Milwaukee, WI, USA
N498CA
BOMBARDIER CL600 2B19
While the flight crew was configuring the airplane for landing and shortly after placing the landing gear selector in the "down" position, they noted a "gear disagree" warning message displayed on the engine indication and crew alerting system that showed that the nose gear and the right main landing gear (MLG) were down and locked but that the left MLG was in transit. The flight crew followed the quick reference handbook directions to troubleshoot the landing gear issue without success. The flight crewmembers then tried to extend the landing gear by pulling the alternate landing gear release handle; however, the left MLG failed to extend, and they subsequently landed the airplane with the left MLG retracted. During postincident activities, the airplane was lifted, and an examination revealed that the left MLG remained in its full-up position within the wheel well. The alternate landing gear release handle was found in its fully extended position. The incident airplane's left MLG uplock pin exhibited signs of slight wear and flat spotting, consistent with in-service usage, and the uplock mechanism latch had wear marks that were within the in-service maintenance wear limits. After the incident, functional ground testing of the airplane's landing gear system found that, although wear was observed on these system components, the left MLG extended as designed when the landing gear was selected down normally and manually. Additionally, the incident airliner's left MLG uplock pin was found properly rigged within its mating uplock mechanism. Therefore, it is unlikely that the worn components alone would have prevented the left MLG from extending during the incident flight. Functional performance testing of the left MLG sidestay actuator showed that it operated within all test specifications. Although an examination of the actuator's restrictor assembly found that it contained two small pieces of aluminum and several particles of debris this contamination did not prevent the actuator from operating during ground tests after the incident. Therefore, it is unlikely that the contamination within the left MLG sidestay actuator would have prevented the left MLG from extending during the incident flight. The force to operate the lever on the bypass valve was measured and found to be 60 lbs, which exceeded the acceptance test procedure -specified force requirement of 40 lbs. However, during the incident flight, the nose and both MLG uplock assemblies did unlock when the alternate landing gear release handle was pulled indicating that the bypass valve functioned during the flight. As previously discussed, neither hydraulic contamination within the left MLG actuator nor any single MLG system component was identified as preventing the left MLG from extending during the incident flight. However, because the left MLG failed to extend when the crew used either the normal or alternate extension systems, it is likely that a combination of several factors contributed to the system malfunction. Some potential factors are: uplock pin rigging and/or wear, uplock mechanism latch wear, hydraulic system pressure and contamination, and bypass valve operation. Even though these items may be within maintenance limits when considered individually, there may be combinations of these factors that result in failure of the gear to extend. When combined with environmental and flight related conditions such as temperature, humidity, and landing gear component deflection due to in-flight loading, the interaction results of all of these factors are difficult to predict and to demonstrate during testing.
HISTORY OF FLIGHTOn September 28, 2010, about 1710 central daylight time, N498CA, a Bombardier CL600-2B19, operated as SkyWest Airlines flight 3074, landed on runway 7 right (07R) at the General Mitchell Airport (MKE), Milwaukee, Wisconsin with its left main landing gear (MLG) retracted. The incident flight originated from Eppley Airfield (OMA), Omaha, Nebraska, at 1538 CDT, with a planned destination of General Mitchell Airport (MKE), Milwaukee, Wisconsin. While on approach to MKE the crew attempted to extend the landing gear and observed an abnormal gear indication. The crew conducted a go-around and attempted to manually extend the gear, but was not able to extend and lock the left main landing gear (MLG), and decided to land before fuel ran low. The flight landed on runway 07R, coming to rest on the right MLG, nose wheel, and the left wing. The captain and first officer, 1 flight attendant, and 39 passengers were not injured. According to interviews with the NTSB, the first officer indicated that he was the flying pilot and the captain was the monitoring pilot during their flight from OMA to MKE. The incident flight departed OMA about 1538 bound for MKE. According to the interviews, after selecting the landing gear down during approach to runway 07R, a GEAR DISAGREE message posted