Human Factor – Aircraft Incident Analysis

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In this assignment, we will discuss, analyze, and critically evaluate the incident involving the aircraft Boeing 737-400 with flight registration number G-OBMM near Daventry on 23 February 1995. We will base our analysis on the report of the Air Accident Investigation Branch (AAIB), Department of Transport, which has the report number 3/96 (EW/C95/2/3).

The reason for selecting this particular aircraft incident is because the report provided by AAIB was comprehensive, including details of the incident sequence, information about the aircraft operator and the Authority. The report also contained clear findings and identified factors that contributed to the incident. Additionally, it included 15 safety recommendations aimed at preventing similar incidents in the future. In this assignment, the SHELL Model, a conceptual framework proposed in International Civil Aviation Organization (ICAO) Circular 216-AN31 (Tzvetomir Blajev, 2009), will be utilized to analyze and explain the causes of the incident and demonstrate how these factors align with the SHELL Model.

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The error module was chosen because it was a conceptual framework proposed by ICAO in Circular 216-AN31 for use in the Human Factors subject (Tzvetomir Blajev, 2009). The incident involved a flight G-OBMM (B737-400) that was scheduled to fly from East Mid Land Airport to Lanzarote in the Canary Island. Prior to departure, during the normal pre-flight check, the flight crews discovered that the hydraulics power circuit breakers (CB’s) had been left open.

The flight officer approached the dispatching engineer on the apron to inquire about the status of the hydraulics power CB’s. The dispatching engineer examined the aircraft logbook to find any maintenance history associated with the open position of the CB’s. Upon inspection, the dispatching engineer discovered that a borescope inspection had been conducted on both engines the previous night. The in charged engineer had signed off on this task, confirming its successful completion.

In terms of the engineer’s concern, the bore scope inspection of the engines did not involve the hydraulic power system. Therefore, there was no reason to leave the hydraulic power circuit breakers (CB’s) open. The dispatching engineer allowed the flight’s first officer to close these CB’s. Flight G-OBMM took off from East Midland Airport on Runway 27 at 1157 hrs. As the flight was climbing to approximately flight level 140, the flight commander observed that both engine oil quantity gauges indicated approximately 15% oil remaining, and it was decreasing continuously.

Simultaneously, the oil pressure gauge is displaying a gradual decrease in oil pressure. At 1204 hrs, the flight commander communicates with London airspace control to request permission to return to East Midland Airport. The London airspace control grants clearance for the G-OBMM to return to East Midland Airport, at an altitude of flight level 180. At this point, both the oil pressure and quantity gauges indicate a reading of zero. Both the flight first officer and commander come to the realization that returning to East Midland Airport may not be possible and request permission from the London airspace controller to land at the nearest airport.

The flight officer declared an emergency condition on the aircraft by saying ‘Mayday’. The London Airspace Controller granted permission for flight G-OBMM to land at London Luton Airport at flight level 120. Flight G-OBMM safely landed at 1214 hrs on Runway 26 at a speed of 170 knots, with no injuries to any flight crew or passengers. Additionally, there was no damage to the aircraft, but both engines were removed for further investigation. Causes of the G-OBMM incident are being investigated.

The Air Accidents Investigation Branch, Department of Transport, provided the Aircraft Accident Report 3/96 which presents the incident involving Boeing 737-400, G-OBMM near Davontry on 23 February 1995. The report identifies several causal factors contributing to the incident including the absence of a refitted High Pressure (HP) rotor drive. Prior to the incident, the aircraft underwent a 750 bore scope inspection for both B737-400 engines at the line maintenance. This inspection task should have been performed by the line maintenance crew.

The HP rotor drive had not been refitted, but the aircraft technical log showed that the bore scope inspection was successfully carried out and signed off by an aircraft maintenance engineer. This led to the loss of oil in both turbine engines for several minutes after takeoff. Additionally, the investigation revealed that the engineer who performed the bore scope inspection did not follow the approved procedures as stated in the Aircraft Maintenance Manual (AMM) provided by Boeing.

The unapproved procedures for the HP rotor drive cover were not refitted and a ground idle engine run was not carried out after inspection. These unauthorized working procedures were unmonitored and uncontrolled by the Quality Assurance department. According to the investigation report, maintenance engineers in the company commonly used wrong and unauthorized procedures for bore scope inspections at turbine engines. The Quality Assurance department did not notice or identify these dangerous procedures, even though they had been used for a significant period of time. The review of operation procedures proposed by the company to obtain European Aviation Safety Agency (EASA) Part 145-Approved Aircraft Maintenance Organization showed limitations in the company’s Quality Assurance ability to demonstrate safety in operation management. However, despite these limitations being detected by the Civil Aviation Authority (CAA) United Kingdom, the CAA still granted authority under EASA Part 145-Approved Aircraft Maintenance Organization for the company to conduct aircraft maintenance activities. In order to analyze all the events that contributed to the incident, the use of the SHELL model is recommended.

