Milwaukee, WI, USA
N866AS
BOMBARDIER INC CL-600-2B19
During final approach, the transport-category airplane's right main landing gear (MLG) did not extend when the landing gear selector was placed in the "down" position. Shortly after, the flight crew noted a "gear disagree" warning message displayed on the engine indication and crew alerting system showing that the nose gear and the left MLG were down and locked but that the right 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 manually extend the landing gear by pulling the landing gear alternate release handle multiple times; however, the right MLG failed to extend, and they subsequently landed the airplane with the right MLG retracted. During postincident activities, the airplane was lifted and an examination revealed that the right MLG remained in its full-up position within the wheel well. The airplane's right MLG uplock pin exhibited signs of slight wear, consistent with in-service usage, and no abnormalities were found with the right MLG uplock assembly. Normal wear patterns were observed on the uplock latch with no discernable depth at the pin contact locations. The uplock pin's slight wear indicates that it might not have always rotated freely when it was in contact with the upper and lower surfaces of the uplock assembly latch as it should have per the system's design. Further, the worn pin could have increased the friction forces between the pin and the latch. Functional testing of the incident airplane's 3A and 3B hydraulic system pumps showed that their output pressures were lower than specified. Analysis of hydraulic fluid samples taken from the No. 3 hydraulic system, including the right sidestay actuator and selected landing gear components, revealed that some of the fluid samples contained particles and fibers that exceeded the in-service size limits specified in the airplane's maintenance manual, and these contaminants likely restricted the hydraulic flow within the hydraulic assembly. It is unlikely that any of these factors (the slightly worn uplock pin, the operation of the hydraulic pumps below their specified operating pressures, or the contamination within the landing gear hydraulic system) would have individually prevented the right MLG from extending; however, it is likely that the combination of these factors prevented the right MLG from extending. In addition, the overall landing gear system design, including modifications made in accordance with an airworthiness directive, did not preclude the consequence that the intermittent combination of these factors prevented the right MLG from extending normally. Postincident examination of the cockpit revealed that the landing gear alternate release handle remained extended about 7 inches. Pulling up on the handle resulted in it moving about 3 additional inches to its fully extended position. The additional handle displacement resulted in the right MLG extending out of its wheel well. When the handle was released from its fully extended position, the handle automatically began to slowly retract, which caused the right MLG to stop extending. The handle had to be pulled and manually held in its fully extended position for the right MLG to extend to its down-and-locked position. During postincident functional testing of the landing gear alternate release handle assembly, the handle's locking mechanism failed to maintain the handle in its fully extended position when a specified retract load was applied to the assembly. Disassembly of the handle assembly did not reveal any mechanical discrepancies. However, the assembly's inner housing and outer slider were found coated with an oily material consistent with lubricant; the origin of the oily material could not be determined. The alternate release handle assembly design specifications do not call for the application of lubricant on the inner housing or the outer slider; therefore, it is likely that the lubricant prevented the handle from remaining in its fully extended position.
HISTORY OF FLIGHTOn June 6, 2011, about 2132 central daylight time, N866AS, a Bombardier CL-600-2B19, operated as Skywest Airlines flight 4443, landed with the right main landing gear retracted on runway 19R at the General Mitchell International Airport (MKE), Milwaukee, Wisconsin. The 2 pilots, 1 flight attendant, and 41 passengers reported no injuries. The airplane sustained minor damage. The scheduled domestic passenger flight was conducted under 14 Code of Federal Regulations Part 121. Visual meteorological conditions prevailed and an activated instrument flight rules flight plan was on file. The flight departed Cincinnati/Northern Kentucky International Airport (CVG), near Covington, Kentucky, about 1951, and was destined for MKE. All airplane occupants evacuated the airplane via the main cabin door. During an interview with the operator's safety staff personnel, the flight crew reported that visual meteorological conditions and calm wind prevailed during the approach. The airplane was on final approach when the gear did not come down, the flight crew informed the tower they needed to go around and flew east over Lake Michigan where they proceeded to complete the quick reference handbook (QRH). They indicated that the gear operated normally on all previous flights. When the landing gear selector lever was first placed in the down position, the flight crew first noticed that the right main gear did not indicate down and locked. They then received the triple chime warning, gear disagree, and proceeded to go around. Upon running the QRH for "Gear Down Disagree" they would receive different indications from right main to both main landing gear unsafe, and they got the nose gear door open warning and oral messages. They also got a No. 3 hydraulic system high temperature caution message. The flight crew stated that about 28 seconds elapsed between the time that the landing gear selector lever was positioned in the down position to the "Gear Disagree" message posting. The flight crew stated that the engine indication and crew alerting system (EICAS) primary page landing gear indications with the landing gear selector lever in the up position, initially, was that all lights were out. When the landing gear selector lever was selected down the first time, the flight crew received nose green, left main green and right main unsafe indications. During the go around, the landing gear selector lever was put in the up position and they received the gear up normal indication. The first officer remembered that the right main would indicate a red unsafe indication very quickly, while the other landing gear would show transit, and then green safe indications. During the third or fourth landing gear selector lever selections, both main landing gears showed unsafe indications, which also happened very quickly. The flight crew stated that the QRH procedures were followed during the attempted manual landing gear extension. They turned the No. 3 hydraulic system off as directed by the QRH and the system pressure subsequently indicated zero. The flight crew pulled the landing gear alternate release