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Guidance concerning Air Navigation in and above the NAT MNPSA

CHAPTER 12

through their level. However, since January 2009, some NAT ATC units are utilising a procedure which permits ATC to clear an aircraft to climb or descend through the level of another aircraft, with as little as 5 minutes longitudinal separation, provided that both aircraft are using GNSS (GPS) for position determination

and reporting. Many NAT aircraft request and are cleared at lesser Machs than 0.85. A 5 minutes in separation between two aircraft flying at M0.80 and experiencing a headwind component of 30 Kts

unusual for W/B NAT flights), will rounding/truncating protocols used by

equate pilots,

to approx 35 FMSs and/or

NMs.

ATC

Furthermore, depending upon

Flight

Data

Processing

Systems

trail (not the (for

“minutes and seconds” to “minutes”), a nominal 5 minutes separation can in fact minutes (it can, of course, also be 6 minutes). In such a circumstance the actual could be less than 30 NMs. In these cases TCAS may register targets.

be close to an actual 4 longitudinal separation

12.6.5

The rule allowing ATC to use this procedure includes a caveat that the climb or descent

needs to be undertaken within 10 minutes of the time that the second aircraft in the pair has passed a common reporting point. Consequently, the pilot of an aircraft cleared for a climb or descent of more than a single flight level, should be alerted to the possibility of a potential TCAS alert by the controller’s use of the conditional phrase “By” or “At or Before” in the clearance received. However, the pilot of the “passive participant” aircraft of the 5 minutes separated pair, if it is the following aircraft, could be presented with a “pop-up” TCAS target without such a warning. The bulletin announcing the introduction of this procedure in the North Atlantic includes the following instruction;- “If there is any concern regarding the proximity of another aircraft, flight crews must not hesitate to clarify the situation and take appropriate action to ensure the safety of the flight.” However, given the air/ground communications methods employed in the NAT, the pilot may not receive a response to such a request for “clarification” prior to the other aircraft passing its flight level. Nevertheless, even at these separations, Resolution Advisories are not anticipated and it is not

expected that pilots will consider deviating from their clearance as “appropriate action”.

NAT Doc 007

81

Edition 2010

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