Description

On 24 May 2012, a Fokker 100 (HB-JVH) being operated by Helvetic Airways on a passenger charter flight (5311) from Kos, Greece to Bern and descending visually downwind right hand for runway 32 at destination came very close in day Visual Meteorological Conditions (VMC) to an EC145 helicopter being operated by Swiss Air Ambulance which was on a Visual Flight Rules (VFR) ferry flight from Lausanne to Zurich and transiting Bern at 4500 feet from southwest to northeast. Both flights were known to ATC and had been given traffic information.

Investigation

An Investigation was carried out by the Swiss AIB (SAIB). No notification of the event reached the SAIB until 5 days after it had occurred. ATC radar and R/T recordings were used to assist the Investigation. It was noted that the conflict had occurred in the Bern CTR which was Class 'D' airspace. ATS was provided by APP and TWR positions both equipped with a radar display with a four second update. It was the TWR position that was working the two aircraft at the time of the conflict and which had cleared the EC145 to transit VFR at 4500 feet Altimeter Pressure Settings on the requested route from the 'FRI' VHF Omnidirectional Radio Range (VOR) to the 'WIL' VOR, one of the two designated VFR transit routes promulgated - the "Southern Transit Route" - see the chart below:

It was noted that in respect of the two available VFR transit routes, the AIP stated (in German) that "unless weather conditions make it impossible or they are instructed otherwise by air traffic control, transit flights should use the published transit routes during airport operating hours" and that the Bern ATM Manual stated that "Usage of the transit routes is not mandatory."

It was confirmed that the F100 was equipped with a serviceable TCAS II and that the EC145 was equipped with a Traffic Advisory System (TAS) which provided three levels of traffic information on the ND. Only the appearance of a target at the highest of these levels, a Traffic Advisory (TA) would be accompanied by an audible Alert and this would include an indication of the relative bearing of the intruder.

Bern was equipped with Short Term Conflict Alert (STCA) but according to the controllers there, the system had been "suppressed for years" to avoid disturbing the controllers at Bern since because it was part of the Zurich West Sector and any alerts would therefore also be triggered on the radar displays for that sector at the ACC.

The F100 had been cleared by Bern APP for a visual approach to runway 32 to join right and was instructed after read back to change to Bern TWR. The Commander (PF) stated that immediately this clearance had been received, he had turned off the AP.

On checking in with TWR, traffic information on transiting helicopter was passed and straight afterwards corresponding traffic information was passed to the helicopter in respect of the F100. A couple of minutes later, both were given updated traffic information. The F100 continued descending downwind right hand from the north.

About a minute after that, the EC145 pilot visually acquired the F100 and at almost the same time, the TAS, which at the sensitivity relevant to the conflict being investigated would generate a TA when a conflicting flight track was less than 30 seconds or 0.55 nm away and the other aircraft transponder was indicating that there was less than 800 feet of vertical separation, was reported to have annunciated "traffic, eleven o'clock, less than one mile, same altitude" (this system generates "same altitude" if there is less than 200 feet of vertical separation).

The EC145 pilot, maintaining 4500 feet QNH, advised TWR that he would "cross behind" the F100 and turned approximately 40° left onto an approximately northerly heading to do so. He subsequently reported that "he did not observe any avoidance manoeuvre on the part of (the F100)".

The F100 First Officer reported visual acquisition of the helicopter to ATC four seconds after the helicopter had made his report in respect of the F100. In doing so, he added to the report "and we're avoiding", adding afterwards that he had done so "of his own accord in order to indicate to the helicopter pilot and the ATCO that the conflict was resolved". As he was making his report, the radar recording indicated that the aircraft was descending through 5000 feet and a TCAS TA occurred. The commander indicated that he had "slightly reduced the rate of descent" in response. Soon afterwards, a TCAS RA 'CLIMB CLIMB' had occurred. The crew assessed the helicopter to be at approximately the same altitude and stated that they could see it in their 2-3 o'clock relative position. The Commander "decided not to comply with the resolution advisory and continued the approach while descending because the First Officer had permanent visual contact with the helicopter and also observed it turning north". The First Officer later stated that the Commander had carried out a minor heading correction to the left. As a result of the continued descent - at approximately 1200 fpm, a reversal RA "DESCEND, DESCEND NOW" occurred as the aircraft passed 4500 feet QNH.

