Destination Forecast at the Alternate Limit with a Single Approach

Two separate questions: does the flight need an alternate, and can the destination be used at all?

Alternate requirement. The forecast ceiling of 900 ft is below the operator's 1,000 ft threshold, and the visibility of 3,000 m is below the 3,200 m threshold. Either one triggers the requirement; both are present. A destination alternate is therefore mandatory.

Destination usability. The approach minimum descent altitude is 620 ft above aerodrome elevation, and the forecast ceiling is 900 ft broken. That is above the MDA, so the ceiling does not by itself preclude the approach. The required visibility is 1,600 m and the forecast is 3,000 m, which also passes. However, several factors erode the margin:

• Gusts. 18 gusting 32 gives an 18-knot gust spread and a crosswind component on runway 235 that must be computed against the aircraft limit. With wind at 240°, the crosswind is small, but the gust factor affects handling on a single narrow runway.
• Temperature/dewpoint. 4 °C with a 1 °C spread is a fog and carburettor/airframe icing risk. Conditions can deteriorate below forecast faster than the forecast suggests.
• Single approach, no straight-in at the alternate. The destination has one non-precision approach and no diversion option at the field itself.

Conclusion. The destination is usable on the forecast but with thin margins, and the alternate is mandatory and must be planned and fuelled. The decision to continue past the destination must be made early, not at the MDA.

Landing performance must be assessed on the actual conditions, not on the book figure.

Distance. Landing distance required is 1,240 m and landing distance available is 1,480 m, a margin of 240 m, or about 19%. That margin must be checked against the operator's required factor for the conditions. A wet runway - rain is forecast - typically requires an increased factor, and a downslope on the landing direction increases the ground roll. If the required factor is applied to 1,240 m and the result approaches or exceeds 1,480 m, the landing is not acceptable.

Runway characteristics. A 1.1% downslope assists deceleration only if the landing is uphill; here the downslope is on the landing direction, which lengthens the rollout. The displaced threshold removes 180 m from the landing distance available, which is already reflected in the 1,480 m figure only if the calculation used the correct declared distance - this must be verified, because using the full runway length would overstate the available distance by 180 m.

Wind. A gust spread of 14 knots on final means the approach speed will be flown with a gust increment, which increases the touchdown speed and the stopping distance.

Conclusion. The margin is thin and depends on the correct declared distance and the correct wet-runway factor. If either check fails, the landing is not acceptable and the flight should plan to the alternate or to a longer runway.

The decision is a diversion to the planned alternate. The reasoning:

• Below minima. The required visibility is 1,600 m and the reported visibility is 1,200 m. The approach cannot be commenced or continued to the MDA. The ceiling at 700 ft is above the 620 ft MDA, so the ceiling is not the blocker - the visibility is.
• No second approach option. With a single non-precision approach, there is no precision approach to fall back on, and a second approach would be flown in conditions that are not improving and are below the published minimum.
• Fuel. Destination plus alternate plus 45 minutes was planned. A hold or a second approach consumes the reserve. The correct action is to proceed to the alternate without delay, declare the situation if fuel becomes a concern, and advise ATC early.

What should have been briefed before descent:

• The specific minima - 620 ft MDA and 1,600 m - stated as numbers, not as 'we'll see how it looks'.
• The diversion trigger. A decision point, expressed as a distance or a fuel figure, at which the crew diverts rather than continuing.
• The alternate and its approach. Which approach, which runway, the minima, and the forecast.
• Task allocation. Who flies, who monitors, who communicates, and who calls the minima.
• The go-around and diversion call. Stated explicitly so that either pilot can make it without hesitation.

The failure mode this prevents is the gradual descent into 'just one more look' that consumes the reserve and removes the diversion option.