Fuel Reserve and the Unforecast Headwind
You plan a VFR cross-country in a Robinson R44 (piston helicopter) from a coastal airfield to an inland destination. Planned distance 185 NM, planned TAS 105 kt, planned wind calm, planned fuel burn 45 L/h. Usable fuel at start 145 L. During the pre-flight weather briefing the forecaster mentions a strengthening headwind component of 20 kt at cruise altitude, which was not in your original plan. Takeoff and landing fuel allowances are negligible for this estimate. Company/ANAC VFR planning requires a 30-minute final reserve plus a 10% trip contingency.
1. Recompute the trip time and fuel with the 20 kt headwind, and determine whether the flight can legally and safely be dispatched as planned.
Read the worked answer
Groundspeed: 105 - 20 = 85 kt.
Trip time: 185 / 85 = 2.18 h ≈ 2 h 11 min.
Trip fuel: 2.18 × 45 = 98 L.
Contingency (10%): 9.8 L.
Final reserve (30 min): 45 × 0.5 = 22.5 L.
Total required: 98 + 9.8 + 22.5 = 130.3 L.
Fuel available: 145 L.
Margin = 145 - 130.3 = 14.7 L, about 20 minutes of additional endurance. The flight is legal on paper, but the margin is thin. The headwind was unforecast, so the wind could strengthen further. A prudent decision is to reduce payload/range, add a fuel stop, or delay. Do not simply accept the plan because the arithmetic passes.
2. Explain why a 20 kt headwind costs more than 20 kt worth of fuel on this leg, and what you would monitor in flight.
Read the worked answer
Headwind increases time in the air, and fuel burn is a rate per hour, not per nautical mile. With no wind the leg takes 185/105 = 1.76 h. With 20 kt headwind it takes 2.18 h, a 24% increase in time and therefore in fuel burned. A tailwind on the return leg does not repay the loss symmetrically because the outbound time penalty is larger than the inbound saving for the same wind.
In flight, monitor groundspeed against plan at each waypoint. If actual groundspeed falls below 85 kt, recompute remaining fuel versus remaining distance. Compare fuel remaining with the sum of remaining trip fuel plus reserve, not with the original total. If the reserve is being eroded, divert or land for fuel before it becomes an emergency.