Latest Departure for a No-Lighting Destination Across a Time Zone
You are flying from Westport (UTC+8) to Easton (UTC+10), 380 NM with a true track of 090°T. TAS is 145 kt. The forecast wind is 250°/25 kt. Last light at Easton is 1845 local (UTC+10). You require a 30 min buffer before last light, and Easton has no lighting. Fuel: 220 L usable, burn 40 L/h, fixed reserve 45 min, taxi 9 L. There is no alternate; if Easton is not available you must return to Westport, which has lighting. Your planned departure is 0500 UTC. The aircraft has a serviceable GNSS and VOR. The route is over land, and the terrain is flat. You have 2 POB. ATC has no radar coverage for the last 60 NM.
1. Compute the groundspeed, flight time, ETA in UTC and local time, and fuel required.
Read the worked answer
Wind 250/25, track 090: the wind is 160° from the track, i.e., 20° off the tail from the left. Tailwind = 25 cos 20 = 23.5 kt; crosswind = 25 sin 20 = 8.6 kt. Drift = arcsin(8.6/145) = 3.4°; GS ≈ 145 cos 3.4 + 23.5 = 168.2 kt. Time = 380/168.2 = 2.259 h = 2 h 16 min. ETA = 0500 + 0216 = 0716 UTC. Easton is UTC+10, so local ETA = 1716. Last light is 1845 local, so the buffer is 1 h 29 min, which meets the 30 min requirement. Fuel = 2.259 × 40 = 90.4 L; reserve = 0.75 × 40 = 30 L; taxi 9 L; total 129.4 L, within 220 L.
2. Calculate the latest departure UTC that still meets the 30 min buffer.
Read the worked answer
The latest arrival is 1845 - 0030 = 1815 local Easton = 0815 UTC. Latest departure = 0815 - 0216 = 0559 UTC. The planned 0500 UTC departure gives a 59 min margin over the latest departure. If the actual wind is weaker than forecast, the flight time will be longer; the latest-departure calculation is only valid for the forecast GS. A 10 kt lower tailwind would increase time by roughly 380/158 - 380/168 = 2.41 - 2.26 = 0.15 h = 9 min, which would still fit within the 59 min margin for the 0500 departure but would make a 0600 departure even later.
3. If departure is delayed to 0600 UTC, what are the options and which do you choose?
Read the worked answer
At 0600 UTC, ETA = 0816 UTC = 1816 local, which is 29 min before last light - 1 min short of the 30 min buffer. The difference is within forecast error, but the buffer is a personal limit and Easton has no lighting. Options: (a) depart and accept a 29 min buffer - not acceptable if the buffer is hard; (b) increase speed to gain 1 min - possible but not reliable, and it increases fuel burn; (c) reduce taxi/fuel weight - negligible; (d) request a level with a better tailwind - unknown; (e) delay to the next day or arrange a lit alternate and carry fuel for it. The best decision is not to depart on the 0600 slot unless you can obtain a lit alternate or a revised forecast that clearly gives a longer buffer. If already airborne, set a decision point: if the ETA slips past 1815 local, divert to Westport (which has lighting) before you are committed. Do not plan to land after last light at an unlit aerodrome.