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Think twice before you go low level flying

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Think twice before you go low level flying

Yes, I am guilty of falling into that unknown zone where the line between life and death is simply luck, Luck that nothing catastrophic will go wrong leaving me no time or space to react. Only a few weeks ago I willingly risked all. I (had to) ferry from Bloemfontein to Lanseria. Look closely and there is the start of the fault, in brackets. I had work commitments, was days behind schedule for a few jobs and the pressure was snowballing and so, off I went. The wind was gusting to 48 knots on the helicopters nose. Dust filled the atmosphere reducing visibility to less than a few kilometers at times and all I could do was crawl along, as low as possible attempting to find the best ground speed to reach destination with adequate fuel reserves. I struggled along for almost three hours, often flying below normal tree top height, a most exhausting journey to say the least and unacceptable in terms of a risk analysis, also, silly considering that I spend virtually all day every day in the gray area of the dead mans curve due to the nature of work.

If aircraft were measured graphically from the ground up, helicopters would rank near the bottom. This isn’t a criticism; it’s just the nature of the beast. Helicopters spend a large portion of time flying close to the ground and, as a result, they are increasingly susceptible to wire strikes or uncontrolled flight into terrain.

Analysis conducted by the United States Helicopter Safety Team ( revealed that approximately 16 percent of all helicopter accidents are attributed to wire or obstacle strikes. In addition, 17 percent of these accidents resulted in fatalities. Power plant failures and flameouts that occurred to all helicopters operating low level (below 150 feet AGL) resulted in fatalities. This unfortunate reality should emphasize greater awareness among helicopter pilots about the dangers of low-level flight.

Wires are strung throughout, whether in controlled or uncontrolled airspace, pilots flying low level must be vigilant because wires often go undetected by human eyes. You might even say they lurk in the shadows waiting to ensnare their victims. Maximizing time and space for see-and-avoid duties is a great strategy for eluding these often near-invisible hazards as well as buying valuable time to react to the sudden shock of an engine failure, since it’s only a matter of nanoseconds that can make the difference between escaping from or being caught in a fatal yet avoidable accident.

Another factor that has very recently contributed to a fatal helicopter accident is low -level turbulence. How many low time pilots truly appreciate and understand what wind and turbulence is capable of?

I have experienced, and so far survived a few terrifying moments of severe turbulence, due to wind/turbulence and sudden gusts and on a few occasions lost partial control of my helicopter for a few seconds.
Brief examples of a couple of my more memorable experiences include:

  • Loss of tail rotor effect in a vertical lift off from thick bush on a mountain into an unexpected wind blowing from the ten o’clock position.
  • Flying into an invisible whirlwind and getting spun violently 180 degrees.
  • Rolling violently to the left after passing downwind of a small hill with a 50knot tailwind, caused by rotor turbulence.
  • Numerous failed attempts at landing in a deep valley as a result of the venturi effect coupled with turbulence, due to lack of aircraft control.
  • Some very scary moments fire fighting on Cape Mountains during a South Easter with turbulence coming from every direction.
  • Running from a severe Namibian dust storm and eventually making a hasty landing to sit it out.

Commercial operators often need to contend with unfavorable weather, private pilots do not. I shall discuss more about wind later, since we mostly have the option to not fly and get caught out as opposed to hitting wires. Both dangers increase at low level, as for wires, Safety devices installed on helicopters such as The Wire Strike Protection System, otherwise known as “wire cutters,” are trusted and proven. This wire chomping mechanism literally cuts through undetected wires coming in contact with the helicopter, provided the wire hits the aircraft at the perfect angle. Although this system does not prevent wire strikes ensnaring the mast, cutting through the intruding danger can be a lifesaver.

Some of the newer detection systems use lasers to alert pilots of potential danger. These high-tech systems allow for greater use and flexibility on both normal and transport category helicopters. Several of these devices also can distinguish between current-carrying wires and non-current-carrying wires, regardless of wire composition and/or diameter. Sadly these items are out of my pay category.

Agricultural, wildlife, air mercy and fire fighting pilots constantly operate low level in wire-infested environments and in unfavorable weather. Aerial work pilots spend enough time at work in this danger zone and should keep out of it while they can during ferry flights. These professionals must focus their attention every second ensuring their jobs get done safely and efficiently. They must constantly keep their heads on a swivel and have a refined situational awareness strategy and instinctively feel and plan for sudden wind changes to make sure their flight paths are clear and constantly maintain a back up plan. This “art” only comes with experience that can only be gained with time.

