Incident Overview
On May 26, 2007, at 1336 Pacific daylight time, Republic Airlines flight 4912 (RPA4912), an Embraer 170 regional jet, and Skywest Airlines flight 5741 (SKW5741), an Embraer Brasilia turboprop, nearly collided in the intersection of runway 1L and runway 28R at San Francisco International Airport (SFO), San Francisco, California. Both aircraft were operating scheduled passenger flights under 14 CFR part 121 on instrument flight plans. No injuries to occupants or damage to either aircraft were reported.
Sequence of Events
SKW5741 was arriving from Modesto, California, and was cleared for a visual approach by Northern California Terminal Radar Approach Control (NCT) before being transferred to SFO tower. At 1332:29, the crew contacted the SFO local controller, reporting they were six miles out on the BRIJJ visual approach. The local controller acknowledged, issued a wake turbulence advisory for a Boeing 757 landing on runway 28L, and cleared SKW5741 to land on runway 28R. According to NCT radar data, the aircraft crossed the runway threshold at 1335:13.
RPA4912, using the radio callsign "Brickyard 4912," taxied to runway 1L and was instructed to taxi onto the runway to hold at 1333:36. The aircraft was cleared for takeoff at 1335:12, and the crew acknowledged.
At 1335:40, during a transmission to an uninvolved aircraft by the local controller, an aural AMASS (Airport Movement Area Safety System) warning was audible in the background. At 1335:44, the local controller attempted to instruct SKW5741 to stop, transmitting, "uh, Skywest HOLD HOLD HOLD."
According to controllers' written statements, SKW5741 came to a stop in the intersection of runways 1L and 28R. RPA4912 lifted off and overflew SKW5741.
Aircraft and Systems
SFO ATCT is equipped with an AMASS that uses radar to track aircraft on and near the airport surface, providing conflict detection and aircraft location information. The system can detect conflicts between aircraft using the same runway and, following a software modification on February 17, 2007, also detects conflicts between aircraft using intersecting runways.
SKW5741's approach was tracked by NCT's ASR-9 terminal radar system at Oakland International Airport, approximately 8 miles north of SFO. The Oakland radar does not provide surface coverage at SFO but detected RPA4912 climbing off the airport immediately after the incident. Comparison of NCT radar time with AMASS radar time indicated the AMASS clock was about 15 to 16 seconds fast relative to the NCT clock. The NCT clock is set and checked every shift, while the AMASS clock derives from the internal computer clock and is more subject to error. Consequently, this report considers the ARTS (Automated Radar Terminal System) clock as authoritative, and AMASS times are corrected accordingly.
The AMASS recorded data for both aircraft, detecting the conflict and alerting controllers at 1335:40. The AMASS targets for the two aircraft merged in the runway intersection at 1335:55. RPA1912 first appeared on the Oakland ASR-9 just south of taxiway V at 1335:59, climbing through 200 feet.
Estimates of Separation
The initial FAA tower report estimated the aircraft missed colliding by 300 feet. However, the Skywest crew estimated the distance as 30 to 50 feet, and the crew of RPA4912 estimated 150 feet, characterizing their estimate as a "guess" because they could not see the Brasilia as they passed over the top of the aircraft.