N960SW – Bombardier Aerospace, Inc. CL-600-2B19 – 2005-06-13 NTSB Accident Report

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 faults are additionally recorded. If the condition is rectified, the EICAS message will be removed from the display.

The landing gear takes from 12 to 16 seconds to complete a cycle. The system allows a maximum of 28 seconds before an unreasonable condition is detected and a gear disagree message is relayed [the DFDR will also reflect if the delay condition is met]. This unreasonable condition encompasses a situation if one or more landing gear is in a contradicting position to that of the landing gear lever.

1.3.6 Manual Release System

The manual release system (MRS) was designed as a fail-safe device in the event of an electrical or hydraulic failure of the landing gear system. The MRS is comprised of the manual release handle (inside the cockpit), downlock assist actuator (only on MLG), release mechanism, landing gear, and landing gear door bypass valves.

The flight crew activates the manual release handle by applying a vertical force of about 45 pounds to achieve an approximate 10 inches of handle travel (leaving about 1.5 inches of extra cable available). There is no means for the flight crew to confirm that they have reached the desired amount of handle movement. The mechanical energy from the handle movement is transmitted by a cable circuit to the NLG and MLG release mechanisms. The movement of the lever releases the hydraulically operated landing gear doors, uplocks, bypass valves, and door-bypass valves. The NLG and doors are assisted to the down (extended) position by spring pressure and aerodynamic flow.

As the handle is pulled, the hydraulic system 3 pressure drops to approximately 2,800 to 2,900 pounds per square inch (psi). Hydraulic system 3 pressure will be restored to 3,000 psi when the handle is stowed.

The manual release system is independent of the normal extension/retraction system with the exception of the NLG and MLG uplocks, as well as the bypass valves.

1.3.7 Quick Reference Handbook

The Bombardier Aerospace quick reference handbook (QRH) contains a landing gear emergency section, which the flight crews are instructed to follow in the event of a gear disagree message on the EICAS. The gear down disagree checklist, pages 13-6 through 13-7, prescribed a seven-step process to help rectify the problem. A complete copy of the QRH landing gear emergency section is contained in the public docket materials for this accident.

In summary, the checklist first required the flight crew to configure the airplane at an airspeed no greater than 200 knots indicated (KIAS) and select the number 2 and 3B hydraulic pumps. The next step advised the flight crew to pull the landing gear manual release handle to "full extension," with a note that the force required to operate the handle is greater than 40 pounds. There was no guidance given as to the distance of travel the handle needs to exceed in an effort to be properly engaged.

If the gear disagree message persisted, the checklist instructed the flight crew to turn "off" hydraulic systems 3A and 3B. After verifying the pressure had bled below 200 psi, the hydraulic systems should be selected back "on". If the gear disagree message continued, the flight crew was advised to repeat the checklist. An additional note indicated that the manual release handle can be cycled more than once and should be pulled again if there was difficulty with the initial pull. The final step instructed the flight crew to use the landing gear up/ unsafe landing procedure checklist if the EICAS still indicated a gear disagree condition.

1.4 METEOROLOGICAL INFORMATION

A routine aviation weather report (METAR) for LAX was issued at 2250. It stated: skies overcast at 900 feet; mist; visibility 6 miles; winds from 260 degrees at 8 knots; temperature 17 degrees Celsius; dew point 14 degrees Celsius; and altimeter of 29.92 inHg.

1.5 FLIGHT RECORDERS

1.5.1 Flight Data Recorder

The aircraft was equipped with a L-3 Communications (Fairchild) Model FA-2100 digital flight data recorder (DFDR), serial number 000167748, which was removed from the aircraft and sent to the Safety Board's Vehicle Recorders Laboratory for readout and evaluation. A complete printout of the DFDR readout is in the public docket materials for this report.

Review of the data revealed that as the aircraft passed through 3,000 feet, the MLG downlock indications changed to a down and locked indication while the nose gear remained in the not down and locked position. The uplock indications reveal that both the MLG and NLG simultaneously changed from the uplock position to the not uplocked position. Shortly after the indications that all landing gear changed to a not in the uplock position, the DFDR shows the gear disagree warning activated.

