes different values or 'NA'.
Weather that matters in IMC
- Freezing rain signals a warm layer above a subfreezing layer and warns of severe structural icing; ice pellets mean that warm layer, and its supercooled drops, sits above you. Clear ice forms with large droplets near 0 to −10 °C, rime with small droplets at colder temperatures.
- Convective hazards include gust fronts, microbursts and low-level wind shear; all argue for a stabilized approach and a low go-around threshold. A small temperature–dewpoint spread with evening cooling suggests radiation fog, while warm moist air moving over a cold surface suggests advection fog.
Navigation and charts
- On low-altitude charts the MEA guarantees obstacle clearance (1,000 ft non-mountainous, 2,000 ft mountainous) plus reception within 4 NM either side of the airway; the MOCA guarantees clearance only, and the OROCA covers off-airway terrain. Check NAVAID status and database currency before relying on an RNAV route.
Worked example
A 280 NM leg at 140 KTAS in still air takes 2.0 hours; at 12 gallons per hour that is 24 gallons. A 45 NM alternate takes 0.32 hour, about 4 gallons. The 45-minute reserve at normal cruise is 9 gallons. Total 37 gallons plus taxi fuel. If that exceeds usable fuel at the planned takeoff weight, the plan changes, not the regulation.
Traps that cost marks
- Applying standard alternate minimums where the chart publishes nonstandard ones.
- Mixing RVR values with statute-mile visibility.
- Using a forecast to justify no alternate while the destination has no usable approach.
- Computing the 45-minute reserve on groundspeed instead of normal cruising speed.