According to the Aircraft Accident Report 3/96, multiple events led to the incidents. Some of these events are connected in a chain where each subsequent event occurred as a result of the previous one. The SHELL Model was used to analyze the contribution of these events to the incident. The report titled “Report on the Incident to Boeing 737-400, G-OBMM near Daventry on 23 February 1995,” provided by the Air Accidents Investigation Branch, Department of Transport, reveals this analysis. As a result, appropriate recommendations have been made by the competent authority to prevent similar incidents in the future.

The first event that contributed to these incidents was being unaware of the Quality Assurance department.

The incident investigation report states that one of the chain events leading to the B-OBMM incident was the widespread use of unapproved procedures for engine bore scope inspection by many company engineers. This behavior persisted for a significant amount of time, but the company’s QA department failed to notice or address it, neglecting to monitor their maintenance crew’s work procedures over an extended period.

According to the report, the CAA identified the QA department’s inability to demonstrate and control quality in the company. This was discovered during the review of the company’s proposal for EASA Part 145 approval. Despite this flaw, the CAA still approved the company for aircraft maintenance and repair. Referring to the SHELL Model, one of the elements that interfaces with the workers is the other people in the company. In this instance, the interface between the QA department and the floor workers was very poor.

The QA department is responsible for monitoring and controlling floor workers on a regular basis to ensure that any errors, whether intentional or unintentional, do not become part of the working culture among maintenance crews. These two events are crucial and serve as the catalyst for subsequent incidents. Granting approval from the CAA would have been contingent upon the company meeting all necessary requirements and the QA department effectively and properly carrying out their responsibilities. This would have prevented the chain of events that led to the incident of both engines losing oil during flight. Another factor contributing to the incident was a shortage of manpower and communication issues among workers. On the night before the incident, the line maintenance engineer, who was supervising the night shift, was extremely busy. This engineer was the only one with approval to conduct engine bore scope inspection that night. Despite five engineers being assigned to the line maintenance night shift, only four were present due to sickness and holiday leave.

Two days before the incident, the night shift supervisor noticed a shortage of man power at line maintenance. The previous night shift supervisor left a note for the day shift supervisor, requesting additional man power for the upcoming night shift. However, the day shift supervisor failed to see the note and did not take any action to address the shortage of man power. As a result of this lack of man power and increased workload at line maintenance, the base maintenance controller volunteered to perform the engine bore scope inspection.

The line maintenance engineer verbally transfers the task to the base maintenance controller without any written handover document. Two main events contribute to this incident: a shortage of manpower and inadequate communication between workers. Regarding the manpower shortage, the company’s Human Resource Management should keep track of staff leaving, available staff, and workload.

According to the SHELL Model, there is a lack of interface between two livewares in this case. Human Resource Management has not successfully managed the balancing of manpower and workload. Additionally, there is a need for proper communication medium among workers, such as handover documents, to ensure clear instructions and information are given. Specifically, the night shift supervisor should have used clear notices to inform the day shift supervisor about the manpower shortage for the upcoming night shift.

The handover of engine bore scope inspection from line maintenance engineer to base maintenance engineer should be done using the proper and approved handover document to state the current situation of the aircraft. This includes the identification of mismatched liveware to liveware interface in the SHELL Model. Additionally, the allocation of the engine bore scope inspection and the outdated knowledge and skill of the Base Maintenance Controller regarding engine bore scope inspection must be addressed. As stated in the Company Maintenance Organization Exposition (MOE), the 750 engine bore scope inspection should be conducted by line maintenance rather than base maintenance.

The task allocation considers that the inspection did not take a long time and the time interval to conduct this task (750 Hours inspection) is difficult to match with other schedule maintenance inspections (SMI) such as the 100 hours inspection. By assigning the engine bore scope inspection to line maintenance, it is entrusted to line maintenance engineers who are highly knowledgeable in this task. This policy of assigning the engine bore scope inspection solely to line maintenance, rather than both base and line maintenance, ensures that only line maintenance engineers possess the expertise required to carry out this task.

The night before the incident, due to a shortage of manpower and a heavy workload at line maintenance, the base maintenance controller volunteered to perform the 750 hours engine bore scope inspection on G-OBMM. The line maintenance supervisor, trusting in the base maintenance controller’s title, simply gave him a verbal handover of the job. While the base maintenance controller had performed many engine bore scope inspections in the past, they were several years ago and this particular inspection was their first for the year.

Furthermore, the Base Maintenance Controller did not understand the task card that explains how to carry out the task. This was because the task card’s layout was designed for engineers who are already familiar with the task. Due to the outdated knowledge, information, and skills, along with unclear instructions from the task card, the Base Maintenance Controller chose to use their old course notes to perform this critical task on both B-OMM engines instead of referring to the approved aircraft maintenance manual.

