Great Lakes Aviation Flight 5150

While on approach to landing, the first officer (the pilot flying) called for the landing gear to be extended. The captain placed the gear handle in the down position and waited for the three landing gear down-and-locked annunciator lights to illuminate. Although the “NOSE” and “RH” annunciators illuminated fully, only the “L” side of the “LH” annunciator for the left main landing gear (LMLG) appeared to be illuminated (each annunciator has two light bulbs). During the landing roll, the airplane began to wobble, and the LMLG collapsed.

An examination of the airplane revealed that the nose landing gear (NLG) actuator end cap, which was found on the runway, failed due to fatigue from multiple origins that propagated from the inside diameter toward the exterior of the cap. The end cap failure, which likely occurred once the NLG reached its down and locked position, allowed most of the hydraulic fluid to be blown out of the NLG actuator, resulting in a decrease in hydraulic system pressure before the LMLG could reach the full down-and-locked position (the LMLG is the last in the system to receive a pressure pulse). No premishap mechanical deficiencies were noted with the LMLG system that would have precluded it from operating normally if sufficient hydraulic pressure had been available. However, because the loss of hydraulic pressure also disabled the manual landing gear extension system, the Quick Reference Handbook (QRH) procedure for the flight crew to manually extend the gear would have been ineffective in securing the LMLG.

The investigation identified safety issues related to the failure mode and inspection procedure for the NLG actuator end cap. Metallurgical examination of the NLG end cap revealed that the fractured NLG end cap’s grain was not in the optimum longitudinal direction. (Grain direction in the metal’s microstructure affects its tensile properties and resistance to fatigue stresses.) The NLG end cap was manufactured before the specifications were revised to indicate a longitudinal grain direction for the part. Also, evidence indicates that the fatigue crack was likely present when the end cap was overhauled in 2008 and during a routine inspection of the airplane in 2010; however, the dye penetrant inspection performed during the overhaul did not detect the crack, which would not have been detectable during the visual inspection of the airplane (because the crack had not propagated to the outside of the end cap). As a result of this investigation, the airplane’s maintenance manual was changed to recommend ultrasonic inspections and overhauls for the NLG end cap at specified cycles.

Another safety issue was identified regarding the illuminated appearance of the LH annunciator in the cockpit. According to the system design, when the LMLG is not down and locked, the indication circuit is not complete, and the LH annunciator lights will not be illuminated in the cockpit. However, examination of the incident airplane revealed that an incorrect lamp module, which did not have a light dam, was installed in the center position. As a result, light from the illuminated "R" (in the adjacent annunciator) was able to bleed over and give the appearance that the "L" was also illuminated, providing the flight crew with an erroneous indication that the LMLG was down and locked.

The investigation examined other factors that influenced the flight crew’s belief that the LMLG was safe and the captain’s decision to continue the approach. The captain noted that, after he placed the gear handle down, the landing gear in-transit light stayed on and that the gear motor continued to run for about 16 seconds before the gear-motor relay circuit breaker popped, but he stated that he was confident that the illuminated “L” indicated that the LMLG was down and locked. The captain also noted that the gear unsafe warning horn sounded just before touchdown, but he stated that he was not concerned about the horn and elected not to go-around and run the QRH checklists because he had confirmed several times that the gear annunciator lights were illuminated, and he had visually confirmed that the landing gear appeared to be down and locked. Both crewmembers stated that their training told them that if at least one light was illuminated for each annunciator, the landing gear was safe. The captain reported that, about 3 weeks before the incident, he had experienced erroneous gear in-transit light and warning horn indications in another airplane that had resulted from a wiring problem.

The QRH procedure, LANDING GEAR MANUAL EXTENSION, states that the failure of the landing gear to fully extend may be indicated by several things, including the red in-transit light and the warning horn; however, it also states: “Because there are two redundant gear-down annunciators for each gear leg, failure of only one green gear-down annunciator to illuminate does not indicate that the gear is unsafe.” The operator’s director of flight standards stated that the company does not train for situations that would involve three illuminated gear down-and-locked lights in conjunction with other gear warnings. As a result of this incident, the operator revised its Flight Standards Manual and training modules.

Verbatim NTSB analysis and probable cause from the NTSB dataset