A Belmont, Calif., couple returning home from celebrating their 48th wedding anniversary died in a plane crash in Livermore. Oskar and Julia Huber died when their plane hit a tree and crashed into a hillside as they flew home from their son's house. After hearing the accident, neighbor Lauren De Vore went out searching with her son, who eventually found the wreckage.

Flying in rain and fog could have been one of the factors involved in the crash, which will be investigated by the National Transportation Safety Board. According to records, there were no issues with the plane, a Piper Cherokee PA-28, which was built in 1973. Julia Huber, 75, had been flying for nearly 30 years. She was at the controls when the plane went down.

According to speculation by Lt. Eric Navarro, who responded to the crash, "the plane hit a tree, disabling it or disabling it even further, and then hit 15 feet below the crest of the hill." Navarro, of the Contra Costa County Sheriff's Office, said the impact of the crash "broke the small plane into several major pieces over a distance of one-quarter to one-half mile."

The weather that night, which included not only rain and fog but also hail and low cloud cover, could have factored into the crash. It was weather Jim Bigelow, another of the Hubers' neighbors and a pilot himself, said "they didn't plan on encountering."

While weather may have been a contributing factor to the crash, it is also likely that pilot error also was to blame. A total of 56 percent of accidents in the last 60 years were caused by human error, with 50 percent caused by pilot error and 6 percent caused by other human error, according to planecrashinfo.com. These can include errors during fueling, as well as maintenance work or while loading cargo on the plane.

In the last 60 years, mechanical failure has contributed to 22 percent of airplane crashes. Sometimes the mechanical failure is the cause of the accident, or is viewed as a contributing factor after some other cause. Occasionally there will be a design error that causes the plane crash, and of course weather, too, can play a part.

A final determination will be made after the FAA and the National Transportation Safety Board finish their investigation.