DCA24FA002

The failure of the alternate gear extension system, which prevented the landing gear from being lowered. The cause of the system failure was a broken wire, due to tensile overload, between the alternate gear extend switch and the alternate extension power pack (AEPP), preventing the AEPP from energizing and supplying hydraulic fluid to the door lock release actuators for the nose landing gear and main landing gear. Contributing to the accident was the loss of the left hydraulic system due to a ruptured left main gear door actuator hose from fatigue, which prevented normal landing gear operation.

This accident occurred when the flight crew of Federal Express flight 1376, a Boeing 757-236, was unable to extend the landing gear during their approach to Chattanooga Metropolitan Airport-Lovell Field (CHA), Chattanooga, Tennessee. The main landing gear (MLG) and nose landing gear (NLG) are hydraulically retracted and extended under normal conditions by the left hydraulic system. To retract or extend the landing gear, a flight crewmember must move the landing gear control lever out of its detent and position it to its up or down position. Lever movement is transmitted through control cables to the landing gear selector valve. Operation of the valve supplies left hydraulic pressure through hydraulic lines to either retract or extend the landing gear and operate the landing gear doors. Shortly after takeoff from CHA, the captain of flight 1376 called for gear up and the first officer (FO) raised the landing gear control lever to retract the landing gear. The flight crew indicated that both the MLG and the NLG retracted to their up and locked position. Digital flight data recorder (DFDR) data showed that 22 seconds after gear retraction the hydraulic fluid quantity and pressure in the left hydraulic system began to decrease. A left hydraulic system low quantity indication and master caution were recorded shortly thereafter. After troubleshooting the hydraulic issue per the procedures in the Quick Reference Handbook (QRH), the flight crew made the decision to return to CHA. While preparing to land, the MLG and NLG did not extend as expected after the landing gear control lever was positioned to its down position. Cockpit voice recorder (CVR) data indicated that a triple chime was audible, and the captain said, “Gear disagree.” The first officer confirmed, “Gear disagree. The gear is not coming down.” The captain contacted air traffic control (ATC) to break off their approach and reported that they had an unsafe gear indication. Following the failed attempt to lower the landing gear, the crew went methodically through the Hydraulic System Pressure checklist of their QRH. Following the procedures, the crew attempted to lower the landing gear using the alternate landing gear extension system. This system uses a dedicated hydraulic circuit within the left hydraulic system to release the uplocks on the landing gear doors and gear when activated. To extend the gear with this system, a flight crewmember would move the ALTN GEAR EXTEND switch (a guarded switch) to the down position. Because the checklist indicated that nose wheel steering would be inoperative following the alternate gear extension and the aircraft would not be able to clear the runway on its own, the crew declared an emergency with ATC. The crew performed the alternate gear extension procedure, and the landing gear did not come down. The crew completed the procedure several more times over the next 7 minutes, including re-completing the Hydraulic System Pressure (L only) checklist in full.  As the crew set the aircraft up for the final approach, they began following the Gear Disagree checklist. ATC cleared the flight to land. The captain briefed the FO that he was planning to aim close to the runway threshold and the FO verbally updated the captain on wind conditions and airspeeds. They agreed that a jumpseat occupant on board the airplane would open the left forward main entry door after they landed. Upon landing on runway 20, the flight crew was unable to stop the airplane, and it slid off the departure end of the runway and impacted localizer antennas before coming to rest about 830 ft beyond the end of the runway.   After the airplane came to a complete stop, the flight crew performed the evacuation checklist in the QRH, and the jumpseat occupant attempted to open the left (L1) door. The door rotated halfway open but would not open fully, and the slide did not deploy. The jumpseat occupant then attempted to open the right (R1) door, but it lodged on the slide pack. The jumpseat occupant used force to open the R1 door, and the slide deployed normally. The flight crew and the jumpseat occupant exited the airplane via the R1 door and slide. Both flight crewmembers and the jumpseat occupant were uninjured. The event was classified as an accident because the airplane sustained substantial damage.  