The pilots' incorrect assumption was due to a "combination of technical and organisational factors, which led them to depart from normal operating parameters", the report said.
The two-man crew "clearly believed the autopilot was not functioning correctly both during and after the event", added the report from the Air Accidents Investigation Branch (AAIB).
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The captain's decision to select an exaggerated nose-down attitude just after the Eurocopter EC155B1 aircraft had taken off from the Clipper gas production platform in the southern North Sea was "made out of concern for the helicopter's apparently poor performance".
This arose "because of the weighing and fuel gauging errors which had existed undetected for some time".
The high rate of descent, in which airspeed reduced below 20 knots (about 23mph) was arrested about 50ft above the sea surface and the captain climbed to normal cruise height, with the helicopter landing at Norwich airport. Described by the AAIB as a "serious incident", the events occurred on the evening of November 6 last year - weeks after four people died in a North Sea Super Puma helicopter accident.
The report said the effect of the weighing and fuel gauging errors was "a major influence on the commander's subsequent actions". The AAIB said a number of safety actions had since been taken.