Washington (CNN) -- Federal aviation authorities say they are investigating a string of events that began Monday when a pilot spilled coffee on a cockpit panel, leading to a false hijack warning and culminating in the plane's emergency landing in Canada.
At issue: Did United Airlines warn pilots not to hold beverages over the plane's electronics-laden console?
The incident occurred on United Airlines Flight 940, a Boeing 777-200 headed from Chicago to Frankfurt, Germany.
According to Canadian transportation officials, the pilot spilled coffee on the aircraft's radio equipment, located on the center console of the cockpit. The crew advised controllers they had communication problems and subsequently reported they were experiencing navigation problems, the officials said.
A United Airlines spokesman confirmed that a crew member's beverage spilled "during a period of light turbulence," but said the pilot remained in contact with air traffic control and followed standard operating procedures.
A source familiar with the incident told CNN that after the pilot switched to a backup radio system, the plane briefly sent out a transponder signal indicating it had been hijacked. The source declined to explain how that happened.
But pilots contacted by CNN said the signal could have been sent if one of the pilots inadvertently hit the transponder knob while cleaning up the spill. Or, they said, it could have happened if the pilot had tried to send a different transponder code indicating the plane had communication problems.
The pilot made the decision to make an emergency landing in Toronto rather than continue the trans-oceanic flight, officials said.
Federal Aviation Administration officials said Wednesday that pilots are allowed to have beverages in the cockpit, noting that many aircraft cockpits have cup holders. But they pointed to a rule prohibiting pilots from "any activity ... such as eating meals" that could distract them during a "critical phase of flight," typically meaning takeoffs and landings.
Drinking coffee could be construed as "eating meals," according to FAA spokesman Les Dorr.
The agency's Certificate Management Office will investigate whether United Airlines has a policy prohibiting crew members from passing liquids over, or placing them on, the console, and whether they've communicated that policy to pilots. The FAA also will investigate whether the spilled liquid in fact caused the communications problem, or if there were other factors, Dorr said.
As unlikely as a coffee spill accident may seem, a similar scenario formed the basis of a 1964 movie, "Fate is the Hunter." In that film, a cup of coffee spills on the plane's center console, leaking into the electronics. The leak leads to a series of events culminating in the plane's crash. The movie, coincidentally, aired Sunday night on Turner Classic Movies, the night before the United mishap.
Mark Weiss, a former B-777 pilot, said a spilled drink is not likely to bring down a plane, pointing to redundant systems in aircraft. But he said it was reasonable for the pilot to land the plane.
Weiss, who works for Spectrum Group, a consulting company, said liquids should not be banned from cockpits. "Particularly on the long flights, you have to have water constantly, you have to continually hydrate your body, otherwise you run into other physiological problems," he said.
The National Transportation Safety Board said it does not plan on investigating the mishap. "It does not appear that this incident warrants an NTSB investigation," spokesman Keith Holloway said.
NTSB records do not show any incidents where spilled substances were a factor in a crash, he said.
CNN's Jeanne Meserve and Chuck Johnston contributed to this report.