## 1 Introduction
### Human Factor in Aircraft Maintenance  
Aircraft maintenance is a complex process, and errors can cause flight delays, cancellations, or, in some cases, incidents and accidents. Over the years, airlines have introduced new technologies to improve maintenance; however, the human factor still plays a crucial role in overall maintenance performance. Some examples of accidents due to human factors are presented below:  
* In June 1990, British Airways Flight 5390 had an accident. The left cockpit windscreen was blown out, and the pilot was partially ejected through the open window. After investigation, they found that 84 out of the 90 bolts that fixed the windscreen were smaller in diameter. Due to a staff shortage during maintenance, the shift maintenance manager tried to help by fixing them himself. However, he never checked the maintenance manual, resulting in the incorrect bolt type being used to fix the windscreen [5].
* In 2003, a Raytheon (Beechcraft) 1900D aircraft lost control shortly after takeoff, leading to a crash caused by improper rigging of the elevator control system [6].
* In 2018, Southwest Airlines Flight 1380's engine failed due to a fractured fan blade. Metal fatigue was the leading cause, and debris was inserted into the engine, resulting in a loss of power. The pilots tried an emergency landing; however, one passenger died [7].
* On 15 January 2023, Yeti Airlines Flight 691, was conducting a scheduled domestic flight from Kathmandu to Pokhara International Airport with 68 passengers and 4 crew members. During the final approach to Runway, the aircraft experienced a dual propeller feathering event, leading to a loss of thrust and subsequent aerodynamic stall. The flight crew inadvertently moved the condition levers to the feathered position, preventing thrust recovery. The aircraft crashed near the Seti River gorge, resulting in the fatalities of all 72 individuals on board. The Aircraft Accident Investigation Commission (AAIC) of Nepal identified high workload, human error, inadequate technical training, and non-compliance with SOPs as contributing factors. Nine safety recommendations were issued to improve crew training, approach procedures, and regulatory oversight to prevent similar incidents in the future [8].
* On 6 December 2023, a Qantas Airbus A380 underwent a scheduled three-day maintenance check at Los Angeles International Airport. During a borescope inspection of the left outboard engine, a 1.25 m nylon turning tool was inadvertently left inside the engine. Despite multiple post-maintenance inspections, the tool remained undetected, and the aircraft was released to service on 8 December 2023. Over the next month, the aircraft completed 34 flights totalling 293.74 h before the tool was discovered during a subsequent maintenance check on 1 January 2024. The Australian Transport Safety Bureau (ATSB) investigation revealed failures in tool accountability procedures, including the lack of a formal lost tool search before releasing the aircraft to service. In response, Qantas Engineering revised its tool control policies, emphasising stricter compliance to prevent similar occurrences in the future [9].  
The Corporate Aircraft Association (CCA) published a paper [10] in 2016 on Aircraft Maintenance Incident Analysis. The paper highlighted various error types in large aircraft maintenance. Approximately 72% of the reported events involved installation errors and failure to follow specific instructions for task completion. A broader analysis of the associated factors causing installation errors (834) showed that poor inspection was the main factor in many cases, including 223 events in total (please refer to Figure 2).