David Learmount/LONDON


The UK Air Accident Investigation Branch (AAIB) is launching a full-scale Inspector's inquiry into an incident involving a British Airways Boeing 737-200, which suffered violent roll oscillations during a test flight following maintenance.

In the test flight on 22 October the BA 737-200 (G-BGJI) was at about 20,000ft (6,100m) with 290kt (540km/h) indicated airspeed when the pilot reported "rapid roll oscillations". The "B" auto-pilot and the auto-throttle were engaged at the time. A 10 November flight test of the aircraft by the AAIB failed to reproduce the control anomaly.

The London Gatwick-based aircraft has been impounded by the AAIB for examination, and Boeing engineers have been asked to assist.

The UK inquiry follows a US National Transportation Safety Board (NTSB) hearing in mid-November, attended by AAIB representatives, concerning two fatal 737 accidents which may have been precipitated by a similar control anomaly. The hearing reviewed the continuing NTSB inquiry, into the September 1994 737-400 crash, at Pittsburgh Pennsylvania, and the apparently similar 1991 Colorado Springs 737-200 accident, for which no certain cause could be found. No new evidence was announced, but it was agreed that full rudder deflection probably contributed to loss of control of the Pittsburgh 737, although the cause of deflection remains unknown.

Because the Pittsburgh 737 was following a 727 on approach, the US Federal Aviation Administration conducted extensive wake-vortex tests. FAA test pilot Lester Berven says that, while flying at less than 190kt with flap setting 1, the 737 was particularly vulnerable to a "rudder hard-over", which would put the aircraft out of control until speed had increased enough to give greater aileron effectiveness.

Meanwhile, the AAIB has agreed with the NTSB to assemble quick-access recorder data from 737s operating in Europe to determine whether 737 rudders have ever exceeded yaw-damper authority, or if there has been any anomalous reaction to pilot rudder input.

Since the Pittsburgh accident, there have been at least 39 US and eight non-US 737 control-anomaly incidents, according to FAA records, most of them involving rudder deflection or roll. All have been contained safely by pilot action.

Source: Flight International