Human Factors In Aviation Case Study

6.4 - Case Analysis: Aviation Human Factors American Airlines Flight 965 Introduction On the unfaithful evening of December 20, 1995 American Airlines flight 965, a Boeing 757, crashed into a high terrain in clear whether during approach to Alfonso Bonilla Aragon international airport in Cali, Columbia fatally wounding 163 people and sparing only 4 people. The airplane was equipped with glass cockpit technology that displays flight information, navigation information and engine information. It was also equipped with a Flight management system(FMS) and a functioning Ground Proximity Warning System (GPWS) Analysis The Aircraft experienced two delays. The first one was when take-off was delayed for 30 minute to allow connecting passengers to board the aircraft. Then after the aircraft began its taxi to the runway, the tower informed them that there was going to be another delay due to heavy seasonal traffic, which caused an additional 1.5 hours delay. The departure was uneventful and the aircraft proceeded on course towards Cali ( Alfonso Bonilla Aragón International Airpor t) from its original destination, Miami international airport about 2 hours behind schedule. During the flight, Captain Nicholas Tafuri voiced his concerns to his co-pilot, First Officer Donald Williams about the delay which meant they might not reach Cali on time. Furthermore, they have a flight the following morning, and they

Investigating incidents by considering the “human factor” is not a new idea. The aviation board has used it for years – and perhaps this is part of the reason why the aviation industry has a high safety record. Work Safe BC investigates accidents initially considered caused by “human error” using the principals of human factors. Download:

The three basic principals of human factor are:
1) People intend a safe outcome.
2) People believe they are safe.
3) The accident investigator considers what the victim was experiencing prior to the accident.

There are two questions to be asked in every accident or when reviewing a near miss:
1) Why did the worker take the action or make the decision?
2) Why did that action or decision make sense at the time?

Jenny Coleman of Work Safe BC explains it this way, “When you hear the term “human error” in an incident investigation, that should be your starting point. Ask, what was the context?”

Coleman spent a couple hours on the phone and via email reviewing some tree service accidents from around the world for the Ontario Arborist applying the principle of human factor. She highlights issues not investigated in the initial reviews or even the inquests. 

Unfortunately, in most cases, we can’t answer the human factor questions because the worker is deceased. The questions are raised in this article to encourage workers to ask the same questions when they face the same or similar circumstances in order to prevent another similar accident. 

Please remember that many tree accidents are similar. Identifiers in these accidents were removed or changed to protect the families and secondary victims. These were all experienced workers, with good equipment, in good order. They were all trained with good safety records until their last day at work. A few did survive to explain the why and how. 

Wrong Technique
A worker was cutting wind throw trees all day. Safety investigators reviewed the scene. All cuts were done in perfect order until the last cut. When he made the last cut, he altered his technique. 

The obvious response here is that Olympic athletes don’t perform optimally for eight hours. Neither do workers. What was this worker focused on at the time of the accident? Were the break periods and water available adequate for the worker’s condition on that specific day? 

There is no evidence to support the idea the worker was fatigued. The more likely horror is this worker died to protect his chainsaw blade. It appears that he altered his standard cut angle to prevent getting his blade from getting dirty. 

Human factor investigators ask: Why did he make this decision? How far did he have to go to get his blade sharpened? Would his day have ended if the blade were dull? Was a back-up chainsaw available? Who owned the chainsaw? What was the consequence of a dull blade? 

Other questions to be asked for those who want to learn from this event are: How can you reduce the cost of a dull chainsaw blade? Can you have a spare chainsaw available? Can you carry a sharpener? How can workers avoid being forced to choose between protecting their equipment and making a less then perfect cut?

Secondary Skills Lacking
The worker lacked the English writing skills necessary to complete the required paperwork for a permit to use a bucket truck in a municipality. While the bucket truck sat unused, the worker climbed a dangerous tree. This worker made a choice that in hindsight we all say we would never do, but human factor principals say we must ask why he made this choice. 

