S. Stewart, F. Koornneef, R. Akselsson, and C. Turner  - HILAS 2009
I’m sure you just penned the question quickly and didn’t give it much thought. Had you given it more thought you have realised that “falling asleep” is a symptom of chronic fatigue. We are just talking about plain old moderate fatigue. The kind which causes you to make “errors” as opposed to “falling asleep”. The kind of errors that make you miss-set altimeters and fly into the ground a few miles before the runway” (www.pprune.org/rumours-news/196539-aircrew-fatigue-2.html, 18 December 2005).

Aviation Authority, is to provide a fatigue-alleviating framework (Flight Time Limitations, FTL) for airlines. These FTL guidelines (Civil Aeronautical Publication, CAP 371, 2004) allow an airline rostering department to conduct rostering practices that minimise flight-crew operational fatigue. Current regulations have been principally designed around long-haul commercial flight operations and do not reflect the high aircraft and crew utilisation practices that characterise LCC operations (BALPA log, 2004; IATA Research Study, 2001). 
S. Stewart, F. Koornneef, R. Akselsson, and C. Turner  - HILAS 2009

An IATA Research Study (IATA, 2001) found that crew fatigue may be affected by the following contributing factors:
. Increased flying hours;
. Unsympathetic rostering practices; and
. Absence of adequate JAA/EU rules on FTL.

The above factors in turn may be influenced by:
Caldwell (2003) defined fatigue as a state of tiredness that is associated with long hours of work, prolonged periods without sleep, or the requirement to work at times that are out of synch with the body’s biological or circadian rhythms. This definition states the link between fatigue risk precursors, circadian rhythm and performance but falls short of associating fatigue performance to an acceptable level of safety.
Fatigue has also been defined as a “change in body physiology, associated with continuous activity, causing a decrease in work performance and characteristic subjective
S. Stewart, F. Koornneef, R. Akselsson, and C. Turner  - HILAS 2009
feelings of tiredness” (Perry, 2000) or “those changes that affect an individual maintaining continued activity” (Jensen, 1995). It is considered to have the status of a “hypothetical construct, an entity whose existence and dimensions are inferred from antecedent and consequent events or variables” (Maher and McPhee, 1994 as cited in the Batelle Memorial Report, 1998). 
The ICAO Fatigue Risk Management subgroup comprising leading fatigue research academics from across the industry cite a definition of fatigue as:
factors of fatigue-related risk include individual and cultural differences in a flight-crew, operational experience levels, route operational hassle factors, and operating within a schedule design that maximizes crew block hour utilization resulting in physical, cognitive, and behavioural manifestations of fatigue, incident occurrence, and degraded threat- and error-management (Caldwell, 1997; Thomas et al, 2006). A study conducted at easyJet (Stewart & Abboud, 2005) has documented flight-crew specific decrements in performance associated with fatigue. easyJet has recognized that continued success, particularly as the company expands to new market areas, demands that managers have
　
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