AD 2021-11-04 Prompted By A Report Of Main Landing Gear (MLG) Retractions After Striking An Obstacle

The FAA is adopting a new airworthiness directive (AD) for certain De Havilland Aircraft of Canada Limited Model DHC-8-400, -401, and -402 airplanes.

This AD was prompted by a report of main landing gear (MLG) retractions after striking an obstacle or severe wheel imbalance after a tire failure. This AD requires inspections for correct height of the lock link over-center stop pin and for correct gaps of the left-hand and right-hand MLG downlock proximity sensors, replacement of the shim if necessary, corrective actions, and installation of a new, improved proximity sensor electronic unit (PSEU) with software changes. The FAA is issuing this AD to address the unsafe condition on these products. This AD is effective July 9, 2021.

Supplementary Information: Transport Canada Civil Aviation (TCCA), which is the aviation authority for Canada, has issued TCCA AD CF-2016-31R1, dated March 24, 2017 (TCCA AD CF-2016-31R1) (also referred to as the Mandatory Continuing Airworthiness Information, or the MCAI), to correct an unsafe condition for certain De Havilland Aircraft of Canada Limited Model DHC-8-400, -401, and -402 airplanes. You may examine the MCAI in the AD docket on the internet at https://www.regulations.gov by searching for and locating Docket No. FAA-2020-0975.

The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 by adding an AD that would apply to certain De Havilland Aircraft of Canada Limited Model DHC-8-400, -401, and -402 airplanes. The NPRM published in the Federal Register on November 2, 2020 (85 FR 69276). The NPRM was prompted by a report of MLG retractions after striking an obstacle or severe wheel imbalance after a tire failure. The NPRM proposed to require inspections for correct height of the lock link over-center stop pin and for correct gaps of the left-hand and right-hand MLG downlock proximity sensors, replacement of the shim if necessary, corrective actions, and installation of a new improved PSEU with software changes. The FAA is issuing this AD to address loss of MLG downlock signal caused by the vibrations from those events, which leads to de-energizing the MLG solenoid sequence valve (SSV) and subsequent removal of hydraulic pressure from the MLG downlock actuator. Loss of the hydraulic pressure in the downlock actuator, combined with the vibrations, can cause the stabilizer brace to unlock and the MLG to subsequently retract. See the MCAI for additional background information.

FMI: www.regulations.gov