Following the Boeing 767 (B767) aircraft's second flight after scheduled maintenance, a pilot maintenance log entry noted that the cabin door L1 slide bustle was hanging down. A line engineer on duty examined and certified that the cause was a weak cable spring. The discrepancy was then placed on deferred maintenance as it was not considered to be an airworthiness defect. The following day, prior to departure, the cabin crew of the aircraft reported the cabin door L1 (forward entry door) bustle was lower than normal. Inspection revealed that the girt bar was not attached to the escape slide girt bar carrier, but was instead in the stowed for maintenance position, rendering the slide inoperative. The line engineer on duty correctly installed the girt bar, thereby returning the L1 door escape slide to operational capability. No maintenance log entry was made concerning the discrepancy, or of the corrective actions.
When maintenance personnel conferred following the L1 door escape slide girt bar rectification, a decision was made to inspect the remaining aircraft door slides for condition. The inspection revealed that cabin doors R1 (forward service door), R2 (rear service door), and L2 (rear entry door) emergency escape slides were inoperative, with the door girt bars also in the stowed position.
Consequently, during 8 and 9 April 2001, the aircraft was operated seven sectors with four cabin door mounted escape slides inoperative. During 9 April 2001, the aircraft was operated one sector with three slides inoperative. If required, the cabin crew could not have successfully activated the escape slides of those doors, nor the automatic opening of the doors during those flown sectors.
B767 cabin door configuration
The cabin door configuration of this aircraft was one forward door and one rear door per side. The doors were arranged in sequential order, numbered from the nose of the aircraft. The doors were identified as per side by lettering indicating `L' for the left side of the cabin looking towards the nose, and `R' for right.
Previous maintenance
Further investigation revealed that the last known maintenance of the escape slides, bustles, and girt bars of doors R1, R2, L1, and L2 was performed sixteen days earlier, during a recent 2C scheduled maintenance check. The C check on B767 aircraft, a major scheduled maintenance check, was performed at 6,000 flight hour intervals, and normally planned for 30 days elapsed time (total days out of service). The incident aircraft was out of service for the 2C check for 41 days.
During that maintenance, the escape slide bustles of doors L1, L2, R1, and R2 were disabled and removed by an apprentice Aircraft Maintenance Engineer (AME) under supervision, to allow repainting, other maintenance in the area, and zone inspections. During disabling and removal of the bustles, the girt bars were removed from the carriers and stowed using special straps on the side of the slide packs.
Door mounted slide normal operation
During normal operation, door arming by the cabin crew would lock the girt bar carrier to the aircraft floor fittings. The girt bar would engage the girt bar carrier, whether the slide was ARMED or DISARMED. The ARMED or DISARMED status of the door-mounted slides was indicated to the cabin crew by witness marks, visible at floor level. The arming witness marks reflected the girt bar carrier position in the floor fittings, not the location of the girt bar itself. Thus, the ARMED indication presented to the cabin crew was the same, whether the girt bar was properly installed in the girt bar carrier or stowed on the slide pack.
When a cabin door was unlocked from the inside while in the ARMED condition, the girt bar (engaged with the carrier) initially caused the slide to release from the door. The unladen door would then open under the influence of heavy springs. The subsequent action of the slide pack falling out of the doorway activated the slide inflation sequence.
Door mounted slide operation with girt bar stowed on the slide pack
With the girt bar stowed on the slide pack, each door would have unlocked if opened from the inside in the ARMED condition, however the girt bar carrier would have remained (disconnected from the door) in the floor fittings. The slide pack would have remained intact and attached to the door. The doors would have needed opening manually (as though disarmed), as the emergency auto-opening feature of the cabin door, before slide deployment, relied on the weight of the slide releasing from the door as the door was unlocked. Manual operation was only applicable if the slide had released from the door, but failed to inflate.
Maintenance documentation (removal of the escape slide)
The maintenance facility's normal practice was to use the company's Maintenance Supplementary Report (MSR) form to document tasks involving removal of items from an aircraft for access and inspection purposes. The removal of four escape slide bustles was noted on the aircraft's MSR form for the maintenance event, however, the apprentice who completed the removal of the bustles did not personally document details of the removal in the MSR form. The apprentice did annotate on a Supplementary Report Card (SRC) the bustles removal in accordance with the company's maintenance manual procedures.
The senior Licensed Aircraft Maintenance Engineer (LAME) responsible for the cabin during the scheduled inspection, and supervising the apprentice, stated that his normal practice was to annotate in the Heavy Maintenance Discrepancy Report (HMDR) form, deactivation of the escape slides. On that occasion, he noted the removal of the slide bustles but not the deactivation of the escape slides. There was no evidence that a hand-over log was employed documenting the work.
Installation of the escape slide
Following completion of the zone maintenance in the area of the slides, another apprentice was tasked with reinstallation of the slide bustles to allow the aircraft to be moved outside for engine runs. The engine runs were normally performed three days before maintenance check completion to identify and resolve any engine related anomalies. Normal Heavy Maintenance practices required the slides be installed and operational during engine runs. The slide bustles, including the girt bars, were not completely installed because of on-going work in the area. It was reported that the aircraft's inspection completion was behind schedule by approximately one week.
The bustles of the four doors were partially installed by a second apprentice and annotated on the SRC as completed in accordance with the applicable maintenance manual section. The installation was not completed as per the maintenance manual at that time, as the girt bars were still stowed and the inflation cylinder regulator safety pins were still fitted. No other documentation was utilised to annotate the partial installation.
Post maintenance inspection of the escape slide
According to aircraft maintenance documentation, two LAMEs conducted independent inspections of the slides following maintenance to the slide during the scheduled check. Both conducted and certified the inspection with the knowledge that slide R1 was not yet fully operational. An independent inspection was not carried out for maintenance actions performed on escape slide bustles at the L1, L2 and R2 doors. No additional entries were annotated in the aircraft documentation to note the non-operational condition of the slides.
Prior to release from the 2C check, the B767 Flight Readiness Schedule, an engineering check list, was used to ensure aircraft completion. The check list contained a requirement to check over-wing emergency exits for correct arming, but for door mounted slides the inspection was only to verify that the escape slides were installed.
Human factors issues
The LAME responsible for cabin certification during the 2C check, reported that this was the first time he had been tasked with such duties.