A further review of the data disclosed the hydraulic pressure for the number 1 and 2 systems remained consistent for the duration of the flight. The number 3 system shows a momentary decrease in pressure at the same time duration as the not uplock indication. The pressure then levels out at 2,900 psi until reference number 422850 (corresponding to time 2208:36) where it drops to 0 psi momentarily then subsequently climbs to about 2,875 psi. The pressure changes are consistent with the manual release system being activated.

No increase of brake pressure was recorded during the gear extension attempt.

1.5.2 Cockpit Voice Recorder

A review of the Cockpit Voice Recorder data revealed that the flight crew followed each step of the QRH. There was no indication that the flight crew ever cycled the landing gear lever, as it is not required in the QRH procedure.

1.6 WRECKAGE AND IMPACT INFORMATION

The airplane came to rest on runway 25L, with the nose supported by the lower fuselage structure and remaining forward NLG doors. The NLG roller was positioned out of the uplock assembly and the tires resting on the runway surface.

The nose section of the airplane was damaged from scraping contact with the runway surface. The NLG forward doors sustained heavy abrasion damage with the remaining edge angled flat with the runway, an indication that they were fully open at the time the airplane made contact with the runway surface. The lower fuselage adjacent to and aft of the NLG wheel well, between stations 220 and 250, was scraped away. The aft door and its respective attachments and links were damaged. The steering manifold along with the electro-hydraulic servo valve, attached hydraulic lines and wiring harness exhibited heavy abrasion. The torque link apex joint sustained light abrasion damage and had an eroded appearance.

1.7 TESTS AND RESEARCH

1.7.1 Airplane Examination

Representatives from Bombardier Aerospace, Messier-Dowty, SkyWest Airlines, the FAA, and the Safety Board initially examined the airplane the day following the accident.

The manual release handle was extended out approximately 10 inches. The NLG tire pressure was measured to be about 140 psi, which the Bombardier representative said was an appropriate inflation pressure. The manual release system was cycled several times by use of the handle inside the cockpit; no anomalies were noted. The uplock pin and claw appeared normal, with slight indications of wear on the center of the pin (.25-inch-long wear mark centered on the pin body and about .001 inches diametrically), consistent with the contact point of the uplock claw. The uplock pin rotated freely and did not appear to be contaminated with any debris. The clearance between the back of the uplock pin to the uplock claw was measured to be .2 inches. No fouling or discoloration was noted on any of the NLG components and there was no evidence of wear or jamming of the NLG body or doors. The shock strut pressure was measured at 450 psi with 5 inches of chrome visible at approximately 70 degrees Fahrenheit, all of which the Bombardier representative said was normal.

Following the initial examination in Los Angeles, the airplane was temporarily repaired by removing the NLG doors and replacing the steering manifold, hydraulic lines and harnesses. The airplane was ferried to SkyWest Airlines facilities in Salt Lake City, Utah, for further examination and testing.

The examination in Salt Lake City occurred on June 17th though 20th, 2005, where numerous NLG systems tests were performed. The NLG system was actuated in many different configurations in an attempt to recreate the extension failure. No discrepancies were noted during the testing.

The following components were removed for additional examination: shock strut (including drag brace), uplock assembly, retract/extension actuator, priority valve, selector valve, bypass valve, PSEU, landing gear lever, and manual release handle. All further examinations were conducted under the auspice of a Safety Board investigator.

1.7.1.1 NLG Assembly

The NLG assembly, drag brace, retract actuator, and uplock were examined at Messier-Dowty on August 15 and 16, 2005. All components were defined as being in good condition and functioned within specification.

