Critical aircraft systems are designed to the fail-safe or redundancy principle, meaning that if the system fails, it either reverts to a safe state, or has an independent system that can complete the selected function. The landing gear system, for example, is designed with features for an emergency extension should the normal gear extension fail. In the past, these features may have included pilot operated hand pumps for a pneumatic or hydraulic release, or a pull-handle connected to a release cable. Nowadays, these systems may be electrically selected and mechanically operated. Pilots can expect that if the normal gear extension fails, the emergency system will work. Then there is Murphy’s Law…
On 9 November 2016, an Airbus A380 aircraft with two flight crewmembers, 25 cabin crewmembers and 345 passengers onboard, departed London Heathrow for Dubai. The departure and initial cruise were without indications of any abnormalities and the flight crew settled in for a quiet sector. 3 hours and 20 minutes into the flight, the electronic centralized aircraft monitor (ECAM) alerted the crew that the green hydraulic system had reached a higher than normal operating temperature. This indicated that the hydraulic fluid temperature had reached 120°C at the engine driven hydraulic pump. Another message shortly after indicated that the hydraulic system had now overheated to a temperature of over 140°C. The flight crew followed the ECAM instructions and isolated the system.
The Airbus A380 is equipped with two independent hydraulic systems, yellow and green, which share the workload and provide redundancy for some of the hydraulically operated systems. The isolation of the green hydraulic system for example results in slower nose gear steering, slower flap and slat retraction, reduced wheel brake function, loss of half of the ground spoilers (which results in a longer landing distance), and the loss of hydraulic pressure for the wing landing gear extension. The loss of the green hydraulic pressure can also result in higher fuel burn if a primary electrical system fails to prevent a possible uncontrolled aileron deflection in-flight. In this case, the flight crew operating manual (FCOM) advises the crew that the fuel burn predictions from the flight management system are no longer considered accurate.
The flight crew reviewed the procedures to lower the landing gear using the free fall extension method and discussed the impact of the system limitations for the rest of the flight and particularly the landing. The crew requested a long approach from ATC and they intended to stop the aircraft on the taxiway after exiting the runway. The ground handling agent was requested to arrange a tow from the taxiway to the passenger terminal.
During the final approach everything proceeded as planned until the aircraft reached approximately 4,000 feet, which was approximately eight minutes prior to touch-down, when the landing gear was selected down.
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The flight crew was expecting a delay to the landing gear extension in the ‘gravity fall’ mode. The landing gear indicated down and locked with the exception of the left wing gear indication which remained unlocked. The wheel page on the systems display confirmed their suspicion by showing that the left wing gear was not down and locked. In a situation where the green hydraulic system is isolated, landing gear retraction is inhibited and the landing gear doors remain open.
According to the ECAM, this can produce a 100% increase in fuel burn, meaning a drastic reduction in flight time should the flight crew have considered entering a hold. The flight crew declared an emergency at approximately 2,500 feet and decided to continue the approach. The aircraft landed safely, but when the flight crew noticed that the aircraft was leaning to the left, they decided to stop on the runway to avoid a turn. Ground crew arrived with a tow vehicle and a fire truck was dispatched. After an inspection of the aircraft, it was towed to the passenger gate, where the passengers disembarked safely.
The aircraft
The aircraft was manufactured in 2011 and had accumulated 20,980 flight hours and 3,543 landings. The last A-check was completed four months, 1,301 flight hours and 216 landings prior to the incident.
The Airbus A380 landing gear system consists of the nose landing gear with two wheels, two body landing gears with six wheels each and two wing landing gears with four wheels each. The forward four body landing gear wheels plus the four wing landing gear wheels are fitted with brakes. The nose wheels and the two rear wheels of each body landing gears are steerable to provide better maneuverability. For redundancy reasons, braking pressure is provided by the yellow system to the forward four wheel brakes of the body landing gear and by the green system to the four wheel brakes of the wing landing gear.
The landing gear extension and retraction system consists of a normal gear extension system and an independent free fall or ‘gravity fall’ system for emergency operation. During normal gear extension, the green hydraulic system operates the nose gear and both wing landing gear extension actuators. The yellow hydraulic system provides pressure to the extension mechanism for the two body landing gears. The complete landing gear can only be retracted with the support of both the green and the yellow hydraulic pressure.
The independent free fall system does not require hydraulic actuation and is controlled by two dedicated free fall control modules. Each of these modules provide an emergency unlock actuator (EUA) located on the gear uplock assembly with independent electrical power to unlock the landing gear. The landing gear doors will remain open when this system is used. According to the FCOM, the gravity landing gear extension can take up to 70 seconds to complete.
Troubleshooting
After passengers had disembarked and the cargo was unloaded, instructions were requested from Airbus to safely tow the aircraft to the maintenance hangar. Troubleshooting commenced the next day in the presence of AAIS investigators and Airbus landing gear system specialists, who arrived from France and provided technical support. While troubleshooting included the reason for the over-temperature of the green hydraulic system, which was subsequently attributed to a faulty temperature sensor, the investigation focused on the failure of the left wing landing gear to extend during the emergency gear extension.
