Incident Overview
On 13 December 2012 at 08:05 UTC, a serious airprox occurred at Khasab Airport, Oman, involving an ATR 42-500 (registration A40-AS) operated by Oman Air as OMA 917 and an Agusta Westland AW139 helicopter (registration DU-139) operated by Dubai Air Wing as DUB 139. The incident was classified as category A (Risk of Collision) by the Directorate General Safety & Aviation Services (DGSAS) and investigated by the Public Authority for Civil Aviation (PACA).
Flight Details
The ATR 42-500 was operating a scheduled passenger flight from Muscat International Airport to Khasab Airport, carrying 43 passengers and 4 crew members. It departed at 07:05 UTC with over an hour delay. At 07:48 UTC, the crew was transferred from Muscat Area Control Centre to Khasab Radar frequency (125.0 MHz) and cleared to descend under VFR. At 07:58 UTC, the crew reported overhead Khasab at 7000 ft QNH and was handed over to Khasab Tower frequency (124.35 MHz) for a visual approach to runway 19. The aircraft was cleared to land at 08:03 UTC with wind from 030° at 9 knots.
Helicopter Actions
The AW139 helicopter was conducting a VFR flight along the coast, routing from Dubai via Khasab city, Habalayn, Diba, and back to Dubai. At the time of the incident, the helicopter was executing a right turn at 1500 ft QNH after clearing a hill west of Khasab, heading east with the intention to cross the extended centreline of runway 19. The helicopter pilot had not established radio contact with Khasab ATC and did not receive a resolution advisory from his TCAS, which lacked that capability.
Traffic Alert and Avoidance
At approximately 1550 ft, the ATR crew received a Traffic Alert (TA) from their TCAS. They queried the tower, which confirmed no known traffic. The crew continued the approach with a normal descent rate (varying between 817 and 1153 ft/min) while visually searching. At around 1300 ft, the crew decided to perform an avoidance maneuver as a safety precaution. The maneuver was executed below 700 ft AGL in mountainous terrain. The helicopter pilot, due to the right turn, did not see the ATR and took no avoiding action. Both aircraft continued their intended flights; the ATR landed safely and the helicopter returned to Dubai.
Communication Issues
The helicopter pilot's initial call was made on Khasab Radar frequency, while the ATR crew was on Khasab Tower frequency. Both frequencies were handled by the same controller. This miscommunication contributed to the lack of coordinated traffic separation.
Minimum Separation
According to the ATR flight crew, the minimum separation was 150 ft vertically and 1 NM horizontally. There were no reports of injury or damage to either aircraft.
Cause as per Investigation
The investigation concluded that the serious incident was mainly caused by the helicopter penetrating the extended centreline of an active runway without prior radio contact while a commercial ATR flight was on final approach. Contributing factors included the helicopter pilot's use of an incorrect radio frequency, the lack of TCAS Resolution Advisory capability on the helicopter, the inhibition of the ATR's TCAS RA below 900 ft, and the inability to execute a go-around due to the mountainous environment.
Safety Recommendations
The investigation issued several safety recommendations, including strict compliance with coordination procedures for traffic entering Muscat FIR from UAE, use of a single frequency for all aircraft in the area, review of AIP information for Khasab Airport, updating of Oman Air's operations manual to mention low-altitude traffic risks, and installation of air traffic radar at Khasab Airport.