You built the navigation log in Lesson 11 and filed the plan in Lesson 12. The plan is a forecast, not a promise. This lesson is the in-flight half: using the log as a living instrument — checking the real flight against it, revising the numbers, and knowing when the fuel picture has become an emergency.
1 — Why the Plan Drifts
Cross a waypoint and the actual numbers rarely match the planned ones. That isn’t a planning error — it’s the world refusing to match the forecast. The usual reasons:
- The wind isn’t as forecast — a stronger or different wind changes both groundspeed and drift, so legs run long or short.
- A different level or routing — a cruising level change, or an ATC re-route around airspace, alters track, wind and fuel.
- A late or slow departure — every downstream ETA slips with the off-block time.
2 — The Progress Check: Speed, Time and Fuel
At each checkpoint you re-derive the flight from what has actually happened. The arithmetic is the same as the nav log, run backwards from real data:
- Actual groundspeed = distance flown ÷ time actually taken × 60. If it differs from plan, every remaining leg re-times.
- Revised ETA = remaining distance ÷ actual groundspeed. Slower than planned → you arrive later and burn more.
- Actual fuel flow = fuel used ÷ time × 60 → project the fuel you’ll have at the destination.
- Off track? Use the 1-in-60 rule (Lesson 11) to revise heading, and the real drift tells you the actual wind for the legs ahead.
Recompute groundspeed from the time actually taken to the checkpoint, then re-time the rest of the route and project the fuel. If the projection at destination drops to the final reserve it is MINIMUM FUEL (advise ATC); if it would land you below the final reserve, that is a MAYDAY FUEL emergency.
3 — Fuel: Minimum Fuel and MAYDAY Fuel
Fuel monitoring has one question: will I land with at least my final reserve? Project the fuel at the destination (or alternate) and compare it to the final reserve (Lesson 8). If the projection won’t close, you act early — slow down for best range, route direct, or divert to a nearer field — never by eating into the reserve. Two standard radio calls mark the line:
- MINIMUM FUEL — an advisory: you are committed to a specific aerodrome and any delay would put you below final reserve. It asks ATC for no further delay. It is not an emergency.
- MAYDAY MAYDAY MAYDAY FUEL — a distress call: your calculated fuel on landing at the nearest suitable aerodrome would be less than the final reserve. This is a declared emergency.
4 — Top of Climb and Top of Descent
Two distances the plan needs but the straight-leg table hides: how far it takes to climb to cruise, and where to start down. Both are just rate, time and groundspeed:
- Distance = groundspeed × (height to change ÷ rate) ÷ 60.
Worked example — climb 4 500 ft (1 500 → 6 000) at 500 ft/min, groundspeed 80 kt: time = 4 500 ÷ 500 = 9 min; distance = 80 × 9 ÷ 60 = 12 NM. That is your top of climb — 12 NM out, level off. For the top of descent, the same maths going down (or the quick 3 : 1 rule — about 3 NM per 1 000 ft): 4 500 ft ≈ 13–15 NM before the field.
5 — Diverting and GNSS Points
When the destination becomes unusable — weather below VFR, a field closed by NOTAM, fuel marginal — you re-plan in flight to a new destination or alternate:
- Re-compute the fuel to the new field and confirm it still leaves the final reserve.
- Re-check the weather and NOTAMs for the new route and field — the conditions that made you divert may reach there too (Lesson 12).
- Pick a usable alternate you can identify and navigate to.
