Description 

On 12 March 2019, a Bombardier DHC8-300 (ZK-NEH) being operated for Air New Zealand by its wholly owned subsidiary Air Nelson on a scheduled domestic passenger flight from Rotorua to Wellington as RLK235 and a second Bombardier DHC8-300 (ZK-NEF) also being operated for Air New Zealand by Air Nelson on a scheduled domestic passenger flight from Gisborne to Wellington as RLK285 lost safe separation in day VMC when the second aircraft failed to follow the first during the approaching traffic sequence after requesting and accepting a visual approach. ATC detected the conflict and issued avoidance instructions and coordinated TCAS RAs followed with safe separation thereby restored. 

Investigation 

An Investigation was carried out by the New Zealand Transport Accident Investigation Commission. Whilst relevant data was downloaded from the FDRs of  both DHC8 aircraft, all relevant CVR data from both aircraft was overwritten after neither recorder was stopped and removed before further use of the aircraft. Recorded ATC data from secondary radar, ADS-B tracking and controller traffic display screen imagery were available.  

The flight crew of the DHC8 aircraft which did not follow its clearance were recorded as a 44 year-old recently promoted PF Captain with a total of 4,200 hours flying experience of which 1,300 hours were on type, most obtained whilst a First Officer after joining the operator in 2016 until promotion three months prior to the investigated event. The flight crew of the other DHC8 aircraft was recorded as a 36 year-old Captain with a total of 4,000 hours flying experience of which 3,400 hours were on type and a 33 year-old PF First Officer with a total of 2,550 hours flying experience of which 500 were on type since joining the operator in 2018.

What Happened

Three aircraft all on IFR flight plans were making their approaches in sequence to runway 34 at Wellington. An Airbus A320 was first and was followed by the two  Bombardier DHC 8-300s which had both requested and been granted visual approaches following a standard right hand pattern commencing with the downwind leg at the ‘TURAK’ waypoint followed by a right turn onto base leg and then right again onto finals (see the illustration below)

Having observed that they had passed abeam the A320 on finals, the first DHC8 crew turned onto base leg. The second DHC 8 crew immediately followed this by advising APP that they were visual with the DHC 8 ahead and after then requesting a visual approach were so cleared. They were instructed to call TWR “when turning base”.

The standard circuit to runway 34 finals for arrivals from ‘TURAK’. [Reproduced from the Official Report]

Just over half a minute later, the first DHC8 turned onto finals and the second one turned onto base leg. Forty seconds later, the change to TWR by the second DHC 8 as instructed by APP had still not been made when both TWR and APP controllers received an STCA based on a collision risk for the projected flight paths of the two DHC 8s - see below the illustration of the controllers’ screens at that time. As can be seen, the STCA was generated by the projected (2 nm) flight path vectors of the two aircraft which were shown to intersect.

The A320 (JST290),the first DHC8 (LNK235) and the second (LNK285) as the STCA occurred. [Reproduced from the Official Report]

It appears that when the APP controller responded to the STCA, the second DHC8 was in the process of (belatedly) changing frequency from APP to TWR and there was no response. The APP controller then contacted the TWR controller by telephone and as the TWR controller was taking this call, the second DHC 8 made its initial call to TWR which went unanswered. When the call finished, the TWR controller immediately broadcast to the second DHC 8 (LINK285) “essential traffic on your left is a Dash 8 on a nine-mile final descending through 2,400 feet, maintain 3,000 feet.”

  

There was no response to this call but it could be seen from the radar recording that the second DHC 8 had made a left turn within 10 seconds of that call. Five seconds after beginning this left turn, it then received a TCAS ‘CLIMB’ RA coordinated with a simultaneous TCAS ‘DESCEND’ RA on the first DHC 8. Both RA’s were followed but only the first DHC 8 advised ATC of their receipt of an RA as procedurally required.

 

Recorded radar showed that after responding to the TCAS RA’s the two aircraft had come within 0.8 nm of each other horizontally and within 475 feet vertically. Both aircraft crews subsequently advised “clear of conflict” to ATC once the TCAS RA had ceased. The first DHC 8 was then instructed by TWR to continue to a landing and the second to change to a radar control frequency where it was subsequently repositioned onto final approach without further event.

Both DHC 8 crews were stood down after completing their flights but neither ensured that the respective CVRs were disabled on their own initiative and, apparently, neither did their operator seek to confirm - by any method - that its own clear procedures in that respect had been followed.

