Limits of "See and Avoid"
Despite good VFR conditions, two aircraft collided with each other in summer 2021. The collision claimed five lives. Both aircraft were equipped with traffic warning systems. How could this accident still happen?
On Saturday, June 12, 2021, an experienced pilot took off from Amlikon airfield at 12.15 p.m. with his ASW 27-18 (HB-3412) for a cross-country glider flight. Around three hours later, a four-seater Robin DR400 (HB-KLB) took off from Neuchâtel for a sightseeing flight to Samedan and Locarno. There were two pilots and two passengers on board. At 17.28 hrs, the two aircraft collided west of Piz Neir at an altitude of 3200 m/M. Shortly beforehand, the pilot of the HB-KLB was still in contact with FIC Zurich. All five occupants of the two aircraft were killed in the subsequent crash.
HB-KLB was not missed
FIC Zurich tried to call Locarno airfield at 6 p.m. and also called HB-KLB twice without success. At 8.05 p.m., the airfield manager at Amlikon airfield contacted the Rescue Coordination Center RCC and reported that HB-3412 was missing. Around an hour later, a Rega helicopter took off and located the wreckage of the HB-3412 and the pilot. The HB-KLB was not reported missing until 9.20 a.m. the following morning. The wrecked aircraft was not discovered until the afternoon of June 13 by the crew of a helicopter that was on site to recover the ASW-27.
Why did it take so long for the accident victim Robin to be found? The STSB examines this question in detail in its investigation report. As the crew of the Robin had not submitted a (non-mandatory) flight plan, the SAR helicopter was not activated after the wreckage of the HB-3412 was found, as no one else was missing. However, the experts from Flarm-Technologys Ltd, who had also been alerted that evening, had already provided indications that the HB-3412 could have crashed due to a collision with another aircraft. Thanks to the Flarm data, Rega was also able to quickly locate the crashed glider. However, the ELT signals (emitted by the HB-KLB) were assigned by the Rega crew to the HB-3412, whose ELT was, however, inoperable after the crash.
Non-functioning systems
Both aircraft were actually equipped with traffic warning systems, but these did not work. The HB-KLB was equipped with a Garrecht TRX-2000 collision warning system. The system combined an ADS-B/transponder receiver with an integrated Flarm module in one device. In terms of its technical design, this device was therefore able to warn of aircraft equipped with either ADS-B out, transponder or Flarm.
However, the Flarm module contained a firmware version that had not been updated and was therefore not functional. Due to the technical design of the collision warning device, the other functionalities of the system (ADS-B/transponder) were also no longer available. Due to the lack of external antennas, the system was not very reliable and the pilot had probably switched it off.
A Garmin GTX 328 Mode S transponder was also installed in the HB-KLB. The radar echoes registered by the FISO during the accident flight were received by this transponder. The powered aircraft was in contact with Zurich Info at 124.7, the glider had no contact with FIS during the entire flight. The transponder position of the powered aircraft was visible on the Zurich Info radar and was tracked by FIS.
Probably not switched on
The HB-3412 was fitted with a PowerFlarm collision warning system, which combined an ADS-B/transponder receiver and a Flarm module in one device. This was able to warn of aircraft equipped with either a Flarm, ADS-B out or Mode S transponder. The Air Avionics VT-01 Mode S transponder, which in this case also had ADS-B out functionality, was also installed in the instrument panel. However, the missing radar echoes and ADS-B data during the entire flight show that the transponder was not in operation. It can be assumed that the transponder was not switched on.
In the present case, if the transponder of HB-3412 had been switched on and in working order, the glider would have been visible at least to the FISO on his radar display. This would have given him the opportunity to give the HB-KLB a corresponding traffic advisory.
For the pilots of the HB-KLB, the technical requirements for being warned of the HB-3412 were not met due to the inoperative or probably not activated collision warning system.
Learning insights
This tragic accident underlines once again how important functioning traffic warning systems with the current international standards Mode S plus ADB-B, plus Flarm in Switzerland, are for flight safety. It is important that these systems not only transmit, but also receive signals and can give the pilot appropriate traffic warnings.
The Mode S and ADS-B systems offer a further safety net as soon as pilots are in contact with the respective FISO and the latter can provide traffic information.
Flying without radio and traffic warning systems means a significantly increased risk of accidents, as shown by the accident and near-accident statistics cited in the report.
Even though the submission of a flight plan is not mandatory for purely domestic flights, its "non-closure" in the evening would have triggered an alarm at the RCC Zurich. The HB-KLB was not officially reported missing until the following morning.
Markus Kirchgeorg/Urs Holderegger
Source:
Final Report No. 2406 of the Swiss Transportation Safety Investigation Board STSB