During basic training, helicopter pilots learn the importance of performing a high and low reconnaissance prior to conducting low-level missions, exercises that usually scare the living daylights out of the student. These exercises are done in ideal conditions to known LZ’s. Older pilots should not get complacent and also constantly identify clear areas that could be used as possible forced landing zones in case of an emergency. Sensory overload close to the ground is normal for low time pilots and during an unexpected emergency can overwhelm even the most experienced pilots. Remember, flying higher can buy time and pushing aircraft and personnel limits is foolish and often deadly.

Towers are a major threat to helicopters. They are used for communications and increasingly to gather wind data for developing new wind farm sites. These slender, hard-to-see structures are supported by nearly invisible guy-wires and often stand slightly below 900 feet above ground level. Only advanced GPS systems display these obstructions and they mostly do not meet any marking requirements.
FAA and NTSB personnel have investigated several accidents involving aircraft colliding with similar, lower towers in the USA and many pilots questioned often report problems of not seeing the towers while flying, until finding themselves uncomfortably close to one. All you can do is keep vigilant especially when flying over higher peaks in poor visibility.

Helicopter pilots can follow several basic procedures to mitigate wire strike accidents. For example, maintain maximum altitude as long as possible and use conservative routes when transitioning from point A to point B. The extra minutes invested following these basic steps will prevent many surprises from happening. Never go straight in to a landing site that is not familiar without getting local knowledge and performing an extremely thorough recce. Recently I made a landing in a confined area where an obvious power line was strung past the southern edge. I selected to approach over the power line to not lose visual and to have a clear overshoot. Only after I had shut down did I discover an invisible telephone wire across the north side of the LZ.

High voltage lines, guy wires, and other low-level obstacles are lethal when mixed with the operational envelope of helicopters. When it comes to maintaining aviation safety, take the path of least resistance and leave the “shock factor” to the wires.

The helicopters ability to transit through, manoeuvre, land and take off from hilly or mountainous terrain is one of the most demanding aspects of helicopter operations. Pilots at some stage are likely to experience this challenging environment and require an understanding of the basic principles, threats, errors and the possible undesirable aircraft states, in order to operate safely. Flying in hilly or mountainous terrain has resulted in a number of fatal helicopter accidents, one all too recently in the Eastern Cape. An awareness of the wind speed and direction is critical in undulating and mountainous terrain because wind follows the surface. If the ground rises, the wind blows upward on the slope and it is referred to as the ‘windward’ side. If the ground slopes away from the wind direction, then the wind blows downwards and is referred to as the ‘leeward’ side. When wind blows over gently rolling hills and valleys it tends to blow smoothly. But when wind blows over a cliff it tends to tumble over the edge in a turbulent manner just like an invisible waterfall. Have you ever watched the tablecloth tumbling over Table Mountain?

When wind is forced through a gap i.e. along a valley and through a ravine then the speed is increased due to the Venturi effect. It goes without saying, the stronger the wind the more violent is the turbulence. On a windward slope turbulence rarely exists and the resulting up-draughts can be beneficial in producing lift and therefore requiring less power to manoeuvre. Eagle’s and Buzzard’s love ridge soaring and are often seen hovering in this region. As a result the windward slope with up-draughts is preferable to operate in whenever possible. On a leeward slope there is generally turbulence and down draughts that can make fight hazardous and this region should be avoided. The area where the up-draught turns to a down-draught is referred to as the ‘demarcation line’. The demarcation line between up-draughting and down-draughting air will, typically, become steeper and move towards the windward edge of the feature as wind speed increases. When flying along a valley it is preferable to fly closer to the windward slope to take advantage of the up-draughts, rather than down the centre of the valley. The leeward slope should be avoided because of down-draughts and potential loss of lift.

The Venturi effect in valleys can cause a significant increase in wind speed possibly doubling the normal wind speed. This phenomenon is also accompanied by a decrease in pressure, which can cause the altimeter to over read the altitude at which the aircraft is flying. A place you want to avoid being in during night flying.

Estimating the local wind speed and direction in hilly and mountainous terrain is difficult, however it is essential and can be achieved by using the following techniques:

  • Smoke
  • Wind farms
  • Wind lanes on water features like dams (smooth surface on the up-wind side of the lake and waves on the down-wind side)
  • Vegetation, long grass, tree movement
  • Cloud movement
  • Fly a 360 degree turn, around a ground reference at a safe height whilst maintaining a constant angle of bank and speed. The resultant drift will indicate the wind direction and strength.
  • Comparing groundspeed to airspeed, visually over the ground or by use of GPS.
  • More subtle tools are possible, like observing the initial take off direction of a flock of birds, or the dust kicked up from hooves of disturbed livestock or wildlife.

