A series of runway foul-ups at Denver International Airport over the past year - including two near-collisions - has forced a major overhaul of the airport's runway- and taxiway-safety program.
It also led the Federal Aviation Administration to send a high-level runway-safety team to DIA last month to review the airport's progress in making safety improvements.
The two near-collisions were among 24 serious runway incursions at U.S. airports - out of more than 61 million takeoffs and landings - in the year that ended Sept. 30.
DIA was the only U.S. airport to have two runway incursions in the government's most serious category, according to FAA data obtained by The Denver Post. The airport also had two less-serious incursions.
On Feb. 2, a DIA snowplow driver drove onto Runway 8-26 in front of a United Airlines jet that had just landed.
A collision was avoided only because the plane's pilots used emergency braking to slow down as the plow crossed in front of the plane.
The snowplow incursion came after the pilot of a small cargo plane on Jan. 5 missed a planned turn onto a taxiway and turned instead onto a runway just as a Frontier Airlines jet was about land.
The Frontier pilots aborted the landing to avoid hitting the cargo plane. The two aircraft came within 145 feet of each other, an FAA report said.
The agency ranked both DIA incidents as "Category A" incursions - the most serious - where "extreme action" is needed to narrowly avoid collision.
Category B incursions are those where "there is a significant potential for collision." Category A and B incidents are deemed "serious" by the FAA.
DIA and Los Angeles International were the only U.S. airports to have two serious incursions. Both of DIA's infractions were Category A incidents, and Los Angeles International's two incursions were Category B.
From 2003 through 2006, DIA did not have a single serious runway incursion.
The snowplow incident led airport officials to ask, "How could this possibly have happened?" said John Kinney, DIA's deputy manager for operations.
It led to a revamping of DIA's driver-training program for those who take vehicles onto the sprawling airfield - especially those with select access to taxiways and runways.
"It was an alarming event that necessitated we look at driver training from A to Z," Kinney said of the February incident, which prompted other safety improvements as well.
The airport added bold, colorful warning signs to intersections where roads for airport vehicles cross or enter taxiways and runways.
DIA is adding grooved rumble strips to the pavement at critical intersections as yet another warning device for drivers.
"This is a complex, intimidating airfield," Kinney said. "It's complex because of its sheer size, number of runways and taxiways" and many roads for emergency vehicles and those who maintain navigational aids.
There were also several less- serious incidents this year in which airport maintenance employees drove across active runways, DIA officials said.
In response to the flurry of incursions, DIA sharply reduced the number of people authorized to drive on the airport's "movement area" - the taxiways and runways - to 340 from 600.
Airport officials also mandated rigorous annual driver training.
The snowplow operator in the February incident was not badged to cross the taxiway and runway without an escort and ignored stop signs and warnings that he was about to enter an active runway, the National Transportation Safety Board said. The plow driver no longer works for DIA.
On a recent tour of DIA's airfield, Kinney acknowledged that some of the airport's warning signs were not large or prominent before the snowplow incident.
That, he said, has been remedied.
At the location where the plow operator drove across Taxiway R and Runway 8-26 in front of the arriving United jet, new signs now stand.
"Do not enter Taxiway R; Turn around; Go back to 98th; Clearance required for red or red striped badges to proceed," one sign reads.
On the other side of the taxiway, the new sign at the road's intersection with the runway has an even more dire message: "Danger. Do not enter."
The 4-foot-by-8-foot sign designating "Runway 8-26" in large white letters on a bright red background also warns: "If lost, stay here, contact your supervisor."
DIA has installed these large billboard-style warning signs wherever roads for airport vehicles meet runways, Kinney said.
Airport workers also painted large red-and-white "35 L - 17 R" signs on the pavement at a "holdbar" - painted markings that require air-traffic-control authority to cross - just before Taxiway EC crosses Runway 35 left/17 right.
On July 31, a DIA electrician mistakenly turned onto EC from an access road, thinking it was a different taxiway farther north.
On EC, the maintenance employee, without permission, crossed the holdbar and then sped across the runway as a United jet was getting into position for takeoff, according to an FAA report on the incident.
The United plane had not started its takeoff roll, so the FAA recorded it as a Category D incursion - one in which there is "little or no chance of collision."
In addition to DIA's two near-collisions, the airport had one other Category D incursion in the recent year.
Last month, FAA acting administrator Bobby Sturgell said his agency would target Category C and D incursions as "precursors" of more serious events.
FAA requires upgrades
To help ensure that pilots stop and get controller permission to enter runways, the FAA has mandated that airports paint extra-wide enhanced taxiway centerline markings to denote the final 150 feet leading up to runway holdbars.
Airports must have the new markings in place by June.
DIA already has completed the task of painting the wider taxiway centerline at 92 runway intersections, Kinney said.
The FAA-led runway-safety meeting in October found that DIA's spacious layout - with no intersecting runways and plenty of separation between them - meant the airport was at a low risk for "wrong runway departures."
It decided "airport complexity" was DIA's only risk factor.
The airport hopes to soon get an updated safety technology called Airport Surface Detection Equipment, Model X, which is designed to give tower controllers early warning of impending collisions between aircraft, or between planes and transponder-equipped vehicles.
In the serious incursions at DIA this year involving the small cargo plane and the snowplow, the tower's existing collision-sensing technology did not warn controllers in a timely way, according to NTSB reports.
Controllers say the tower's shortage of radar displays for the current collision-avoidance system contributed to the incursion involving the small cargo plane in January.
On Thursday, the NTSB's board recommended that the aviation industry develop devices that provide "a direct warning to the cockpit" of a looming collision.
"A system being installed at airports by the FAA provides warning to air traffic controllers, but not to the flight crews," the NTSB said, "a situation that greatly reduces the amount of time that pilots have to react to an impending incursion."