Phraseology

This section focuses on the radio telephony (RTF) specifically required for PBN.  This is not the only phraseology stakeholders should be aware of and complete guidance of all appropriate RTF is given in PANS ATM (ICAO Doc 4444).

Chapters 6 and 12 of PANS ATM (ICAO Doc 4444) details the radiotelephony (RTF) phraseology agreed at the global level and which is designed to ensure safe and unambiguous instructions are issued and understood.  Pilots, controllers and other personnel operating close to aircraft or the movement surfaces need to learn and apply the agreed phraseology.

ICAO provides awareness training material on SID/STAR clearances which can be found here.

DEPARTURE

A basic clearance to depart on a published RNAV or RNP SID will be given by ATC:

C/S CLEARED (designation) DEPARTURE

If the aircraft is unable to fly the designated SID, for whatever reason, the pilot should respond with:

UNABLE (SID) DUE TO [reason e.g. AIRCRAFT TYPE]

Should the controller be informed that the aircraft is unable to take the allocated SID, the he/she should issue a conventional SID; all aircraft are capable of flying a conventional SID.  Under the new PBN Implementing Regulation, aerodromes may continue to provide conventional procedures until 6 June 2030.  From June 2030 onwards, conventional procedures will have to be withdrawn except as contingency measures.


If no speed or altitude restrictions are stipulated by the controller, the aircraft will comply with the published constraints for that procedure. 

Prior to departure or once airborne, should the controller wish to place a level restriction on the departure he/she can clear the aircraft as follows:

  • 'C/S CLIMB VIA SID TO (level)' 

The aircraft will comply with all altitude and speed constraints that have been published up to that altitude.  Once the cleared level has been reached the pilot is to ensure that the explicit instruction is followed as it overrules a published constraint. 

Therefore, should the controller wish to alter any of the published requirements of the SID, he/she can cancel either the height or speed constraint as follows:

  • 'C/S [CLIMB VIA SID TO (level)], CANCEL LEVEL AND/OR SPEED RESTRICTION(S)'

The aircraft will follow the lateral profile of the SID and the pilot is required to comply with the ATC issued altitude and/or speed control instructions as applicable. 

The controller can stipulate at what point the constraints can be lifted if there are factors which limit when the altitude or speed constraints are to be applied:

  • 'C/S CLIMB VIA SID TO (level)], CANCEL LEVEL AND/OR SPEED RESTRICTION(S) AT (point(s))'

Remember, an explicit instruction issued by ATC overrides the published constraint for that procedure.


The controller may elect to provide the aircraft with a shorter routing by providing the pilot with a ‘Direct’ to a subsequent waypoint on the procedure.  The controller will use the following phraseology:

  • C/S CLEARED [or PROCEED] DIRECT TO (waypoint)

The use of PBN in Terminal airspace does not change existing responsibilities:

When vectoring an IFR flight and when giving an IFR flight a direct routing which takes the aircraft off an ATS route, the controller shall issue clearances such that the prescribed obstacle clearance will exist at all times until the aircraft reaches the point where the pilot will resume own navigation. When necessary, the relevant minimum vectoring altitude shall include a correction for low temperature effect. (PANS ATM Para 8.6.5.2)

Therefore, it does not relieve pilots of their responsibility to ensure that any clearances are safe in respect to terrain clearance.

It does not relieve the controllers of their responsibility to assign levels which are at or above established minimum flight altitudes.

The pilot remains responsible for terrain clearance and ATC will assign levels consistent with the Minimum Flight Altitude (MFA).


If the pilot accepts the amended clearance, he/she will select that waypoint in the flight plan list and execute the Direct To.  Controllers should be aware that any intermediate published altitude or speed restrictions will not be complied with.  Similarly, should the Direct To be to a waypoint which is the initiation of a Radius to Fix (RF) turn or to the end of that turn, the aircraft will ‘Fly By’ the waypoint and not perform a ‘managed’ transition.


Controllers should only make use of ‘Direct To’ instructions to waypoints on the assigned SID/STAR.

If the aircraft is to be directed to a waypoint not on the SID/STAR then it will take time and effort for the pilot to locate that waypoint in the NAV Database. 

It is recommended that the controller radar vectors rather than use waypoints not associated with the procedure.

Equally, pilots are operationally not allowed to create a user defined or manually created waypoints in terminal airspace. 

Finally, as the computer dynamically removes active waypoint once they have been passed, ATC should only direct an aircraft to a future waypoint on the procedure.

