Incident Overview
On December 25, 2009, at approximately 2000 central standard time, an Aerospatiale ATR 72-212A airplane, registration N494AE, operated by Executive Airlines, Inc. as Eagle Flight 4756, experienced a flight control system malfunction while en route to Dallas/Fort Worth International Airport (KDFW) in Dallas, Texas. The flight originated from Midland International Airport (KMAF) in Midland, Texas at 1827. The two pilots, two flight attendants, and 41 passengers were not injured. The flight was conducted under 14 CFR Part 121 on an instrument flight rules flight plan, with night visual meteorological conditions at the time of the incident.
Sequence of Events
During level flight with the autopilot engaged, the flight crew received a pitch mistrim message. Following checklist procedures, they disengaged the autopilot and found that fore and aft movement of both control columns was stiff with limited travel, allowing only about 1 inch of movement in pitch up and down. They attempted the Jammed Elevator procedure twice but could not uncouple the elevators. Coordinating with company maintenance, they slowed to 180 knots and regained increased elevator control.
On final approach, after configuring for landing, the elevators again felt jammed. The crew executed a go-around and declared an emergency. During a second landing attempt, the control columns remained partially jammed. They performed a shallow approach and landed smoothly. While taxiing to the gate, the elevators were still jammed; after parking and engine shutdown, full control was regained.
Maintenance Findings
Post-flight inspection revealed that the brackets holding the left elevator down limit stop had fractured and separated from the hinge fittings. The separated down limit stop restricted left elevator movement. The left elevator hinge assembly and pitch uncoupling mechanism (PUM) disconnect switch were sent to the NTSB Materials Laboratory for examination.
Examination showed fractures in the inboard and outboard L-brackets attaching the lower stop to the hinge fitting. The inboard bracket fractured at the bend; the outboard bracket rotated about 90 degrees and fractured near the nut plate. The lower stop shim stack, composed of four layers totaling 0.15 inch (3.8 mm), exceeded the maximum allowable thickness of 0.062 inch (1.6 mm). The shim stack specification required a solid shim of 0.030 inch and a maximum 0.032 inch stack of laminated shims. A section of the laminated shim layer in contact with the hinge fitting was missing; the adjacent solid shim was deformed and torn. Layers were splayed between brackets.
The inboard L-bracket fracture surface showed features consistent with high-cycle fatigue initiating from the inner bend. Curved striations indicated unidirectional bending fatigue.
The PUM disconnect switch was found stuck in the closed position, which is normal when elevators are coupled. Electrical continuity tests matched a closed switch. The reason for the switch failure could not be determined.
Related Procedures
Normal procedures require gust lock engagement during preliminary cockpit preparation and parking, and release before takeoff. The operator's procedures caution that the gust lock must be engaged when moving on the surface or parked unattended. Flight data recorder (FDR) data from the incident airplane showed that on about 9 of 92 flights, the elevators were in a position consistent with proper gust lock engagement before takeoff and after landing. On many other flights, the elevators were in contact with the lower stops during taxi and other times, indicating the gust lock was not properly engaged, allowing repeated stress on the stops and brackets.
Subsequent Actions
On July 30, 2010, the operator issued Safety Alert Bulletin 2010-005 reminding flight crews and mechanics of gust lock engagement procedures. On July 1, 2010, the European Safety Agency (EASA) issued Airworthiness Directive AD 2010-0138 mandating inspections of all ATR-42 and ATR-72 airplanes. Effective December 15, 2011, the Federal Aviation Administration (FAA) issued Airworthiness Directive FAA-2011-0721 mandating similar inspections.