Investigators looking into what caused a Delta jetliner to skid off a LaGuardia Airport runway last month while landing in a snowstorm said thrust reversers -- which help slow an aircraft -- were applied differently.
The latest update from the ongoing investigation showed engine pressure ratio, which measures the amount of thrust being produced by a jet engine, was higher on the left side of the plane than on the right side.
"It's a contributor as to why the pilot went to the left," said John Goglia, an independent air safety consultant and a former National Transportation Safety Board member. "We don't know if that is the only reason."
On Thursday federal investigators said they found the engine reached peak reverse thrust of 2.07 engine pressure ratio on the left, and 1.91 EPR on the right, between six and seven seconds after touchdown.
The difference, Goglia said, is substantial.
Thrust reversers act as a brake using the engine's exhaust, Goglia said, and, at high power setting, it exerts considerable force on the back of the plane.
"When it's uneven, the nose of the airplane tends to follow the engine with the higher power," Goglia said. "On dry pavement, the tires have good grips and resist that force. On slippery surfaces, it's very easy to get the nose of the airplane to swing around."
Delta Flight 1086, carrying 127 passengers and five crew members, took off from Hartsfield-Jackson Atlanta International Airport shortly after 9 a.m. on March 5.
After it touched down, the aircraft began to veer off at about 3,000 feet down Runway 13, according to the NTSB. At about 4,100 feet, the plane's left wing struck a perimeter fence, located on top of a berm, and destroyed about 940 feet of the fence.
Twenty-three passengers suffered minor injuries, the NTSB said.
There was significant damage to the airplane, including to the fuel tank, nose landing gear and the underside of the fuselage from the front of the airplane to the area of the left front passenger door.
The ongoing investigation also determined that Runway 13 had been plowed 20 to 25 minutes before the accident. On the day of the crash, the Port Authority, whose job it is to clear the landing strip, said the runway had been plowed just minutes before the accident.
Before Delta 1086, an MD-88, crashed, four planes had landed safely on Runway 13 from the time the runway was cleared of snow at 10:40 a.m. to the time Delta 1086 went off course at 11:02 a.m., the NTSB said.
The plane that landed three minutes before Delta 1086 was another Delta MD-88 aircraft.
"It's a good indication there was no problem with the runway," Goglia said.