FAA mandates stabilizer inspections

The U.S. Federal Aviation Administration (FAA) has required inspections of the horizontal stabilizer on certain Boeing 737 Next Generation (737NG) aircraft due to concerns about potential mechanical issues that could lead to a dangerous condition known as aeroelastic flutter.

Scope of the inspection directive

The airworthiness directive AD 2026-20-14, published on October 19 in the Federal Register, will take effect on November 23, 2026. It applies to specific models of the 737NG, including the 737-600, -700, -700C, -800, -900, and -900ER, but not the newer 737 MAX models.

Function of the horizontal stabilizer

The horizontal stabilizer, located at the rear of the aircraft, helps maintain the plane’s longitudinal balance. Its adjustment involves pivot joints and a control screw, and airlines will need to measure any excessive movement in these components, which could affect the system's performance under aerodynamic forces.

Risk of aeroelastic flutter

The FAA warns that if the issue is not corrected, it "could lead to a phenomenon of aeroelastic flutter, which could result in a loss of control of the aircraft." This occurs when structural movements interact with aerodynamic forces, potentially causing sustained and amplified oscillations that could destabilize the plane.

Required inspection procedures

Operations must follow Boeing’s Alert Requirements Bulletin 737-55A1104 RB, dated June 12, 2025. This includes measuring the play in the left and right joints of the stabilizer and the control screw. Depending on the results, airlines may need to perform repeated checks or repair or replace worn parts.

Timeline and clarification

The effective date of November 23 is not a deadline for inspecting the entire fleet. Instead, the applicable deadlines are determined by the bulletin. The FAA emphasized that the directive’s effective date must be retained, even if the bulletin uses its publication date as a starting point.

Industry feedback and FAA response

During the consultation, Canadian North requested extending the initial threshold for inspections from twelve to thirty-six months to align with heavy maintenance schedules. United Airlines sought to space out inspections further after installing new parts. The FAA rejected these proposals, stating the current deadlines are necessary for safety, though it left room for alternative solutions if properly justified and approved.

Cost and additional considerations

The FAA estimates that inspecting the play in the stabilizer will take six hours per aircraft, costing about $510 per plane at an hourly rate of $85. For the 1,987 affected U.S. aircraft, the total estimated cost is $1,013,370. However, the final cost may be higher due to potential additional checks or the replacement of worn parts, for which the FAA has no definitive data.