UNABLE RNAV

If the pilot is unable to fly area navigation, then by that nature the aircraft can not deliver PBN; the key enabler for PBN is the ability to perform area navigation. The three causes of the aircraft not being able to deliver area navigation are an avionics failure, the aircraft is not certified or the crew is not operationally approved for the operation.

The pilot should inform ATC by transmitting:

  • UNABLE RNAV DUE TO (reason, e.g. NAV SYSTEM FAILURE or AIRCRAFT TYPE)

Unable RNAV means that the aircraft will have to be conventionally managed.  This may be achieved by instructing the aircraft to fly inbound/outbound on a specific VOR radial; this would be subject to the aircraft being fitted with that sensor, the NAVAID being operational and the navigation sensor still working and feeding to a navigation display.  Alternatively, the controller can issue radar vectors to maintain safe separation.


UNABLE RNP

Whereas the causes of ‘UNABLE RNAV’ are relatively simple to understand and the impact clear, an ‘UNABLE RNP’ alert can be caused for a number of reasons and this does not automatically mean that the aircraft is unable to perform area navigation; however, what level of continued performance will need to be established by ATC. The pilot should inform the controller immediately the UNABLE RNP caption illuminates in the cockpit by transmitting:

  • UNABLE RNP (specify type) DUE TO (reason, e.g. LOSS OF RAIM or RAIM ALERT)

As there is no automatic downlinking of this alert, if the pilot fails to transmit this information then ATC will have no idea that there is a possible navigational problem with the aircraft.

If the cause of the UNABLE RNP caption is due to a navigation computer failure then this would have the same impact as UNABLE RNAV and require an alternative, conventional form of navigation.


However, losses of the primary navigation infrastructure (GPS) due to signal interference, constellation failure or a loss of RAIM could cause the avionics to declare UNABLE RNP.  If the State has considered in its contingency measures, that a redundant DME/DME infrastructure is to be maintained and provided the aircraft is fitted with DME/DME as an alternative positioning source, then those aircraft would still be able to perform area navigation with a performance accuracy of +/-1 NM at least.  It should be noted that the loss of GNSS will impact RNP approach operations as DME/DME is not able to support a performance of +/- 0.3 NM or lower.  More information on GNSS reversion can be found in the space-based infrastructure section of Understanding PBN. 

If the loss of the GNSS signal is the cause of UNABLE RNP, the controller will need to assess if this is an individual aircraft issue or a wider problem affecting multiple or all aircraft within the sector.  Multiple aircraft declaring UNABLE RNP would indicate either wide area inference, possibly deliberate, or an issue with the satellite constellation.  This information will need to be communicated throughout the ATM system.

A Loss of RAIM alert will be issued by the avionics when the number of visible satellites falls below 5.  This impacts the availability of the integrity function of the receiver and is not necessarily indicating the loss of the GPS signal.  Similarly, a RAIM alert indicates that the receiver has detected erroneous satellite signal(s) and that the position estimation should be cross-checked.  In both cases these alerts do not necessarily mean that the aircraft cannot continue to perform area navigation.

Another possible cause of UNABLE RNP illuminating could be due to pilot inattention.  If the pilot allows the Actual Navigation Performance (ANP) to exceed the Required Navigational Performance (RNP) for that leg of the operation then an alert will be given.  The probability of the alert not occurring in normal circumstances is one time in 10 million per flight hour (10-7). 

In abnormal situations or due to navigation degradation, the system will declare UNABLE RNP when the avionics is unsure of its position.  The boundary of this alert is 2 x RNP for that operation.   Therefore, when the uncertainty in position is sensed to be at or greater than 2 time the RNP the pilot should be alerted; the probability of this alert not being given is one time in 100 000 per flight hour (10-5).  Controllers should understand that, in this circumstance, the avionics does not know where the aircraft is and it is perfectly possible that the aircraft’s true position still appears to be meeting the requirements of the airspace.

Therefore, controllers need to establish what the impact of the UNABLE RNP is.  Pilots should provide clarity as to the cause of the warning if it is known.  Airspace designers should consider the loss of GNSS and area navigation capabilities and develop appropriate mitigations that will then support contingency measures.

Not all causes of UNABLE RNP are due to the loss of GNSS.  However, if there is a specific GNSS issue then there is specific phraseology for both pilots and ATC to use.

