Point of No Return with a Deteriorating Destination
You are departing Alpha (sea level) for Bravo, 300 NM to the west, in a single-engine aeroplane with 185 L usable fuel. Before start, fuel is 180 L. Taxi allowance is 10 L, cruise burn is 36 L/h, and the fixed reserve is 30 min. Cruise TAS is 125 kt. The forecast wind is 270°T at 30 kt, giving an outbound groundspeed of 95 kt and a return groundspeed of 155 kt. Bravo has no suitable alternate; if Bravo is not useable, you must return to Alpha, which has fuel and lighting. One hour after departure, the destination forecast deteriorates to marginal VFR with a 1,500 ft cloud base and rain; Bravo has no instrument approach. You are 95 NM from Alpha. A private airstrip, Charlie, lies 40 NM ahead, but its strip is 800 m long, soft, and has no fuel. You have 3 POB and no liferafts; the route is over land.
1. Calculate the PNR time and distance from Alpha.
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Fuel available for flight after taxi and reserve = 180 - 10 - (30/60 × 36) = 180 - 10 - 18 = 152 L. Endurance = 152/36 = 4.222 h. For a PNR, let t be outbound time; return time = (95t)/155. Then t + 95t/155 = 4.222, so t(1 + 0.6129) = 4.222, t = 2.617 h = 2 h 37 min. PNR distance = 95 × 2.617 = 248.6 NM from Alpha. This assumes the forecast winds hold and that Alpha remains available. The PNR is not a destination; it is the last point at which a return to Alpha still leaves the fixed reserve.
2. At the one-hour point, with 95 NM flown, determine remaining fuel and decide whether to continue to Bravo, divert to Charlie, or turn back to Alpha.
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At takeoff fuel = 180 - 10 = 170 L. After 1 h, fuel used = 36 L, remaining = 134 L; reserve = 18 L, so 116 L is available. Return to Alpha: 95 NM at 155 kt = 0.613 h = 36.8 min; fuel = 36.8/60 × 36 = 22.1 L; landing fuel = 111.9 L. Continue to Bravo: 205 NM at 95 kt = 2.158 h; fuel = 77.7 L; landing fuel = 56.3 L, which is 1 h 34 min above reserve. Charlie: 40 NM at 95 kt = 25 min; fuel 15 L, but the strip is soft, 800 m, and has no fuel, so it does not solve the weather problem and leaves you stranded. The decisive point is that Bravo has no approach and the forecast is marginal VFR; if Bravo is not useable after 2.1 h, you will be near or beyond the PNR and will not have fuel to return. At 95 NM you are still well inside the PNR, so turning back to Alpha is the safe choice. Continuing is only justified if you have a reliable report that Bravo is VFR.
3. Explain how the decision changes if Bravo reports 3,000 ft broken and 10 km visibility.
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With 3,000 ft broken and 10 km, Bravo is VFR (assuming the applicable VFR minima are met) and the main reason for turning back disappears. Continue: 205 NM at 95 kt = 2.16 h, fuel 77.7 L, landing fuel 56.3 L, which is above the 18 L reserve and gives about 1.5 h of holding. However, you will cross the PNR at 248.6 NM from Alpha, i.e., after another 154 NM / 95 kt = 1.62 h. Beyond that point, if Bravo deteriorates unexpectedly, you cannot return to Alpha. Set a hard decision point at the PNR: obtain an updated Bravo report before crossing it; if the report is not VFR, turn back. Charlie remains a poor option because it has no fuel and a soft strip. The fuel calculation is based on the forecast 95 kt; a stronger headwind would reduce the margin, so monitor actual GS and fuel flow.