THE US FEDERAL Aviation Administration has issued an airworthiness directive (AD) designed to prevent Raytheon Aircraft Beechjet 400 and Mitsubishi MU-300 Diamond aircraft from suffering un-commanded nose-down pitch at certain flap settings during icing conditions.

The Beechjet is based on the Diamond design, which Beech Aircraft acquired from Mitsubishi.

The AD results from icing tests, which were used to demonstrate, that ice accumulations on the horizontal stabiliser may cause the aircraft to pitch down.

Icing tests conducted on a Beechjet 400, in a wind tunnel demonstrated that under certain conditions, "run-back ice" may build up on the aircraft's tail-plane, aft of the heated element of the leading edge. The ice caused the test aircraft to pitch down at landing-flap settings beyond 10°.

Tail-plane icing had occurred during a flight of a Beechjet 400A, but the pilot was able to land without incident, with the flaps set at 10°. Inspection revealed that the tail-plane had triangular accumulations of run-back ice.

The AD revises instructions about special operating procedures during icing conditions.

Source: Flight International