Los Angeles, CA, USA
N960SW
Bombardier Aerospace, Inc. CL-600-2B19
The airplane was damaged during a landing with the nose landing gear partially extended. As the airplane descended toward the destination airport, the crew moved the landing gear handle to the extend position. The pilots noted a "gear disagree" warning message displayed on the Engine Indicating and Crew Alerting System (EICAS) that showed the main landing gear were down and locked, and the nose gear was not fully extended. The pilots opted to land at an alternate airport and began to troubleshoot the gear indication with use of the airplane's Quick Reference Handbook (QRH). Despite their efforts, which included using the manual release handle, the nose gear failed to extend. The airplane touched down on the main landing gear and the nose lowered to the ground as the speed dissipated. Review of the CVR found that the flight crew followed each step of the Quick Reference Handbook checklist. The flight crew did not cycle the landing gear lever, nor was that action part of the QRH procedure at the time. The digital flight data recorder (DFDR) revealed that the main landing gear transitioned into the down and locked position but the nose gear remained partially extended between the uplocked and downlocked positions. A further review of the data disclosed the hydraulic pressure remained constant throughout the extension and did not show the pressure changes that should have occurred with normal system operation. The only pressure changes noted in the duration of the flight were consistent with the manual release system being activated. No increase of brake pressure was recorded during the gear extension attempt, indicating that there was no hydraulic backpressure on the retract side of the NLG actuator. The components of the nose landing gear (NLG) system were tested, revealing no anomalies or malfunctions that could have precluded normal operation. Several simulations were conducted exploring various potential problem scenarios; however, none of the simulations produced the hydraulic system and brake pressure readings on the accident flight's DFDR data. A review of several maintenance discrepancy databases for events where the NLG failed to extend into the downlock position found that the operator did not have a higher event rate of gear disagree messages than the rest of the worldwide fleet. Simulations and static analysis on the friction force of the uplock was performed, specifically detailing the interaction between the pin and the latch. As a result of the testing, areas of potential problems with the NLG extension systems (both normal and manual) under adverse conditions were found to include: friction within the system and the sequencing of the hydraulic pressure application. Bombardier subsequently modified the QRH procedure, adding another step to specifically recycle the gear selector handle.
1.1 HISTORY OF FLIGHT On June 12, 2005, about 2226 Pacific daylight time, a Bombardier CL-600-2B19, N960SW, operated by SkyWest Airlines, Inc., as United Express flight 6543, contacted the runway with the nose landing gear partially extended at the Los Angeles International Airport (LAX), Los Angeles, California. The airline transport pilot and first officer, 1 flight attendant, and 14 passengers were not injured; the airplane sustained substantial damage. Instrument meteorological conditions prevailed for the 14 CFR Part 121 scheduled domestic passenger flight that was operating on an instrument flight rules (IFR) flight plan. The flight originated from LAX at 2127, with a planned destination of Santa Barbara Municipal Airport (SBA), Santa Barbara, California. A review of the SkyWest Airlines Aircraft Flight Log disclosed that the accident flight was scheduled to be the flight crew's fourth trip of the day, corresponding to the airplane's eighth flight. The 31-minute accident flight was scheduled to depart from LAX at 2125, and terminate in SBA at 2156. The accident flight crew indicated that the preflight inspection was normal and that no maintenance discrepancies were noted. During post accident interviews, the flight crew stated to a National Transportation Safety Board investigator that the takeoff and en route portions of the flight to SBA were normal and uneventful. The captain stated that as the airplane entered the vicinity of the airport, he began the landing checklist and called for the first officer to extend the landing gear to the down and locked position. Upon manipulating the landing gear lever into the down position, both pilots heard the nose landing gear (NLG) doors open almost immediately. Several seconds after the landing gear lever was selected to the down position, the pilots noted a "gear disagree" warning message displayed on the