Incident Overview
On a flight from Rankin Inlet, Northwest Territories, to Winnipeg, Manitoba, Northwest Territorial 962 (NWT962), a Boeing 737-210, was cruising at flight level (FL) 310 under instrument flight rules (IFR) via Churchill and then direct to Winnipeg. At the same time, Northwest 69 (NW69), a Boeing 747-451, was en route from Detroit, USA, to Kansai, Japan, also at FL310 under IFR via Red Lake, Ontario, and airway NCA20.
Sequence of Events
As NWT962 approached the southern boundary of the Winnipeg North High (North) sector, the radar controller observed an unidentified target converging with NWT962. The controller identified the target as NW69 and, at 1455:19 central daylight saving time (CDT), re-cleared NWT962 to FL290. NWT962 began descending from FL310 at 1455:58 CDT when the horizontal separation was 9.5 nautical miles (nm) from NW69. The aircraft leveled at FL290 at 1456:32 CDT with about 3 nm horizontal separation. The minimum required separation in Class A airspace is 5 nm horizontal or 2000 feet vertical.
The flight paths crossed about 160 nm northeast of Winnipeg at 1456:48 CDT, near the boundary between the North sector and the Winnipeg East (East) sector. At the crossing point, the aircraft had about 500 feet horizontal and 2000 feet vertical separation. NW69 crossed from the East sector into the North sector, but the East controller had not yet handed off the flight to the North radar controller.
Air Traffic Control and Procedures
The East sector controllers determine aircraft position via flight progress strips and radar. There was no posted time estimate for a co-located position for the two flights. To recognize the conflict, one would need to use NWT962's Gimli estimate to calculate the time of its crossing of NCA20, NW69's route. Neither the East data controller nor the radar controller recognized the conflict from the flight progress strips. The East sector was staffed by a radar controller and a data controller until 1449 CDT, when the data controller left for a break and the radar controller assumed both positions. Traffic was judged light at that time. The East controller was taking a position estimate by telephone when the North controller indicated he was descending NWT962 and that NW69 should remain at FL310.
The East controller had planned to hand off NW69 at or before the sector boundary, but usually delayed handoffs on NCA20 due to potential communications difficulties with the remote communications outlet (RCO) at The Pas. The East controller considered it desirable to reduce unnecessary workload by delaying handoffs until aircraft were within range of The Pas.
The North radar controller's instruction to NWT962 at 1455:22 CDT was: "Territorial nine six two maintain flight level two nine zero. Start descent now." NWT962 replied: "Territorial 962 we're leaving three one zero." At 1455:52 CDT, NWT962 asked: "Centre it's nine sixty two. Do you want us to start our descent now?" The North radar controller replied: "Territorial nine six two affirmative. Two nine zero right now please." The NWT962 crew was not expecting a new clearance and discussed the situation for about 30 seconds, during which the horizontal separation closed from 19 nm to 12 nm. Neither crew member recalled hearing the clearance limit of FL290 or the urgency.
Conflict Detection and Systems
Both aircraft were equipped with Traffic and Collision Avoidance Systems (TCAS). NWT962's TCAS activated when descending through 29,200 feet at a range of 5 nm from NW69, with about 35 seconds to closest point of approach (CPA). NW69's TCAS issued an aural alert about 40 seconds before CPA. In both cases, TCAS indications corresponded with visual sighting of the other aircraft.
The radar system at Winnipeg ACC (RAMP) is not capable of conflict advisory or resolution. A conflict advisory system was developed but had deficiencies requiring corrective action.
Staffing and Handoffs
The Winnipeg North specialty was staffed with three controllers and two supervisors. At 1422 CDT, two North Low controllers observed the North sector controller experiencing heavy workload; one moved to assist, but received no status briefing. The shift manager considered staffing sufficient for the day's traffic.
Key Findings
The North radar controller set his indicator module (IM) to show only data blocks of aircraft under his control, omitting data tags for adjacent sectors. The North IM was set to 256 nm scale, the maximum. The two aircraft were at appropriate altitudes for their direction of flight and converging at a relative angle of 120° when the loss of separation occurred.