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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