Postaccident inspections of the landing gear system found that hydraulic fluid was leaking from the left landing gear door actuator retract hydraulic hose. Inspections also found that the engine indication and crew alerting system (EICAS) indicated that the left hydraulic system had only 32% fluid quantity remaining after the MLG door retraction shortly after takeoff, which is considered fully depleted. After the hose was replaced as part of this investigation and hydraulic fluid was added to the system, the landing gear extended using the normal extension system. A review of the left hydraulic system found that a leak from this hose could result in the left hydraulic system losing the hydraulic pressure required to overcome the internal locks within the door actuator. When there is a loss of left hydraulic system pressure, the landing gear doors cannot be opened, and the landing gear cannot be lowered using the normal gear extension system. Analysis of the failed hydraulic hose revealed that it had multiple broken wire strands along its length and a rupture in its polytetrafluoroethylene (PTFE) inner liner adjacent to the cluster of broken wire strands. The cause of the broken wire strands most likely originated from an overload event as evidenced by the necking down of the wire strands and a reduction in their area. There were no signs of fatigue on the broken wires. Postaccident electrical system inspections of the alternate extension system found no electrical continuity between the alternate gear extend switch and the alternate extension power pack (AEPP). A visual examination of the alternate extension system wiring revealed a break (open) in a wire between the circuit breaker and the alternate gear extend switch. The failed wire was sent to the Boeing Equipment Quality Analysis (EQA) lab for further examination. Analysis of the wire’s fracture surfaces showed a reduction in area and circumferential cracking of the coating, consistent with tensile loading. No obvious defects or anomalies were observed on the fracture surfaces. The wire was inside a wire bundle, so the probability that maintenance personnel could have detected this fault is low. There is no inspection interval for the inside of the wire bundle, which would likely have identified the break in the wire. Since the accident, FedEx has implemented a 275 Flight Hour check on the alternate extension system, including performing a general visual inspection while the NLG and MLG doors are open while on the ground. Findings from the NTSB’s investigation indicate different reasons for the jumpseat occupant’s difficulty opening the L1 and R1 doors to deploy the respective evacuation slides. Postaccident examination of the R1 door found that the R1 bannis latch (which releases the slide pack when an armed door is opened) did not conform to the then-current configuration of the release cable assembly. Specifically, the assembly is supposed to have three links added with two spacers and hardware, as required by Federal Aviation Administration (FAA) Airworthiness Directive [AD] 86-09-09 by reference to Boeing Service Bulletin (SB) 757-25A0058, dated April 18, 1986. Instead, the R1 bannis latch on the accident airplane had only one link and lacked other required hardware, which caused the slide pack to jam before the jumpseat occupant was eventually able to force the door to open. Postaccident examination of the L1 door found that a misrouted deployment strap caused the L1 door not to fully open, prohibiting the occupants from using that door for evacuation. After the CHA accident,

Landing Gear / Hydraulic System: The Boeing 757 is equipped with three independent hydraulic systems: the left, right, and center. The left and right hydraulic systems are each powered by an engine-driven pump (EDP) and an alternating current motor pump (ACMP). The center system is powered by two ACMPs. The hydraulic system incorporates a power transfer unit (PTU), which has a hydraulic motor pump and transfers hydraulic power from the right system to the left system. The PTU is automatically activated when the left EDP pressure is low or the left engine fails. When activated, the PTU powers a dedicated portion of the left hydraulic system, which operates the flaps and slats, landing gear, and nose wheel steering. If the PTU fails to develop pressure in the left system (as expected following the loss of left system hydraulic fluid), the pressure switch for the PTU senses the lack of system pressure and will command the PTU off. The PTU cannot be controlled by the flight crew in the flight deck. The MLG and NLG are hydraulically retracted and extended under normal conditions by the left hydraulic system. The MLG and NLG can also be powered as mentioned above by the right hydraulic system through the PTU. To extend the landing gear, a flight crewmember must move the landing gear control lever out of its detent and position it to its down position. Lever movement is transmitted via control cables to the landing gear selector valve. Operation of the valve supplies hydraulic pressure through the extension hydraulic lines to extend the landing gear and operate the landing gear doors. When there is a loss of left hydraulic system pressure (for example, due to a loss of fluid), the landing gear doors cannot be opened, and the landing gear cannot be lowered using the normal gear extension system. In such a case, the landing gear may be extended using the alternate landing gear extension system. This system uses a dedicated hydraulic circuit within the left hydraulic system to release the uplocks on the landing gear doors and gear when activated. To extend the gear with this system, a flight crewmember would move the ALTN GEAR EXTEND switch (a guarded switch) to the down position. When the switch is moved to down, two alternate extension control relays are energized: one relay supplies power to a latching circuit and the other sends power to the alternate extension power pack (AEPP) to start the alternate