Air navigation book11/4/2022 ![]() The ICAO MNPS working group declared a volume of airspace in which it authorised a reduction in separation between parallel tracks, initially to 60 nautical miles and eventually to 30 nautical miles, on the basis of statistical studies showing that the probability of a hazardous navigational infringement was acceptable. Even in the afternoon westbound flights the sheer volume of aircraft wanting to fly the tracks available was approaching the capacity available. There were operational implications of having widely spaced tracks, because only a few aircraft could get adequately close to the desirable jetstreams that would help propel them eastbound on night flights. The 747 and smaller aircraft that were INS- retrofitted were plying the North Atlantic by 1973 and track-keeping with an unprecedented accuracy. This is a military-derived technology, autonomous to the extent that it does not need radio guidance and has a time-dependent position-fixing error characteristic, which at the time of introduction was about 1 nautical mile per hour. In 1970 the Boeing 747 entered service and brought a new navigation system that had been trialled over the Pacific Ocean, called the inertial navigation system (INS). 1950) had concluded that 120 nautical miles was the lowest acceptable separation. An early task for the ICAO had been to assess the probability of two aircraft on adjacent tracks wandering close enough together to represent an unacceptable risk, and the study (circa. As it came into range of the second Loran system, the aircraft would regain spatial position data and the error level diminished as the coastal fringes were reached. Therefore, even in the days when it was usual for jet airliners to ply transAtlantic, an aircraft could often deviate significantly off-track. After one hour relying on the radar the error could be up to 25 nautical miles, and after a further hour as much as 30 nautical miles. This error was time-dependent, and was about 5 nautical miles per hour on a jet airliner. The aircraft position was known to within about 20 nautical miles at the point where the crew might turn their attention to the Doppler radar. ![]() This was a downward-looking radar that had a number of beams, in which the Doppler shift from ground returns could be analysed to determine the aircraft's groundspeed and ground-based track (as distinct from the compass- measured heading).Ī navigational-error audit of this navigational process was not flattering. To combat this, the crew had a device called a Doppler radar. Its HF radio reception could fail to give an accurate position as the Heaviside layers from which radiation was reflected in the ionosphere would dissipate with solar excitation. It worked well, although night-time crossings (the best-suited in scheduling terms for eastbound operations) often fell foul of a basic inadequacy of Loran. This was a practice that had stemmed from earlier generation piston-engine era operations. There was a point of no return (PNR) that was the critical point from where, once passed, their least fuel to an aerodrome option was to press ahead. The flight engineer would control the fine-tuning of engine fuel settings and the three-man crew monitored their progress against fuel predications on what was called a 'howgozit' chart. Seasoned navigators could cope with the inaccuracies, and in many cases the airlines use pilots who were also trained as navigators, so they would periodically check their position on Loran. There were two Loran chains (sets of master and slave radio transmitters) on either side of the North Atlantic, and their high-frequency (HF) radio signals presented a position report to the aircraft crew that was very accurate near the coastlines but increasingly less accurate as they flew towards the mid-Atlantic. ![]() Until around 1970 the acknowledged way of navigating the first- generation (narrow-body) jet airliners across the North Atlantic had been by using a hyperbolic area-navigation system called Loran. ![]() This led to new legislation in the late 1970s. ![]() An ICAO committee was set up at that time to assess a North Atlantic Minimum Navigation Performance Specification (MNPS). Within the airspace community there was an equivalent conundrum in the early 1970s, when jet airliners were clogging the available capacity for aircraft operations over the North Atlantic. In many respects the FANS committee addressed a well-trodden path. Mike Hirst, in The Air Transport System, 2008 Airspace safety ![]()
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