A Boeing 757's "unnecessarily fierce descent" during a collision-avoidance manoeuvre injured 20 passengers and crew members, four of them seriously, according to Taiwanese investigators.

Three passengers were left with broken bones and a fourth was knocked unconscious as the pilot put the Far Easten Air Transport aircraft into a dive that at one point exceeded 12,000ft/min. Ten other passengers and six flight attendants had light injuries and cabin fittings were damaged.

During the manoeuvre, a response to a "descend" command from the TCAS collision-avoidance system, the aircraft was subjected to negative forces of up to 1.06g for about four seconds, followed by positive 2.48g for two seconds as the pilot arrested the descent and levelled, reports David Kaminski-Morrow for Flight International.

Taiwan's Aviation Safety Council, in its inquiry into the November 16, 2006 incident, has concluded that the evasive action was "excessive".
 
The 757 had been approaching Jeju in Korea when an Inchon area controller instructed it to descend. As the jet neared its new altitude, the controller realised it could conflict with a Thai Airways International Boeing 777-300 cruising in the opposite direction.

With the jets separated by 22.6km horizontally, and just 17m vertically, both aircrews received initial traffic advisories warning of one another's presence. The controller, also aware of the potential threat, started instructing the 757 to halt its descent before he realised the aircraft had already passed below the level of the other aircraft, and decided to let it continue. But the 757's crew misunderstandend the mixed message and levelled the aircraft.
 
Thirteen seconds after the traffic advisory, the TCAS ordered simultaneous collision-avoidance commands, telling the 757 pilots to descend and the 777 crew to climb.

"The [757's] flight crew did not complete the TCAS resolution advisory standard operation procedures and commenced an excessive high-rate descent," says the ASC inquiry. "The induced negative g-force resulted in the occupants' injury."

It says that maintaining a normal 1500ft/min descent rate would have provided enough vertical clearance between the aircraft, and points out that the controller was also resolving the conflict in the horizontal plane by instructing the 777 to make a right turn.

Although the 757's crew had illuminated the seat-belt sign a few minutes before the encounter, several passengers were still moving around the cabin.

Investigators say the conflict arose partly from the use of non-standard phraseology and a failure, both by the 757 pilots and the controller, to follow proper TCAS procedures.