Engine Indicating and Crew Alerting System (EICAS), accompanied by a simultaneous warning chime. The primary page on the EICAS system displayed that the main landing gear (MLG) were down and locked with green box indications, and the nose gear was showing a red box, indicating that the nose landing gear was not fully extended. The captain opted to divert back to LAX in consideration that SBA was closing shortly and that LAX has longer runways and more emergency facilities. After receiving a clearance and maneuvering the airplane back toward Los Angeles, the pilots began to troubleshoot the gear indication with use of the airplane's Quick Reference Handbook (QRH). Following the checklist, the first officer pulled the manual release handle located between the pilot's seats, during which time the gear indication remained unchanged. With the EICAS continuing to display the unsafe gear indication, and the airplane approaching LAX, the captain declared an emergency. The airplane touched down on runway 25L, making initial contact with the MLG. The airplane continued to roll on the MLG, and as the ground speed dissipated the nose lowered. The nose contacted the runway surface and the airplane slid to a stop. All the crew and passengers egressed the airplane through the main cabin door without further incident. According to a SkyWest Airlines maintenance manager who responded to the accident, the airplane came to rest on the nose landing gear doors; the NLG uplock pin was released from the uplock assembly. The nose of the aircraft was lifted from the runway surface with air bags. As the nose was raised from the runway, the NLG extended freely from the wheel well, and continued to the down and locked position without any outside force (hydraulic or manual). The nose tires continually remained in contact with the runway surface while being recovered. The aircraft was then towed to the United Airlines ramp area. 1.2 PERSONNEL INFORMATION 1.2.1 Captain The captain was hired by SkyWest Airlines on March 23, 1996. A review of information on file with the Federal Aviation Administration (FAA) Airman's Certification Division, Oklahoma City, Oklahoma, revealed that the captain was issued an airline transport pilot certificate on December 22, 1996, with ratings for airplane single engine land, multiengine land and instrument airplane. The captain additionally held an airplane and powerplant mechanic certificate. He was last issued a first-class medical certificate on January 17, 2005, with the limitation that he must wear corrective lenses while exercising the privileges of his airman certificate. The captain received a type rating for the CL-65 on August 4, 2001. SkyWest Airlines records indicated that the captain had accumulated 11,728 hours total flight experience. He had amassed 4,636 hours in the CL-65 series airplane, of which 3,229 hours he was pilot-in-command (PIC). He had flown about 189, 55, and 5 hours in the 90 days, 30 days, and 24 hours, respectively, before the accident. The captain's last CL-65 proficiency check was marked as completed on March 12, 2005; his last competency check and route check marked as completed on September 06, 2004; and his last simulator check marked as completed on September 15, 2004. 1.2.2 First Officer The first officer was hired by SkyWest Airlines on December 2, 2002. According to the FAA, he held a commercial pilot certificate with ratings for airplane single engine land, multiengine land and instrument airplane. He held a first-class medical certificate that was issued on October 14, 2004, without limitation. SkyWest Airlines records indicated that the first officer had accumulated 3,145 hours total flight experience. He had amassed 1,768 hours in the CL-65 series airplane and had flown about 245, 72, and 5 hours in the 90 days, 30 days, and 24 hours, respectively. The first officer's last CL-65 proficiency check was marked as completed on May 20, 2004; his last competency and simulator check was marked as completed on May 13, 2005; and his last route check was marked as completed May 20, 2005. 1.3 AIRCRAFT INFORMATION 1.3.1 Airplane, General The Bombardier Canadair Regional Jet (CRJ) CL-600-2B19 is a twin-engine turbo fanjet. It is certified under the bilateral certification treaty with Canada in effect since 1971. The CRJ-100 (also known as the CRJ-200 and CL-600-2B19) is included in FAA Type Certification A21AE; it was certified January 21, 1993. The accident transport-category airplane, serial number 7853, was manufactured in 2003. Skywest Airlines was the sole operator of N960SW since its delivery from Bombardier in October 2003. The airplane was configured with 53 seats; 50 of which were passenger seats located in the main cabin. At the time of this writing there are approximately 740 CL-600-2B19 series airplanes operated in the United States. SkyWest Airlines has the third largest fleet consisting of about 125 airplanes. 