gear extension. The ALTN GEAR EXTEND switch is a momentary switch, spring-loaded to the OFF position. When energized, the AEPP supplies hydraulic fluid to the door lock release actuators for the NLG and MLG and a hydraulic pressure switch commands off the AEPP electric motor pump. These actuators extend and mechanically release the internal locks in the door actuators. When activated, the doors and landing gear begin to free-fall to the down-and-locked position. This system is also used to open the landing gear doors for maintenance procedures in or near the wheel well areas. Doors: The accident airplane was converted from a passenger configuration to a cargo configuration for FedEx in 2015. The airplane had two floor-level (Type I Exit) doors behind the cockpit (L1 and R1) that allowed entry to the airplane and a means of evacuation if necessary. Doors 2L, 2R, 3L, 3R, 4L, and 4R were deactivated as part of the cargo conversion. The conversion also included installing a large main deck cargo door on the forward left side and two lower deck cargo doors on the forward and aft right side. The door system consisted of a door, a door handle, an arming lever, a slide/raft pack, a girt bar, and an emergency power assist system (EPAS). Once the door is closed using the door handle, a flight crewmember slides the arming lever over into the red area, arming it, and the door armed indicator pops out on the door. The girt bar is attached to the floor fittings by the bar locks when the door is armed. If the door is opened while armed, the slide/raft will deploy. This is a complex system that begins when an occupant rotates the door handle. Once the door reaches a certain point during opening, the EPAS, a pressurized system with one bottle per door that assists the door opening in an emergency, is triggered, which then aids in the door rotating outward. The EPAS only discharges when the door is in the armed position, and does not discharge during normal door operation. Postaccident examination of the EPAS found that both the left and right door power-assist bottles were depleted. The girt bar, which secures to the floor fitting during the closing and arming process, has an escape slide harness latch assembly attached to it and the harness securing the slide/raft pack to the door. This is a directionally sensitive latch system that will only release because of a horizontal pull, which would be initiated if the door was opening with the girt bar still secure in the floor fittings. When the door is mostly open and the escape slide latch releases, the harness no longer supports the slide/raft and the slide pack begins to drop downward. This downward motion then pulls the cover release pins on the slide/raft pack, allowing the slide to drop and the pressure bottleor inflation to discharge. The slide is then inflated, allowing the occupants to evacuate. On October 4, 2023, about 2347 eastern daylight time (EDT), FedEx flight 1376, a Boeing 757-236, sustained a failure of its left hydraulic system shortly after takeoff from CHA. The airplane turned back to CHA, and, while preparing to land, the landing gear failed to extend normally. The landing gear also failed to extend using the alternate extension system. The flight crew declared an emergency and the airplane sustained substantial damage during the emergency landing. The two flight crewmembers and a jumpseat occupant onboard the airplane were not injured. The flight was operating under the provisions of Title 14 Code of Federal Regulations (CFR) Part 121 as a non-scheduled domestic cargo flight from CHA to Memphis International Airport (MEM), Memphis, Tennessee. According to postaccident flight crewmember statements, the airplane had no maintenance issues before the flight, and the push-back, engine start, and taxi were all uneventful. The captain was the pilot flying, and the FO was the pilot monitoring. CVR data indicated that the flight was cleared for takeoff by the CHA local control (LC) controller at 2221:54 EDT. The Runway Awareness Advisory System (RAAS) announced “On runway two zero” at 2222:58, and the engines were audible accelerating to high power shortly afterward. As the aircraft accelerated, the FO made the following callouts: thrust set, eighty knots, V1, rotate, and positive rate. DFDR data showed that the gear weight-on-wheels transitioned from ground to air at about 2223:42. The captain called for gear up at 2223:44 and the FO raised the landing gear control lever to retract the landing gear. DFDR data showed that the landing gear lever handle was moved to its up position at 2223:47. The flight crew indicated that both the MLG and the NLG retracted to their up and locked positions. The completion of nose gear retraction was audible in the flight deck at 2223:58. DFDR data indicated that 11 seconds later, at 2224:09, the hydraulic fluid quantity and pressure in the left hydraulic system began to decrease. A left hydraulic system low quantity indication and master caution were recorded shortly thereafter (figure 1) At 2224:17, CVR data indicated that the captain said, “Left hydraulic quantity.” At 2224:32, a caution beeper was audible while the FO communicated with the CHA approach controller. The captain called for flaps up and asked the FO to ask ATC to “see if we can just maintain runway heading.” A click consistent with flap handle movement was audible at 2224:47, followed by another caution beeper at 2224:52, and the captain stated, “Fl