Miscommunication between flight attendants aboard a Northwest Airlines flight from Osaka to Honolulu on Dec. 25, 2007, resulted in serious injury to one of them during extreme turbulence, according to a National Transportation Safety Board report released today.

Another flight attendant and two passengers suffered slight injuries when Northwest Flight 16 encountered the rough patch of air 1,500 miles west of Honolulu. The other 291 people aboard the Airbus A330-323 were uninjured.

The flight crew had changed course to avoid thunderstorms when the pilot advised the lead flight attendant to make sure all flight attendants took their seats and remained seated and buckled in for about 15 minutes, the NTSB report said.

The lead attendant walked to the aircraft's galleys and told flight attendants to be seated for the next 15 minutes, but two attendants in the aft galley misunderstood her instructions and believed they had 15 minutes before they were to be seated, the report said.

Both were standing 12 to 13 minutes later when the plane hit a brief area of severe turbulence without warning while cruising at an altitude of 38,000 feet. The turbulence caused the plane's autopilot and autothrust to disconnect, and the flight lost approximately 1,000 feet of altitude before the pilots regained control, the report said.

The two attendants were knocked to the floor during the episode, with one suffering two fractures in her neck.

The flight continued to Honolulu and landed without further incident.