The warnings indicated the Airbus A300 cargo plane was descending at a rate outside normal parameters given its altitude, Sumwalt said, but investigators haven't made any determination on the actual cause of the crash into an Alabama hillside.
"We haven't ruled anything in, haven't ruled anything out," he said.
The aircraft went down less than a mile from the end of Runway 18 at Birmingham's airport before dawn Wednesday. UPS has identified the victims of the crash as Capt. Cerea Beal, Jr., 58, of Matthews, N.C., and First Officer Shanda Fanning, 37, of Lynchburg, Tenn.
Landing on the runway can be tricky for pilots, an expert said, particularly those flying big jets like the twin-engine UPS cargo carrier. Sumwalt said the plane was being flown by the captain — who had 8,600 hours of flight experience, including 3,200 hours in an A300 — but investigators don't know whether Beal or Fanning had ever before landed on Runway 18.
"We're going to do our best to find out," he said.
Sumwalt said investigators will analyze the airplane's weight to determine whether it should have attempted a landing on the runway, the shorter of two runways at Birmingham Shuttlesworth International Airport.
With a large hill and trees at one end, the runway lacks the electronics for a full instrument landing. That forces pilots to make key judgments about altitude while aiming a descending aircraft at a runway that's 5,000 feet shorter than the airport's main runway, which was closed for maintenance work at the time of the crash.
Some pilots simply avoid landing on Runway 18 when possible, said veteran commercial pilot Ross Aimer.
"When I heard they were using Runway 18 it caught my attention because of that hill," said Aimer. "It's sad, but it didn't surprise me."
Aimer, a retired United Airlines captain, is now chief executive of Aero Consulting Experts, a firm based in Los Angeles.
The NTSB previously said a preliminary investigation didn't reveal any evidence of engine failure before the plane struck trees about one mile away from the end of the runway. It crashed into the bottom of a hill less than a quarter mile after hitting the trees.
The A300, which weighs about 172,700 pounds when empty, was at the end of a 45-minute flight from Louisville, Ky., to Birmingham when it went down. A flight summary from flightaware.com, which tracks airplanes, shows the aircraft made a descent in steps, which Aimer said is a "dive and drive" method common on runways with the same navigational guidance as Runway 18.
Sumwalt said the aircraft went down during its first landing attempt. Sumwalt said investigators have not found any problems with the runway's lights or navigation system, which typically provides pilots with information about their lateral position but not about their altitude, unlike those on runways where pilots can land using only instruments.
National Weather Service records from the morning of the crash show the plane would have descended through overcast conditions to only a few clouds at 1,100 feet. Within seconds after the plane hit a tree and at least one turbine sucked in wood, the twin-engine plane crashed.
It hit the base of that large hill mentioned by Aimer, who said he had landed on Runway 18 about a half-dozen times, including on some flights as a cargo pilot.
Located near the southern tip of the Appalachian foothills, Birmingham's airport is nestled in a low spot between Red Mountain to the south and hills that lie at the northern end of Runway 18, which is 7,000 feet long. The main runway is 12,000 feet long and runs east and west, meaning pilots don't have to negotiate the rough terrain.
The NTSB said the longer runway was closed for maintenance work on its lights early Wednesday, leaving the shorter runway as the only path to the ground. Runway 18 is an approved runway with a valid approach, Aimer said.
"It is definitely legal, but it I had a choice I'd use another runway first," he said.
A key task for investigators will be determining why the UPS jet was low enough to hit trees. The impact sheared off pieces of the aircraft and sent them crashing onto two homes along with large pieces of limbs.
Keenen Brown, 17, said he witnessed the crash while getting ready for work before dawn. Brown, who lives with relatives across the street from the crash site, said it was unusual to see such a large aircraft attempting to land on the runway.
"I saw the sky turn orange and I looked up and I saw it in the air on fire," Brown said. "I watched it hit the ground and dirt flew up. This whole area just shook."
Aimer said the flames could have been shooting from the plane after it struck the trees.
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