337 Yeti Airlines flight 691 crash – Human Factors for General Aviation Pilots

Max discusses the critical role of human factors in aviation safety, prompted by the tragic crash of Yeti Airlines flight 691 in Nepal in 2023. He emphasizes that pilot error is a significant contributor to aviation accidents, accounting for about 80% of them, underscoring the importance of understanding and mitigating human factors to enhance flight safety. He highlights the significance of human factors, which can affect any general aviation aircraft.

The episode centers on the crash of Yeti Airlines flight 691, an ATR 72, which crashed during a visual approach to Runway 12 at Pokhara International Airport. The flight crew had already completed two legs between Kathmandu and Pokhara earlier in the day. The accident occurred in VMC, with the flight crew initially cleared to land on Runway 30, but later requesting a change to Runway 12 without providing a reason.

During the approach, the Pilot Flying (PF) disengaged the autopilot and called for flaps to be set to 30 degrees. However, the Flight Data Recorder (FDR) did not record any flap movement. Instead, both propellers entered a feathered condition, resulting in a loss of thrust. The cockpit voice recorder (CVR) captured the crew performing the before-landing checklist without noticing the incorrect flap position. Despite increasing the power lever angle, both engines remained at flight idle, contributing to the loss of thrust.

As the aircraft descended, the crew struggled to manage the situation. The PF handed control to the Pilot Monitoring (PM), who also noticed the lack of engine power. The aircraft entered a left bank, and the stick shaker activated, warning of an impending stall. Despite attempts to regain control, the aircraft crashed, killing all 68 passengers and four crew members.

The investigation into the crash revealed several human factors contributing to the accident. The use of noise-canceling headsets by the PF may have reduced his ability to detect auditory cues indicating engine issues. Additionally, the high workload and distractions from operating into a new airport contributed to ineffective Crew Resource Management (CRM) and checklist adherence. The new Pokhara airport’s challenging approach, with a tight circuit and no published instrument procedures, further increased the crew’s workload.

Trescott delves into the SHELL model used to analyze human factors in the accident. The SHELL model examines the interaction between Software, Hardware, Environment, and Liveware (people). The analysis highlighted that the crew’s high workload and distractions, coupled with the challenging visual approach, led to critical errors. The proximity of the propeller condition levers and flap handle on the ATR 72’s center pedestal also contributed to the inadvertent feathering of both propellers.

In discussing workload, Max emphasizes its impact on pilot performance. High workload can lead to increased errors, task degradation, and poor performance. He explains that workload is influenced by task difficulty, the number of tasks in parallel or series, and the time available to complete tasks. Pilots can mitigate high workload by starting descent and approach planning earlier, slowing down the aircraft, and increasing flight path length. These strategies provide more time to complete tasks methodically, reducing the risk of errors.

Trescott also highlights the effects of high workload, such as attentional narrowing and task shedding. Under high workload, pilots may focus excessively on one task, neglecting others, leading to errors and decreased situational awareness. He shares an anecdote about a glider pilot who, under high workload and anxiety, fixated on the landing area and neglected airspeed monitoring, resulting in a stall and crash. This story parallels the Yeti Airlines crash, where high workload and distractions led to the crew’s inability to manage the aircraft properly.

The episode continues by discussing how high workloads can affect decision-making processes. High workload can lead to rapid decisions made without considering all factors, options, or complexities. When pilots are under high workload, they might simplify decision criteria to reduce their workload quickly. This can increase the chance of errors as tasks might be performed hastily and without thorough consideration. High workloads can also lead to autonomous routines or checks being underprioritized, which may cause critical steps to be overlooked.

Task engagement and concentration on a single task are benefits of high workload, but they also pose risks. The captain on the Yeti flight, despite his experience, may have allowed some tasks to become too routine, not giving them the attention they required. Processes that require executive control—such as concentrating, calculating, or performing unfamiliar tasks—can add significantly to workload. When these tasks are combined with existing ones, especially under time constraints, the risk of excessive workload increases.

Trescott explains that task difficulty is often the most challenging workload driver to reduce. However, pilots can manage time limitations by allowing more time for tasks, which helps to avoid workload buildup. Effective CRM practices, such as delegating tasks to colleagues, can also help manage workload. Emergency situations are particularly challenging, but creating time and managing tasks between crew members can help.

Recognizing high workload is critical, but individuals might not always realize it in the moment. Changes in time perception and task absorption can make high workloads less obvious. Therefore, relying on individuals to recognize and manage their workload is unreliable unless they have specific training. Recognizing changes in situations before workload increases and noticing high workload in others are more effective triggers for managing workload.

High workloads can lead to decreased situational awareness, reduced attention span, slower reaction times, increased errors, auditory exclusion, tunnel vision, physical stress responses, impaired decision-making, communication breakdown, and task shedding. Understanding these responses is crucial for developing strategies to mitigate risks associated with high workload.

The episode concludes with a reflection on the experience of the pilots on Yeti Airlines flight 691. The crew was experienced, but the new airport’s challenges and distractions contributed to errors. The check captain’s familiarity with the controls and potential overconfidence might have led to complacency. Trescott warns that overconfidence and complacency are key factors that can erode safety benefits from experience.

Trescott emphasizes the importance of strategies to catch and correct mistakes, highlighting that all pilots are human and fallible, and proactive measures are essential for safety.

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Mentioned on the Show
Yeti Airlines flight 691 Final Report
Flight-crew human factors handbook
Episode #283 Wings to Rotors: How to add a helicopter rating

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