Guidance concerning Air Navigation in and above the NAT MNPSA
In-Flight – Entering and Flying in MNPS Airspace
One automatic altitude-control system should be operative and engaged throughout the
cruise. This system should only be disengaged when it is necessary to retrim the aircraft, or when the aircraft
encounters turbulence and operating procedures dictate.
When passing waypoints, or at intervals not exceeding 60 minutes (whichever occurs
earlier), or on reaching a new cleared flight level, a cross-check of primary altimeters should be conducted. If at any time the readings of the two primary altimeters differ by more than 200 ft, the aircraft’s altimetry system should be considered defective and ATC must be informed as soon as possible.
To prevent unwanted TCAS/ACAS warnings or alerts, when first approaching any cleared
flight level in NAT RVSM airspace, pilots should ensure that the vertical closure speed is not excessive. It is considered that, with about 1500 ft to go to a cleared flight level, vertical speed should be reduced to a maximum of 1500 ft per minute and ideally, to between 1000 ft per minute and 500 ft per minute. Additionally, it is important to ensure that the aeroplane neither undershoots nor overshoots the cleared level
by more than 150 ft, manually overriding if necessary.
Abnormal operational circumstances (e.g. engine failures, pressurisation problems, freezing
fuel, turbulence, etc.), sometimes require a pilot to change level prior to obtaining a re-clearance from ATC. Such a timely re-clearance is more difficult to obtain in oceanic or remote areas where DCPC are not necessarily available. This is indeed the case in NAT MNPS Airspace, in which the majority of ATS communications are conducted indirectly through a third party radio operator, utilising HF, SATCOM Voice or GP/VHF facilities. As previously indicated, extreme caution and vigilance should be exercised when executing any such (uncleared) level changes, as the potential collision risk (particularly in the OTS) is
It must also be recognised that even under normal operations when using such indirect
communication methods, there does exist the potential for misunderstanding between pilot and controller regarding the detail of any issued clearances or re-clearances. Occasionally, such “ATC Loop Errors” can lead to an aircraft being flown at a level other than that expected by the controller. In such circumstances separation safety margins may be eroded. To avoid possible risks from any of the foregoing situations, it is therefore essential in NAT MNPS Airspace that pilots always report to ATC immediately on leaving the current cruising level and on reaching any new cruising level.
The Strategic Lateral Offset Procedures (SLOP) described in Chapter 8, paragraph 8.5, have
been established as a standard operating procedure in the NAT Region to assist in mitigating the potential
risks of any of the foregoing height deviations or errors.
The following equipment failures must be reported to ATC as soon as practicable following
loss of one or more primary altimetry systems; or
failure of all automatic altitude-control systems
The aircraft should then follow the appropriate procedure described in Chapter 12 - “Special
Procedures for In-Flight Contingencies”, or as instructed by the controlling ATC unit.
NAT Doc 007