For those reading this article, it may be challenging to recognize that 4 out of 10 Canadians struggle with literacy. Because you are reading this article, you are likely in the 60% category, but there are 9 million adults, right here in Canada, who will turn the page after reviewing the photos. Statistically, on a crew of five, two will have reading challenges. They can’t comfortably read pesticide labels. They can’t follow written safety instructions and it is statistically proven that they won’t acknowledge this concern or ask for help. We also know this: if they passed the certified arborist exam without solid reading skills, they are highly intelligent with a strong memory. 

Those with literacy challenges have options depending on whether the person is the “boss” or the “worker.” First, make sure someone on the crew reads pesticide labels and safety instructions out loud. Have a secretary or number 2 who will complete forms and paperwork. Never assume your partner can read. 

Seeing Grey
On bright sunny day, a climber trimming a tree cut his own line. 

Coleman stated the obvious – and something arborists should never forget – tree work above ground level is a highly complex task. It involves monitoring weather and wind, knowing chainsaw position, how the tree is shifting, where the branches are falling, and so much more. A novice will go through each step. An expert will focus on specific features. Extensive experience allows the expert to focus on the task and specific details will jump out at him. This type of accident usually happens to the experienced knowledgeable professional, not the novice.

What was the worker’s context in this case? Was the line sufficiently different then the surroundings? Should workers wear two lines in these circumstances? How can the line be positioned so it will not be accidentally cut? How can safety lines be made more secure? 

For those who want to understand how it is possible for a careful worker to cut his own line and how our attention resources work, there is a great YouTube video Work Safe BC uses:

There is another key factor to consider here. The eye can take from several seconds to several minutes to adjust to a change in lighting conditions. Think of the experience of walking in to a building on a bright sunny day. The timing of this change alters with age, Vitamin A levels and eye health.  

During the delay as your eyes adjust, retinal (an aldehyde) and opsin (a protein) are recombining into rhodopsin, a pigment in the retina of the eye. During the period when a climber leaves bright sunlight and enters the shade of a canopy, the eyes cannot differentiate between the colour of a flame orange climbing line and a brown branch. 

Equipment Failure
A worker was cutting a tree that was leaning in the “wrong” direction. The sledgehammer broke. He left the tree and continued working down the row. The first tree fell. 

This is yet another case where a worker died because his equipment failed. Why did the worker believe the first tree was not a hazard? Was the worker aware of the wind speed and weather conditions? How far did he have to go to get his sledgehammer repaired or replaced? What was the pressure to get the job done? 

In this specific situation, there was an eyewitness. There was evidence the witness tried to warn the worker but as the worker was correctly wearing ear protection and using a chainsaw, he did not hear the warning. Arborists must wear hearing protection. It is the law. Realizing that sound is a critical early warning sign in accidents, all must be reminded of this additional challenge to safety. Visit:

There are many examples of arborists continuing to work following some type of equipment failure. Looking at human factor, we again take the position that the worker believed the job could be completed safely. This is the cost of experience and confidence. Likely the worker had taken chances before and the outcome was positive. 

Another example is a worker who experienced chainsaw failure. He chose to dig around the tree and attempted to push it over with a backhoe. He lived, but he lost his business and reputation. 

Climbing “Danger” Trees 
In forestry, it is sometimes said before an accident, “If you don’t hear my chainsaw….” The statement implies that although something went wrong, the worker is safe. There is no cry for help. There is no emergency just a bland statement that implies: If you notice I have stopped working, come over and see if I need help to perhaps carry the equipment back to the truck. Urban tree workers have similar beliefs. Coleman says, “Workers believe they have the skills and equipment to overcome challenges.”

One worker died because he didn’t know the exact location of the tree rot. The tree broke below where he was tied. Did he have the equipment to accurately assess the severity and location of the tree rot? How was the tree assessed? Did more then one person assess it? 

