Alaska Airlines Audio, Pilot Went 'Little Overboard' in Attempt to Take Down Plane

New audio from the flight that was almost taken down by an off-duty pilot details the terrifying incident -- and it reveals a huge understatement from the flight's actual pilot ... who described the guy as going "overboard."

The recording starts not long after 44-year-old Alaska Airlines pilot Joseph Emerson allegedly tried to shut off the engines aboard the Horizon Airlines jet. Emerson was traveling in a jump seat in the cockpit as a courtesy -- the same company manages Horizon and Alaska Airlines.

You can hear a pilot saying they got Emerson out of the cockpit and into the back where he was subdued ... asking for law enforcement to meet them instantly when they land in Portland, where the plane was diverted after Emerson's actions.

The pilot tells air traffic control Emerson was handcuffed and has calmed down a bit from his original outburst, but also says the threat level was initially a 4, which is the highest ... often signifying a breach of the cockpit.

As we reported, Emerson was arrested and booked Monday on 83 counts of attempted murder, for each person aboard the plane, 83 counts of reckless endangerment and endangering an aircraft. The flight was bound from Everett, WA to San Francisco.