A crash of two planes near Gillespie Field that killed three men last year was caused by pilot and flight-instructor error and a failure of air traffic controllers to warn pilots, according to a study by the National Transportation Safety Board.
That "probable cause" of the crash over La Mesa was published this week, nearly 14 months after the collision.
In July, an NTSB safety recommendation said the failure of air traffic controllers to tell pilots about computer warnings contributed to the accident and at least 10 other crashes nationwide since 2002.
The San Diego Union-Tribune reported in January that the Federal Aviation Administration said computer alarms flashed and beeped at air traffic controllers for 51 seconds before the collision, but that the controllers never warned the pilots.
In February, the NTSB released details about improperly installed computer equipment at the Gillespie tower, which might have limited the ability of some controllers to notice the warnings.
On Feb. 8 last year, a Cessna 172 and a Cessna 182 took off from Gillespie Field in El Cajon. They were being monitored by the FAA's Gillespie tower and by TRACON, the Southern California Terminal Radar Approach, an FAA facility near Miramar Marine Corps Air Station.
This week's report concludes the investigation into the crash by the NTSB. The agency found the probable cause of the crash to be "the pilots of both airplanes' failure to maintain an adequate visual lookout due to their relative flight paths, which limited the available visual cues."
Other factors, it added, "were the failure of the air traffic controller(s) to issue a conflict alert to the Cessna 172 after repeated visual and aural warnings of an impending collision and the task load of the certified flight instructor."
The NTSB has not identified the men flying the Cessna 172, two Swedes who were living in San Diego County. But relatives and friends of the victims say the pilot-in-training was Michael Rangeby, 23, and his flight instructor was Anders Sigurdson, 22.
The Cessna 182 was piloted by 68-year-old William Kupiec, an orthodontist from La Jolla.
In July, the NTSB said a conflict-alert tone -- a high-pitched "wah-wah" that lasts at least five seconds -- could be heard in an audiotape of communications between a TRACON radar controller and the Cessna 172 for "about 40 seconds" before the collision. TRACON controllers have said they did not notice the alarm.
During conflict alerts, controllers watching their radar screens also see a red "CA" flashing over the call signs of the aircraft.
An NTSB factual summary of the crash published in February said improper computer settings at the Gillespie tower made it unable to receive "aural conflict-alert warnings on departing traffic."
The NTSB's final report explains that because the Cessna 172 was on an instructional flight, one of the two people aboard was "most likely wearing a view-limiting device" (a type of hood) that allowed him to look at the gauges and instruments in front of him but not out the windows.
During such training, the instructor keeps watch out the windows for obstacles such as structures or other aircraft.
The agency said investigators were unable to determine whether the instructor -- Sigurdson, sitting in the right front seat -- could have seen the Cessna 182 just before Kupiec's plane struck the right rear corner of the Cessna 172. The NTSB found that even without wearing the hood, the pilot-in-training would not have been able to see Kupiec's plane from his left front seat.
However, Kupiec could have seen the Cessna 172 before the collision. He was "within about 6 degrees of its left view angle," the report said.