Terminal Operations

This page will look at aspects of PBN in terminal airspace considered important to ATC.

Operational ATCOs regularly use expressions such as contingencies, back up, fall back, reversion (plan B!).  The generic meaning to be attributed to this variety of informal terms is that due to some ‘issue’, ATM operations cannot continue normally and ATCOs have to do something ‘different’. Reasons for these issues causing ‘non-normal’ situations can include equipment failure such as a glide path inoperative; partial or total surveillance system failure; depressurisation experienced by an aircraft; hijack or aircraft’s loss of navigation function.  Often, contingency has a negative impact on traffic flow i.e. causing less runway or sector throughput or reduced air traffic flow rate.

When controllers are trained and receive their ratings, they are required to be familiar with contingency procedures developed for their Units. These procedures can be detailed in local instructions/regulations for the particular unit and 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 that should be added to such contingency measures is to cater for cases when GPS is unusable.

Select here to see more on the loss of GPS signal.

 

Discuss:

  • Minimum expected navigation performance for terminal operations should be +/- 1NM 95% of the flight time, with or without OPBMA.
  • Aircraft required to have databases, procedures loaded from there.  Pilots cannot alter data in Nav DB (so if procedure is incorrect, a waypoint coordinate cannot be corrected) and pilots are not operationally permitted to manually create waypoints in Terminal airspace (danger of human error).

PBN SIDs/STARs and IAPs normally require multiple waypoints to define the Instrument Flight Procedures (IFPs).  Therefore, in 2009 ICAO published a requirement that waypoints supporting SIDs/STARs and Approaches could be designated as either:

  • a unique 'five letter' pronounceable name code (5LNC) –a waypoint considered of strategic importance to ATC and requiring a globally unique identity
  • a five character alpha-numeric – for tactical use within the terminal airspace

A waypoint over a ground-based NAVAID will use the NAVAID's three letter identifier.

Alpha-numeric naming should adhere to the following principles:

  • Waypoint unique to one aerodrome
  • The State applies the same convention to all aerodromes
  • The code should consist of not more than three numbers and the letters are taken from the aerodrome's ICAO designator. eg DM424
  • The rules and convention should be published in the State's AIP.

Where 5 letter name codes are managed through the ICAO iCard system, there is no such provision for management of tactical waypoint naming.  Therefore, controllers should be aware that it is possible for aerodromes in neighbouring States to have similar named alpha numeric waypoints in their terminal airspace due to the fact that the aerodromes both have the same two end letters in their ICAO code.

Finally, where alphanumeric codes consisting of numbers less than 360 are used to designate tactical waypoints, e.g. LL125, controllers must ensure that in busy RTF periods if a ‘Direct To’ is issued the whole code is transmitted and not shortened to the number only.  This should ensure that the flight crew do not confuse the instruction as to fly a heading, altitude or even a speed.  It is highly recommended that designers only use alphanumeric codes with a value of 400 and more to avoid this possible confusion.

PBN implementation provides ATC the possibility for the systematic use of 'DIRECT TO' in the overall management of TMA traffic.  All RNAV 1 and RNP certified aircraft are able to execute 'DIRECT TO' to waypoints in the Navigation database.

Where appropriate, ATC could consider 'DIRECT TO' as an alternative to radar vectoring for RNAV 1 and RNP capable aircraft cleared on RNAV/RNP instrument flight procedures. One flight deck advantage is that the use of 'DIRECT TO' will ensure the flight crew still has 'distance to go' information.to support a managed vertical profile.

However, both pilots and controllers must understand that 'DIRECT TO' instructions are not radar vectors.  Therefore, the use of Performance Based Navigation (PBN) in Terminal Control Areas (TMAs) does not change existing terrain clearance responsibilities. It does not relieve:

  • Pilots of their responsibility to ensure that any clearances are safe in respect to terrain clearance.
  • Air Traffic Control (ATC) of its responsibility to assign levels which are at or above established minimum flight altitudes.

The pilot:

  • Is responsible for terrain clearance.
  • Must ensure flight operations conform to published Minimum Flight Altitudes (MFAs)
  • Must inform ATC of any inability to accept a clearance or instruction on the basis of terrain clearance issues.  The pilot always has the right to reject the 'DIRECT TO' and remain on the cleared procedure should he/she be concerned about terrain clearance.

The controller:

  • shall issue clearances such that the prescribed obstacle clearance exists when an IFR flight is being radar vectored by ATC or is given a direct routeing off an ATS route.
  • must assign levels in ATC clearances consistent with MFAs.

Controllers should be aware that aircraft may not be able to comply with a ‘Direct To’ instruction if:

  • There is a navigation problem
  • The waypoint specified is too close
  • The angle of turn and/or is too great
  • The waypoint is not displayed in the FMS for pilot selection
  • The waypoint is not part of the procedure
  • The SID/STAR not assigned

If pilots are unable to comply and will probably request radar vectors

In addition, controllers should be aware that large turns or high aircraft speeds may result in aircraft overshooting the next leg.

Use of ‘Direct To’ to a future waypoint on the published procedure

Once the aircraft has been cleared for a SID/STAR, if ATC elects to issue a 'DIRECT TO' a waypoint that is part of the SID/STAR, the following applies:

  • The pilot will select the specified waypoint in procedure list on the FMS.
  • The FMS and Navigation Display (ND) are updated maintaining all details of the route from the 'DIRECT TO' waypoint onwards.
  • The aircraft continues with the SID/STAR after reaching the waypoint.

The aircraft is expected to meet any level or speed restrictions if published, provided the cleared level makes this possible.

Be aware that a 'DIRECT TO' could shorten track miles to the waypoint, which could have an impact on the aircraft's ability to meet level and speed restrictions.

Use of ‘Direct To’ to a waypoint not on the published procedure

If the aircraft has been cleared for a SID/STAR and ATC decides to issue a 'DIRECT TO' a waypoint that is not part of the cleared procedure, the following applies:

  • Waypoints within Terminal Airspace which are not held in the navigational database are not allowed to be created by manual insertion.
  • It will take time for the pilot to retrieve the waypoint from the database.
  • The clearance for the SID/STAR is cancelled and previously loaded SID/STAR is dropped by the RNAV system.
  • No further routeing is maintained or displayed.
  • The aircraft requires explicit routeing after the waypoint from ATC.
  • If no further explicit routeing information from ATC, RNAV systems will revert to "present heading mode" after reaching the waypoint. That means that the aircraft will continue on from the waypoint on the heading it is on when it arrives there, unless otherwise instructed.

Therefore, aircraft reaction could be significantly delayed and there is a high chance of errors being created.  Therefore, ATC should consider the use of radar vectors if routeing away from the SID/STAR is necessary.

Finally, as area navigation systems dynamically update the 'active' waypoints and as waypoints are passed, they are removed from the 'active' waypoints list.  Therefore, ATC should make use of 'DIRECT TO' instructions only for waypoints on the assigned SID/STAR and 'DIRECT TO' should only be used for waypoints ahead of the aircraft.

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