Human Factors

Human Factors is a science in its own right. (In the modern age is usually also associated with Non Technical Skills – HF/NTS.)  It has evolved from the early Cockpit Resource Management (CRM) in the 1980s, to Crew Resource Management in the 1990s – also CRM but now with a recognition that is was more than just pilots involved with the safe operation of an aircraft – to today’s requirement to integrate HF/NTS into SMS.

Personnel in aviation are highly trained in Technical skills – flying or maintaining aircraft – but it is the cognitive ability of successful HF/NTS that allows them to be highly proficient in the employment of these skills.  HF/NTS includes understanding and awareness of;

  • CommunicationMechanics work over assembly of aviation engine
  • Leadership (and ‘followship’)
  • Teamwork
  • Stress
  • Fatigue
  • Alcohol and Other Drugs
  • Situational Awareness
  • Decision making
  • Airmanship
  • Threat and Error Management
  • …(others – this is not an exhaustive list)
Human Factors can be considered as ‘smudges’ on a virtual window that separate Reality from Perception as detected through cognitive and physical senses

Human Behaviour

Human Factors is about human performance, behaviour, error, and an understanding of how humans interact with procedures, technology, environment and other humans.

Ultimately it is unsafe behaviours and actions that lead to accidents.

Behaviours are influenced by multiple inputs, commencing in the formative years when our personalities are being formed.


The SHELL Model

The SHELL The model is used by ICAO to identify where HF breakdowns might occur.

Adaptation of the SHELL model
Adaptation of the SHELL model

The encompassing culture needs to be considered to expand the scope and the and understand the full spectrum of HF influence, both positive and negative.

Breakdowns between the human – the Liveware component in the middle – and any of the surrounding components, can lead to HF consideration and possible error.


Consider the table below;

SHELL table 1

The above Breakdowns / Failures are just examples and possible outcomes.


Consider the following table with the same Breakdowns and Failures but different outcomes;

SHELL table 2

Clearly the consequences, cultural influences, and potential causes can be greatly varied. They may be the result of active or latent failures which may only become evident depending on the investigation of the event.

As as exercise, discuss each example above  to make the listed consequences, cultural influences, potential causes and latent/active failures fit, or alternatively consider different scenarios resulting in other outcomes.

Other components of the SHELL model will also interact. For example; A weather incident (Environment) causes a ground equipment failure (Hardware) that has consequences for pilots (central Liveware). However it is clearly an E-H problem rather than an HF issue as it did not involveL-H or L-E application.

The Reason Model

The Reason model of accident causation describes how the breach of multiple system defences could result in an accident.   Professor James Reason also argued that single point failures in complex systems like aviation should not be consequential.  The defence failures (breaches) could be both active or latent failures.

.An Active failure could be described as something that a conscious decision (regardless of the motivation), resulted in a defence layer being breached.

Example; maintenance crews using work-a-rounds to achieve operational efficiency when they know a procedure might be contrary to SOPs.

A Latent failure is more insidious, it lies in wait  and  unknown until discovered.

Example; An organisational manual that details company procedure, that happens to be contradictory to OEM manual perhaps prohibiting such action.  The OEM manual was not thought to be checked on the assumption that the SOP writer had already done so.

Reason Model adaptation

The PEAR model – an HF model for Engineers

Although appropriate and applicable to maintenance, both the SHELL and Reason model were conceived and fitted for flying operations.  Use of both in the maintenance environment tended to be a ‘bolt on’.

The PEAR model was developed for HF specifically with maintenance in mind. It has been widely used in both EASA/FAR and FAA for HF training for maintenance personnel.  It has now been adopted by CASA for HF training for CASR Part 145 organisations.


The aim of the models – is to generate thoughts and discussions about the HF breakdowns involved in an incident.  It is not necessarily to neatly categorise where the breakdown belongs.  This will become apparent in Exercise 1


CASA Drama – Crossed Wires (From the CASA YouTube channel)

CASA Drama – The Right Connections (From the CASA YouTube channel)


What the Experts say: Human Factors within an Organisation (From the CASA YouTube channel)

CRM the old way!

A different perspective – the Elaine Bromiley case – UK 2005

Elaine Bromiley was a 37 year old UK mother of two who was admitted for routine elective sinus surgery on the recommendation of her GP. She did not survive.

At 0835 she was put under general anaesthetic and the anaesthetist set about trying to administer oxygen through a laryngeal mask. He was unable to fit the mask and tried several sizes. He proceeded to try and intubate but encountered obstruction from the soft pallet. Realising the seriousness of the situation he call another anaesthetist and an ENT surgeon. The three doctors worked feverishly to try and get oxygen to Elaine. At 0847 her heart rate and Sats were both down to 40 and deep cyanosis was setting in. The senior theatre nurse observed Elaine’s colour and retrieved a tracheoscopies kit and announce this to the doctors. The doctors looked at her but none spoke.  Another nurse noted the symptoms and immediately notified ICU for an inbound patient.  Again, the doctors ignored her and the room was cancelled. They continued a further 10 minutes before they managed to secure airway, over 20 minutes without oxygen for Elaine. She died 13 days later when her husband made the decision to turn off her life support, having been in a coma since the operation.

It is a tragic outcome for a team of very experienced, well-meaning professionals doing their level best dealing with an unanticipated complication.  The doctors when interviewed were collectively flabbergasted when it was pointed out that 22 minutes had elapsed without Elaine having oxygen – they simply couldn’t believe it. This is the human element acting under stress; time compression and a loss of our situational awareness. The doctors simply lost track of time.

The other crucial aspect is that others in that team had the critical insight that could have averted tragedy. But for whatever reason was unable to bring that information to the key decision makers in a way that would change the trajectory of this occurrence.  Elaine Bromiley’s husband Martin was an airline pilot, who happened to specialise also in HF.  In the sad aftermath of having been told with his two young children, ‘Martin, we are terribly sorry…there were complications…it is simply one of those things…’ He naturally asked when the investigation would be so that the lessons learnt could be used and other families would not have to suffer as they had. He was told they was never an investigation unless there is litigation involved.

As a group – let’s discuss the HF failures of this case study

Human Factors – Antarctica

Casey to Wilkins




Track maintenance using GPS
Track maintenance using Radar