The objective of this research study is to elaborate on the cause of the incident involving a Boeing 757-21B that occurred on the last 7th of September, the year 2011, in the cruise of Bulgarian airspace. This report includes the failures found, underlying issues, contributing factors that prompt the incident, and recommendations related to the aircraft incident.
This study aims to discuss the electrical issues of the Boeing 757-21B that caused its electrical instruments to dysfunction.
The Boeing 757-21B’s left AC electrical bus lost power during its flight from Leeds Bradford Airport to Cyprus – Larnaca Airport. A thin haze of smoke appeared on the flight deck during this incident. After an emergency landing, the source of the electrical fault was found at the left pylon bulkhead.
The crimp terminal at the D1114J connector was found corroded due to the moisture build-up from the stripped wires. To avoid these situations, it is recommended to conduct a visual pre-flight inspection and the proper tightening of the connector’s internal component to minimize the equipment being corroded.
A serious incident involving a Boeing 757-21B on September 7th of the year 2011, due to the dysfunction of multiple flight instruments on the aircraft.
During its mid-flight, the warning lights triggered a power failure, and the pilot took cautions of the power loss in the left generator. The person in charge resets the electrical bus, and the power was temporarily restored. After a few minutes, the power went off again and a thin smoke appeared on the flight deck.
An attempt to power the aircraft’s left AC electrical bus from the APU bus was unsuccessful and the aircraft diverted to Kavala Airport in Greece, where a normal landing was carried out. After the aircraft investigation, they found out that the crimp terminal at the D1114J connector of the left power generation system located at the left pylon bulkhead was already corroded. However, the source of the smoke from the flight deck was not clearly identified.
The following statements are the problems found after inspecting the aircraft:
- – The aircraft’s flight instruments failed to function because the left AC electrical bus lost its power.
- – The smoke appearance on the flight deck.
- – The loose back shell of the connector due to the stripped threads
The factors to be considered in making a recommendation are:
- – Flight hours of the aircraft and the schedule of the aircraft inspection
- – Proper maintenance troubleshooting actions
- – Costs of the repair actions and replacements of the part
- – Checking of wiring in accordance with the aircraft’s Fault Isolation Manual
- – The AC BUS OFF and L GEN OFF warning captions on the lower EICAS display
- – The amount of occurrence of an ‘L AC BUS OFF’ event before the incident flight
- – Its previous incident report and other factors indicated in the aircraft’s Technical Log
RECOMMENDED CONCEPTS AND ACTIONS TO ADDRESS THE RELATED AIRCRAFT PROBLEM
- – Disassemble the connector of the damaged crimp terminals. Check the wiring of any stripped threads is there in the connector’s backshell.
- – There are four main steps to know more about the electrical dysfunction that happened in the aircraft. First, the mechanic must search for signs of an open circuit, especially at the pin of the D1014P plug. Checking of wiring in accordance with the aircraft’s Fault Isolation Manual is a must in searching. Second, check the D1114J wiring at the left pylon bulkhead and other electrical equipment near it.
- – Use specialized equipment to test the electrical system. Equipment called the power probe electrical tester is needed to check if the electrical signals are functioning properly. It also identifies an open circuit and detects whether the electrical system has a resistive or bad ground. The mechanic can trace and identify the true source of the electrical problem by the use of this equipment. Lastly, fix, repair, or change the damaged area.
- – After finding out the cause of the aircraft failure, which is in this case, the crimp terminal at the D1114J connector that helps generate power, replace the wiring loom and do an operational check. Make sure that the AC power generator is still serviceable after the equipment replacement. In the process of installing the wiring, the mechanic should consider using the conduit method to drain any moisture by putting a 1/8-inch drain hole must be made at the lowest point in each run of the conduit.
The mechanic, aircraft manufacturer, and/or a certified repair station are responsible for implementing the action plan to know the cause of the dysfunction of the aircraft. The pilot and other people involved in the event are also needed to pass the information about the occurrence of the incident.
The post-incident maintenance action includes the visual inspection of the circuit breaker and the AC power generator. Then, the examination of the equipment near the smoke location and the power generator system will follow, to found out if any damaged electrical equipment is presented. After finding the causes of the aircraft failure, fix or replace the stripped threads and other wirings involved.
The mechanic will then check the crimp terminals if any connectors are still in good condition and with no sign of damage presented. Then, they will check if the AC power generator can properly function after the operation.
In terms of the financial assessment of these maintenance actions, since the repair station probably have the equipment needed for repairing the aircraft, the labor cost and the prices of the replaced item will be the total cost of the post-incident maintenance. A basic electrical wire terminal cost about 13.99 US dollars per set and connector wiring pins costs 980 US dollars. Therefore, the minimum price of the required actions is 992.99 US dollars plus the maintenance fee and other miscellaneous expenses.