T-handle as the QRH directed. The T-handle had slipped back a few inches from the fully extended position, which they had achieved during the manual extension. The EICAS page exhibited red hash marks for the right main landing gear the entire time on the approaches. During one approach, the indications showed both main landing gears were unsafe and a low approach was performed to confirm which of gear were extended. The tower reported that the nose and left main landing gear were down. While returning for the last approach and landing, the left main landing gear subsequently indicated green safe and the right main landing gear still indicated unsafe. During the alternate landing gear extension, the first officer initiated the QRH. He reported his seat height typically is in a very low seat position and he did not reposition its height. He tilted the seat-back forward. He then moved the seat to the aft position. He pulled the T-handle from his seat and felt like he had complete ability to apply all his strength to move the T-handle. The first time he pulled the T-handle it was an abrupt pull and he felt the T-handle reach the stop. The T-handle then sank back a few inches as if a spring was pulling it back down. The gear did not extend and he pulled the T-handle multiple times holding it at the stop for 5 to 10 seconds each time. The only sound that was heard was oil bypassing under their feet. The first officer, who was right-handed, was using his right hand but at one point used both hands to pull the T-handle. He told the captain that the T-handle would not stay up and the captain decided that he would try manually extending the T-handle himself. The captain's seat was low and all the way to the aft position. This is the position he always sits when flying. He reached over with both hands, pulled the T-handle and held it there for ten seconds, and then re-pulled the handle a second time even harder. He could feel the T-handle hit a stop at full extension. He then took back the controls from the first officer. Neither crewmember got out of their seats to pull the T-handle. The flight crew estimated that the elapsed time between the first landing gear selector lever down command and the airplane's touchdown was about 35 to 45 minutes. The airplane sustained minor damage to its right wing tip, right flap assemblies, and right flap pylon assemblies. The right main landing gear door was up, and the right main gear was in its wheel well. PERSONNEL INFORMATIONThe captain held an airline transport certificate with a multiengine land airplane rating and commercial privileges for single engine land airplanes. He held a first class medical certificate with no limitations. The operator reported that he had accumulated about 8,618 hours of total flight time, which included about 438 hours in the Bombardier CL-600-2B19. The captain had flown about 187 hours in the last 90 days, 69 hours in the last 30 days, and 6 hours in the last 24 hours. The first officer held a commercial certificate with single-engine land, multiengine land, and instrument airplane ratings. He held a first class medical certificate without limitations. The operator reported that he had accumulated about 5,156 hours of total flight time, including about 2,997 hours in the Bombardier CL-600-2B19. The first officer had flown about 197 hours in the last 90 days, 44 hours in the last 30 days, and 6 hours in the last 24 hours. AIRCRAFT INFORMATIONThe airplane was a Bombardier Canadair model CL-600-2B19, twin engine, transport category Regional Jet (CRJ), with serial number 7517. It was manufactured on June 15, 2001. The CRJ had a maximum takeoff weight of 53,000 pounds. The engines were General Electric model CF-34-3B1 engines that delivered 8,900 pounds of thrust each. The airplane was on a continuous airworthiness maintenance program. The last service check was conducted on June 1, 2011. At the time of the incident, the airplane had 24,969.4 flight hours and 20,132 flight cycles. The airplane was configured with 53 seats, of which 50 were passenger seats located in the main cabin. The airplane was equipped with a retractable tricycle landing gear system that comprised 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 nose landing gear (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 no. 3 hydraulic system. The MLG system comprises a selector valve, run-around and bypass valve, a left and right MLG sidestay actuator and uplock mechanism. 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. Two of the three NLG doors are operated hydraulically and are sequenced to operate independently of the NLG position, while the other (single 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 is 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 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 run-around 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 alternate 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 alternate extension system is controlled by the vertical movement of a T-shaped alternate release handle. To extend the landing gear manually, a flight crew member must pull up on the alternate release handle. Movement of the T-handle is transmitted by a cable circuit to the NLG uplock 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, this will result in a landing gear disagree aural warning being 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 cancelable 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 is also ON at all times during the flight. According to the airplane's 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. METEOROLOGICAL INFORMATIONAt 2052, the MKE weather was: wind 230 degrees at 7 knots; visibility 9 statute mile; sky condition few clouds at 5,500 feet; temperature 31 degrees C; dew point 20 degrees C; altimeter 29.77 inches of mercury. AIRPORT INFORMATIONThe airplane was a Bombardier Canadair model CL-600-2B19, twin engine, transport category Regional Jet (CRJ), with serial number 7517. It was manufactured on June 15, 2001. The CRJ had a maximum takeoff weight of 53,000 pounds. The engines were General Electric model CF-34-3B1 engines that delivered 8,900 pounds of thrust each. The airplane was on a continuous airworthiness maintenance program. The last service check was conducted on June 1, 2011. At the time of the incident, the airplane had 24,969.4 flight hours and 20,132 flight cycles. The airplane was configured with 53 seats, of which 50 were passenger seats located
The failure of the right main landing gear (MLG) to extend normally, which resulted from the combination of a slightly worn uplock pin, the operation of the hydraulic pumps below their specified operating pressures, and contamination within the landing gear hydraulic system. Also causal to the accident was the failure of the right MLG to extend manually using the alternate gear selector handle due to the improper use of lubricant within the alternate release handle assembly, which prevented the handle from remaining in its fully extended position.
Source: NTSB Aviation Accident Database
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