Approach Minima, Fog and a Late QNH Change
You are inbound to an aerodrome at 06:10 local time in late autumn. The TAF issued at 04:00 forecasts visibility 6,000 m with no significant weather until 08:00. The actual METAR at 06:00 reads: wind calm, visibility 600 m, RVR 550 m, vertical visibility 100 ft, fog, temperature 6 C, dewpoint 6 C, QNH 1015. The aerodrome has a precision approach to runway 09 with a decision altitude of 200 ft and RVR minima of 550 m. The temperature/dewpoint spread is zero. Sunrise is at 07:20. Your alternate is 40 NM away with a TAF of visibility 10 km and no cloud below 5,000 ft.
1. Explain why the fog formed despite the TAF, and assess whether the approach is legally and safely possible.
Read the worked answer
Temperature and dewpoint are both 6 C, so the air is saturated and the spread is zero. With calm wind there is no mixing, and radiative cooling overnight has brought the temperature to the dewpoint, producing radiation fog. The TAF issued at 04:00 did not capture the rapid saturation because the spread was probably small but positive at issue time and the model expected some mixing or a slight temperature rise.
Legality and safety: the reported RVR of 550 m equals the RVR minimum, and the vertical visibility of 100 ft is below the decision altitude of 200 ft. Vertical visibility is not the same as a cloud base, but it indicates the surface visibility is restricted by fog and the pilot will not see the runway environment at DA. The approach is at the absolute limit and the reported conditions are at or below minima. A safe decision is to divert to the alternate, which has 10 km visibility and no low cloud, rather than attempt an approach at minima with no margin. If you do attempt it, you must be prepared to go around at DA and must have the alternate fuel available.
2. How does the expected sunrise affect your decision, and what would you do if you had to hold?
Read the worked answer
Sunrise at 07:20 will begin to heat the surface and, with calm wind, may dissipate the radiation fog, but the process takes time and is not guaranteed. The fog may lift to low stratus rather than clear, and the visibility may improve slowly. Holding until after sunrise is a reasonable option only if you have fuel for the hold plus the diversion to the alternate plus reserves.
Decision: if fuel permits, hold for 30 to 45 minutes and obtain a new METAR and a pilot report before committing to another approach. If the RVR does not improve above 550 m and vertical visibility does not rise above the DA, divert. Do not descend below DA without the required visual reference, and remember that the reported RVR is an instantaneous value that can change between the outer marker and touchdown. The safest plan is to divert to the alternate unless there is a clear, reported improvement.