1.7.1.2 PSEU

The PSEU was sent to Eldec Corporation for the extraction of non-volatile memory and examination of unit. After receiving the downloaded data, the unit was functionally tested. According to the manufacturer, the PSEU was in serviceable condition and performed to the requirements of the acceptance test procedure/plan (ATP). The following information was gathered from the PSEU logic (written as fault code, description, and definition of code as confirmed by Eldec):

-C 114 Prox 21: unreasonable FAR (recorded 28 seconds after gear down selection)

Nose gear downlock #1 (proximity sensor indicates not down and locked)

-D 112 Prox 35: unreasonable FAR (recorded 28 seconds after gear down selection)

Nose gear downlock #2 (proximity sensor indicates not down and locked)

-C 11D Prox 30: unreasonable FAR (recorded 30 seconds after gear down selection)

Left nose door closed (proximity sensor indicates left nose door not closed)

-C 11E Prox 31: unreasonable FAR (recorded 30 seconds after gear down selection)

Right nose door closed (proximity sensor indicates right nose door not closed)

-C 11F Prox 32: unreasonable FAR (recorded 30 seconds after gear down selection)

Nose door uplock lock (proximity sensor indicates nose doors not up and locked)

-C 135 Discrete Input 14: unreasonable GND (recorded 30 seconds after gear down selection)

Left nose door open (micro-switch indicates left nose door was open)

-C 136 Discrete Input 15: unreasonable GND (recorded 30 seconds after gear down selection)

Right nose door open (micro-switch indicates right nose door was open)

1.7.1.3 Valves

The bypass and NLG selector valves were tested at Tactair Fluid Controls, Inc., on August 11, 2005. The components tested in accordance with the ATP requirements. No anomalies were found.

The priority valve was examined at PneuDraulics, Inc., on November 09, 2005. A functional check and subsequent teardown of the component revealed no anomalies. The valve was in good condition with no contaminants found inside the unit. The valve tested in accordance with the ATP requirements.

1.7.2 System

A review was conducted of several discrepancy databases to compile documentation of similar events related to NLG extension. Specifically, investigators sought out instances where the NLG failed to extend into the downlock position upon the activation of both normal and manual release systems. A review of all prior events revealed that 68 percent of the total extension gear disagree reports were related to the NLG, most of which were a result of slow extension. SkyWest Airlines did not have a higher event rate of gear disagree messages than other CRJ operators.

Bombardier performed a detailed root cause analysis using the fishbone method in an effort to ascertain a common failure point in both the normal and manual release systems. A copy of their research and findings is contained in the public docket for this accident.

Upon completion of the initial analysis, Bombardier determined that three possible scenarios could lead to the accident gear-disagree event:

-Uplock not releasing the NLG body: claw not releasing, latch lever not releasing, insufficient input received on normal and manual operation

-NLG mechanical jam: drag brace or shock strut assembly

-Hydraulic blockage: back pressure or hydraulic blockage on "up" side of actuator or suction on "down" side of actuator

Further testing was performed to investigate these possibilities. Present to these examinations were representatives from the Safety Board, Transportation Safety Board of Canada, Transport Canada, Bombardier Aerospace, Messier-Dowty, and SkyWest Airlines.

A retraction simulation revealed that the NLG uplock pin could be located anywhere between the uplock and 15 inches away from the uplock and not leave any indication on the DFDR of increased brake pressures when the NLG is retracting from that position. This result is similar to the accident scenario (and other reported events), which indicates that the NLG was likely somewhere in this range of travel after the flight crew attempted to extend the landing gear.

Another test was performed in an effort to simulate a hydraulic lock by capping the up line in the actuator. The testing revealed that with a hydraulic blockage, the NLG body will move between 0 and 5 inches away from the uplock and stay in that position due to the hydraulic fluid trapped in the actuator. This condition resulted in an increase in pressure in the hydraulic line between the NLG selector valve and the NLG actuator; it also showed an increase of brake pressure on the DFDR. Another test to simulate hydraulic lock was performed by capping the down line in the actuator. This test revealed that with a vacuum, the NLG would extend almost to the downlock position, requiring only about 5 pounds of force to lock. It was determined that suction and internal leakage would allow the NLG to extend.

A test was performed to evaluate the NLG system if the manual release handle was not pulled completely. During the test the manual release handle was pulled until the NLG body released from the uplock. After it was released, the handle was pushed in about 1 inch and locked in that position. This resulted in a hydraulic lock between the bypass valve and the actuator stopping any further movement of the NLG; this condition resulted in an increase of brake pressure on the DFDR.

