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8.7.1 The navigation performance of operators within NAT MNPS Airspace is monitored on a
continual basis. The navigation accuracy achieved by NAT MNPS aircraft is periodically measured and
additionally all identified instances of significant deviation from cleared track are subject to thorough
investigation by the NAT Central Monitoring Agency (CMA), currently operated on behalf of the NAT SPG
by the UK National Air Traffic Services Limited.
8.7.2 When a GNE is identified, follow-up action after flight is taken, both with the operator and
the State of Registry of the aircraft involved, to establish the reason/cause and to confirm the approval of the
flight to operate in NAT MNPS Airspace. The format of the (navigation) Error Investigation Form used for
follow-up action is as shown at Attachment 1. Operational errors can have a significant effect on the
assessment of risk in the system. For their safety and the safety of other users, crews are reminded of the
importance of co-operating with the reporting OAC in the provision of incident information.
8.7.3 The overall navigation performance of all aircraft in the MNPS Airspace is continually
assessed and compared to the standards established for the Region, to ensure that the TLS is being
maintained
NORTH ATLANTIC MNPSA OPERATIONS MANUAL CHAPTER 9
NAT MNPS 53 Edition 2005
Chapter 9: RVSM Flight in MNPS Airspace
9.1 GENERAL
9.1.1 The aircraft altimetry and height keeping systems necessary for flying in RVSM airspace are
capable of high-performance standards. However it is essential that stringent operating procedures are
employed, both to ensure that these systems perform to their full capabilities and also to minimise the
consequences of equipment failures and possible human errors.
9.1.2 As is the case with lateral navigation systems, technical failures of altimetry and/or height
keeping systems are extremely rare within the NAT MNPSA. However, less rare in the NAT MNPSA are
situations in which an aircraft is flown at a level other that cleared by ATC. ATC Loop Errors, when there is
a misunderstanding or miscommunication between ATC and the pilot over the actual cleared level,
unfortunately do occur. In an SSR environment ATC are alerted to any such error immediately the aircraft
departs from the cleared level. Furthermore with Direct Controller Pilot Communications (DCPC) the
controller can instantly intervene to resolve the situation and/or to provide potential conflict warnings to
other traffic. In the NAT MNPSA SSR coverage is very limited and regular air/ground ATC
communications are conducted via a third party radio operator, most commonly using HF.
9.1.3 Severe turbulence in the NAT MNPSA is uncommon but mountain waves in the vicinity of
Greenland and clear air turbulence associated with jet streams are not unknown. Aircraft encountering such
conditions can inadvertently depart from their cleared levels or the pilot may elect to change level to avoid
the effects of the turbulence. Other circumstances also occur in which the pilot will be forced to change
level, before an appropriate ATC re-clearance can be obtained, e.g. power or pressurisation failure, freezing
fuel, etc. Again, without surveillance or DCPC, there can be a significant lag between the aircraft’s
departure from its cleared level and any possible action from the controller to provide separation from any
other potentially conflicting traffic.
9.1.4 It must be appreciated that the NAT MNPSA is the busiest oceanic airspace in the world.
Furthermore, NAT traffic is comprised of a very wide range of aircraft types, flying a wide range of sector
lengths and carrying a significant range of loads. As a result, optimum flight levels vary over the whole jet
cruising range and nearly all the flight levels of the core tracks of the OTS, during peak hours, are fully
occupied. Also, the Mach Numbers flown can vary significantly (e.g. typically between M0.78 and M0.86),
resulting in up to 20 mins variation in NAT transit times. Given that the nominal longitudinal separation
standard employed in the NAT MNPSA is 10 mins, one consequence of the foregoing is that it is rare for any
NAT OTS flight to transit the NAT without overtaking, or being overtaken, by another aircraft at an adjacent
level on the same track. It will therefore be seen that an on-track departure from cleared level in the NAT
MNPSA will involve a significant risk of conflicting with other traffic. Furthermore, given the extreme
accuracy of lateral track keeping provided by modern LRNSs (e.g. GPS) such conflict risk can translate to a
collision risk. It is primarily with this in mind that the Strategic Lateral Offset Procedure (see
“Strategic Lateral Offset Procedure (SLOP)” above in Chapter 8) has been established as a standard
　
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