Darwin SB20 at Lugano on Sep 10th 2010, near collision with military aircraft
Last Update: February 15, 2013 / 16:32:11 GMT/Zulu time
The Swiss BFU have released their final report reporting, that the minimum separation between the two aircraft was 0.5nm at 200 feet vertical associated with "high risk of a collision". The investigation into the serious incident concluded:
The serious incident is attributable to the convergence of an Air Force training aircraft with a civil commercial aircraft, because the flight instructor allowed the trainee to control the aircraft in a manner that endangered the commercial aircraft. The instructor and the trainee pilot were not aware of any threat.
Contributing to the serious incident is the fact that the air traffic control officer in the Lugano aerodrome control center did issue the crossing clearance without having an adequate overview over the situation regarding possible danger.
The systemic cause of this serious incident is that the crew of the training aircraft had an inadequate awareness of the overall situation because they had insufficient knowledge of the consequences of the utilisation concept of class D airspace with regard to the method of the operation of TCAS, the onboard aviation safety net.
The following have been identified as systemic contributing factors of the serious incident:
- The Air Force flight mission included elements which were not sufficiently error-tolerant concerning flying in the Lugano control zone.
- Some inaccurate aids were used for the preparation and the execution of the flight mission.
- The exchange of information between the Air Force and the Lugano aerodrome control centre concerning a series of special flights was not effective.
- The extent of the Lugano control zone, the location of the reporting points, the speed limits and the operating procedures also in an airspace of class D were not effective.
The BFU reported: "The Lugano Airport control zone is Class D airspace. In this airspace, traffic information between IFR and VFR traffic is assured as an air traffic control service. Alternate route recommendations are issued at the request of crews. In addition, no separation minima exist between IFR and VFR traffic and between VFR and VFR traffic. Crews themselves are responsible, according to the principle of "see and be seen", for maintaining sufficient separation from other aircraft."
A Swiss Confederation Pilatus PC-7, callsign A939, conducting a flight within the annual selection for military pilots was flown by a trainee pilot in the front seat, an instructor was in the back seat. 24 trainees were seeking to be selected for becoming military pilots, all of them had received the same flight from Locarno via Monte Ceneri, Tesserete, Melide, Mendrisio and other waypoints back to Locarno. The students had received the information a few days prior to the flight and were to prepare the selection flight independently and on their own. The trainee was to operate the flight as independently as possible, the instructor would just evaluate the trainee's performance and intervene only to avoid hazardeous situations.
The trainee had contacted Lugano tower just prior to entering the control zone, the aircraft was at 3500 feet QNH, and received traffic information identifying the Darwin Saab 2000 turning base and another private aircraft on downwind, the trainee had both aircraft in sight and reported them in sight, so that the tower controller cleared the Pilatus to cross the control zone at 3500 feet and report over E. At that point the Pilatus was just inside the control zone, the Saab was descending through 3900 feet and turning base, the Saab's TCAS system generated a traffic advisory and a few seconds later the resolution advisory, while the Pilatus did not feature a TCAS - and the trainee pilot was not aware of TCAS in general and the impact of his operation on TCAS systems in other aircraft. The crew of the private aircraft had heard the clearance to cross the control zone at 3500 feet and decided to not descend below 4000 feet, but received a TCAS "Monitor vertical speed" resolution advisory to maintain 4000 feet, too. The Pilatus descended to 3000 feet to increase vertical separation.
Following the near collision another selection flight was sent into a holding pattern in order to facilitate the landing of the Darwin Saab 2000.
The investigation conducted a survey amongst 241 pilots of all ratings including flight instructors regarding knowledge of class D airspace, rules and consequences as well as TCAS operation and found the results "noteworthy" with a large proportion of the surveys, including flight instructors, returned being incorrect.
The BFU analysed: "It should be noted, however, that the issuing of the crossing clearance for the A939 involved the following risks as a result of the combination of the following points:
- the clearance was issued at a time when two aircraft being operated under instrument flight rules were on the airport circuit and at an altitude which corresponded roughly to that of the A-939.
- the PC-7 and the two civil aircraft exhibited potentially conflicting flight paths.
- the arrival of an aircraft via MEZZO is not visible from the control tower for topographical reasons. An aircraft becomes visible to the ATCO just before crossing the approach axis of runway 19."
The BFU analysed that at the point the traffic advisory was generated in the cockpit of the Saab 2000, the aircraft were 0.7nm apart, both trainee and instructor of the Pilatus did not perceive the distance as low or dangerous. The BFU continued: "the crew of the A939 were not aware that in the event of a convergence with aircraft which are equipped with a collision warning system, optical and acoustic resolution advisories are generated which the crew must follow." The trainee had no knowledge of TCAS at all, the instructor had marginal knowledge of TCAS only.
The instructor held the opinion, that the trainee had reported on tower frequency 2.2nm before reaching the control zone and thus had fulfilled the criteria listed on the visual charts to obtain a crossing clearance. The BFU continued: "It should, however, be noted that at the airspeed of 175 kt the ATCO only had approximately 40 seconds to clear or refuse a crossing. Depending on the traffic conditions and frequency occupancy, this time is not sufficient for the ATCO to undertake a situation analysis and give an appropriate clearance. The fact that the PC-7 was already inside the CTR when the crossing clearance was issued shows that the crew was not aware of this problem. If the clearance had not been given by the ATCO, the crew would no longer have been able to react in a timely manner and would clearly have penetrated the CTR. The whole process proves that even given an initial radio call as provided for on the visual approach chart (VAC) there is hardly any margin left for a safety-conscious and prudent entry into the CTR, if the maximum speed of 180 KIAS indicated on the VAC is adopted."
The BFU further analysed: "When the crew of SWR75PE received the "climb, climb" resolution advisory, they immediately initiated a climb and followed the missed approach procedure. They therefore acted in accordance with the system and contributed to resolving the dangerous situation."
With regards to procedures at Lugano the BFU analysed: "The serious incident has shown that the extent of the control zone, the location of the reporting points, the speed limits and the operating procedures are insufficiently coordinated."
In addition the military crew were operating with a hand drawn chart showing the extent of the control zone in 2004, which had not been udpated to current status. The BFU analysed: "As the serious incident indicates, the crew of the A939 were insufficiently aware of the extent of the CTR, otherwise it is difficult to explain why the flight instructor tolerated the actual intrusion of the trainee into the CTR before the latter had even received a crossing clearance." and continued "A detailed analysis of the 24 PC-7 flights on that day yields the same conclusion. All pilots made contact with Lugano control tower. It is surprising that in 11 cases the first radio contact was made so late that the aircraft at its current speed would already have been inside the control zone if a crossing had been refused by the ATCO."
With regards to handling of IFR and VFR traffic in class D airspace the BFU analysed: "It is especially striking that a majority of the pilots questioned were apparently ensconced in a form of false security; depending on the rating level up to 75% of pilots are of the incorrect opinion that air traffic control ensures adequate separation between mixed instrument flight rules and visual flight rules traffic. Even 70% of the airline transport pilots, who mostly have many thousand hours of flying experience, shared this incorrect view. Also, the fact that no separation minima exist between IFR and VFR traffic is known to less than half of the pilots questioned."
This article is published under license from Avherald.com. © of text by Avherald.com.
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