Parallel Offset Route. A parallel track to the left or right of the designated or established airway/route. Normally associated with Area Navigation (RNAV) operations.
Parallel offset enables the aircraft to fly a path parallel to, but offset left or right from, the original active route (parent route). All performance requirements and constraints of the original route are applied to the offset route. It is applicable only for en-route segments and is not foreseen to be applied on standard instrument departures (SIDs), standard instrument arrivals (STARs) or approach procedures. The execution of a parallel offset includes sharper turns compared to what is normally done in the en-route phase of the flight. The intercept angle of the new flight path is normally 30 degrees.
The capability to implement parallel offset is a mandatory requirement for RNP4 and A-RNP certification.
Examples of parallel offset application include:
An alternative to radar vectoring. This is normally done on a controller's initiative. For example, if a fast aircraft is following a slow aircraft on the same track, an instruction by ATC to fly oﬀset by several nautical miles can allow the faster aircraft to overtake and be climbed/descended through the slower aircraft. The deviation used is normally the horizontal separation minimum and additional safety buffer may be added at the controller's discretion. For example, if the separation minimum is 5 NM, an instruction for parallel offset would most likely require the aircraft to fly 6 or 7 miles to the side of the track. It should be noted however that although many P-RNAV certified aircraft have this function, it is not a requirement for e.g. RNP2 and RNP1 certification and therefore there is no guarantee that an aircraft is capable of executing the manoeuvre.
Example of controller employing parallel offset to separate aircraft
Avoidance of wake turbulence. This is normally done on a pilot's initiative. When the pilot suspects that the turbulence being experienced is caused by the wake of the preceding aircraft flying the same route they may ask the controller to deviate slightly to the left or right in order to avoid it. This situation is normally associated with smaller deviations, usually up to 3 miles to the left or right of the airway.
Example of parallel offset use to avoid wake turbulence
Weather avoidance. Pilots may sometimes request to avoid convective clouds (CBs) by flying a parallel offset instead of a heading change. The magnitude of the deviation may be much greater than what is normally expected in the separation or wake turbulence scenarios and may often be more than 10 or 15 nautical miles.
ICAO Doc 4444 PANS-ATM Chapter 12 defines the following phraseology to be used in parallel offset operations:
ADVISE IF ABLE TO PROCEED PARALLEL OFFSET - used by the controller to determine whether such a manoeuvre is feasible.
PROCEED OFFSET (distance) RIGHT/LEFT OF (route) (track) [CENTRE LINE] [AT (significant point or time)] [UNTIL (significant point or time)] - the instruction format for starting a parallel offset.
Example: PRE0405 proceed offset 5 miles left of track until abeam SNA VOR
CANCEL OFFSET (instructions to rejoin cleared flight route or other information)
Example: PRE0405 cancel offset, proceed direct to SNA VOR
REQUEST OFFSET (distance) RIGHT/LEFT OF (route) (track) [CENTRE LINE] [DUE TO WAKE TURBULENCE/TO AVOID WEATHER] - a pilot request to perform a parallel offset. Note that while this is commonly used format it is not a part of the ICAO Standard Phraseology and therefore variations do exist.
Example: PRE0405 request 2 miles left of track due to wake turbulence