What happened
On January 8, 1997, a serious loss of vertical separation occurred within the RIDAS holding pattern in the Helsinki approach area. The incident involved three aircraft: two SAAB 340A models, registered as OH-FAA and OH-FAB, and an MD-81, registered as OY-KHL.
Heavy snowfall and winter maintenance requirements at Helsinki-Vantaa Airport had caused significant delays, forcing aircraft into holding patterns. During the sequence, the crew of OH-FAA descended through the altitude of OY-KHL and eventually reached the same altitude as OH-FAB. The investigation established that the aircraft were flying in instrument meteorological conditions (IMC) within clouds at the time of the event.
The investigation
The OTKES investigation utilized SSR radar data from the Global Environment Monitoring System (GEMS) and analyzed radio communication recordings. The investigators examined the sequence of altitude clearances and the quality of the verbal exchanges between the Helsinki approach controller and the various flight crews.
Analysis of the transcripts revealed a high frequency of non-standard phraseology. Specifically, the investigation found that the controller and pilots frequently omitted required information, such as aircraft callsigns or the intention to leave a specific flight level. The investigation also identified instances of overlapping transmissions and