Testing was also performed to simulate a combination of improper rigging of the uplock pin, a jam of the uplock pin, and a worn uplock. The NLG retract actuator was pressurized to 3,000 psi, prior to activating the NLG uplock actuator. With this condition the NLG uplock roller released with 300 psi of pressure, but the NLG latch did not rotate to release the NLG uplock pin. The pressure at the uplock was increased to 3,000 psi and there was still no movement of the NLG, which remained in the uplock latch. The manual release handle was pulled to full extension and there was no additional rotation of the uplock latch. When the hydraulics were switched off, the NLG uplock pin came out of the latch and released the NLG into the down and locked position. Based on the results of this test, further research was performed on the uplock and its interaction with the NLG.

Bombardier performed simulations and static analysis on the friction force of the uplock, specifically looking at the interaction between the pin and the hook. They determined the friction coefficients that were required to stop rotation of the uplock latch at various positions of the uplock pin. Additionally, they assessed the impact of a groove, or imperfection, on the uplock latch simulating a worn condition. With the introduction of this anomaly, the direction of the contact force changed between the latch and the pin, reducing the moment at the bushing (change in moment arm). In short, a worn groove of 0.010 inches on the latch would result in about 1/2 the friction moment necessary to lock the uplock latch. This worn condition was never reported in service of the accident airplane.

As a result of the testing, Bombardier Aerospace identified areas of sensitivity of the NLG systems (both normal and manual), under adverse conditions, to include: friction within the system and the sequencing of the hydraulic pressure application.

1.8 ADDITIONAL INFORMATION

1.8.1 QRH Change

Following the accident, Bombardier changed the QRH to reflect an additional action. Another step to be taken by the flight crew in the event of a gear disagree message was added that dictates the cycling of the landing gear lever. Specifically, the added wording instructs the flight crew to cycle the landing gear lever when the manual release handle is in the extended position. It later states that the flight crew should restow the handle after confirmation that the landing gear is down and locked.

1.8.2 Prior Events

SkyWest Airlines has reported a total of three events where the flight crew experienced difficulty extending the NLG using both the normal and manual extension systems (airplane serial numbers: 7595, 7795, 7827). During those events the problem was rectified prior to landing by bleeding the number 3 hydraulic system pressure (as per the QRH) or cycling of the landing gear lever.

Bombardier found 13 events reported and confirmed where the normal and manual extension procedures did not initially result in a NLG downlock condition. Summaries of those events are attached to this report in the public docket.

1.8.3 Subsequent Event

On March 12, 2006, a Bombardier CL-600-2B19 (serial 7404), N856AS, operated by Atlantic Southeast Airlines, received a gear disagree message on the EICAS while on approach to the destination airport. The EICAS display and corresponding warning chime, indicated that the NLG was not in the down and locked position. The flight crew followed the QRH (updated from the QRH used by the accident flight crew) for the gear disagree checklist. They cycled the landing gear lever, bled the hydraulics, and finally pulled the manual release handle; the EICAS continued to display the gear disagree message. After cycling the manual release handle several times, the flight crew kept the handle in the extended position while cycling the landing gear lever up and then down. The landing gear subsequently extended in the down and locked position.

According to Bombardier Aerospace, a review of incident airplane's DFDR reveals that the manual release handle was not in the pulled position during the hydraulic system bleed down as required per the QRH sequence of steps. A representative from Bombardier stated that selecting landing gear lever to the "up" position while the manual release handle is pulled will have the same effect as the bleed procedure: it removes the hydraulic pressure from the extend line of the retract actuator and NLG uplock actuator. A detailed account of this event is contained in the public docket for this accident.

As a result of the incident, Delta Airlines maintenance examined the airplane at their Macon, Georgia, facilities. The airplane was positioned on jacks and numerous landing gear swings were performed; the problem could not be replicated. The cause of the failure was not determined and all components of the nose landing gear system passed all functional tests. At the time of this writing the airplane has not had any further issues with the landing gear system.