1.3.2 Maintenance, General At the time of the accident N960SW had accrued about 4,126 hours of flight time. SkyWest Airlines maintained the airplane on a continuous airworthiness basis following the Bombardier maintenance inspection program. The engines' total time were also about 4,126 hours, as they were the original engines equipped on the airplane at manufacture. The most recent continuous airworthiness check was conducted on April 26, 2005, which consisted of an A-2 inspection. 1.3.3 Nose Landing Gear System Components 1.3.3.1 System The nose landing gear (NLG) system is electrically activated and hydraulically operated by hydraulic system number 3. It is comprised of double-wheel and aft-extending (forward retracting) landing gear body. The NLG system encompasses a landing gear lever, proximity sensor system, extension/retraction actuators, uplock assembly, selector valves, priority valve, bypass valves, restrictors, downlock, and check valves. 1.3.3.2 Landing Gear Lever The airplane cockpit is equipped with a landing gear control panel. The control panel is located beneath the EICAS panels in the middle of the center pedestal (between the pilot seats). A landing gear operating lever is installed in the control panel, which when manipulated, sends an electrical extension or retraction command to the proximity sensor system; it additionally sends ground signals directly to the selector valves. The handle consists of a two-position control lever, and is designed to move up or down to select landing gear position; it is held into position by two mechanical detents. 1.3.3.3 Proximity Sensor Electronic Unit The proximity sensor system operates by identifying the position of targets and giving a resulting command. The system is comprised of the Proximity Sensor Electronic Unit (PSEU), multiple proximity sensors, and numerous switches. The PSEU receives information via proximity sensors to determine the position of the nose landing gear. The PSEU serves several functions within the NLG system: -determines if the airplane is on the ground or in the air -monitors for gear up, down and transit status -determines if the landing gear doors are open or closed -provides commands to extend or retract the landing gear -provides commands to open and close the NLG forward doors -records faults of any contradictions or discrepancies 1.3.3.4 Actuators A single piston, double-acting hydraulic actuator is utilized in the NLG system to hydraulically move the NLG body into the extended or retracted position. The actuator assembly includes a piston, cylinder, restrictor, and several hydraulic seals. The cylinder body is comprised of two hydraulic ports: one for fluid to extend the actuator piston, and the other used to permit fluid to the retract port (on the opposite side of the cylinder). Restrictors installed in the system control the speed of the actuator during extension or retraction. The forward NLG doors are equipped with a similar type of actuator to position the doors in the extended or retracted (open or closed) position. 1.3.3.5 Uplock The uplock assembly, which is attached to structure in the nose wheel well, locks the NLG body in the retracted position. The locking mechanism (uplock claw) consists of a latch and latch lever assembly. The latch lever assembly is designed with a roller, which engages a shoulder of the latch to secure it in the up and locked position. The latch-lever pivots on bushings located in the body of the unit. A hydraulic actuator assembly releases the mechanical uplock, permitting the NLG to be extended. When the actuator is fully pressurized, the piston extends resulting in the uplock lever moving. When the uplock lever is moved, the roller mechanism lifts out of engagement with the latch. A combination of the actual weight of the NLG body and springs acting on the uplock latch further turn the latch to release the uplock pin; the NLG is designed to then freely fall to an extended position. Although the uplock is normally released hydraulically, there are two co-axial mechanical release levers installed on the body of the uplock. One is used as the release mechanism during emergency gear landing extension, following an extension/retraction system malfunction, while the other is a manual release used during maintenance. The uplock pin underwent a design improvement in November 2004, removing the possibility that an over torqued pin could cause the pin to stop rotating. At the same time the uplock pin bracket was redesigned with low friction bushings at the pin interface. The accident airplane did not have this modification/upgrade. The airplane maintenance manual (AMM) specifies that the rigging between the root of the uplock claw and the NLG uplock pin is a distance of 0.125 to 0.205 inches (which was applicable to the uplock pin installed on the accident airplane). 