Troubleshooting
The process started with an initial visual inspection of the landing gear components, which did not reveal any damage or abnormalities. The aircraft was then raised on jacks to test the emergency free fall extension. After the aircraft weight-on-wheel systems were reset, the left wing landing gear extended successfully.
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With the landing gear now extended and secured it was safe for the team to conduct a number of system tests and detailed visual inspections.
The inspections identified broken wires of the left wing landing gear harness for the free fall system connectors to the Channel A and Channel B EUAs.
Inspections of the other landing gear looms identified similar damage to wires at the right wing landing
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The damaged wires were repaired and the left wing uplock assembly and the two free fall control modules were replaced on advice from Airbus. A subsequent emergency free fall extension test was successfully conducted.
Wiring examination
The damaged wires were sent to the Airbus testing facility in Toulouse, where an initial detailed visual inspection was conducted, followed by the removal of the wire insulation and a subsequent tomographic examination and analysis of the electrical conductor. The inspection found that the three command wires to the EUA of the Channel A and Channel B looms were broken. The deformation of the wire strands indicated that a repetitive mechanical load caused the fracture of the conductors. Signs of arcing between the wire ends and damage to the insulation were also evident. The final tomographic examination confirmed the initial results and determined that the wires failed due to flexure endurance fatigue, which occurs when the wire loom is subject to frequent movements.
Airbus determined that:
“These repetitive aerodynamic solicitations of the harness are most likely the root cause of the damage we observe on these wires and result in rupturing conductor’s strands. Once ruptured, arcing is generated between both sides of the ruptured strands that leads to degradation of the insulation.” [Airbus]
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Airbus Alert Operator Transmission
The incident and its initial findings prompted Airbus to issue an Alert Operator Transmission (AOT) to inform other operators of the incident and findings. AOT A32R009-16 recommended that Airbus A380 operators perform two tasks to verify the integrity of the emergency landing gear free fall system. The first task required a landing gear gravity extension test with the aircraft on the ground to verify the proper system function. The second task included the visual inspection of all wires at the 24 EUA positions for damage, followed by a gravity extension test. A reporting card provided Airbus with an overview of the A380 global fleet status. The initial responses from 75 A380 inspections identified 37 aircraft with single or multiple wire damage.
The damage was primarily found at the left and right wing landing gear EUA looms. Final results from 154 aircraft inspections identified wire damage relating to the topic of the investigation on 63 aircraft. The findings confirmed that looms on the left and right wing landing gear unlock systems were affected and the damage appeared to be unrelated to aircraft age or to a previously introduced production modification to reduce loom lengths.
Flight tests
Flight Tests
In order to further investigate and document the dynamics of the landing gear wiring during in-flight gear operation, Airbus undertook flight tests with cameras installed in the wheel bays monitoring the uplock mechanism and the wiring looms. These tests have shown that loom movements from wind effects are present at the wing landing gear positions, while the body landing gear and nose landing gear looms showed minimal or no movement. This verified the findings of the fleet inspection as per the AOT.
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Investigation analysis
The investigation concluded that a faulty hydraulic temperature sensor provided an overheat indication of the green hydraulic system, which required the flight crew to isolate the system. This resulted in the flight crew being faced with a number of aircraft system limitations for the completion of the flight. One of these limitations was the utilization of the emergency landing gear free fall system for landing.
Damage to the independent Channel A and Channel B EUA wires resulted in the failure of the uplock assembly to release the left wing landing gear prior to landing.
An examination and analysis identified that the wires were most likely damaged by repetitive wind loads as a result of the installation design, which provides insufficient support for the wiring looms when they are exposed to wind effects during gear extension and retraction. This finding was supported by tomographic inspection results and film footage from the flight tests.
When the investigation team became aware of the extent of the initial findings, two prompt safety recommendations were issued ahead of the publication of the final report. It was recommended that EASA ensures that the root cause of the wire damage be determined by Airbus and that a design improvement for the elimination of possible future wire damage and emergency landing gear freefall system failures be developed.
Results
As a result of the incident and the initial findings, Airbus promptly published an All Operator Transmission to determine the full extent of the problem and to identify the affected wires.
The result of these inspections, an analysis of the damaged wires, and visual evidence from the flight tests resulted in the issue of Service Bulletin SB A380-92-8103 to modify the installation design of the EUA looms on the A380 fleet. The Service bulletin was issued in May 2017.
Further recommendations to Airbus included a request for a review of relevant sections in the FCOM to provide additional advice for flight crew and the provision of supporting towing instructions for pilots, engineers and ground personnel.
Recommendations were also issued to the operator relating to the publication of towing instructions to their staff and communication within their Network Control Centre.
Please refer to the GCAA AAIS website for a complete copy of the investigation report at:
https://www.gcaa.gov. ae/en/epublication/pages/investigationreport.aspx