on the Engine Indicating Crew Alert System (EICAS) along with an associated aural alert and a triple chime. The flight crew performed a go around and received radar vectors from air traffic control (ATC) while trouble shooting the problem. After executing the go around, the captain selected the gear back to the up position and initiated the "GEAR DISAGREE" procedure located in the Quick Reference Handbook (QRH). According to the interviews, fuel was critical so they asked for radar vectors back to final approach for runway 07R. During final approach and after pulling the landing gear manual release T-handle (the gear selector lever remained up), the captain took control of the airplane and asked the first officer to verify a note in the checklist regarding the landing gear manual release T-handle.. After discussing their interpretation of the note, the flight crew decided to leave the landing gear manual release handle in the extended position and proceeded to land. The captain stated that the notes in the checklist implied to him that the landing gear could come back up if he stowed the manual extension handle. He said about that time, the tower asked them if they wanted to do a fly by. He said they had already talked to ATC and declared an emergency so they did the fly by over runway 07R and the tower controllers confirmed that the left main gear appeared to be up. The captain used right aileron to keep the left wing up as long as possible. The left wing eventually dropped to the runway and the airplane came to a stop at the runway's left edge. PERSONNEL INFORMATIONThe captain was a certificated airline transport pilot with a multi-engine rating. His most recent first class Federal Aviation Administration (FAA) medical certificate, dated May 11, 2010, bore no limitations. He reported 6,313 hours total flight time with 4,048 hours in the Bombardier CL600-2B19. The first officer was a certificated airline transport pilot with a multi-engine rating. He held a first class medical certificate that was issued on September 20, 2010. He reported a total of 5,042 flight hours with 2,966 hours in the Bombardier CL600-2B19. AIRCRAFT INFORMATIONThis Bombardier Canadair Regional Jet CL-600-2B19 was manufactured in 2003 and was identified with serial number 7792. It was equipped with two General Electric (GE) CF-34-3B1 engines and was configured with 53 seats; 50 of these seats were passenger seats located in the main cabin. SkyWest Airlines leased the airplane from GE Capital Aviation Services (GECAS) on May-17-2007. At the time of the incident, the airplane had accumulated about 17,101 flight hours and 15,754 flight cycles. The airplane was equipped with a tricycle landing gear system that comprises two main landing gear (MLG) assemblies mounted on the inboard part of each wing, and a nose gear assembly mounted directly below the flight compartment. Both MLG retract inward into recesses in the wing and center fuselage, and the NLG retracts forward. The landing gear system, operated by a selector lever, is electrically controlled by a proximity sensor electronic unit (PSEU) and hydraulically operated by the Number 3 hydraulic system. The MLG system comprises a priority valve, selector valve, run-around and bypass valve, a left and right MLG sidestay actuator and a left and right uplock assembly. The NLG system comprises a selector valve, extension/retraction actuator, uplock assembly, downlock, nose selector valve, and priority valve, bypass valves, restrictors, and check valves. Both MLG are secured in their up and locked position by their respective uplock assembly, which is attached to structure in the main wheel well. Each uplock assembly consists of a latch, an uplock lever, an actuator, and a manual release lever. When up and locked, the MLG uplock roller (which is attached to the gear) is positioned and held in place by the uplock latch. Two forward NLG doors are operated hydraulically and are sequenced to operate independently of the NLG position, while the other (single aft NLG door) is mechanically linked to the nose gear position. During extension, the forward doors open 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 down and locked), the forward doors close and remain in that configuration until a retraction command is selected. The cockpit was equipped with a landing gear control panel, which contains a landing gear selector lever. When the selector lever is manipulated, an electrical command is sent to the PSEU to extend or retract the landing gear. Both MLG are extended in the outboard direction by their respective MLG sidestay actuators and are hydro-mechanically locked in place for landing. Each gear is retracted by the MLG side stay actuator in the inboard direction and mechanically locked in the MLG wheel wells during flight by their respective uplock mechanism. During normal landing gear extension, when the landing gear selector lever is placed in the gear down position, the