The US Federal Aviation Administration (FAA) has mandated inspections of the horizontal stabilizer mechanism on Boeing 737 Next Generation aircraft after several operators reported unwanted nose-up and nose-down oscillations. The regulator estimates that the measure affects 1,987 US-registered aircraft and aims to identify and correct excessive freeplay that could compromise flight safety.

Airworthiness Directive AD 2026-20-14 was published on October 19 and takes effect on November 23, 2026. It applies to certain 737-600, -700, -700C, -800, -900 and -900ER aircraft identified in the relevant Boeing bulletin. The Boeing 737 MAX is not covered by this directive.

The issue concerns excessive freeplay in the assemblies that support the horizontal stabilizer and allow its position to be adjusted. This tail surface plays an important role in longitudinal stability and aircraft trim, and excessive freeplay can allow unwanted movement. The FAA warns that, if left uncorrected, the condition could lead to flutter, a dangerous aeroelastic vibration that can result in loss of aircraft control.

Operators will be required to measure freeplay in the stabilizer’s left and right pivot hinges and in the jackscrew mechanism used to adjust its position. Depending on the findings, further measurements, repairs or replacement of worn parts will be required. The procedures and compliance times are set out in Boeing Alert Requirements Bulletin 737-55A1104 RB, dated June 12, 2025, which is incorporated into the directive.

During the public consultation, some airlines sought more flexible deadlines to better align the inspections with scheduled maintenance. Canadian North proposed extending the initial inspection threshold from 12 to 36 months, citing better alignment with heavy maintenance visits and additional time to procure parts and arrange the work. United Airlines requested longer intervals between repeat inspections for aircraft fitted with new stabilizer hinge bearings.

The FAA rejected both requests, explaining that freeplay must be monitored on original components as well as replaced or repaired parts. Individual extensions may nevertheless be approved if an operator submits sufficient evidence demonstrating that an alternative schedule would maintain an acceptable level of safety.

In its response to comments from Alaska Airlines, the regulator also clarified that freeplay must be checked again after parts are repaired or replaced, with measurements confirmed to be within the prescribed limits before the aircraft returns to service.

The FAA estimates that the basic measurement procedure will require six work-hours, costing $510 per aircraft, or approximately $1.01 million across the affected US fleet. The final cost could be higher once additional inspections and corrective work are included. The regulator cannot yet estimate how many aircraft will require replacement parts and does not have reliable pricing data for those components.