VLM F50 at Friedrichshafen on Apr 21st 2016, near collision with private aircraft
Last Update: October 3, 2018 / 15:15:31 GMT/Zulu time
A private Piper was at 4000 feet MSL about 20nm east of the aerodrome inbound to Friedrichshafen on a different frequency (VFR control) when the VFR controller instructed the aircraft to join the final approach course runway 24 and subsequently handed the aircraft off to tower. Tower instructed the aircraft to turn right towards waypoint OSCAR and remain outside the control zone.
A short term collision alert activated at both the approach controller's desk as well as at the tower. Approach provided traffic information about an unknown VFR traffic to OO-VLF. The crew of OO-VLF looked out for the conflicting traffic but were unable to pick up the aircraft due to sun glare (position and reflection of sun) and thus could only identify the position of the conflicting traffic via their TCAS display. The commander decided to turn right 90 degrees.
At the same time the pilot of the Piper had established visual contact with the Fokker and advised ATC accordingly. However, due to the avoidance maneouver initiated by the Fokker the separation between the aircraft reduced to 100 feet vertical and 0.5nm horizontally.
Both aircraft continued for safe landings at Friedrichshafen.
Special procedures applied as the annual aviation trade show "AERO Friedrichshafen" was in progress between Apr 20th and Apr 23rd 2016. ATC Approach Control services were provided by Swiss ATC provider Skyguide, which is why Switzerland's SUST investigated the occurrence rather than Germany's BFU or Austria's Austrocontrol (who is responsible to provide Aerodrome Control Services).
On Oct 3rd 2018 Switzerland's SUST released their final report concluding the probable causes of the serious incident were:
The serious incident is attributable to a dangerous convergence of two aircraft flying on a converging course in airspace class E during an aviation exhibition, during which time the commercial aircraft flying under instrument flight rules was in contact with the approach control, while the light aircraft flying under visual flight rules was in radio contact with the aerodrome control.
The dangerous convergence arose from the concurrence of the following factors in chronological order:
- The operational concept consisting of the simultaneous approach of traffic under visual and instrument flight rules during the trade fair entailed systemic risks.
- The pilots of both aircraft were not in radio contact with the same air traffic control unit.
- The traffic guidance within the aerodrome control service concerning the light aircraft approaching under visual flight rules was coordinated inadequately.
- The traffic alert and collision avoidance system on board the commercial aircraft did not generate a resolution advisory due to a lateral avoidance manoeuvre.
- The traffic information provided by the aerodrome control to the pilot of the light aircraft was given too late.
- The pilots of both aircraft only acquired a late visual contact of each other.
The current classification of the airspace, in which the dangerous convergence took place, contributed to the occurrence of the serious incident.
The SUST analysed:
There is no indication of pre-existing technical defects, which could have caused or influenced the serious incident.
The analysis of the Multi Radar Tracking (MRT) readings show that the conditions for the emission by the traffic alert and collision avoidance system (TCAS) on board of the VLM of a traffic advisory (TA) between 08:14:27 UTC and 08:14:55 UTC as well as of a resolution advisory (RA) between 08:14:41 UTC and 08:14:54 UTC were satisfied.
However, the horizontal miss distance (HMD) for a RA of 0.2 NM between the two aircraft was never reached during the convergence, therefore not producing a RA in the present case. The analysis also demonstrates that the TCAS on board of the VLM reacted according its specifications and that the height over the ground did not play a role.
The SUST analysed that the sudden turn by the Piper following the tower instruction to turn right and remain outside the control zone, which effectively turned the Piper straight towards the Fokker, surprised the approach controller. The Piper pilot also did not remain outside the control zone, the Piper "did not circumnavigate the CTR of Friedrichshafen, as required by the operational guidelines in force during the aviation exhibition, thereby establishing a key condition for the dangerous convergence".
The SUST wrote with respect to the approach controller: "Having observed the fluctuations of altitude and heading of the VFR traffic and the fact that the commercial aircraft was already at the minimum vectoring altitude (MVA) of 4000 ft AMSL, he considered it impossible to give any reliable heading change or descent clearance to VLM."
The SUST analysed:
The information concerning the position of the sun shows how the flight crew of VLM encountered strong glare from the sun, while on the base leg, all the while attempting to spot the conflicting traffic on a collision course. It is therefore understandable that the pilots kept track of the situation based on the traffic information and the TCAS display. However, it must be said that due to the low azimuth resolution of the TCAS, the display of the traffic situation is limited, particularly on an IVSI. In addition, experience has shown that the convergence of a conflicting traffic in relation to ones own aircraft position is frequently misinterpreted. By way of example, a perpendicular convergence of two aircraft on a horizontal flight is displayed on the TCAS of the respective aircraft as a convergence with an angle of 45 degrees.
As the flight crew was expecting further instruction from the controller, a traffic advisory TA was triggered by the TCAS on board of the VLM at approximately 08:14:53 UTC. Subsequently, both pilots expected a resolution advisory RA. The MRT recordings reveal that a RA was not triggered, in accordance with the HMD concept because, after the initiation of the right turn at 08:14:42 UTC, that is before the TA, the threshold value of 0.2 NM for a RA was never reached.
Turning right towards Friedrichshafen Airport seemed obvious to the flight crew, yet, in doing so, they lost the possibility of seeing the Piper which was between their eleven to twelve o’clock position. In view of the inaccuracies in azimuth and the incorrect interpretation of a conflicting traffic, avoidance manoeuvres based on information displayed by the TCAS entail high risks. In the present case, the underlying safety net withheld the TCAS avoidance order.
Adding to the unfavourable visual conditions for all pilots, both aircraft were approaching each other with a constant angle (constant bearing) between 08:14:10 UTC and 08:14:42 UTC, meaning that the respective position of the other aircraft remained unchanged from the perspective of the pilots. It is, therefore, not surprising that the pilot of Piper saw the large silhouette of the Fokker 50 only late as he initiated a right turn, thereby altering the constant convergence bearing. This indi-cates that the traffic information given by the aerodrome controller at 08:14:50 UTC, only four seconds before the closest point of approach, came too late.
This article is published under license from Avherald.com. © of text by Avherald.com.
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