selector lever module sends an electrical extension command to the proximity sensor system and provides electrical signals to command the MLG and the NLG selector valves to their gear down position. When the MLG selector valve transitions to its gear down position, the valve is designed to allow no. 3 hydraulic system fluid, from the priority valve, to be ported, via the runaround and bypass valve, simultaneously to the uplock assembly and the extend side of the sidestay actuator for each MLG. The hydraulic pressure causes each uplock assembly to unlatch and release the MLG assembly. When unlatched, an uplock sensor (on the uplock mechanism) provides an input to the PSEU, which in turn signals the data concentrator units (DCUs) to generate an amber 'IN TRANSIT' gear indication on the EICAS display for each of the gear. When hydraulic pressure is supplied to the extend side of the gear actuators (sidestay), the actuator extends causing each MLG to extend to its full down and locked position; the extension rate is controlled by a restrictor in the actuator up line. The landing gear manual release system provides the flight crew with another means to extend the landing gear in the event that an electrical or hydraulic failure within the landing gear system prevents the landing gear from being extended normally. The manual extension system is controlled by the vertical movement of a T-shaped manual release handle. To extend the landing gear manually, a flight crew member must pull up on the manual release handle. Movement of the T-handle is transmitted by a cable circuit to the NLG release mechanism and to the MLG release mechanism. For the NLG system, the mechanisms activates the NLG door bypass valve and the NLG bypass valve and releases the NLG uplock and nose door lock. For the MLG system, rotation of the interconnect lever results in three actions: 1) the displacement is transmitted by two cables to the release levers on the left and the right uplock mechanism to unlock the uplock mechanism permitting the gear to extend by gravity and 2) re-positions the runaround and bypass valve into bypass mode and 3) positions the assist valve to pressurize the assist actuator. In bypass mode, the runaround and bypass valve connects the extend pressure from the selector valve and both extend and retract pressure from the sidestay actuators and the extend pressure of the uplock assembly to an independent return line. The extend pressure of the uplock assembly is sent to the return line via the selector valve and check valves. The removal of all hydraulic pressure from the uplock mechanisms and sidestay actuators is designed to allow the gear to free-fall regardless of the position of the MLG selector valve. The activation of the downlock assist selector valve results in the valve porting no. 2 hydraulic system pressure to the MLG downlock assist actuators to assure down locking of the main gears after free-falling. The landing gear indication system provides the status of each landing gear position on the landing gear display area on the EICAS primary page. The primary page contains three rectangles that will change color depending on the position of the landing gear. When a gear assembly is "up and locked", its respective rectangle will be colored white and display "UP". When the indication system detects that a gear assembly is not "up and locked" or "down and locked", its respective rectangle will transition to amber and when a gear assembly is "down and locked", its respective rectangle will be colored green and display "DN". If any landing gear remains in transit for longer than 28 seconds, the amber intransit indication of that affected gear will change to red (gear unsafe). Simultaneously a red 'GEAR DISAGREE' message will be displayed, accompanied by a 'GEAR DISAGREE' aural warning message. When any landing gear assembly remains in its up and locked position for longer than 6 seconds after the landing gear has been commanded down, a landing gear disagree aural warning will be annunciated along with an EICAS red gear disagree warning message. This warning will also be annunciated when any landing gear assembly remains in its downlock position for longer than six seconds when the landing gear has been commanded up. If the landing gear selector lever remains "UP" during the manual extension, the EICAS immediately displays a "GEAR DISAGREE" message and the master warning illuminates and the corresponding cancellable voice message sounds. The landing gear indication and warning system comprises a PSEU and multiple proximity sensors and switches located within the control system. The PSEU logic analyzes inputs from these various proximity sensors and switches to determine the status of the landing gear and doors. Its output is displayed on the EICAS system (primary page) and master caution/warning panel on the glare shield. The No. 3 hydraulic system is an independent hydraulic system that