When Palm Springs International Airport added a second parallel runway in 1991, planes began landing on the taxiway between the two landing strips. Twenty taxiway landings took place in 1995 alone, although most involved small private planes rather than commercial jets.
Steve Zehr, systems aviation director at Palm Springs International, said the airport in the late 1990s painted several "TAXI" and "TAXIWAY" labels on the taxiway and added a serpentine center line.
"The incidence of errant landings dropped dramatically," Zehr said.
Airports in Las Vegas and Tucson took similar precautions, with similar success.
But Matt Cavanaugh, manager of the safety and standards branch of the FAA's Pacific Northwest office in Renton, said: "The flight tests we've done have verified that when it's wet and shiny, you cannot see markings."
"The water on the surface will obscure those markings," agreed Alaska pilot Wilkes.
Max Tidwell, head of runway safety in the FAA's Pacific Northwest office, said such markings work best at airports where pilots fly past the airport before landing, allowing them to look down and see the taxiway.
"At this airport all you have are straight-in approaches," said Coates, manager of operations at Sea-Tac. "You don't have an opportunity to fly beside a taxiway and look out a window and say, 'What's that?' "
The FAA says it tested paint on the taxiway last February and concluded it would not be visible.
The NTSB said the FAA painted only small strips on the taxiway's dark asphalt shoulders, rather than on the bright concrete surface, and didn't try running a serpentine line down the taxiway.
"Given this information, the Safety Board questions how the FAA could determine that these measures were ineffective," the NTSB wrote in its Aug. 8 letter.
No matter how Taxiway Tango is marked, electronic instruments can guide pilots precisely to Runway 16R.
When the coordinates for Sea-Tac are dialed in to a plane's navigation system, it indicates whether the final approach is on target.
But on clear days pilots generally opt for visual approaches rather than relying on their instruments. Both airlines and air traffic controllers prefer this because it allows shorter distances between incoming planes. Kathryn Vernon, air-traffic hub manager for the FAA in Seattle, says most airlines' policy is that pilots should check their electronic guidance anyhow when doing a visual approach.
Yet even highly experienced pilots occasionally forget to look back at their instruments once they see what they think is the runway.
"You pick it up, you get locked in and you land," Wilkes said.
The FAA's Max Tidwell said the agency and Sea-Tac officials are trying to correct the taxiway problem at that early point in pilots' decision making, so they do not let their eyes deceive them.
Coates said he thinks the cumulative effect of all of the measures taken by Sea-Tac has greatly diminished the possibility of another airplane landing on Taxiway Tango. But he said he, air traffic controllers, the FAA and the NTSB will keep a close eye on the situation this winter.
Because as long as Tango remains at Sea-Tac, more errant landings remain a possibility.
"The only 100 percent solution is to remove that taxiway," Wilkes said, "and I don't believe that's going to happen."
FAA AND NTSB -- WHO DOES WHAT?
--FAA issues and enforces regulations that apply to airlines, airports and air traffic controllers.