Ground Operations

Whilst it is the pilots’ responsibility to ensure that they and their aircraft meet the airspace requirements, the ATCo does not want the extra workload imposed when an airspace user declares that it is not able to accept the clearance given ‘due to RNAV type’.

This is especially pertinent when clearing aircraft on Instrument Flight Procedures (SIDs and STARs especially).

There are two aspects to limiting this possible workload increase.  The first is by limiting (or eliminating) mixed mode operations by use of an airspace mandate.  The second is by providing the controller with awareness of the aircrafts’ capabilities.

Mixed Mode Operations versus a Mandate

From a perspective of ATC, to have a single level of capability operating in the airspace would mean that, subject to serviceability, all aircraft would be able to accept the appropriate clearances.  However, operators do not want to be forced into fitting or retro-fitting new equipment onto their aircraft without a very clear cost benefit argument.  Unfortunately, due to:

• the inability to move SIDs/STARs due to environmental and even political limitations;
• the need to retain the largest route spacing for the lowest navigation performance;
• the inability, in some operations, to reap the benefits of reduced obstacle clearance criteria for more high performing navigation specifications,

means that capacity, environmental and efficiency benefits of PBN are not necessarily realised.   Furthermore, AUs who have advanced avionic capabilities wishing to benefit from their investment leads to pressure on the Service Providers to support Best Equipped Best Served (BEBS) or Most Capable Best Served (MCBS).

More information on this topic can be found here.

Flight Plan Information

Since 2012, the ICAO flight plan provides the ATM system with information on aircrafts’ PBN capabilities, if the filed flight plan is correctly filled out.  If this information was to be extracted from the PBN field of Item 18 and provided to the ATCo on either the electronic flight strip or the radar display, the controller would have enhanced situational awareness of each aircraft’s PBN capabilities and could issue clearances appropriately.  There are, however, limitations with providing all the current PBN information and it is possible that the ATC systems are unable to read the Item 18 information as their systems truncate the ‘Other Information’ section of the flight plan.

For more information on the PBN elements introduced in Flight Plan 2012, click here.

ATCos should be aware that all aircraft are capable of conventional navigation, providing they have the sensor fitted to the aircraft.  This sensor issue is certainly becoming more evident as more and more aircraft are rolling of the production line without an ADF capability.  Therefore, these aircraft are unable to fly NDB procedures.

Although NDB is not support PBN in any form, the PBN manual allows for a conventional missed approach, which can be navigated using NDB.  Clearly, with no ADF fitted to an aircraft, if a RNP APCH procedure has NDB guidance for the missed approach, this approach cannot be flown by that aircraft.  This could be one reason why, on issuing a clearance, the pilot responds with ‘Negative due to RNAV or RNP type’. 

Click here to see more on phraseology specific to PBN.

Conventional procedures will be phased out over this decade, so that by June 2030, the only conventional operation within terminal airspace would be for contingency

Another cockpit limitation that controllers should be aware of is that pilots are not operationally permitted to manually create waypoints in terminal airspace; the pilot is allowed to use waypoints that are in the aircraft’s database only.  Therefore, if a PBN procedure is not coded in the aircraft’s navigation database then the pilot should not accept a clearance for that procedure.  Again, the controller would probably be informed that the pilot is ‘Unable (the SID/STARs name) due to RNAV/RNP type’.  Furthermore, once cleared on a PBN procedure, if the controller wants the aircraft to fly to a waypoint that is not in the designed procedure, the controller should radar vector.  This is because to locate a waypoint not in the aircraft’s flight plan is a workload issue for the flight deck.  However, the use of ‘Direct to’ a future waypoint on the flight plan is fully encouraged but controllers should give consideration to the aircraft’s vertical profile.

Finally, again due to cockpit workload and also to ensure the smooth flow of aircraft taxiing for T/O, a late change of clearance should be avoided wherever possible.  To provide the ATCo with an appreciation of the workload impact on the flight deck select here to see a late change of departure clearance.

Critical NAVAIDs

Some NAVAIDs may be critical for the flight operations of a procedure or at a specific airport. The outage of a critical NAVAID, through either failure or maintenance, will mean that aircraft cannot navigate on that procedure and therefore the procedure becomes unavailable for operations.  This may have a major impact on capacity and/or accessibility to the aerodrome.  Controller awareness of these critical NAVAIDs and the impact of an outage must be understood.  Furthermore, timely notice of the outage should ensure controllers do not clear aircraft on a procedure that is ‘unflyable’.

Contingency Procedures

When ATCOs are trained and receive their ratings, they are required to be familiar with contingency procedures developed for their Units. These procedures can cater for a variety of abnormal situations such as radar failures, a blocked runway, particular maintenance routines or severe weather. An additional abnormal situation should be added to such contingency measures is what actions to be taken when GPS is unusable.

In the past, some ATCO units provided a panel indication of the status of NAVAIDS. A RED light indicated unusable and GREEN indicated usable. So if a particular VOR was unserviceable, it would show a red light, and the controllers would know not to issue any clearance that relied on the use of that VOR. As the number of SIDS/STARS have increased to over a 100 at some major airports, the situation has become more complex.

With PBN operations, the ATCO also needs to know whether GNSS is usable and the scale and duration of the period when it is not usable. Currently, this information will reach the controller either through NOTAM, pilot reports or, possibly in the future, utilising aircraft derived information such as ADS-B and integrated into information that is usable by ATC.

When ATC is aware that GPS is unusable, the controller should inform the pilot who would then decide which approach to conduct as an alternative, or whether or not diversion to another aerodrome is required. When the GPS is unusable, the ATCO should not:

• Clear an aircraft for an RNP APCH: An RNP APCH requires GPS. If EGNOS is not working the pilot knows that the aircraft cannot fly an RNP APCH to LPV minima.
• Clear an aircraft for a procedure that is authorised using GPS position only. Here the reference is to PBN SID/STAR, ATS or free routes which have been published/predicated only on GPS, either because there is no available DME infrastructure or because the use of available DMEs is not suitable;
• Clear a GPS only equipped aircraft for a procedure which is authorised using GPS or DME position.

For more information on this refer to the European GNSS Contingency/Reversion Handbook for PBN Operations (PBN Handbook 6)

Minimum Operational Network (MON)

The MON is a reduced ground-based navigation infrastructure that can efficiently support the continuation of PBN operations or alternative contingency operations in case GNSS becomes unusable.