WASHINGTON, D.C. – The NTSB has launched an investigation to determine why a commercial jetliner and a small light airplane came within an estimated 300 feet of colliding over San Francisco on Saturday.

At about 11:15 a.m. PDT on March 27, the crew of United Airlines Flight 889, a B777-222 (N216UA) destined for Beijing, China, carrying 251 passengers and a crew of 17, was cleared to takeoff from San Francisco International Airport on runway 28L and climb to an initial altitude of 3,000 feet.

The first officer, who was flying the aircraft, reported that after the landing gear was retracted and the jet was at an altitude of about 1,100 feet, the tower controller reported traffic at his 1 o’clock position.

Immediately following the controller’s advisory, the airplane’s traffic collision avoidance system issued an audible alert of “TRAFFIC TRAFFIC.”

The pilots saw a light high wing airplane, an Aeronca 11AC (N9270E), in a hard left turn traveling from their 1 o’clock to 3 o’clock position. The first officer pushed the control column forward to level the airplane. Both crew members reported seeing only the underside of the Aeronca as it passed to within an estimated 200-300 feet of the 777.

TCAS then issued an “ADJUST VERTICAL SPEED” alert, followed by a “DESCEND, DESCEND” alert. The first officer complied and the flight continued to Beijing without further incident.

An NTSB investigator is traveling to San Francisco to begin the investigation.