What defines a 'near miss'..?

[Good Old Bakes]

Can anyone tell me how close two aircraft approaching each other would need to be apart for it to be classed as a 'near miss'?

I've just watched two aircraft 'merge' with only 1,000 feet or so between them (26,000 ft and 27,000 ft). I would have thought that's pretty close for two planes traveling towards each other at 400mph plus?

[The Sage of Milton]

Can anyone tell me how close two aircraft approaching each other would need to be apart for it to be classed as a 'near miss'?

I've just watched two aircraft 'merge' with only 1,000 feet or so between them (26,000 ft and 27,000 ft). I would have thought that's pretty close for two planes traveling towards each other at 400mph plus?

That's no problem. It happens all the time. You may even hear the controller say something like "Air Canada 868, maintain FL350 on reaching. Traffic 1000 feet above, opposite direction."

[LEMD]

Reduced vertical separation minima or minimum (RVSM) is the reduction of the standard vertical separation required between aircraft flying between FL290 (29,000 ft) and FL410 (41,000 ft) inclusive, from 2,000 feet to 1,000 feet (or between 8,850 and 12,500 m from 600 m to 300 m). This therefore increases the number of aircraft that can safely fly in a particular volume of airspace.
Historically, standard vertical separation was 1,000 feet from the surface to FL290, and 2,000 feet above FL290. This was because the accuracy of the pressure altimeter (used to determine altitude) decreases with height. Over time, air data computers (ADCs) combined with altimeters have become more accurate and autopilots more adept at maintaining a set level, therefore it became apparent that for many modern aircraft, the 2,000 feet separation was too cautious. It was therefore proposed by ICAO that this be reduced to 1,000 feet.
In the US this program was known as the Domestic Reduced Vertical Separation Minimum (DRVSM).

source: wikipedia

[Tony15]

I have often seen two aircraft travelling trans Atlantic almost together. What is the minimum distance they can approach each other and fly "in company"?

[LEMD]

North Atlantic Tracks (NAT) are trans-Atlantic routes that stretch from the northeast of North America to western Europe across the Atlantic Ocean. They ensure aircraft are separated over the ocean, where there is little radar coverage.

Entrance and movement along these tracks is controlled by special Oceanic Center air traffic controllers to maintain separation between airplanes. The primary purpose of these routes is to provide a Minimum Time Route (MTR). They are aligned in such a way as to minimize any head winds and maximize tail winds impact on the aircraft. This results in much more efficiency by reducing fuel burn and flight time. To make such efficiencies possible, the routes are created daily to take account of the shifting of the winds aloft. North Atlantic Tracks are published by Shanwick Center (EGGX) and Gander Center (CZQX), in consultation with other adjacent air traffic control agencies and airlines.



Prior to departure, airline flight dispatchers/flight operations officers will determine the best track based on destination, aircraft weight, aircraft type, prevailing winds and Air Traffic Control route charges. The aircraft will then contact the Oceanic Center controller before entering oceanic airspace and request the track giving the estimated time of arrival at the entry point. The Oceanic Controllers then calculate the required separation distances between aircraft and issue clearances to the pilots. It may be that the track is not available at that altitude or time so an alternate track or altitude will be assigned. Planes cannot change assigned course or altitude without permission.
Contingency plans exist within the North Atlantic Track system to account for any operational issues that occur. For example, if an aircraft can no longer maintain the speed or altitude it was assigned, the aircraft can move off the track route and fly parallel to its track, but well away from other aircraft. Also, pilots on North Atlantic Tracks are required to inform air traffic control of any deviations in altitude or speed necessitated by avoiding weather, such as thunderstorms or turbulence.
Despite advances in navigation technology, such as GPS and LNAV, errors can and do occur. While typically not dangerous, two aircraft can violate separation requirements. On a busy day, aircraft are spaced approximately 10 minutes apart. With the introduction of TCAS, aircraft traveling along these tracks can monitor the relative position of other aircraft thereby increasing the safety of all track users.
Since there is little radar coverage in the middle of the Atlantic, airplanes must report in as they cross various waypoints along each track, their anticipated crossing time of the next waypoint, and the waypoint after that. These reports enable the Oceanic Controllers to maintain separation between planes. These reports can be made to dispatchers via a satellite communications link (CPDLC) or via High Frequency (HF) radios. In the case of HF reports, each aircraft operates using SELCAL (Selective Calling). The use of SELCAL allows an aircraft crew to be notified of incoming communications even when the aircraft's radio has been muted. Thus, crew members need not devote their attention to continuous radio listening. If the aircraft is equipped with Automatic Dependent Surveillance, (ADS-C & ADS-B), voice position reports on HF are no longer necessary, as automatic reports are downlinked to the Oceanic Control Centre. In this case, a SELCAL check only has to be performed when entering the oceanic area and with any change in radio frequency to ensure a working backup system for the event of a datalink failure.

Additionally from 10 June 2004 the Strategic Lateral Offset Procedure (SLOP) was introduced to the North Atlantic airspace to reduce the risk of mid-air collision by spreading out aircraft laterally. It reduces the risk of collision for non-normal events such as operational altitude deviation errors and turbulence induced altitude deviations. In essence, the procedure demands that aircraft in North Atlantic airspace fly track centreline or one or two nautical mile offsets to the right of centreline only. However, the choice is left up to the pilot.

[Tony15]

Thanks LEMD for your comprehensive answer, I think that covers my question.
Regards, Tony.