N407AW

On December 14, 2008, about 1700 Eastern Standard Time, Air Wisconsin Airlines flight 3919, a Bombardier CRJ Cl-600-2B19, N407AW, made an emergency landing at Philadelphia International Airport (PHL), Philadelphia, Pennsylvania, with the left main landing gear in the retracted position. The aircraft was being flown as a repositioning flight from Norfolk International Airport (ORF), Norfolk, Virginia, to PHL under Title 14 CFR Part 121. There were no injuries to the two pilots and one flight attendant on board the aircraft. The previous day, December 13, 2008, the aircraft had arrived in Norfolk, Virginia, to have scheduled maintenance preformed. During maintenance, both the right and left main landing gear uplock assemblies were replaced as required by the air carrier's airplane component time limitation (20,000 cycles). After maintenance was performed, the airplane was released and was being repositioned to prepare for a revenue flight.

During the repositioning flight the flight crew received indications of a landing gear problem prior to landing and stated that after unsuccessfully attempting to lower the left main landing gear, they elected to land with the nose and right main landing gear indicating in the "down and locked " position. In addition, upon arrival in the airport environment, air traffic control tower personnel visually confirmed that the left landing gear was not extended. The crew then made an emergency landing at PHL on runway 27L, with the airplane touching down on the nose and right main landing gear, and subsequently coming to rest on the left MLG and the left wing tip. The crew evacuated the airplane. There was damage to the left wingtip, aileron, and flaps, however no other damage was noted.

Examination of the flight data recorder (FDR) data showed that both MLG received signals simultaneously, with the right MLG signal indicating "Up and Locked", and the left MLG signal indicating "Not Up and locked. See the Flight Data Recorder Specialist's Report.

Post-incident examination of the airplane revealed no anomalies with the right MLG uplock assembly installation; however the investigation revealed that the upper attachment bolt used to mount the left MLG uplock assembly to the structure was installed and cotter pinned, but did not engage the uplock assembly. The lower bolt was installed and did engage the uplock assembly. Such installation permitted the uplock assembly to pivot inboard about the lower attachment bolt.

As a test, the uplock assembly was positioned close to the design orientation, the lock could be released and the main landing gear lowered. When the assembly was pivoted downward as found in the incident investigation, the uplock release cable no longer released the lock since the lock release lever contacted the upper bolt and would not allow the lever to travel fully and release the landing gear.

The investigation revealed that at the maintenance facility two separate mechanics had been assigned to replace each of the uplock assemblies on the airplane.

The installation of both assemblies could not be completed during the midnight shift (2130 to 0730), and the mechanic who replaced the left uplock departed when the shift was over. The mechanic who replaced the right uplock stayed through the morning to finish the installation and departed prior to completing the functional check and manual gear swing on the airplane. Both mechanics signed off the work package as being complete having made a verbal turnover to the day shift supervisor, inconsistent with Air Wisconsin, Inc., policies and procedures (Air Wisconsin GMM Chapter 21, Section 9 - Work Interruption Procedures).

The investigation also revealed that the mechanic that had installed the left gear uplock assembly had never before done such installation, nor had he received pertinent training. The mechanic who replaced the right uplock assembly stated that he had replaced uplocks on other airplanes but not the accident type airplane.

Additionally, removal and installation of the uplock assemblies is a required inspection item (RII) and must be inspected by an RII inspector. The inspection noted discrepancies in the installation of the left gear assembly installation, which were corrected by the mechanic (at the request of the inspector) who stayed past his shift and installed the right uplock assembly; however the discrepancies were not properly documented.

The investigation revealed that the RII inspector performed a functional check and manually extended the landing gear, however the RII inspector stated that he could not actually see the gear being extended from his position during the test as he was only watching the flight instrument panel for the indication of a full extension.

Further discussions with the RII inspector revealed that he did not follow the manual extend procedures outlined in the manuals, but instead followed an abbreviated procedure. In addition, the RII inspector did not use a flash light or inspection mirror as part of the inspection to see in the darkened MLG wheel-well. Furthermore, the RII inspector assigned to check the mechanics work had no previous experience inspecting an uplock assembly and he did not recall if he had ever moved and replaced one during his work as a mechanic.

The investigative team reviewed the Air Wisconsin, Inc. removal and installation maintenance manual for the MLG Uplock Assembly (MM 32-32-05 page 401-407, dated April 20, 2004). The contents of the maintenance manual were the same as the Bombardier CRJ MLG Uplock Assembly manual. Both manuals outline instructions to install both bolts through the uplock assembly and to the aircraft structure. It was noted that the instructions did not mention of the spacer located between the uplock assembly attachment lugs and that the orientation of Figure 401 was confusing when referenced to install the left uplock assembly. The instructions also failed to reference a nearby hydraulic line cover that had to be removed in order to remove the uplock assembly attachment bolts.

For further details, see the NTSB Airworthiness (Systems and Structures) Factual Report; and the NTSB Maintenance Records/Human Factors Factual Report.

Based upon findings of the investigation, the NTSB promulgated Safety Recommendation A-10-96 and A-10-97, dated May 28, 2010.