The fatal collision on Friday evening between a large airliner and a small plane on the ground at Los Angeles International Airport, one of the nation's busiest, comes as no surprise to aviation experts who have been issuing warnings about dangerously congested runway conditions nationwide.
"We know that the two planes did collide on the (ground)," said Elly Brekke, a regional spokeswoman for the Federal Aviation Administration. "There is a lot of traffic at LAX but we have not experienced a problem" before.
"We don't know yet what the circumstances were behind that collision," Brekke said.
USAir Flight 1493 was landing on Runway 24L shortly after dark when it collided with a SkyWest commuter flight as it was preparing to take off.
The safety records of USAir and SkyWest are not considered poor. USAir is ranked 11th among the 49 major airlines in fatal accident rates.
In the last several years, pilots and air traffic controllers have reported hundreds of near-misses on runways at crowded airports around the nation. In 1989, the National Transportation Safety Board warned that ground accidents posed "a high potential for catastrophe."
The runways and taxi areas at Los Angeles International Airport are so congested that the airport is one of fewer than a dozen in the country that has a ground collision avoidance system intended to warn traffic controllers and pilots of potential ground collisions.
"There are runway incursions that occur from time to time. This is a very rare instance," said Carl Battis, an American Airlines pilot.
However, Battis added, the layout and separation of runways at Los Angeles International is "something pilot groups have been very concerned about in recent years."
Potential ground hazards, according to the FAA, are created by a wide range of conditions and the many aircraft, vehicles and people on the ground. Aviation experts blame forgetfulness, miscommunication between the cockpit and control tower, attention lapses and pilot failures to see runway markings for most reported runway errors.
The most recent major airline collision took place on the ground between two Northwest airliners at Detroit Metropolitan Airport last December. The world's worst aviation accident occurred on a runway in the Canary Islands in 1977, when two Boeing 747s collided in the fog, killing 583.
In Friday's fatal crash, National Transportation Safety Board officials worked into the night trying to piece together the sequence of events, said Fred O'Donnell, FAA spokesman at Los Angeles International Airport.
Four hours after the 6:04 p.m. crash, Brekke said the collision apparently occurred on the eastern third of Runway 24L.
The cause of Friday's crash, according to interviews with aviation experts, could have been any of the following:
Pilot error. The larger 737 could have run into the smaller commuter or landed on top of it. The SkyWest pilot was "instructed to hold for takeoff but where they were beyond that, I don't know," Brekke said.
After the collision, the USAir pilot would have had great difficulty controlling the path of the plane with the smaller craft crushed against its belly.
Another airline pilot, Jim Tilmon, suggested a slight downgrade on that runway would have further hampered the pilot's efforts to slow the plane.
"It's amazing when you consider the fact this accident could have been so much worse. . . . We already can applaud the crew," Tilmon said. When pilots face emergencies, "you hope you're going to do the right things. Well, apparently all those people did."
Mechanical error. The 737 could have had mechanical problems and skidded into the SkyWest plane after both were on the ground. The FAA discounted reports from witnesses that the USAir jet's landing gear may not have been down.
"The (USAir) pilot did not report any type of emergency to the control tower," said Brekke. If the landing gear failed to operate properly, she said, "normally there would be an indication in the cockpit," such as a warning light.
She said investigators will be reviewing air traffic communications tape.
Control tower error. Planes approaching the airport operate under the guidance of air traffic controllers at the Los Angeles Terminal Radar Approach Control (TRACON) facility, located in a hangar on the south side of the airport. Controllers observe the aircraft they are guiding by radar--the facility is windowless.
Brekke said both planes were in radio communication with the tower just before the crash.
As planes prepare to land at L.A. International, control is transferred to the tower, which is atop the administration building at the east end of the passenger terminal complex. The controllers in the tower observe the planes visually. However, at night, only the lights of planes preparing to land can be seen from the tower.
Separating the traffic on the ground is the responsibility of so-called "ground controllers" in the tower. They direct the movement of planes along the airport taxiways and on the ramp areas surrounding the passenger gates.
Sabotage. This has been a grave concern of late because of the Persian Gulf War. But an FBI spokesman said there was no evidence of terrorism in the crash.
"There's no indication of any foul play," said agent Fred Reagan of the Los Angeles FBI office. "No threats were involved. We believe this was accidental."
SOUTHERN CALIFORNIA'S WORST AIR CRASHES
A chronology of Southern California's worst air disasters: Dec. 7, 1987: A former USAir ticket agent smuggles a gun aboard a PSA BAe-146 jetliner, kills a fellow passenger and attacks the cockpit crew, after which the jetliner crashes near Paso Robles. All 46 people aboard die. USAir had recently bought PSA.
Aug. 31, 1986: An Aeromexico DC-9 and a Piper PA-28 collide over Cerritos, resulting in 82 deaths, the worst accident involving flights to Los Angeles International Airport.
Aug. 24, 1984: A Beechcraft on a training flight hits a Wings West twin-engine propjet northwest of San Luis Obispo, killing 17.
Sept. 25, 1978: A Pacific Southwest Airlines Boeing 727 and a Cessna 172 collide over San Diego, killing 144 people, the worst air disaster in California.
March 1, 1978: During takeoff from Los Angeles International, a Honolulu-bound Continental Airlines DC-10 tips on one wing when several of its tires blow out. Its fuel tanks rupture and fuel ignites. Four people die and seven are injured.
Jan. 9, 1975: A Cessna 150 collides with a Golden West Airlines Dehavilland Twin Otter over Whittier, killing 14.
Jan. 18, 1969: A United Airlines Boeing 727 plunges into the Pacific Ocean near Marina del Rey, killing 37.
Jan. 13, 1969: A Scandinavian Airlines (SAS) DC-8, on a landing approach, crashes eight miles out from Los Angeles International into the Pacific, breaking in half, killing 15. Thirty people survive in a portion that remains afloat.
June 25, 1965: An Air Force C-135 carrying 72 Marines and a crew of 12 crashes into a mountain within a minute after takeoff from El Toro Marine Base. All 84 aboard die.
Feb. 1, 1958: A Military Air Transport Service DC-6 collides with a Navy P-2V Neptune over Norwalk, killing 48.