The stated recommendation will prevent the connector’s wiring to vibrate extensively during flights, which will lessen any electrical damage. It will prevent the cause of corrosion to the connector’s internal components because it will lessen the moisture entering the connector. After all, the back shell of the connector has fixed threads. These steps will ensure that the intermittent disconnection of the left AC electrical bus will function properly to avoid failure in the power generation system and faulty wirings.
The factors below contributed to be aircraft’s electrical failures:
- – Moisture that enters the connectors due to the stripped threads not properly crimped or attached on the bulkhead causes the wirings to vibrate extensively.
- – The open-circuit at the left pylon bulkhead
- – Corroded crimp terminals that result in the AC electrical bus to lost power
- – Proper tightening of the connector’s internal component
- – The condition of the left Generator Control Unit (GCU) and the Bus Power Control Unit (BPCU)
The following recommendations and concepts from the Final-term modules are applied to the Boeing 757-21B incident:
- – The clamps of the wires should be spaced close enough together, so the wiring bundle does not sag or vibrate excessively. Insulation problems can cause short circuits, malfunctions, or inadvertent operation of equipment.
- – Crimping the terminal around a wire to secure the wire and provide a high-quality electrical connection.
- – The appropriate power distribution of the AC power generator system
- – Faults on the distribution system should have the minimum effect on system functioning and should constitute a minimum possible fire risk.
- – Warning Lights that alert the flight crew of unsafe conditions and are accordingly colored red.
- – Periodic inspections of the wire for chafing and hardware security, corrosion of components, and general condition of the circuit should be performed during routine inspections.
To prevent furthermore disturbance of the flight and incidents, the following alternatives should also be considered:
- – Visual and another pre-flight inspection method
- – Proper
- – Make sure that the warning and indicating lights are working.
- – Repairing or replacing the damaged wiring and equipment
- – Replacing of the corroded materials
- – Material of the wires and the type of electrical cables used.
- – Coated protection of the wiring to prevent oxidation.
- – The use of aluminum wire to the area that vibrates extensively will cause it to break.
- – The wiring installation methods use the conduit method with 1/8 drain hole to drain moisture that causes the corrosion of the metallic materials.
- – The electrical wiring should be at least 6 inches above the fluid line.
- – The clamps of the wiring should be spaced close enough together, so the wiring bundle does not sag or vibrate excessively.
- – If there is less than the 1/4-inch clearance between the bundle and the hole, a protective rubber grommet must be installed.
- – Shielding of the wires that are connected to aircraft ground through a crimped-on ring terminal.
- – The use of equipment such as the power probe electrical tester is needed to check if the electrical signals are functioning properly.
The following statements should be considered to examine the alternatives:
- – The location of the wirings where extensive vibration normally occurs will determine what kind of wiring installations should be used to avoid stripping of the wire.
- – The cost of the repair of the damaged equipment. if it is not economical, replace the left Generator Control Unit (GCU) and the Bus Power Control Unit (BPCU) to avoid further disturbance of power in the left AC electrical bus.
- – The intermittent nature of the connections, especially around the D1114J connector. This defect is difficult to diagnose due to its location.
- – The condition of the AC power generation system if the engine experience often electrical overheating fumes.
The stated Boeing 757-21B incident is due to the electrical dysfunction of the left AC power generation system due to the corroded crimp terminal at the D1114J connector located in the left pylon bulkhead. The aircraft’s equipment was affected by the extensive vibration of the wirings that lead to the threads being stripped. It is recommended to assess the methods of wiring installation and its protection to avoid the oxidation of the metallic equipment.
The mechanic, aircraft manufacturer, and/or a certified repair station should consider the recommendations stated in this research study to avoid further incidents involving the AC power generator system. They will identify the nature of this problem easily by starting at the crimp terminals and wirings of the electrical systems. With the help of the stated recommendations, the proper electrical wiring and the cause of the stripped threads will not be overlooked to prevent electrical failures of the instruments.
The following concepts prevent furthermore disturbance of the flight and incidents:
- – The mechanic or other personnel involve will know more about the nature of the failure of the AC power generator system due to the corroded materials.
- – The study elaborated on the appropriate wiring installation methods to avoid this kind of incident.
- – The readers will understand the use of equipment such as the power probe electrical tester to check the electrical signals during the pre-flight inspection.
- – The overlooked problems are listed in this study to remind the readers about the possible moisture build up in the electrical systems due to the improper location of the wirings.