Changes to the Clearance
Sometimes, certain circumstances call for changes to be made. No controller or pilot is infallible, and even with the best conflict prediction software, it is occasionally necessary to alter a pilot's flight level, speed, or sometimes their entire route.
Unlike domestic control, where these characteristics can be altered without any changes to the flight plan, all changes to an aircraft's altitude, speed and position whilst under Oceanic control are considered modifications to the oceanic clearance and must be handled accordingly.
Let's take a look at the process.
Clearance revisions
There are two primary ways that a clearance can be revised, ‘pilot request’, and ‘controller instruction’.
A pilot can request a change to their speed, route or flight level. The following phraseology examples demonstrate typical pilot-controller exchanges:
Re-route clearance:
BAW123: Shanwick Radio, Speedbird 123 requesting a re-route after 57/20 via 58/15, GOMUP, GINGA.
EGGX_FSS: Speedbird 123, amended route clearance, Shanwick re-clears Speedbird 123 after 57/20 to re-route via 58/15, GOMUP, GINGA.
Meaning: The pilot shall follow their previous route until reaching 57/20, whereby they are cleared to re-route direct 58/15, GOMUP and GINGA.
Level clearance - no restrictions:
ACA123: Gander Radio, Air Canada 123, requesting climb to FL340.
CZQX_FSS: Gander Radio clears ACA123, climb FL340.
Sometimes, the controller may ask the pilot to report when leaving their current flight level, and/or when reaching their cleared flight level. In this case, the pilot should report when they have commenced their climb/descent, or when they have reached their cleared level..
ACA123: Gander Radio, Air Canada 123, reached FL340.
Level clearance - with restriction:
Sometimes the controller must issue a restriction with the level revision. This can be geographically-based (i.e. before/after a certain point) or time-based (i.e. before/after a certain time).
ACA123: Air Canada 123, requesting climb to FL340.
CZQX_FSS: Gander Radio clears ACA123, climb FL340 to be level at or before 2048z. Report leaving, report reaching.
Meaning: The pilot is cleared to climb to FL340, but must do so before 2048z. They must report leaving their current flight level, and report reaching the new flight level.
Speed clearance - no restrictions:
BAW123: Speedbird 123, requesting increase Mach .89.
EGGX_FSS: Speedbird 123, Shanwick clears Speedbird 123 increase Mach .89.
Meaning: The pilot is cleared to increase their speed to Mach .89.
Speed clearance - with restriction:
Just like level clearances, controllers sometimes need to issue restrictions with the speed revision. These are usually a combination of geographically and time-based instructions. It is unusual for a pilot to ask for speed clearances in this way, and as such, it is nearly always controller-initiated.
EGGX_FSS: Speedbird 123, amended speed clearance, Shanwick clears Speedbird 123 cross 52/20 at or before 1623z.
Meaning: The pilot shall adjust their speed accordingly to reach 52/20 before 1623z. It is then assumed that the pilot will resume the Mach number they received in their clearance thereafter.
All of the above exchanges regularly occur without the pilot initiating the request. More often than not, the controller needs to make an amendment without the pilot specifically asking for the change. This is the ‘controller instruction’ method.
Sources
NAT Document 007