Introduction
New Zealand is a mountainous country with terrain and weather strongly influencing almost all types of flight operations. Especially for general aviation, because most flights are operating at low altitude and relatively close to terrain. Statistics has shown that NZ general aviation has experienced an unacceptably high rate of terrain related accident. Flying in close proximity with terrain is challenging for both general aviation and commercial operations. Lack of training and awareness may leads to incident or accident. The most famous accident in New Zealand is NZ 901, which collided with Mt Erebus on a scenic flight on 28 November 1979.
Traditional low flying training does not prepare pilots adequately for operations in more demanding terrain and weather environment. Recently NZ CAA has added Terrain Awareness and Mountain Flying as new syllabus which are required PPL (Private Pilot Licence) and CPL (Commercial Pilot Licence) training.
In order to improve mountain flying and terrain awareness, there are four essential components need to be considered:
- Terrain: Size, shape, slope and elevation
- Weather: Visibility, temperature, icing, turbulent, wind strength and direction
- Aircraft: Performance, loading, configuration and speeds
- Pilot: Training, experience, currency, decision-making and situational awareness
Terrain
The size and scale of terrain can be difficult to judge, it is even more difficult when flying at low altitude and with no real horizon. The gradient and slope of valley floors or glaciers is often underestimated by pilots. The gradient of most glaciers will exceed the climb performance of general aviation aircraft. Therefore it is important to carefully study the charts during planning.
The route which is planned, may not necessarily be a straight line, but will more likely to depend on geographic condition and weather condition. Plan to position the aircraft to utilise up-flowing air where possible, while also being aware of right-of-way principles in considering opposite-direction traffic in a valley.
Weather
All mountainous terrain can be subject to severe and rapidly changing weather conditions. Weather is therefore a very important consideration when flying in the mountains. Wind at low level is playing an important role for mountain flying. Turbulence, downdraughts and wind shear can often be experienced. The upper-level wind will not always give a clear expectation of wind at low level. Downdraughts and turbulence will generally be found on the lee side of features and will increase in severity and extent with increase in wind strength. If the wind speed is less then 15 knots, it will generally follow the shape of mountains. On the other hand, if the speed exceeds 15 knots, turbulent condition and wind shear may appears. Therefore when the wind is above 15 knots, flight below the ridgelines in the lift side of the valley could be more comfortable, and pilots should make the full use of the updraughts at all times. But flight at lower level is unwise without the specific knowledge, training and experience. If a downdraught is encountered, turn out of the down-flowing air as soon as possible and try to locate the adjacent up-flowing air. Do not try to outclimb downdraughts.
Clouds can often be used as an indication of weather when mountain flying. Knowing different types of cloud and the process of cloud formation can be useful to judge the weather condition. One of the most obvious cloud types is lenticular cloud. It is formed when the wind blows at or near right angles to a mountain range, which causes air to raise on the windward side, with sufficient moisture, the crest of these waves may form a lenticular cloud. It is a good indication for any mountain waves. Generally, smooth strong updraughts can be found on the windward side of these clouds, and severe turbulence and downdraughts can be expected on the downwind of these clouds.
A good up-to-date weather briefing is essential. Weather in the mountains can change very quickly. Actual weather reports and reports from other pilots are a great help. Generally, conditions for flight in mountainous areas will be better in the morning. In the afternoon there can be more cloud build-up and stronger winds.
Aircraft
It is vey important to know the aircraft performance and its limitation when mountain flying. Factors such as turning radius, aircraft loads, speeds for best angle or rate of climb and the aircraft configuration which can be used under poor visibility condition are important to know and may reduce pilot’s stress and work load significantly under certain conditions. Aircraft performance may also be different depends on ambient conditions such as the loading of aircraft, the density and pressure altitude, outside air temperature and turbulence condition.
Pilots
Mountain flying is a demanding task for every pilot and it requires extensive knowledge, experience and skills to ensure safety. Many traps and illusions have caused pilots to false judge the conditions and make poor decisions in the past. In order to increase pilot’s situational awareness, it is required for pilots to understand these factors such as lighting and illusions and develop strategies to overcome them through training and experience.
