The National Transportation Safety Board (NTSB) has asked FAA to issue an airworthiness directive ensuring that operators adopt a Boeing service bulletin for inspecting parts from 737 aircraft main landing gear for corrosion and cracking.

Four instances of damage to the main landing gear actuator beams -- the latest on an Aer Lingus 737-500 last August -- prompted the safety board to issue the two related recommendations, which call for inspection and rework and repair of the beams, if necessary.

NTSB is urging the directive "because experience has shown that, unless a service bulletin (SB) is required by FAA, some operators will likely choose not to take action," NTSB said.

Boeing cautioned in the SB that a break in the main landing gear actuator beam could cause damage to the beam arm, hydraulic tubing and flight control cables, and noted that damage to the control cables could result in loss of control of the plane.

In the case of the Aer Lingus plane, the pilot heard a loud noise when the landing gear was retracted on takeoff, but he continued with the flight since there were no abnormal indications in the cockpit. The flight crew heard the loud noise again on landing.

The board said all four instances of actuator beam damage were traced to improper overhaul procedures, happening within a 10-year timeframe specified in Boeing's bulletin. Part of Boeing's guidance to operators recommended reworking the retract actuator beam with an organic finish to increase durability. Because "time to failure" can't be accurately predicted and the current condition of the beams can't be determined without inspection, FAA should mandate scrutiny of the beams on the 737s specified in Boeing's guidance as soon as possible, the board said.