Airports set among mountains operate under constraints that have no equivalent at sea level. The terrain removes options that flat-country flying treats as always available.
Terrain shapes the approach path
An instrument approach normally descends along a gentle path with clear ground beneath it. In a valley, that path may be blocked by rising terrain on one or both sides.
Approaches are then designed to follow specific tracks, sometimes curving, with steeper descent angles and higher minimum altitudes than a standard procedure would use.
Because the path is tighter, the weather conditions in which an aircraft may attempt it are correspondingly more restrictive.
A missed approach needs somewhere to go
Every approach must have an escape route if the crew cannot land. At a flat airport this is simply a climb straight ahead, which is uncomplicated.
Surrounded by peaks, the escape has to climb along a defined route with turns, and that route determines how much performance the aircraft must have available.
Aircraft that cannot meet the climb requirement on one engine may be excluded from the airport entirely, regardless of runway length.
Thin air reduces performance
At elevation the air is less dense, so engines produce less thrust and wings generate less lift at a given speed. Takeoff runs lengthen and climb rates fall.
High temperatures compound the effect, which is why performance at a mountain field is worst on warm afternoons rather than in winter.
Operators respond by restricting payload, departing early in the day, or using aircraft with performance margin for these conditions.
Local weather changes faster than forecasts
Valleys generate their own wind patterns, and terrain drives cloud formation that can close an airport within a short period, then clear as quickly.
Mountain waves and turbulence near ridges add a further layer, which is why some fields effectively operate only in the calmer morning hours.
Schedules at these airports are built around those windows, and afternoon services are often the first to be canceled.
Crews are qualified for the specific field
Some mountain airports require pilots to complete airport-specific training before operating there, because the procedures depend on local knowledge rather than general technique.
That requirement limits which crews an airline can assign, and a disruption cannot always be solved with whichever crew happens to be available.
The narrow qualification pool is one reason mountain routes recover slowly after a bad weather day.