Guidance concerning Air Navigation in and above the NAT MNPSA
carriage of dual GPS to allow the required on-board performance monitoring and alerting which would be necessary for the closer track spacing then envisaged.
Note: For more detailed information on RNP see ICAO Document Doc 9613 – ‘Performance Based Navigation Manual’
Obviously, there are several combinations of airborne sensors, receivers, computers with
navigation data bases and displays which are capable of producing like accuracies, and which with inputs to automatic flight control systems provide track guidance. However, regardless of how sophisticated or mature a system is, it is still essential that stringent navigation and cross checking procedures are maintained if Gross Navigation Errors (GNEs) are to be avoided. A GNE within NAT Airspace is defined as a deviation from cleared track of 25 NM or more. Some of these errors are detected by means of long range radars as aircraft leave oceanic airspace. Other such errors may also be identified through the scrutiny of routine position reports from aircraft.
All reported navigation errors in North Atlantic airspace are thoroughly investigated.
Records show that navigation equipment or system technical failures are now fortunately rare. However, when they do occur they can be subtle or progressive, resulting in a gradual and perhaps not immediately discernible degradation of performance. Chapter 11 of this Manual provides guidance on detection and
recovery when such problems are encountered.
Unfortunately, human failings produce the vast majority of navigation errors in the North
Atlantic Region. As indicated above, while the flexible OTS structure and the employment of a 60 NM lateral separation standard, provide for highly efficient use of NAT airspace, they also bring with them a demand for strictly disciplined navigation procedures. About half of NAT flights route via an OTS track and a large portion of the remaining random flights follow routes that at some point approach within one or two degrees of the outermost OTS tracks. One consequence of this is that a single digit error in the latitude of one significant point of an aircraft’s route definition will very likely lead to a conflict with another aircraft which is routing correctly via the resulting common significant point. Ironically, the risk of an actual collision between two aircraft routing via a common point, as is the case when such errors are made, is
further exacerbated by the improved technical accuracy of the modern navigation equipment employed.
Today in North Atlantic operations the predominant source of aircraft positioning
information is that of GPS. This includes aircraft that use stand-alone GPS equipment and aircraft where GPS positioning information is integrated into the system navigation solution (e.g. a GPS / IRS mix). The accuracy of GPS navigation is such that the actual flight paths of any two GPS equipped aircraft navigating to a common point will almost certainly pass that point within less than a wingspan of each other. Given that the North Atlantic is the most heavily used oceanic airspace anywhere in the world, it must therefore be
appreciated that even a single digit error in just one waypoint can result in a significant conflict potential.
The importance of employing strict navigation system operating procedures designed to
avoid the insertion of wrong waypoints or misunderstandings between pilots and ATC over cleared routes cannot be over-emphasised. The principles embodied in many of the procedures described in this Chapter are aimed squarely at the prevention of such problems.
Many of the procedures listed in this Chapter are not equipment specific and others may not
be pertinent to every aircraft. For specific equipment, reference should be made to Manufacturers' and
operators' handbooks and manuals.
There are various references in this material to two pilots; however when carried, a third
crew member should be involved in all cross check procedures to the extent practicable.
NAT Doc 007