According to Mode S downlink data, the RA annunciated in the F100 lasted 29 seconds. The RA was not reported to ATC. According to downloaded FDR data, there was no change in the rate of descent and no marked change of course.

It was found that the descending F100 had crossed from left to right 0.7nm ahead of the EC145 at 75 feet below its unchanged altitude of 4500 feet QNH.

The concerns of the Investigation included that:

• the F100 crew had ignored clearly-stated Operations Manual guidance in respect of TCAS TA response.

• Helvetic Airways had not taken any action on receipt of the Commanders internal report which had clearly stated that the RAs had not been followed.

• although ATC action in this event was compliant with procedures, the risk in issuing potentially conflicting clearances to a combination of IFR and VFR traffic cannot necessarily be effectively mitigated by the issue of even good traffic information.

• see-and-avoid is not necessarily a reliable means of judging collision risk and it results in a level of risk far greater than following TCAS RAs even in relatively good in-flight visibility and clear of cloud.

• STCA alerts which could have been made available to controllers at Bern had been suppressed by Skyguide

The formal statement of the Causes of the conflict was:

• The serious incident is attributable to the fact that there was a dangerous convergence of a commercial aircraft and a helicopter despite mutual visual contact, because no appropriate avoidance manoeuvre had been performed.

• The limited effectiveness of the "see and avoid " principle was identified as the systemic cause of this serious incident.

• The fact that air traffic control cleared the commercial aircraft for a visual approach to runway 32 created a condition that allowed the two flight paths to cross with dangerous proximity.

• The fact that the crew of the commercial aircraft did not comply with the TCAS resolution advisory reduced the minimum separation of the convergence and thus exacerbated the hazard.

It was also determined that whilst the following were neither causal nor contributing, they were, in the context of the Investigation recognised as Risk Factors:

• the ground-based short term conflict alert system (STCA) was not available to Bern air traffic control.
• the crews had insufficient knowledge of air traffic control services in class D airspace.

Safety Action taken as a result of the event by Skyguide was noted to have included the restoration of STCA at Bern with effect from 17 October 2013.

Three Safety Recommendations were made as a result of the Investigation as follows:

• that the European Aviation Safety Agency (EASA) should, in cooperation with other relevant international organisations, verify the extent to which the use of traffic alert and collision avoidance systems (TCAS) and the "see-and-avoid" principle can be better coordinated, particularly in airspace without established separation criteria. [No. 489]

• that the Federal Office of Civil Aviation should, together with Skyguide, take simple and effective measures to ensure that at least the protective envelope of commercial aircraft, in which resolution advisories will be triggered by the traffic alert and collision avoidance system is not violated, particularly in Class 'D' airspace and other airspace used by commercial aircraft without established separation criteria. [No. 490]

• that the Federal Office of Civil Aviation should, together with the air navigation service provider Skyguide, take all necessary measures to allow existing safety nets to be offered to the air traffic control units affected. [No. 491]

The Final Report was completed on 3 November 2014 and approved for publication by the Swiss AIB on 18 November 2014. The event was classified as an ICAO Category 'A' AIRPROX as well as a Serious Incident.

Further Reading

• Loss of Separation
• Traffic Information
• Airborne Collision Avoidance System (ACAS)
• ACAS Resolution and Traffic Advisories
• Collision Avoidance
• Loss of Separation - ATCO-induced Situations
• Loss of Separation - Pilot-induced Situations
• See and Avoid
• Pilot Perception
• ATC Expectation Bias
• Decision Making