Mountain Flying

The following gives an overview of issues to consider when flying near high terrain and in mountainous areas. It is not a comprehensive guide to the subject, and pilots are strongly recommended to seek local knowledge and instruction as appropriate to the location.

Terrain clearance

It is recommended that enroute Visual Flight Rules (VFR) flights always have a terrain clearance of between 500 ft and 1000 ft above ground level. Over mountainous areas, 2,000 ft provides a greater margin to account for descending air created by turbulence, downdrafts and mountain waves.

Descending air associated with mountainous areas may exceed the aircraft climb performance. Significant mountain wave can be encountered downwind of even relatively low hills such as the Pennines.

Flying abroad

Mountain ranges such as the Alps may be too high to cross above surrounding terrain without oxygen and a turbocharged aircraft. Flying below the peaks is acceptable in some circumstances, however you must plan your route carefully and it is recommended to receive training on mountain flying in the relevant areas before doing so.

If planning a route through the Alps or areas of similar terrain:

• Check the local VFR routes and confirm the minimum recommended VFR crossing altitudes.
• Navigation information should be confirmed via review of the relevant ICAO chart for the state and guidance in the relevant AIP.
• Consult the relevant weather forecasts. Some alpine states such as Switzerland and Austria publish specific mountain route forecasts, known as ‘GAFORS’ – these can be found via the national aviation weather services and may be available via moving map software.
• Information and data from moving map devices should be checked against official sources of information.

Weather

Weather conditions in mountain ranges can be severe and change rapidly. You should have a backup plan and be flexible when faced with unforecasted inclement weather. Lowering cloud bases, turbulence and descending air all pose a risk when flying near terrain.

Mountain weather is normally better in the mornings. In the afternoon cloud cover will often increase and winds become stronger. Familiarise yourself with the conditions for mountain wave and rotor turbulence – winds aloft above 25 kts will create challenging conditions.

Navigation

Situational awareness is important in any flight but is critical when flying in mountainous terrain where conditions can change rapidly.

Moving map devices are an important tool for situational awareness, but you must be able to navigate by conventional means in the event of device failure. GPS signal may be disrupted by terrain, and tablet computers can quickly overheat from exposure to sunlight.

Study the ICAO chart for your flight, identifying terrain elevations and clear landmarks that will help you remain on track. Stick to recommended VFR routes where published and any local procedures. Mountains and valleys will look remarkably similar from the cockpit, and it is easy to make navigational errors that may put you on a collision course with terrain.

Shadows can obscure terrain features, making it appear that you are further from terrain than you actually are. Valley flying should only be conducted in good weather and lighting conditions.

Human Factors

A higher workload can impact your mental capacity to make decisions or handle new tasks or problems. If you are inexperienced in mountain flying, the physical and mental demands may be high, and steadily erode the capacity for sound judgement and action. This can be mitigated by taking appropriate instruction in mountain flying.  

Hypoxia can also play a part in higher altitude mountain ranges. Physical and mental function is normally adequate below 10,000 ft, although health conditions or smoking may affect this. Above 5,000 ft you may experience some loss of mental capacity and degradation of eyesight. Above 10,000 ft you should use supplementary oxygen in compliance with NCO.OP.190.

Fatigue and dehydration are other factors to consider when flying in mountainous regions. Glare is greater due to clouds and snow-covered terrain as there are less atmospheric pollutants. The use of cabin heating is more likely due to the cooler air at altitude. It is strongly recommended to carry an active carbon monoxide (CO) detector to ensure no dangerous gases are present in the cockpit. From January 2025, it will be a requirement for UK registered piston engine aircraft to carry an active CO detector when passengers (who are not qualified pilots) are onboard.

Aircraft Performance

High terrain requires consideration of climb and cruise performance. During the summer, density altitudes may be considerably higher than you have previously experienced. The reduction of aircraft performance in ‘hot and high’ conditions can take pilots by surprise.

Most non-turbocharged piston aircraft experience a significant reduction in climb performance with density altitudes above 5,000 ft. Ensure you know your rate of climb and associated climb gradient to ensure terrain and obstacle clearance in the prevailing conditions.

Follow any applicable guidance in the Aircraft Flight Manual or Pilot’s Operating Handbook – for example regarding leaning the fuel/air mixture control to ensure optimum power. If conditions dictate, leaning is often permitted at any altitude and may be required during the climb – leaning is not something only reserved for high altitude cruise.