Mid-air collisions are rare because they are separated by air traffic control, technology on-board large planes and well-defined visual flight rules in uncontrolled air space where many smaller aircraft operate. However, most mid-air collisions are fatal. The Wairarapa crash is one of a handful in New Zealand during the past 30 years and another close to an airfield when aircraft are in the most critical stage of flight - take off and landing. Figures from the United States show more than half of all collisions occur during approach, descent and landing or at take-off.
Mid-air collisions in NZ include:
• In July 2010 two people were killed when training aircraft crashed near Tauonui Aerodrome near Feilding.
• In February 2006 two Massey University training pilots were killed when their planes, flying close together, clipped each other. An investigation concluded the pilots could not see each other as a result of the planes' inherent blind spots and because one, which for much of the time was climbing, was headed directly towards the sun.
• In November 1993 four men died when a traffic-spotting plane and the Eagle police helicopter collided above central Auckland. A Transport Accident Investigation Commission report later said neither pilot saw each other in time to take evasive action in the uncontrolled airspace.
What contributes to mid-air collisions
The ''see and avoid'' principle breaking down
For most small aircraft flying outside controlled airspace the pilots are responsible for maintaining a safe distance from other aircraft. This is the "see and be seen" principle, otherwise known as VFR or Visual Flight Rules.
A 2013 Transport Accident Investigation Commission report following a mid-air collision in 2010 and a near miss the same year found that factors that limit the see-and-avoid principle include: "diffusion of responsibility", where a pilot might assume that another pilot in the cockpit is keeping watch; cockpit workload, where pilots are tempted to keep their heads down rather than look out their windows; aircraft design features or ergonomics, where cockpit layout and windshield obstructions prevent proper views outside the aircraft; pilots' visual blind spots; and pilots not being able to actually see aircraft owing to sun glare. International data shows that a loss of situational awareness and the failure of, or failure to use, proper see-and-avoid techniques were significant factors in a high number of occurrences, the commission found.
The US-based Aircraft Owners and Pilots Association says all aircraft have blind spots. High-wing aircraft have reduced visibility of aircraft above them, and can have their view of traffic blocked when making turns in the pattern as the wing is lowered in the direction of the turn. Low-wing aircraft have a large blind spot beneath them that may obscure conflicting traffic when descending into the pattern or while on final approach. A Mid-Air Collision (MAC) is an accident where two aircraft come into contact with each other while both are in flight.
How to avoid collisions
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The guiding principle of air traffic control is to keep aircraft apart. State-owned enterprise Airways controls all domestic and international airspace but the Civil Aviation Authority determines which airfields have controllers dependent on air traffic volume and the type of traffic. Though rules of separation vary depending on the airspace in which a passenger aircraft is flying, in general, air traffic controllers and pilots are required to maintain a horizontal distance of 5 nautical miles between two aircraft flying at the same altitude. For altitudes at and below 29,000 feet, vertical separation must be maintained at a minimum 1000 feet. For altitudes above 29,000 feet vertical separation must be maintained at a minimum of 2000 feet. Air traffic controllers create a ''protective bubble'' around the aircraft which increases in size as the aircraft climbs and gets faster.
What about uncontrolled airspace?
For most small aircraft flying outside controlled airspace the pilots are responsible for maintaining a safe distance from other aircraft. This is the "see and be seen" principle otherwise known as VFR or Visual Flight Rules.
All passenger aircraft are equipped with Traffic Alert and Collision Avoidance Systems, commonly referred to as TCAS. This system indicates the relative altitude, distance and bearing of transponder-equipped aircraft within a selected range, generally up to 60km.
Colour-coded symbols and aural warnings called traffic advisories (TAs), the system indicates which aircraft pose a potential threat. Since the early 1990s in the US – and since 2000 in Europe – passenger aircraft cannot fly without a traffic collision avoidance system (TCAS). The aircraft constantly transmits its identifier via radio, along with some other information such as altitude, speed and direction. TCAS was developed over decades following a series of disasters including:
• In 1956, a mid-air collision between a United DC-7 and a TWA Super Constellation over the Grand Canyon killed all 128 occupants of both airliners. By the 1960s and early 70s, prototype collision avoidance systems were available but gave numerous unnecessary alarms when tested in busy terminal areas.
• In 1978, Pacific Southwest Airlines (PSA) flight 182 collided with a four-seat Cessna 172 near San Diego Airport, killing 144.
• In 1986 when an Aeroméxico DC-9 collided with a four-seat Piper Archer over Cerritos, California, causing 82 fatalities including 15 on the ground. Soon after, the FAA mandated that airliners in US airspace be equipped with TCAS. Other countries followed. Studies show the risk of a mid-air collision is reduced by 90 per cent with TCAS. Since the introduction of TCAS in the United States in 1989, no US commercial air carrier collisions have occurred.
What about technology in light aircraft?
The is similar technology available for the general aviation sector but is not compulsory. The cost of equipment that can be retrofitted to aircraft ranges up to $20,000 but is falling. Bill McGregor, executive officer for the agriculture and helicopter divisions of Aviation NZ, said transponding equipment to help avoid collisions will be compulsory on all aircraft within the next five years. However, these would be for signals sent out and only those light aircraft with equipment to receive them would have the same protection as passenger planes. He said pilots using an uncontrolled airfield should signal their intentions on common radio frequencies and have a good idea of what each other's standard flight plans would be.