How close is TOO close? Experts reveal the dangers of planes flying near each other after a Luton-bound jet narrowly missed four gliders

The pilot of an airliner narrowly missed hitting four gliders while landing at Luton Airport, it has emerged in a report. The incident, which took place on April 27 close to Leighton Buzzard, Bedfordshire, was investigated and showed the 'gliders appeared to be operating in, or very close to, controlled airspace at the same level as their aircraft'.The report from UK Airprox Board said the controllers 'looked at one another in disbelief' when the pilot confirmed what was occurring.Near-miss incidents like this in aviation can be terrifying, as demonstrated in this case.There are other examples of it happening, too, with French investigators currently probing a near-miss incident in Nice. So, just how dangerous is it when planes fly too close together?'Most are assessed as low-risk, and collisions between commercial airliners are extremely rare precisely because of these overlapping safeguards. But the risk is not zero,' says Erika Josefsson, legal strategist and air passenger rights expert at AirHelp. When two aircraft get close to each other, it can be seriously riskyHowever, Erika adds: 'Even where there is no collision, sudden avoidance manoeuvres can cause injuries to passengers and crew who are unbuckled or moving around the cabin.' There can be real dangers, too. Monica Badra, founder and CEO, of aviation software company Aero NextGen, told the Daily Mail: 'In terms of danger, a near miss where two aircraft come within a few hundred feet of each other is extremely serious. 'The 1978 Pacific Southwest Airlines mid-air over San Diego and the 2002 Uberlingen collision over Germany, which killed 71 people, are examples of what happens when the system fails. Both are studied extensively in aviation safety training.'What the public often sees reported as near misses are usually loss of separation events where the system worked exactly as designed. Every such event is investigated and fed back into safety improvements.'David Martin, managing director at specialist tooling supplier Heamar, added: 'They're taken extremely seriously, but the term covers incidents with very different levels of risk.'UK classifications range from situations where there was no collision risk through to Category A incidents, where a serious risk of collision existed. A reported near miss therefore doesn't mean disaster was seconds away.'What passengers don't see is the engineering infrastructure supporting all of this. Collision-avoidance, navigation and flight-control systems rely on aircraft being maintained to exceptionally precise standards.'What is classified as a near miss?A near miss in aviation, known as a Near Midair Collision (NMAC) or an Airprox, is when two or more aircraft get dangerously close without actually coming into contact with each other. But the official body investigating such instances says there is a clear distinction between a near miss and an 'Airprox', the official term. According to the UK Airprox Board: 'An Airprox is a situation in which, in the opinion of a pilot or air traffic services personnel, the distance between aircraft as well as their relative positions and speed have been such that the safety of the aircraft involved may have been compromised.'It adds: 'The term "near miss" is in common usage but has no relevance to Airprox. If a pilot or controller is of the opinion that the distance between aircraft as well as their relative positions and speed was such that the safety of the aircraft involved was or may have been compromised then he or she may report an Airprox.'How close together must planes be for it to be considered a risk?It is usually when the distance between planes is less than 500ft.However, the Airprox Board says: 'As the definition of an Airprox makes clear, if the pilot or controller feels that the distance between aircraft as well as their relative positions and speed have been such that the safety of the aircraft involved was or may have been compromised then a report can be filed. An Airprox is when the distance between aircraft and other flying conditions make a situation dangerous 'In some cases, the aircraft are miles apart but an Airprox is filed because the required separation standard has not been met. 'The judgement as to whether or not to file an Airprox is for the pilot and/or controller to make.'David expanded on what this means: 'To put the distance into perspective, UK aviation rules generally require aircraft under air traffic control to maintain defined separation.'Vertical separation is typically at least 1,000 feet up to around 41,000 feet, while surveillance-based horizontal separation can commonly be five nautical miles, reducing to three nautical miles in some high-density airspace.'That means aircraft can still be several miles apart when a required safety margin is breached. Importantly, a loss of separation doesn't automatically mean the planes were seconds from colliding; the direction they are travelling, their altitude and how quickly they are converging all determine how serious the situation actually is.'What happens in this situation?Reports are investigated thoroughly, starting with information gathering: radar recordings, transcripts of what was said over the radio at the time and reports from each of the pilots and/or controllers involved.Further investigation is then carried out, and a comprehensive report is presented to the Airprox Board for assessment.David adds: 'If aircraft do get dangerously close, there are several layers designed to stop the situation from escalating; for example, air traffic controllers can immediately instruct pilots to change altitude or direction, but commercial aircraft also carry collision-avoidance technology known as TCAS.'This works independently of ground-based air traffic control, and if it detects a developing collision threat, it can issue pilots with a Resolution Advisory telling them to climb or descend.'What's interesting is that the system looks at more than distance. It calculates factors including the other aircraft's range, altitude and closure rate to determine how quickly a potential conflict is developing. Two aircraft could still be some distance apart, but if they are rapidly converging, the situation can become much more urgent.'