Aircraft Maintenance: Human Performance Limitations

• Humans are capable but have limitations, such as performing poorly at night or in the cold
• The intention of this chapter is to overview physical and mental human performance characteristics affecting aircraft maintenance engineers in their working environment
• These include vision, hearing, information processing, attention, perception, memory, judgement, and decision-making

Human Performance Limitations

• Human performance limitations must be considered for aircraft maintenance and workplace safety
• Mechanical components have limitations, and engineers also have capabilities and limitations
• Human performance can degrade under conditions like stress, similar to how rivets fail under force
• Humans require tools, vehicles, clothes, air conditioning, computers, and PPE because of their physiological and psychological limitations
• People can fail to function properly due to fatigue, cold, workplace accidents, and other physical ailments, as well as mental errors, limited perception, and poor judgement
• Human performance is affected by social and emotional factors, which can be detrimental to aircraft safety
• An AME is central to aircraft maintenance, so understanding body and mental processes and their influence on work effectiveness is important

Vision: Factors Affecting Clarity of Sight

• Eyes are sensitive in the correct conditions like clear air and good light
• The eye has 1.2 million nerve cells, making it 24 times more sensitive than the ear
• Visual acuity is the ability to discriminate sharp detail at varying distances
• Factors limiting visual acuity include physical factors, environmental factors, the nature of the object being viewed, and the ingestion of foreign substances

Physical Factors (Lens Imperfections)

• Long sightedness is caused by a shorter than normal eyeball, causing blurred vision at close objects
• Convex lenses can overcome long sightedness by bending light inwards before the cornea.
• Short sightedness occurs when the eyeball is longer than normal, causing blurred distance vision
• Concave lenses overcome short sightedness by bending light outwards before it reaches the cornea.

Physical Factors (Ageing)

• As a person ages, the lens becomes less flexible, causing presbyopia (long sightedness)
• After 40, spectacles may be needed for near vision, especially in poor light conditions
• Fatigue can temporarily affect the lens' ability to accommodate, resulting in blurred vision for close work

Physical Factors (Imperfections)

• The blind spot is a visual deficiency where the optic nerve enters the retina
• Cataracts involve clouding of the lens, usually associated with UV light exposure and ageing
• Astigmatism causes objects to appear irregularly shaped due to a misshapen cornea
• Glaucoma is a build-up in pressure of the fluid within the eye, which can damage the optic nerve and cause blindness
• Migraines are severe headaches that can cause visual disturbances

Environmental Factors (Light)

• Vision can be improved by increasing lighting, but too much illumination can cause glare
• Older people are more affected by glare than younger people
• Moving from a bright to dimmer environment reduces vision until the eyes adapt
• In low light, looking slightly to one side allows images to fall on the part of the retina with many rods.

Environmental Factors (Airborne Particles)

• Airborne particles like dust, rain, or mist can interfere with light transmission, distorting vision
• This is worse when wearing spectacles, as they get dirty, wet, misted up, or scratched

Contact Lenses

• Contact lenses should be work for a period of 8-12 hours
• Extended wear may have negative effects such as dryness and irritation, particularly in dry or dusty conditions
• Goggles should be worn to exclude dust

The Nature and Angle of the Object Being Viewed

• Visual cues compare known object sizes to unknown ones; small objects are associated with being farther away
• Larger objects are easier to see than smaller ones, but close proximity is not always better
• An appreciation for the micro and macro view is importnat
• Harder to distinguish edges and shapes of objects with poor contrast against their background
• Movement and relative motion, as well as distance and angle increase visual demands
• A vibrating object or viewer will result in an unclear view.

Vision Colour

• Inability to see particular colours can be problematic for colour recognition
• Colour vision is important for recognising components, distinguishing wires, using diagnostic tools, and recognising lights
• The most common colour vision deficiency is distinguishing between red and green, while confusing blues and yellows is rare
• Ageing causes colour vision changes because of lens yellowing, reducing discrimination in the blue-yellow range.

Night Vision

• Central vision is poor at night or with low illumination, but clearer by looking slightly to one side
• Those with perfect day vision may still be myopic at night, which isn't always recognised and presents a hazard due to false confidence from daytime vision

Angle of viewing

• Objects are not as clear when viewed from an angle, and background changes may affect clarity
• Parallax is the way an object seems to change its position when you own point of observation changes
• Instrument reading can suffer from parallax error if the instrument is not read directly from in front

Eyesight detriments

• Vision can be affected by using certain drugs and medications, drinking alcohol, and smoking cigarettes
• Carbon monoxide builds up in the bloodstream, reducing oxygen to the eyes and impairing sensitivity
• Alcohol can have similar effects, even hours after drinking

Vision for aircraft maintenance

• Vision is critical, especially for inspection tasks
• Age and eye problems can gradually affect vision, so regular testing is important
• Airline companies and airports may set eyesight standards not related to maintenance safety
• Individuals must recognise when their vision is affected and consider the consequences of working with impaired vision

Ambient and Task Lighting

• Maintenance organisations must provide well-lit areas for aircraft inspections or supplementary lighting
• Individual maintenance workers must ensure adequate illumination for inspection tasks

Hearing

• The performance of the ear is associated with the range of sounds that can be heard
• The audible frequency range for young people is 20-20,000 Hz, with the greatest sensitivity at 3000 Hz
• Volume or intensity of sound is measured in decibels (dB)

Noise

• Noise can have negative effects in the workplace and can be annoying, interfere with verbal communication, cause accidents, be fatiguing, and damage hearing
• Intermittent and sudden noise is more disruptive than continuous noise, with high frequency having a greater impact
• Noise tends to increase errors and variability rather than affecting work rate

Hearing Impairment

• Hearing loss can occur from short or long duration noise
• The degree of impairment is affected by the intensity of the noise
• May be temporary or permanent
• Hair-like cells convert sound to electrical signals in the inner ear
• Recovery gradually decreases as additional exposure is added, hearing loss becomes permanent
• Workplace safety regulations state that employers must assess hearing risks, inform employees, and provide ear protection devices and inform them about their use

Hearing Protection

• Employers must reduce noise exposure, mark dangerous zones, and exclude people where possible
• Other work areas must be isolated from severe noise
• Hearing deteriorates naturally, which is called presbycusis, affecting high-pitched sounds first, gradually from age 30 onward
• Natural decline can occur sooner if there is noise induced hearing loss.

Hearing Protection

• Hearing protection is available by using ear plugs or earmuffs
• Noise is reduced by up to 20 dB using ear plugs and 40 dB using earmuffs
• Must be used consistently and effectively even if they interfere with communication
• Best practice is to reduce noise or move away from workers

Aircraft hearing awareness

• Engineers must understand that ears have limited ability to protect when conducting aircraft maintenance
• Engineers should be given and use hearing protection
• Ears do not always recover from noise exposure and noise will damage ears gradually and be irreparable