What happened
On June 10, 2010, at 13:09, a serious air proximity incident occurred southeast of Pori aerodrome involving two Beechcraft A36 Bonanza aircraft, registrations OH-BBN and OH-BBM. The aircraft OH-BBN was performing a flight test for an instrument rating course, while OH-BBM was conducting a training flight.
Air traffic control had cleared OH-BBN for an instrument approach to runway 30 from an altitude of 1,700 feet. Simultaneously, OH-BBM, approaching from the direction of Turku, was cleared to maintain a flight level of 70 or above until passing a specific fix, after which it was permitted to descend to 2,700 feet. However, the pilot of OH-BBM incorrectly read back the clearance, acknowledging an altitude of 1,700 feet. Following this error, the aircraft descended to the incorrect altitude.
As a result, both aircraft passed the initial approach fix for runway 30 at nearly the same altitude. Radar recordings indicated that the minimum lateral separation was approximately 0.1 nautical miles (180 m) and the minimum vertical separation was only 100 feet (30 m). Neither crew observed the other aircraft during the encounter.
The investigation
The investigation established that the incident was caused by a misinterpreted altitude clearance readback. The investigation found that the pilot of OH-BBM flew according to the incorrectly acknowledged altitude. This error went unnoticed by both the flight instructor and the air traffic controller, the latter of whom inadvertently confirmed the incorrect readback as correct.
Several contributing factors were identified. The air traffic controller was working alone during a period of very heavy traffic, which likely impaired their ability to closely monitor readbacks. Additionally, the presence of visitors in the control tower at the time of the incident served as a distraction. The investigation also noted that the approach charts for Pori aerodrome required certain fixes to be reported only upon request, which is inconsistent with the procedural control methods used at the station.
Findings
- The primary cause was the pilot of OH-BBM flying at the incorrectly acknowledged altitude.
- The air traffic controller failed to detect the error and inadvertently validated the incorrect readback.
- High traffic volume and the controller working alone contributed to the loss of situational awareness.
- Distractions from visitors in the tower impacted the controller's concentration.
- The investigation noted that the controller's visibility of the movement area was significantly hindered by the placement of computer monitors in the tower.