1.3.3.6 Valves The selector valves are solenoid-operated, four-way, three-position type valves, which control the flow of hydraulic fluid to the extend or retract side of the NLG door actuator and NLG actuator. The landing gear lever position and resulting PSEU command will activate the solenoid in the valve (must additionally have the landing gear lever ground). The priority valve is a relief valve, which ensures that the flight controls have hydraulic pressure priority during airplane operation. The valve is designed to close at a certain psi, starving the landing gear of fluid and permitting the flight control systems to function normally. The bypass valve allows hydraulic fluid to flow out of the up port of the NLG actuator back to the return pressure line. 1.3.3.7 Doors The NLG system is equipped with three doors used to prevent damage to the system components within the wheel well and increase aerodynamic efficiency when the landing gear is in the retracted position. Two of the doors are operated hydraulically and hinged horizontally along the nose landing gear bay; they are independent of the NLG. The remaining door is located aft of the other NLG doors and hinged perpendicular, mechanically attached to the NLG. During extension, the forward doors open several seconds before the NLG is released from the uplock. The nose gear assembly will then extend, simultaneously opening the rear door. Upon reaching full extension (when the NLG is downlocked), the forward doors close and remain in that configuration until a retraction command is selected. 1.3.4 Nose Landing Gear Extension As the flight crew selects the landing gear lever into the down position, an electrical signal is sent to the PSEU. The PSEU in turn will determine the ground condition (in air or on ground) and verify that all the required input signals are present and satisfied prior beginning to transit. If all conditions are met, the PSEU sends a signal to energize the forward gear door selector valve to the down (door open) position. As the selector valve for the NLG doors opens, hydraulic fluid moves the piston in the door actuator, extending the forward doors to the full open position. The PSEU recognizes that the door extended condition has been met and energizes the NLG selector valve. This allows hydraulic fluid from the number 3 hydraulic system to be directed through the priority valve to the selector valve. The selector valve opens, allowing hydraulic fluid to flow to the NLG uplock actuator and to the extend side of the NLG actuator. The selector valve additionally connects the retract side of the NLG actuator to the return pressure line. As the NLG is released from the uplock assembly, the NLG actuator extends, and with the aid of gravity, the NLG will reach the extended position where it is mechanically locked. The PSEU senses the gear-down indication and will energize the NLG door selector valve, closing the forward nose landing gear doors. Throughout the process, the PSEU updates the EICAS gear icons, graphically displaying the gear position on the screen for the flight crew. During the NLG extension, hydraulic pressure builds up in the retract line between the NLG selector valve and the actuator. This pressure increase is due to the presence of flow controls within the line, allowing a control of motion in the NLG. This is accomplished by providing a backpressure on the retract side of the NLG actuator. This back pressure creates a command pressure to the brake metering valve, enabling a brake command pressure to be sent to the MLG brakes. During the retraction, the pressure in the retract line also creates this brake command pressure to stop the main wheels from spinning when the landing gear are retracted. This increase in brake pressure is recorded on the digital flight data recorder (DFDR). 1.3.5 Gear Disagree A function of the PSEU is to monitor for unreasonable conditions (gear disagree), while recording faults in both the extension and retraction phases. This information is sent to the EICAS, enabling the flight crew to be aware of positions and activity. Corresponding codes are numerically recorded in the PSEU non-volatile memory, recording an activity snapshot during a given period of disagree mode. The EICAS displays the landing gear system's position and faults associated with unreasonable conditions detected; the fault
failure of the nose landing gear to extend to the down and locked position for undetermined reasons.
Source: NTSB Aviation Accident Database
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