DURHAM, N.C. -- A flight instructor on Tuesday said the preliminary report on last month's fatal plane crash explains what happened but not why it happened.

• The October 20 plane crash happened at night with a relatively low cloud deck
  
• An experienced pilot said ATC traffic suggests confusion during the final approach
• The NTSB said its final report will take 12 to 24 months

Chris O'Connor is a flight instructor and a pilot who flies a small plane on a cargo route out of Raleigh-Durham International Airport four days a week. He said his route usually puts him back at RDU after dark, using the same runway Dr. Harvey Partridge was trying to reach on the night of October 20. O'Connor said under those circumstances, air traffic controllers typically give pilots a set of waypoints and headings until they are close enough to land using the airport's runway lights. He said a set of red and white lights indicate to pilots whether they are too low or too high.

“They typically don't conduct instrument approaches into Runway 32,” he said. “That night, the weather was marginal, meaning that to get into Runway 32 might require, initially, a portion of instrument approach,” he said.

Harvey Partridge and his wife, Patricia, died when their plane hit trees in William B. Umstead State Park, about 1.2 miles short of RDU's Runway 32. The NTSB's preliminary report shows clouds that night were overcast at 1,400 feet, but there was no precipitation and visibility was greater than 10 miles.

Investigators wrote Partridge was having trouble conducting an instrument-based landing and air traffic controllers tried to help him by turning up the runway lights. O'Connor said Partridge might have become confused while trying to sync up his instrument and visual approaches.

“The challenge that I think that the pilot had that night was maintaining a visual on the runway and the transcript of the ATC communications indicates that there was some confusion and he was not able to easily find the runway visually,” he said.

O'Connor said weather and the dark of night probably were the biggest factors, but the preliminary report doesn't answer why the crash happened. He said the answer could be anything from running out of fuel, which he considers unlikely, to controlled flight into terrain, where a pilot accidentally flies into an obstruction.

The NTSB said it will issue a final report sometime in the next 12 to 24 months.