Fatigue is a Safety Threat

Two aviation occurrences in 1999, one of them a fatal mustering incident and the other a wheels-up landing, highlight some of the potential hazards of fatigue on flying performance.

fatigue is a safety threat

Mustering accident

A newly-licensed private pilot was fatally injured at Mindaroo Station in Western Australia when mustering sheep with a Cessna 172. The accident happened late in the afternoon at the end of more than eight hours of low-level flying following nine days of intense flying activity.

During the nine days, the pilot had flown 68 (tachometer) hours. The flying was both mentally and physically demanding, involving sheep spotting and low-level mustering.

The pilot, who had no formal low-level or mustering training, had to manoeuvre the aircraft in conditions that were sometimes turbulent, and was operating under constant aircraft noise and vibration. On the day of the incident, he had taken no more than a short break, which included refuelling after about four hours of flying.

It is quite possible that he was unaware that fatigue had affected his flying performance.

The pilot had exceeded the flight duty times normally permitted for a commercial operation (dealt with in Section 48 of the Civil Aviation Orders). Although these requirements are not mandatory for private operations such as this one, they are a guide to flying limits.

In the absence of any formal duty time requirement, the pilot was responsible for determining his own daily flying limitations. This was done in conjunction with the property owners, property manager and the mustering party. A typical day started at 0700 local time and the pilot worked through the day until just before last light.

Wheels-up landing incident

In this incident, the pilot of a Cessna 210 had forgotten to re-engage the landing gear circuit breaker, which had popped during the flight.

On the morning of the incident, the pilot woke at 0530 local time and started his tour of duty at 0630. The pilot had flown an Instrument Flight Rules (IFR) check flight for 2.3 hours in the morning and his performance was considered to be above average.

The pilot departed on a Visual Flight Rules (VFR) charter towards the end of the tour of duty. The pilot had pulled the circuit breaker after it popped to prevent damage to the electric motor that had continued to run. This procedure was in accordance with the Cessna 210 Operating Handbook recommendation.

On final approach, the pilot selected the landing gear down but forgot to re-engage the landing gear circuit breaker and the landing gear did not deploy. The investigation revealed that the pilot did not recall hearing the landing gear warning horn nor did the pilot notice the status of the landing gear indicator lights.

The investigation concluded that the pilot was probably suffering from a transient fatigue-related memory lapse and, unlike the incident at Mindaroo Station, was not suffering severely from accumulated fatigue. "The pilot reported that he was very tired on the day of the occurrence and he had been for some time leading up to the incident," the ATSB report said.

During the investigation, the pilot's work and rest history for the 14 weeks before the incident was examined using a computerised fatigue algorithm developed by the Centre for Sleep Research.

The results demonstrated that the pilot probably wasn't suffering severely from cumulative fatigue. Of more significance was that the pilot had been on duty for more than 12 hours and had been awake for almost 14 hours.

Effects of fatigue

Research has shown that the effects of fatigue are similar to moderate alcohol consumption. On-the-job performance loss for every hour of wakefulness between 10 and 26 hours is equivalent to a .004 per cent rise in blood alcohol concentration. Eighteen hours of wakefulness is usually considered to be equivalent to a blood alcohol concentration of .05. A person who has been awake for this length of time will act and perform as if they have consumed .05 of alcohol.

The result is significantly delayed response and reaction times, impaired reasoning, reduced vigilance and impaired hand-eye coordination.

The article 'Pilot Fatigue and the Limits of Endurance', Flight Safety Australia (April 1999), reported that fatigue makes a pilot less vigilant and more willing to accept below par performance, and a pilot begins to show signs of poor judgement. It reported that expert research into fatigue had established that it degrades a pilot's:

  • Muscular strength and coordination
  • Vision and perception
  • Memory
  • Performance monitoring
  • Error management
  • Decision making
  • Motivation and attitudes
  • Communication
  • Ability to cooperate.

But the greatest single threat is being unaware that it is happening.

Before the mustering incident at Mindaroo Station, the pilot had been talking to the ground mustering party by radio as well as flying the aircraft (possibly below 500 ft). The ATSB investigation found that he had worked very long hours in a highly demanding job in which he was inexperienced.

He had received minimal training that would help him to understand the visual illusions associated with low-level flight. The investigators considered that in the absence of specific training for low level flying operations, he was probably unaware of the appropriate techniques to safely manoeuvre an aircraft at low level.

According to the ATSB Occurrence Brief (number 199903464) a human factors report noted that the pilot had worked long hours in a job in which he was inexperienced and that he probably found this type of flying both physically and mentally demanding. The report concluded that at the time of the incident the pilot was suffering from the effects of fatigue, possibly impairing his ability to safely operate the aircraft.

According to the Centre for Sleep Research's 1999 report to the Neville Committee Fatigue and Transportation it has been difficult for researchers to determine all the factors that cause and contribute to fatigue; and "determining the relative importance of these factors under different conditions has also been problematic".

However, research had concluded that when a person works long hours, for more than say 50 hours a week, there is increasing competition between restorative sleep and the other activities of daily living.

Non-work factors contribute to overall fatigue by a reduction in the opportunity for sleep and recovery. These include social factors and domestic arrangements (for example working away from home) sleep disorders and shift work.

"For example, the same roster could have quite different effects according to social circumstances," the report stated. "A 12-hour night shift might have very different consequences for an 18-year old single male living on his own compared to a 35-year old single mother of two toddlers without access to 24-hour childcare facilities.

"Taken together, both employees and employers have clear responsibilities with respect to managing fatigue. The basic responsibilities of both parties relate to ensuring that adequate sleep can be obtained between shifts so that fatigue does not reach dangerous levels during shifts. Thus, lack of sleep causes fatigue and sleep allows recovery from fatigue.

"Employers have a duty of care to provide safe work schedules that permit adequate time for an employee to sleep, rest and recover as well as fulfil their social and domestic responsibilities.

"Employees also have a duty of care to use their time away from work in a safe and responsible mannerto ensure that they obtain sufficient sleep and recovery in order to complete their work duties in a safe and responsible manner."

How safe are you?

There are many flying organisations operating with exemptions from the requirements of CAO 48 issued by the Civil Aviation Safety Authority (CASA).

Whether you are working to the flight and duty time guidelines under CAO 48, or under an exemption, how safe are you? Are there other factors in your life that may make you more tired than usual?

Remember, the onset of fatigue is insidious.

Type: Educational Fact Sheet
Author(s): Sarah-Jane Crosby
Publication date: 10 April 2000
Related: Fatigue
Last update 07 April 2014