Why It happened

The issue of responsibility for safe separation of aircraft given visual approach clearances was central to the development of the loss of safe separation. The airspace involved was all Class ‘C’ and the weather conditions being reported at the airport at the time included a surface visibility of 20 km. The request for and approval of visual approaches was therefore not in itself causal. The requirements of ICAO PANS-ATM Doc 4444 Edition 16 in force at the time in respect of successive visual approaches were noted as stating that:

“separation shall be maintained by the controller until the pilot of a succeeding aircraft reports having the preceding aircraft in sight. The aircraft shall then be instructed to follow and maintain own separation from the preceding aircraft.” 

In including this requirement, the New Zealand MATS (Manual of Air Traffic Services) stated that the controller is responsible for ensuring that “there is no possibility of incorrect identification” when giving clearance to operate by visual separation.

In terms of the controller responsibility for separation between successive IFR flights granted visual approach approval, it was noted that whilst the APP controller was responsible for determining the traffic sequence and would also normally be responsible for separation between aircraft until landing, an exception to this existed when an IFR aircraft has been cleared for a visual approach and has agreed to follow a preceding IFR aircraft, in which case the flight crew of the following aircraft then take over responsibility for separation.

The release of an aircraft on a visual approach from APP to TWR was flexible, thus allowing TWR to resolve any conflicts with VFR traffic also in the CTR. It was noted that in respect of IFR traffic cleared for a visual approach by APP, the TWR controller “takes responsibility for providing visual separation within 5 nm of the (runway) threshold, but only to ensure that the runway is clear and available for each aircraft sequenced to land” in which case a clearance to is issued.  

Given that the origin of the conflict between the two DHC 8s was visual identification by the second DHC 8 crew of an Airbus A320 as a DHC 8, it was observed that “this may seem improbable given that the A320 is a low wing, twin-engine jet aeroplane while a DHC 8 is a high wing, T-tail, twin-engine turboprop aeroplane”. In respect of this error, it was also noted that:

• The pilots who made this error were visually ‘identifying’ an aircraft which was exactly the same as the one they were flying and being operated by their own airline in identical livery.
• These pilots were also well aware of where aircraft would be when on each leg of the standard approach pattern when runway 34 at Wellington was in use.
• They were locally based and had both made many previous approaches to the same runway, including visual ones.
• Visual clearance from cloud was assured and the horizontal visibility was well in excess of the minimum required for flight by visual reference.
• Both pilots confirmed that they were aware from passively monitoring other ATC clearances for traffic approaching the same runway that there were at least two preceding IFR aircraft in the approach sequence before they went visual.    

The Investigation sought to examine the other “circumstances and factors that increased the likelihood of the event occurring and those that were intended to prevent the incident” and identified three Safety Issues that it was considered “could have the potential to adversely affect future operations” as follows:

• Although it was normal procedure at the time to provide IFR flights which had requested and been granted approval to conduct visual approaches in sequence behind other aircraft their position in that sequence, reporting that the aircraft to be followed was “in sight” - in this case wrongly - was accepted as sufficient. It was however noted that crews can always ask for this information.  
• Allowing a IFR flight approved to make a visual approach to change from APP to TWR based on their future position meant the APP controller was unable to establish immediate communication with a flight which was still “within their control sector” when the STCA occurred. 
• It was a matter of significant concern that neither of the flight crew involved took action after landing to isolate the 2 hour CVRs on their aircraft as required by both the State Safety Regulator and the SOPs of their operator.     

Finally, in respect of aircraft operator SOPs relevant to the investigated event, it was noted that as a wholly-owned subsidiary of Air New Zealand at the time (it was subsequently absorbed fully into the parent company) all “Standard Operating Policies” (such as CVR protection following a potential Serious Incident) were already common to both operations.

Safety Action taken or to be taken as a result of the event and known to the Investigation was noted as including the following:

• A revised MATS Advisory Circular was promulgated on 31 December 2020 which included new sections outlining previously unstated potential risks arising from visual approaches and describing best practice to prevent a recurrence of the investigated event from an ATC perspective.
• As part of the integration of Air Nelson operations with those of parent Air New Zealand since the event, DHC8-300 pilot initial and recurrent type technical training will, from an unspecified date in 2022, include specific reminders to pilots about isolating the CVR after a serious incident and examples of such serious incidents which replicate those in Attachment C of ICAO Annex 13.

The Final Report was approved by the Commission on an unspecified date in February 2022 and published on 14 April 2022. No Safety Recommendations were made.

Related Articles

• Loss of Separation
• Visual Approach
• TCAS RAs
• Confirmation Bias
• IFR