A GNSS outage can be caused by an avionics failure on-board the aircraft. This will affect this aircraft alone.  The pilot can communicate the problem using the following RTF call:

  • BASIC GNSS UNAVAILABLE [DUE TO (reason, e.g. NAV SYSTEM FAILURE)]

The term basic, although still in PANS ATM (ICAO Doc 4444), is redundant and is considered not necessary.   ATC will need to ascertain whether the loss of the primary positioning source will affect aircraft performance.

ATC should understand that if the aircraft is performing a RNP approach, where the performance is required to be less than +/- 1 NM, then a missed approach should be expected.

The loss of the GPS signal is across a wide area could occur if there is interference on the frequency, there is a space weather event or if the constellation suffers a failure or human error.  Once ATC is aware of the dimensions of the affected area then this information can be relayed to the pilots using the following phraseology:

  • GNSS REPORTED UNRELIABLE

or

  • GNSS MAY NOT BE AVAILABLE [DUE TO INTERFERENCE]

and can be amplified with:

  1. ...IN THE VICINITY OF (location) (radius) [BETWEEN (levels)]

or

  1. ...IN THE AREA OF (description) (or IN (name) FIR) [BETWEEN (levels)]

If ATC has prior knowledge of a GNSS outage, this could be due to a military exercise or other publically notified reasons, the controller can inform the flight crews as follows:

  • BASIC GNSS (or SBAS, or GBAS) UNAVAILABLE FOR (specify operation) [FROM (time) TO (time) (or UNTIL FURTHER NOTICE)]

The pilot can inform ATC when the aircraft is not receiving a signal or service from an augmentation system by transmitting:

  • GBAS (or SBAS) UNAVAILABLE

Finally, ATC can confirm that the aircraft is again able to navigate using the space-based infrastructure by using the following phraseology:

  • CONFIRM GNSS NAVIGATION

and once the GNSS signal is available and the aircraft is able to navigate with it, the pilot is expected to respond as follows:

  • AFFIRM GNSS NAVIGATION

ATS routes are coded into the Navigation Database as Airways Records.  These records, which are defined in the Industry standard ARINC 424, allow a performance requirement to be coded for each leg and how the aircraft will transition to the next leg as it approaches the waypoint.  The aircraft can fly over, fly by or, if coded, execute a Fixed Radius Transition (FRT). 

FRTs can enable reduced spacing between routes to be maintained on both the straight and turning segments of the ATS route; this can provided strategically designed separation enabling capacity growth within the target airspace. 

If the airspace designer has planned for FRTs on a specific ATS route, then how the route clearance is given can influence how the aircraft will transition between the legs of that cleared route.  The cause of this is how the route data is extracted from the navigation database.  If the controller gives a clearance as:

  • C/S CLEARED [or PROCEED] (waypoint) VIA (waypoint)

or

  • C/S CLEARED [or PROCEED] (waypoint) DIRECT (waypoint(s))

The pilot may pull out the waypoints from the Navigation Database in the cleared sequence to create the ATS route.  In this case, any waypoint transitions designated as FRT in the Airway Record will not be created and executed and the aircraft will ‘Fly By’ the waypoint. 

However, if the pilot loads the ATS route by calling out the route from the Nav DB using the designated route title, then the performance requirements and FRTs associated to that Airways Record will be loaded.  Therefore, if the route has been correctly coded by the Datahouse, and ATC gives a clearance along that ATS route as follows:

  • C/S CLEARED ON (ATS route designator) TO (waypoint) 

Then provided that the ATS route has been designed correctly (taking account enough leg distance prior and post the waypoint for the FRT to be executed), aircraft capable of executing FRT will demonstrate consistent and highly repeatable turn performance.

FRT is still very immature, and is not exercised in the en-route environment today.

SID/STAR phraseology allows ATC and aircrew to communicate and understand detailed clearance information that would otherwise require long and potentially complex transmissions.

ICAO has provided core phraseology that positively reinforces that the lateral, vertical and speed requirements embedded in a SID or STAR will continue to apply, unless explicitly cancelled or amended by the controller

The terrain clearance responsibilities prescribed in ICAO Doc 4444 (PANS-ATM) 8.6.5.2 do not change.

While pilots and ATS providers are expected to comply with the revised phraseology, in unusual or unforeseen circumstances it may not be possible to apply the phraseology as intended. Should this happen, pilots and ATS personnel are still expected to use plain language, which must be as clear and concise as possible.

ICAO has a training package considering different possible scenarios, which can be found here.

 

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