Max range display is the guaranteed (worst-case headwind) full-tank range (ISS-099, v0.51.0)

lib/sim/movement.test.ts

@@ -5,6 +5,8 @@ import {
   burnRateConstant,
   FUEL_DENSITY_KG_PER_L,
   fuelAtWeighted,
+  guaranteedRangeKm,
+  MAX_WIND_BURN_FACTOR,
   maxRangeKm,
   positionAt,
   progressAt,
@@ -234,3 +236,22 @@ describe('maxRangeKm (v0.28.0)', () => {
     expect(() => maxRangeKm(BURN, 0, TANK)).toThrow();
   });
 });
+
+describe('guaranteedRangeKm (v0.51.0, ISS-099)', () => {
+  it("MAX_WIND_BURN_FACTOR is windBurnFactor's actual maximum over the weather domain", () => {
+    // Full headwind at max wind: exactly +20% — the constant must track the factor function, not a copy.
+    expect(MAX_WIND_BURN_FACTOR).toBe(windBurnFactor(30, 90, 90));
+  });
+
+  it('is maxRangeKm under worst-case headwind burn (ATR ≈ 3696 km ±5%)', () => {
+    const r = guaranteedRangeKm(BURN, WE, TANK);
+    expect(r).toBeCloseTo(maxRangeKm(BURN * MAX_WIND_BURN_FACTOR, WE, TANK), 12);
+    expect(r).toBeGreaterThan(3696 * 0.95);
+    expect(r).toBeLessThan(3696 * 1.05);
+  });
+
+  it('zero fuel → zero range; rejects a non-positive empty weight', () => {
+    expect(guaranteedRangeKm(BURN, WE, 0)).toBe(0);
+    expect(() => guaranteedRangeKm(BURN, 0, TANK)).toThrow();
+  });
+});

lib/sim/movement.ts

@@ -137,3 +137,16 @@ export function maxRangeKm(burnPerKm: number, emptyWeightKg: number, maxFuel: nu
   const rho = FUEL_DENSITY_KG_PER_L;
   return Math.log(1 + (rho * maxFuel) / emptyWeightKg) * ((emptyWeightKg + rho * maxFuel) / (rho * burnPerKm));
 }
+
+// Max headwind burn multiplier over the weather domain (v0.51.0, ISS-099): the upper end of
+// windBurnFactor's [0.8, 1.2] range — a full 30 kt headwind (the max in lib/weather.ts) ÷ 150.
+export const MAX_WIND_BURN_FACTOR = 1 + 30 / 150;
+
+// Guaranteed full-tank range (v0.51.0, ISS-099): the maxRangeKm closed form under worst-case
+// headwind burn — any leg within this distance is flyable in ALL weather, since windBurnFactor
+// never exceeds MAX_WIND_BURN_FACTOR and requiredFuelFor grows with the burn rate. This is what
+// the UI displays as "Max range", so a spec-possible leg always passes flyPlane's fuel guard
+// (requiredFuelFor ≤ maxFuel under the same wind-adjusted burn).
+export function guaranteedRangeKm(burnPerKm: number, emptyWeightKg: number, maxFuel: number): number {
+  return maxRangeKm(burnPerKm * MAX_WIND_BURN_FACTOR, emptyWeightKg, maxFuel);
+}