Mountain Waves are defined as oscillations to the lee side (downwind) of a mountain caused by the disturbance in the horizontal airflow caused by the high ground. The wavelength and amplitude of the oscillations depends on many factors including the height of the high ground above the surrounding terrain, the wind speed, and the instability of the atmosphere. Formation of mountain waves can occur in the following conditions:

  • Wind direction within 30 degrees of the perpendicular to the ridge of high ground and no change in direction with height.
  • Wind speeds at the crest of the ridge in excess of 15kts, increasing with height.
  • Stable air above the crest of the ridge with less stable air above and below that stable layer.

Vertical currents within the oscillations can reach 2,000ft/min. The combination of these strong vertical currents and surface friction may cause ‘rotors’ to form beneath the mountain waves, resulting in severe turbulence. Mountain Waves are associated with severe turbulence, strong vertical currents, and icing. The vertical currents in the waves can cause significant fluctuations in airspeed potentially leading, in extremes, to loss of control. (Been there!) Loss of Control can also occur near to the ground prior to landing or after take-off with a risk of terrain contact or a hard landing if crew corrective response to a down-draught is not prompt.

Local knowledge of the conditions which tend to cause the formation of mountain waves enables forecasting of potential wave propagation. Lenticular Clouds (lens shaped clouds) can form in the crest of the mountain waves if the air is moist. Roll Clouds can also occur in the rotors below the waves if the air is moist. These clouds are a good indication of the presence of mountain waves but, if the air is dry, then there may not be any cloud to see.

Fohn Effect is a warm dry wind that blows down the lee side of a mountain. When a large air mass is forced up and over a mountain range (Orographic Lift ), clouds and precipitation form as the air rises and cools adiabatically. Like on the Kirstenbosch side of Table Mountain. When the air mass reaches the top of the mountain it has lost a significant amount of its water content and so has a much lower dew point. As the air then begins to descend down the lee slope of the mountain, and the air pressure increases, it warms adiabatically, as seen from Camps bay. The resultant wind is dry and warm giving clear conditions at airfields on the lee side of the mountain range. As well as creating a warmer climate, these dry winds can be a cause of wild fires during the summer months, also known as Berg winds. Another aspect when crossing a mountain range and flying into a warm headwind is that a pilot approaching from the leeward side of a mountain can only see the silhouette of the top of a cloud, but he cannot see the full extent of the cloud on the windward side. This phenomena happens often flying over the escarpment from Johannesburg to Hoedspruit.

Cumulonimbus clouds and other clouds of vertical development typically produce heavy rain and thunderstorms, especially when the air is forced up due to Orographic Lifting. The cumulonimbus convection currents produce strong and unpredictable winds particularly up-draughts and down-draughts which are extremely dangerous and aircraft should avoid flying in the vicinity of a cumulonimbus cloud giving at least a ten nautical mile gap, whilst keeping well clear of the dark grey scud clouds.

In mountainous areas turbulence is often encountered. This can either be mechanical turbulence (due to the friction of the air over uneven ground at low levels), or thermal turbulence (due an air temperature instability at mid levels). Turbulence affects the behaviour of the aircraft in flight and increases the threat of retreating blade stall, vortex ring and LTE as the ground and air speed fluctuates. For helicopters equipped with teetering rotor systems (Robinsons and Bell 206 types) there is the additional danger of main rotor mast bumping and rotor/tail strike.
As a refresher from study days, the severity of turbulence:

  • Light turbulence:
    is the least severe, with slight, erratic changes in attitude and/or altitude.
  • Moderate turbulence:
    is similar to light turbulence, but of greater intensity - variations in speed as well as altitude and attitude may occur but the aircraft remains in control all the time.

Severe turbulence:

is characterised by large, abrupt changes in attitude and altitude with large variations in airspeed. There may be brief periods where effective control of the aircraft is impossible. Loose objects may move around the cabin and damage to aircraft structures may occur.

Extreme turbulence:

is capable of causing structural damage and resulting directly in prolonged, possible terminal loss of control of the aircraft.

