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from communicating with each other, whether directly or by means of an intercommunication system
(“intercom”), and can also interfere with voice communication between ground and aircraft. When sound
pressure levels within cockpits and communication systems rise, the voice must be raised in order to
communicate against the noisy background, and if the interference becomes excessive, speech
intelligibility becomes adversely affected or lost altogether. This is auditory masking or “drowning out”
by noise; it lasts only whilst the noise is present. It represents the inability of the auditory system to
separate the different tonal components, and tends to be worse when the conflicting frequencies are
similar.
Apart from controlling noise sources, efforts must also be made to limit the entry of noise into the
communication system. The position can be further improved by selecting the best possible characteristics
for a communication system and by the use of special vocabularies (as standard ICAO phraseology for
Aeronautical Telecommunications, described in detail in Annex 10, Volume 2, Chapter 5). Apart from
engine and aerodynamic sources, noise can be generated by the cabin air conditioning system, by
electronic equipment within the cockpit, by certain types of oxygen regulators, and by the individual’s
breathing into a “live” microphone. The degree of interference will depend upon the relative frequencies
and strengths of the voice or tone signal and the ambient noise level.
6 IMC: Instrument Meteorological Conditions, i.e. weather with reduced visibility where only flying in accordance
with the Instrument Flight Rules (IFR) is allowed.
7 +Gx: Acceleration (G) is a change in velocity either in direction or in magnitude. It is described in three axes in
relation to the human body, x, y and z. Each axis is described as positive (+) or negative (–). +Gx is a forward
acceleration with a transverse anterior-posterior (chest to back) resultant force.
ICAO Preliminary Unedited Version — October 2008 II-1-18
To guide the medical examiner in the proper assessment of applicants for medical certification,
speech tests in neutral noise as well as aviation noise have been described elsewhere in this manual (see
Part III, Chapter 11).
FLIGHT CREW WORKLOAD AND
ITS EFFECTS ON PERFORMANCE
Fatigue
Many working and environmental conditions lead to fatigue, affecting people in a multiplicity of ways.
Individual responses to fatigue are significantly different.
Fatigue may be transient and/or cumulative. Transient fatigue is normally experienced by a healthy
individual following a period of work, exertion or excitement, and it is normally alleviated by a single
period of sleep. Cumulative fatigue may occur after delayed or incomplete recovery or as the after-effect
of more than normal amounts of work, exertion or excitement without sufficient recuperation.
Workload fatigue, as it affects flight crews, may have a significant effect in reducing performance.
Some of the causes contributing to workload fatigue are the cockpit layout, the hours of work and other
specific factors as follows: beginning and end of last flight, duration of rest time between present and last
flight, duration of sleep during this rest period, the time of commencement of pre-flight briefing,
problems arising during briefing, delays preceding departure, timing of flights, meteorological conditions,
quality and quantity of radio communication, visibility during descent, glare and protection from sun,
turbulence, and technical and personal problems. One Contracting State found that what flight crew
described as “hassle”, meaning anything that caused a non-routine situation, was fatiguing.
Continuous technological developments are being pursued; seating, instrumentation, lighting, cockpit
design, climatic conditions in the cabin and radio communications equipment are being further improved.
An important contributing factor to fatigue in aviation operations is the disruption of circadian
rhythms. Time zone displacements without sufficient adjustment time might seriously impair the
performance of personnel engaged in aviation duties. Many organic functions are periodic - their rhythm
determined by both internal and external phenomena - for instance sleep-wake cycles, respiration, body
temperature, endocrine functions and physical and psychological performance. All these functions show a
24-hour cyclic pattern. Transmeridian flights crossing time zones affect the specific patterns and
periodicity for travellers.
One of the most common causes of fatigue in aviation has to do with the scheduling of flight crews.
Mental and physical conditions might influence the appearance and severity of fatigue, the end result
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Manual of Civil Aviation Medicine 1(66)