In this case, there were three chain events that led to the incident. These events included the company’s policy, the unapproved task handover procedures, and the unapproved procedures for carrying out the job. According to the SHELL Model, the software to liveware interface should match correctly to ensure accurate and clear task information. Despite MOE’s statement that line maintenance crews should conduct the 750 hours engine bore scope inspection, the company should have provided updated training to all maintenance crews.

The company must ensure that when publishing the task card, they use a universal and standard layout, language, and content to prevent any misinterpretations of the instructions. Additionally, the handover of tasks from the line maintenance engineer to the base maintenance Controller should be done using the appropriate medium, such as a handover document, to provide accurate and clear information and instructions. Furthermore, it is completely unacceptable for the base maintenance engineer to use their old course notes instead of the approved aircraft maintenance manual for conducting the bore scope inspection.

If you consider his job title, the Base Maintenance Controller, who holds the highest rank in base maintenance, should not have made that foolish decision. He was aware that the task was not approved procedures to be carried out. 4. 5) The engine bore scope inspection, which should have occurred in a suitable environment, was conducted at night by the Base Maintenance Controller according to the investigation report. According to the SHELL Model, the environment is another crucial factor that significantly impacts the worker’s performance in carrying out an outstanding job.

The good surrounding environments encompass various factors such as lighting, temperature, and weather. Therefore, it is crucial for the engine bore scope inspection to have optimal lighting and accessibility for the individual performing the task. This ensures that the person can fully concentrate, focus, and satisfactorily complete the inspection. However, due to limited lighting and potential sleepiness during nighttime inspections, the base Controller conducting the bore scope inspection is prone to making numerous mistakes, which can lead to incidents. In response, Aircraft Accident Report 3/96 by the Air Accidents Investigation Branch, Department of Transport, presented recommendations to prevent similar incidents in the future. One recommendation specifies the need for a correct and approved aircraft maintenance manual.

After conducting the investigation, the Civil Aviation Authority (CAA) emphasized the importance of engineers using and referring to the approved aircraft maintenance manual for all aircraft maintenance work. Any deviation from this manual indicates that engineers are illegally signing off the certificate of release to service (CRS) (Safety Recommendation 96-28). Additionally, the company needs an effective and reliable system to monitor and control the available manpower in order to handle the workload and eliminate any manpower shortages. This is necessary to prevent employees from working under pressure and stress (Safety Recommendation 96-29). CAA should review the requirements set by EASA Part 145 regarding manpower and qualified staff within the company to ensure that the staffing level matches the workload. CAA should also monitor the company’s ability to demonstrate safe and quality operations within a specific time period (Safety Recommendation 96-30). Finally, CAA should provide advice based on these recommendations.

CAA’s recommendation (Safety Recommendation 96-31) states that the company should be advised on the policy regarding maintenance and repair work on aircraft engines. This policy requires that any task or job that impacts the airworthiness of the engines should be carried out by different individuals at different times. Additionally, the company should adhere to the standard format for documenting and publishing information, such as task cards and maintenance instructions, to ensure clarity and prevent misinterpretation by base and/or line maintenance personnel (Safety Recommendation 96-32). The company should also have a clear understanding of its responsibilities in this regard.

The company should clearly and comprehensively provide a list of responsibilities for base and line maintenance to prevent individuals from performing maintenance jobs that are inconsistent with their roles and responsibilities (Safety Recommendation 96-34).

The company should review and redefine the roles and responsibilities of the Quality Assurance (QA) department to ensure that correct and approved maintenance practices are followed during both day and night shifts, as well as to monitor the company’s engineering products (Quality Recommendation 96-36).

CAA should ensure that all task cards produced by aircraft maintenance organizations or manufacturers contain a full description and instructions for the work and potential risks, rather than just referring to other documents (Safety Recommendation 96-1).

6)CONCLUSION. The G-OBMM aircraft incident mentioned in aircraft incident investigation report number 3/96 (EW/C95/2/3) resulted in no fatalities, injuries, or damage to the aircraft and its engines. It was determined that the incident was caused by human error. The incident was influenced by various potential events that were like ticking time bombs waiting to occur. According to the SHELL Model, predicting and managing the behavior and performance of individuals (liveware) is challenging due to their mood and external factors. To achieve optimal performance, all elements – software, hardware, liveware, and environment – must be effectively aligned. The recommendations provided by the Air Accident Investigation Branch (AAIB) at the end of their investigation report will only be effective if the involved parties take appropriate actions and precautions to prevent a similar incident from happening again in the future.

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Human Factor – Aircraft Incident Analysis. (2017, Feb 18). Retrieved from

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