Different lighting conditions can create definition and depth perception problems for pilots. The position of the sun can cause areas of deep shadow, especially in valleys it is difficult to see. This can occur in the early morning or late in the day when the sun is low on the horizon. When flying over larger amount of snow surface, perception problems may often be experienced by whiteout or brightout. Pilots must be aware of potential perception problems and be prepared to turn back before the safe visual cues start to disappear.
The most common illusion experienced by pilots is called false horizons. The frequent lack of a defined external horizon can create aircraft attitude and airspeed problems. When flying among the mountains, or anywhere the horizon is not visible, the pilot must learn to imagine that horizon. The real horizon is where the sky meets the sea. In mountains, visualise where this line is as if the mountains were transparent, and superimpose it on the mountains. This skill requires both experience and training.
Useful Techniques For Mountain Flying
Ridge crossing
Ridge crossing is one of the most common skills used when mountain flying. The basic rule for crossing a saddle or pass is to cross at an angle that allows for an escape route if needed. The shallower the approach angle, the easier it is to turn away if necessary. Try to approach at 45 degrees, or a shallower angle if possible, with the ridge or saddle on your left. This approach offers:
• The best view of the approach including the other side of the ridge
• A shallow approach angle and therefore shallow escape angle.
The escape route must be obstacle-free through a shallow angle downhill and downstream in anticipation of any sink. Choose a knife-edge ridge which can be crossed in minimum time rather than a flat ridge, which requires a longer period of time to cross. Always ensure adequate terrain clearance before crossing a ridge by judging the perspective with surrounding terrain.
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| The picture of ridge crossing (image embedded from NZ CAA on 20 September 2011) |
Valley flying
Normal recommended technique when flying in a valley is to stay on the lift side so that the aircraft is flying in smoother updraughting air. If a 180-degree turn becomes necessary, it is then being made into wind, requiring less distance over the ground. However, this technique needs to be balanced with the recommended right-of-way practice of flying on the right hand side of a valley. This is very important in areas of high traffic density and on any commonly used VFR route.
In narrow valleys, always position on one side of valley (either the upwind side or right side of valley). Under no circumstances should position in the middle of the valley. Leaving maximum space to turn also means less bank angle is needed, therefore less loading and less stall speed. Make regular check during the turn and constantly change bank angle and power to ensure use all available space to turn.
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| The picture of valley turns (image embedded from NZ CAA on 20 September 2011) |
Recommendation
- Study the charts carefully for terrain, pass heights, and good reporting points. Prominent peaks make good reporting points.
- Don't go when the upper wind are forecast over 25 knots. Winds will be much stronger over mountain passes.
- It is recommended to set a personal limit and complete an I’M SAFE checklist before every flight. Ensure to have some sorts of flying following and alerting system in place. Filing a VFR Flight Plan with Airways is generally the best option. Unfortunately, this option might not always be available due to operations may be taken out side the control airspace. Therefore it is pilot’s responsibility to maintain an adequate level of communication and/or to have other types of flight following system on board.
- Survival equipments should be carried on every flight and it is important to ensure that there are enough warm clothing, water and food carried for every person on board. A passenger briefing would be appreciated if any passenger were carried. Particular attention should be paid to any aspects pertinent to flying in the mountains, eg. Effects of possible turbulence-loose articles secured, seatbelts firmly fastened, location of sick bags.
- Finding a useable horizon can be difficult. Visualise where the sky meets the sea and superimpose this horizon on the terrain as if the mountains were transparent.
- If you find yourself in a downdraught, keep the nose down to maintain a safe airspeed, and alter your flight path to fly out of the downdraught.
- Fly on one side of valley, not down the middle. Flying on the lift side will keep you in the upgoing air and provide room to turn around if need be.
- Rely on good decision-making, not performance. That is constantly use the brain and common sense to keep out of trouble rather than rely on aircraft performance to get you out.
- Lastly, and most importantly, always have an escape route.
Reference:
Massey University School of Aviation (March 2011). Pilots Standard Operating Procedures
Civil Aviation Authority of New Zealand (June 2006). Safety Publication for General Aviation
