CEN11IA341

The fatigue failure of the nose landing gear (NLG) end cap, which resulted in insufficient hydraulic pressure to secure the left main landing gear into the down and locked position. Contributing to the fatigue failure was the NLG end cap’s non-optimum grain direction and the inadequate inspection procedure performed during overhaul.

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.

HISTORY OF FLIGHT On May 17, 2011, approximately 0645 mountain daylight time, N218YV, a Beech BE-1900D, sustained minor damage when the left main landing gear (LMLG) collapsed during landing roll out on Runway 35L at Denver International Airport (DEN), Denver, Colorado. The airline transport pilot rated captain, commercial rated first officer and the nine passengers were not injured. The airplane was registered to Raytheon Aircraft Credit Corporation, Wichita, Kansas, and operated by Great Lakes Aviation, Cheyenne, Wyoming. An instrument flight rules flight plan was filed for flight 5150 that originated at Pueblo Memorial Airport (PUB), Pueblo, Colorado, about 0607, destined for DEN. Visual meteorological conditions prevailed for the scheduled passenger flight conducted under 14 Code of Federal Regulations Part 121. According to the captain, while on approach to DEN, the first officer (pilot flying) called for the landing gear to be extended. The captain placed the gear handle in the down position and waited for all three landing gear down-and-locked annunciator lights to illuminate green. However, only the “NOSE”, “RH”, and “L” illuminated. The "H" light for the left gear did not illuminate (there are two bulbs per annunciator, one for the "NO", "SE", "L", "R", and both "H's"). Due to the redundancy built into the annunciator lights, the Captain was confident that even though the “H” light was not illuminated; the fact that the “L” was, indicated that the gear was down and locked. He also noted that the landing gear in-transit light stayed on and the gear motor continued to run for approximately 16 seconds before the gear-motor relay circuit breaker popped. The captain said he confirmed several times that at least one light was illuminated on all three landing gear down-and-locked annunciator lights prior to landing and stated, "All training tells us that indication is 3 down and locked gear." Just before touchdown, the gear unsafe warning horn sounded. The captain said he was not concerned about the horn and elected not to go-around and run the Quick Reference Handbook (QRH) checklists, since all three landing gear down annunciators were illuminated. Plus, he had previously looked out the window and saw what appeared to be a down and locked gear. Upon landing, and after a few seconds during the rollout, the airplane began to "wobble" and the left main gear collapsed. The airplane came to rest on the left side of the runway and all nine passengers exited from the left main door. There was no fuel spill or fire. PERSONNEL INFORMATION The captain held an airline transport pilot rating for airplane multi-engine land. His last Federal Aviation Administration (FAA) first class medical was issued on August 24, 2010. At the time of the incident, the captain reported a total of approximately 3,250 total hours; of which, 2,800 hours were in a Beech 1900D. The first officer held a commercial pilot certificate for airplane single and multi-engine land, and instrument airplane. His last FAA first class medical was issued on February 3, 2011. At the time of the incident, the first officer reported a total of approximately 950 total hours; of which, 85 hours were in a Beech 1900D. AIRCRAFT INFORMATION The Beech BE-1900D is equipped with a retractable tricycle landing gear system. Extension and retraction of the NLG and both main landing gear (MLG) is accomplished by the action of individual hydraulic actuators installed on each landing gear assembly. The MLG actuators retract for gear extension and extend for gear retraction while the NLG actuator extends for gear extension and retracts for gear retraction. Hydraulic pressure for the system is supplied by a hydraulic power pack located in the left wing leading edge inboard of the nacelle and associated plumbing for the normal extend, normal retract, and emergency extend modes. Control of the system is accomplished through the landing gear handle located to the left side of the center pedestal on the pilot’s inboard sub panel. The landing gear handle has two detents, UP and DN. When the handle is placed in the UP position, power is supplied to the hydraulic pump motor and to a gear-up solenoid that allows fluid to flow through the normal retract side of the system. When the handle is placed in the DN position, power is supplied to the hydraulic pump motor and to a gear-down solenoid that allows the fluid to flow through the normal extend side of the system. Once the landing gears are fully extended, an internal mechanical lock in each of the actuators holds the landing gear in the down position. The lock will also activate a down-position switch in each actuator that will interrupt current to the hydraulic pump motor once all three landing gears are in the down and locked position. In the event that all three landing gears are not down and locked, the hydraulic pump motor will continue to run for about 14 seconds before the LANDING GEAR RELAY circuit breaker pops and interrupts current to the pump motor. In the event of a system malfunction or failure, the pilots can extend the landing gear manually through the use of a hand pump located in the cockpit. The manual extension system utilizes a different reservoir and plumbing than the normal system. Visual indication of the landing gear positions is provided by two red lights located in the landing gear handle and six green lights (two bulbs for each annunciator) located next to the landing gear handle. Illumination of the red lights in the handle occurs when the landing gear are in-transit and illumination of the green lights occurs when the gear are in a down and locked position. A gear-up indication occurs when none of the lights are illuminated. In addition to the down-position switches internal to the actuators, there are two position switches located in each wheel well to provide either an up-position or a down-position indication. In order to get either a gear-up or gear down and locked indication two of the three switches have to be in the correct position. When the down-position switch in the wheel well and the down-position switch in the actuator are both closed, current is supplied to the two green down and locked lights in the landing gear position annunciator assembly corresponding to the appropriate landing gear (NOSE, RH, or LH). When the down-position switch in the wheel well is open and the up-position switch in the wheel well is closed, no current is supplied to any lights. For any other combination of switch positions, current is supplied to the two red in-transit lights in the gear handle. Based on the wiring, there are no combinations of switch positions that can supply current to both the in-transit and down and locked lights simultaneously. Each individual landing gear indication and the in-transit indication have two bulbs installed in a parallel circuit for redundancy. If one bulb should burn out, the pilot will still see the correct indication. According to the Model 1900D Illustrated Parts Catalog, Chapter 31-10-00-03, the landing gear position annunciator assembly is composed of three different lamp modules; P/N 90-42292-001 for the NOSE indication, P/N 90-42292-002 for the L and R indications, and P/N 90-42292-003 for the H and H indications. The lamp module part number is stamped on the top of each module by the manufacturer. According to Hawker Beechcraft, the lamp modules are sold complete and there are no provisions in their manuals for replacing or changing the face plates. The individual light bulbs can be changed from the rear of the module without removing the face plate. In addition, a small wall, also called a light dam, is placed between each annunciator bulb to keep individual light bulb illumination concentrated on a specific annunciator. The operator performed an examination of the incident airplane landing gear position annunciator assembly. A lamp module ma