Guidance concerning Air Navigation in and above the NAT MNPSA
RVSM FLIGHT IN MNPS AIRSPACE
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.
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 immediately when 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 Voice communications are usually conducted via a third party radio operator.
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.
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 Procedures (see “Strategic Lateral Offset Procedures (SLOP)” above in Chapter 8 have been established as a standard
operating procedure in the NAT Region.
For flight through the NAT MNPS Airspace the aircraft and the operator must have the
appropriate State Approvals for both MNPS and RVSM operations. The crew must be qualified for flight in RVSM airspace and all aircraft intending to operate within NAT MNPS Airspace must be equipped with altimetry and height-keeping systems which meet RVSM Minimum Aircraft System Performance Specifications (MASPS). RVSM MASPS are contained in ICAO Doc 9574 (Manual on Implementation of a
NAT Doc 007