What happened
While climbing through 26,000 feet, the aircraft began experiencing rapid pitch up and down oscillations. The captain took manual control of the airplane and successfully leveled off the aircraft. However, the pitching continued, prompting the crew to engage the autopilot, which stabilized the flight.
After slowing the aircraft to 230 knots, the crew turned off the autopilot, at which point the pitch oscillations resumed. The crew declared an emergency and returned to Newark International Airport in Newark, New Jersey. The aircraft landed without incident.
The investigation
Maintenance crews replaced the left inboard elevator parallel engage solenoid shut off valve, after which no further system discrepancies were observed. An examination of the faulty valve identified an electrical short between coil 1 and coil 2.
Upon disassembly of the unit, investigators found hydraulic fluid located underneath the case assembly and around the coil assembly. The investigation also revealed that the orange-colored lead for coil 1 and the brown-colored lead for coil 2 had fused together through the insulation, creating the electrical short. It could not be determined how the hydraulic fluid entered the valve or if it played a role in causing the short.