After a fatal accident, identifying the human factors that caused or contributed to it is a combination of playing detective and guesswork. Identifying the human factors that could contribute to an accident saves lives. 

Site Survey
One of the best examples of a site survey going wrong occurred when a worker cut a tree in the approved manner and all was going well. He took a step back. A mother bear with cub appeared from a den behind where the worker was standing. She struck him behind the leg and he fell forward in to the path of the falling tree. (His injuries were minor.)

Why didn’t this worker do better site survey? Because he believed he had done a good assessment of the area. Depending on the specific site, a complete survey can be next to impossible. Is there a red ant’s nest? I know of workers who were “attacked” by hummingbirds defending a nest. Another worker was swarmed by Asian ladybeetles and too late he discovered he had an allergy to their bites. Site surveys are complex! 

For interest sake, here is a list of British Columbia bear incidents: 

  • Worker witnessed a bear attack colleague. 
  • Worker was startled by a bear and twisted ankle. 
  • Worker shot bear with pepper spray. Mist went into worker’s eyes and he fell off the truck. 
  • Worker strained knee turning to get away from a bear. 
  • Worker tripped against a log and was injured when chased by a bear.
  • Worker was chased by a bear and jumped off a 10-foot cliff. 
  • Worker tripped and fell backing away from a bear. 
  • Worker fell running from a bear. 
  • Worker was startled by a bear, lost his footing and fell. 
  • Worker was chased by a bear, fell into ditch and landed on a large boulder.

Natural Threat Responses
Bears and other animals are not always the culprits. Another reported case occurred when a worker climbed a 5-foot folding ladder to do some minor pruning. He climbed just two steps when his head struck a wasp nest. He came down the ladder in a panic and was injured due to a trip and fall. He had no allergy to the stinging insects and no reaction to the two stings he endured. He was injured due to his natural response to a threat. 

When the body perceives a threat or severe stress, many changes happen automatically. A threat can be anything from a branch cracking to a swarm of hornets. Mike Dennis is a retired OPP officer who in two separate occasions faced severe life threatening events and lived to teach others how to survive. These principles are also fundamental in flight training and road safety. 

Dennis describes the process as “tachy psyche.” Under severe stress you will have auditory exclusion or you will not hear sounds except those directly related to the threat. (In the case of hornets you will hear increased buzz but you will not hear a co-worker calling with help.) Your thought process will appear to slow down. Your outer extremities will go numb. Your heart rate will increase. You body will prepare to try to run away. 

This response is the body’s way of preparing you to survive. Training before the emergency starts is survival. Control the response and you increase your potential to win. 

Dennis said visualization each and every time is key. Before you turn on the chipper or pull the cord on the chainsaw, take a moment to role-play in your mind how you will respond if an accident occurs. Professional downhill skiers shut their eyes at the gate. They imagine each twist and turn as they descend the hill. Then they start. If a branch cracks, if your glove catches, if a swarm of hornets appears, how will you react? “Always formulate in your mind a winning and reasonable response. If you haven’t prepared, you will likely freeze under severe threat.”

Under threat, “you will focus on the danger.” Before you start to work, “practice moving your eyes scanning for options,” said Dennis. If a threatening situation happens you must be prepared to shift your eyes from the danger and look for options and other potential problems. 

More information on tachy psyche syndrome is at Especially take note of the sections on pre-visualization and things to remember. 

Here are some forestry hazard alerts to review:

The following rules for creating a safety conscious workplace were adapted from an article by Robert Baron in the AeroSafety World Magazine. 

  • Arrive at work prepared to work.
  • Respect your peers.
  • Be part of the team to make safety number one.
  • Be assertive when necessary for safety.
  • Draw a line between right and wrong.
  • Do not follow unsafe practices.
  • Even if it is legal and technically “safe,” is it morally wrong?
  • Don’t compromise safety for speed.

As Transport Canada says to pilots: “Learn from the mistakes of others because you will not live long enough to make them all yourself.”  


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