  • Turbulence can be experienced anywhere and without warning, therefore it should always be anticipated, especially in hilly and mountainous terrain. Pilots should always be prepared for turbulence by keeping a positive grip on the flying controls and reducing the airspeed to the recommended RFM ‘turbulent airspeed.’
  • In days with no or little winds, orographic up-draughts or down-draughts are not very significant; therefore the effect of heating the ground by the sun can produce an inversion with associated up-draughts on the sun-side and down-draughts on the shadow-side of a mountain.
  • The same effect can be experience by day and night. During the day, the air heated by the ground creates an ascending air mass (anabatic wind). This breeze can be apparent from about a half hour after sunrise. At night, the air close to ground cools creating a down-draught (katabatic wind). This night breeze can start an hour before sunset and can continue throughout the night.
  • Remember where there is rising air, there will also be descending air!
  • When flying through hilly or mountainous terrain, the route should be planned taking the local meteorological conditions into account avoiding adverse weather as described earlier. When crossing mountains, especially in strong winds, you should clear the top of the mountain by at least 500 feet . If you are unable to achieve a safe clearance, consider an alternative route or a diversion.
  • When crossing a range of hills or mountains with cloud on the top, it is better to approach parallel to the top of the range in order to see the extent of the cloud. If the cloud cover appears to be extensive beyond the high ground, consider an alternative route or a diversion.
  • If flying along a valley it is preferable to fly closer to the windward slope rather than along the centre of the valley. The leeward slope should be avoided for transiting because of down-draughts and potential loss of lift. If it is necessary to fly on the leeward side, it is advisable to fly at Vy in order to optimise the power margin.
  • Special consideration should be given to the threat of power/cable wires, which are often strung across valleys sometimes without any notice to pilots. One such wire is strung very high across a valley south east of Barberton from an old mine.
  • The best escape route when flying along a valley is normally to perform a 180 degree turn. Therefore if continued flight along the valley is deemed inappropriate, e.g. due to low clouds, or obstacles, an early decision with sufficient height and space to turn back is essential to ensure a successful turn.
  • Before undertaking flight in hilly or mountainous terrain a risk analysis should be conducted in which the Threats, Errors and Undesired Aircraft States are identified and managed with the appropriate mitigating actions.
  • A Threat is defined as an, event or errors, which occur beyond the influence of the flight crew, increasing operational complexity and which have to be managed to maintain safety margins.
  • Errors are defined as actions or inactions by the flight crew that lead to deviations from organisational or flight crew intentions or expectations.
  • Undesired Aircraft States are defined as flight crew induced aircraft positions or speed deviations, misapplication of flight controls or incorrect system configuration, associated with a reduction in safety margins.
  • Example:
    Turbulence, wind-shear, up- and down-draughts.
    Error: Flying at inappropriate speeds and low level through deep valley.
    Undesired aircraft state: Retreating blade stall, LTE, Vortex Ring, mast bumping, momentary loss of control
  • Mitigating Action: Fly at appropriate turbulence speed / Vy keeping wings level with a firm grip on controls. Leave the danger area as fast as possible.

If you wish to ensure a safe and enjoyable flight in, around, or over hills or mountains, you must develop the skills, collect the knowledge and appreciate the factors involved. Above all, know your own limitations and those of the aircraft and stick to them.

In summary, when conducting operations in hilly or mountainous terrain consider the following:

  • Be aware of aircraft performance and limitations.
  • File a flight plan or notify someone of your intentions.
  • Study the navigational charts carefully ? do not rely on GPS.
  • Get up to date weather information for a go no-go decision.
  • Don’t go when winds are stronger than 25 knots or 7 meters/second.
  • Fly at a safe altitude.
  • Be aware of the wind direction and speed.
  • Monitor for signs of change in weather.
  • Be aware of the psychological and physiological effects of mountain flying.
  • Always plan an escape route.
  • Be aware of wind-shear and recovery actions to be taken.
  • Most importantly: Before undertaking flight in hilly or mountainous terrain receive appropriate training from a qualified flight instructor who is experienced in mountain flying techniques.

Above all, every time you take off, pause for a moment to consider all the dangers awaiting your flight. Run through a mental risk analysis of each phase of your flight and look for areas where you are able to reduce your exposure to risk. Slow everything down and remind yourself that if you are unlucky, things will go terribly wrong, very fast and only you can give yourself another chance by building in a safety margin, plus another safety margin and keeping a focused thought that your engine can fail, at any moment. An invisible wire can appear at any moment. An undesirable wind can get you at any moment. Think safety!!

Written by John Bassi, CEO and Chief Pilot at Bassair Aviation

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