On 21 August 2010, an ATR72-200 being operated by Golden Air on scheduled passenger service from Stockholm Bromma to Ängelholm and a Jetstream 32 being operated by the AIS Airlines, a Dutch professional pilot training organisation from Jönköping to Stockholm Bromma received and responded to co-ordinated TCAS RAs in day Visual Meteorological Conditions (VMC). Controller intervention during the RA response was ignored. The location of the conflict was approximately 30 nm north-east of Jönköping.
An Investigation was carried out by the Swedish AIB using ATC recordings and an interview with the controller. It was established that traffic in the area, which was under the control of Stockholm ACC, was light and that the usual en route separation standard of 5nm horizontally or 1000 feet vertically was prescribed in the Class ‘C’ airspace where the conflict occurred.
The experienced controller involved was found to have cleared the Jetstream to climb to FL150 after forgetting that the opposite direction ATR72 was at FL140 and would conflict. She had then become distracted when turning away from her radar screen in order to arrange the hand-off of the Jetstream and a second aircraft to another sector being controlled from an adjacent position in the same ACC.
The ATR72 was then heard reporting a TCAS climb and this prompted the controller to look at her radar screen where it was apparent that the STCA had been activated by the proximity. As the ATR72 climbed from its cleared level FL140 to FL146, the controller attempted to continue active controlling by issuing a radar heading and provided traffic information. She then issued an instruction to the JS31 to stop climb and descend to FL130 to which the response was “TCAS descent” - this had commenced at FL137. Both flight crews “managed the TCAS alerts in an appropriate manner” with the result that the minimum vertical distance of 300 feet was recorded as occurring at a horizontal distance of 3.24 nm (see the recorded radar screen display below). Thirty three seconds elapsed between the ATR 72 reporting a TCAS climb and the same aircraft reporting “Clear of Conflict” indicating the cessation of the RA. The Investigation observed that the STCA alert would have appeared on screen ninety seconds before separation minima were breached, considerably earlier than the TCAS RA activation and noted that “the fact that STCA alarms are not always perceived in a timely manner has also emerged in previous investigations”.
The Investigation assessed that “the design of the STCA alarm was a risk factor as it was not immediately capable of catching the attention of the operator and only made use of the sense of sight”. However, it was decided that no safety recommendation was necessary in this regard as the ANSP had “announced that they intended to deploy a new air traffic control system in 2012, including a modified visual presentation of STCA and a sound warning”. This intent was noted to have followed request to the ANSP in May 2011 from the supervisory authority, the Swedish Transport Agency, for “a statement on the measures taken as a result of several incidents in which STCA alarms had not been immediately noticed” and related findings against the ANSP following an audit by the same Agency in August 2011. The Investigation noted that audio warnings for STCA were then introduced at both Swedish ATCCs - Malmö in February 2013 and Stockholm in April 2013.
Radar tracks with the minimum vertical distance of 300 feet at a horizontal distance of 3.24 nm; PHDCI is climbing, GA 551 is level (reproduced from the Official Report)
It was noted that despite the requirement to “search for conflicts using MTCD” the controller involved made no mention of utilising Medium Term Conflict Detection (MTCD) or the associated tools ‘Flight Leg’ (FLEG) and the ‘Conflict And Risk Display’ (CARD) which depending on what is selected can generate information in a red box on the radar screen showing any MTCD conflicts and risks.
It was further noted that the response to the TCAS alerts by the controller was not in accordance with SOPs. It was considered that it may be possible to explain this “by the controller being in a situation involving high stress levels and by her not having had practical exercises in handling this type of conflict during training” and noted that at the time “the controller did not clearly perceive that it was a TCAS alert”.
The Investigation was told by the ANSP “the reason no practical exercises (on TCAS RA response) take place during initial training or local training is that there are difficulties in logically and credibly creating situations that generate TCAS alerts”. However, it was considered that as “ICAO guidelines on training in this regard recommend that practical exercises be carried out (it can be assumed that they have) not identified such difficulties that it would not be considered useful to have practical exercises for this, for example, in a simulator”.
In the opinion of the Investigation, “recurrent practical training of situations with TCAS alerts would probably increase the chances of correct actions under operational conditions” and therefore “the possibilities for introducing practical exercises of this nature should therefore be reconsidered”.
The Investigation concluded that the Causal Factors leading to the conflict were:
- The air traffic controller's focus was on an early handover of flights to another sector.
- Air traffic control's aids for noticing the conflict were not capable of breaking the mental picture that the controller had of the situation in the sector.
The following Risk Factor was also identified:
- Air traffic control attempted to modify the aircraft flight paths during the TCAS alert.
One Safety Recommendation was made as a result of the Investigation as follows:
- that The Swedish Transport Agency should ensure the training at suppliers of Air Traffic Services with respect to procedures for TCAS RAs is improved. [RL 2013: 11 R1]
The Final Report of the Investigation was completed on 11 June 2013.