What happened
On the morning of the incident, a runway incursion involving two aircraft took place at Denver International Airport (DEN) in Colorado. At 0-7:25 MST, the ground controller instructed Key Lime Air flight 4216, an Swyer Sw4, to taxi toward runway 34 using taxiways M and AA.
The pilot of the Swyer Sw4 reported that blowing snow had reduced visibility. Additionally, taxiway SC was covered in snow, which prevented the pilot from identifying the centerline lighting. While attempting to locate the lights, the pilot followed blue taxi lights that led the aircraft onto runway 35L. Airport surface detection equipment (ASDE) confirmed that the Swyer Sw4 entered runway 35L at taxiway M2 at 07:27:06.
At 07:28:10, the ground controller queried the pilot regarding his position. The pilot stated he was abeam Signature, at which point he realized the aircraft was on a runway.
Simultaneously, Frontier Airlines flight 297, an Airbus A319, was cleared to land on runway 35L. The crew of the Airbus A319 reported that weather conditions had deteriorated, necessitating a full instrument approach. The visibility was approximately 1/2 mile with blowing snow. Although the crew visually confirmed the runway was clear while still in the clouds, they did not see the other aircraft until they were between 50 and 100 feet above the ground. The Swyer Sw4 was located approximately 2,000 feet down the runway. The crew immediately initiated a go-around.
The investigation
The investigation utilized recorded airport surface detection equipment (ASDE) to track the movement of the aircraft. The crew of the Frontier Airlines flight provided testimony regarding visibility and the impact of wind and prop wash on the visibility of the runway incursion. The Airport Surface Detection Equipment System (ASDE) provided an aural and visual alert for an occupied runway at 07:28:17, and the tower controller issued a go-around instruction to the Airbus A320 four seconds later. The aircraft avoided a collision by approximately 50 feet.
Findings
At the time of the incident, weather conditions included 600-foot overcast ceilings, 1,100-foot broken clouds, and visibility of 1/2 mile with light snow and mist. Winds were recorded at 030 degrees at 12 knots, with a runway visual range of 5,500 feet on runway 35L.