supplies the landing gear system, braking system, and certain flight control systems with hydraulic pressure. This hydraulic system comprises two alternating current motor pumps, identified as ACMP 3A and ACMP 3B, to generate hydraulic power (3000 psi), a pressure manifold, and a return manifold. Pressure generation comes primarily from ACMP 3A. However, because the system No. 3 accumulator had been removed from the airplane in accordance with the requirements of FAA Airworthiness Directive 2010-22-012, ACMP 3B was also ON at all times during the flight. According to the MLG uplock component maintenance manual, the published wear limits for the uplock assembly's latch was: 1. The maximum permitted wear limit at overhaul is 0.003 in. (0.07 mm) on either or both upper and lower wear surfaces. Between overhauls, the maximum permitted wear limit can be a further 0.003 in. (0,07 mm), for a total of maximum 0.006 in. (0.15 mm) per surface. 2. If the wear on either upper or lower surface is greater than 0.006 in. (0.15 mm), replace the latch. The hydraulic pumps are replaced on condition of failure to deliver 3000 ± 200 psi hydraulic pressure to the respective hydraulic system. AIRPORT INFORMATIONThis Bombardier Canadair Regional Jet CL-600-2B19 was manufactured in 2003 and was identified with serial number 7792. It was equipped with two General Electric (GE) CF-34-3B1 engines and was configured with 53 seats; 50 of these seats were passenger seats located in the main cabin. SkyWest Airlines leased the airplane from GE Capital Aviation Services (GECAS) on May-17-2007. At the time of the incident, the airplane had accumulated about 17,101 flight hours and 15,754 flight cycles. The airplane was equipped with a tricycle landing gear system that comprises two main landing gear (MLG) assemblies mounted on the inboard part of each wing, and a nose gear assembly mounted directly below the flight compartment. Both MLG retract inward into recesses in the wing and center fuselage, and the NLG retracts forward. The landing gear system, operated by a selector lever, is electrically controlled by a proximity sensor electronic unit (PSEU) and hydraulically operated by the Number 3 hydraulic system. The MLG system comprises a priority valve, selector valve, run-around and bypass valve, a left and right MLG sidestay actuator and a left and right uplock assembly. The NLG system comprises a selector valve, extension/retraction actuator, uplock assembly, downlock, nose selector valve, and priority valve, bypass valves, restrictors, and check valves. Both MLG are secured in their up and locked position by their respective uplock assembly, which is attached to structure in the main wheel well. Each uplock assembly consists of a latch, an uplock lever, an actuator, and a manual release lever. When up and locked, the MLG uplock roller (which is attached to the gear) is positioned and held in place by the uplock latch. Two forward NLG doors are operated hydraulically and are sequenced to operate independently of the NLG position, while the other (single aft NLG door) is mechanically linked to the nose gear position. During extension, the forward doors open 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 down and locked), the forward doors close and remain in that configuration until a retraction command is selected. The cockpit was equipped with a landing gear control panel, which contains a landing gear selector lever. When the selector lever is manipulated, an electrical command is sent to the PSEU to extend or retract the landing gear. Both MLG are extended in the outboard direction by their respective MLG sidestay actuators and are hydro-mechanically locked in place for landing. Each gear is retracted by the MLG side stay actuator in the inboard direction and mechanically locked in the MLG wheel wells during flight by their respective uplock mechanism. During normal landing gear extension, when the landing gear selector lever is placed in the gear down position, the selector lever module sends an electrical extension command to the proximity sensor system and provides electrical signals to command the MLG and the NLG selector valves to their gear down position. When the MLG selector valve transitions to its gear down position, the valve is designed to allow no. 3 hydraulic system fluid, from the priority valve, to be ported, via the runaround and bypass valve, simultaneously to the uplock assembly and the extend side of the sidestay actuator for each MLG. The hydraulic pressure causes each uplock assembly to unlatch and release the MLG assembly. When unlatched, an uplock sensor (on the uplock mechanism) provides an input to the PSEU, which in turn signals the data concentrator units (DC
The failure of the left main landing gear (MLG) to extend normally and manually for reasons that could not be conclusively determined.
Source: NTSB Aviation Accident Database
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