Aviation Maintenance Human Factors and Visual Physiology

The Dirty Dozen: Common Human Factors in Aviation

The Dirty Dozen represents the 12 most common human error preconditions that lead to accidents or incidents in aviation maintenance:

• Lack of communication
• Distraction
• Lack of resources
• Stress
• Complacency
• Lack of teamwork
• Pressure
• Lack of awareness
• Lack of knowledge
• Fatigue
• Lack of assertiveness
• Norms

Consequences resulting from normal aging or abnormal medical conditions are classified as Latent Medical or Environmental Conditions (LMEC). The FAA estimates that 25-40% of a mechanic's time is spent on paperwork. Human factors are divided into four topic areas: People, Environment, Actions, and Resources (the latter being the broadest category, encompassing everything a mechanic needs to perform the job). Approximately 80% of maintenance incidents are caused by human factors, such as repetitive tasks, fatigue, environmental conditions (snow, poor lighting), and exposure to fumes.

Human Visual Physiology

The human eye is a complex organ. The primary colors that are difficult to distinguish for those with color vision deficiency are red and green.

Structure of the Eye

1. Cornea: The clear window at the front of the eye; it acts as the primary focusing element, providing 70-80% of total focusing ability.
2. Iris: The colored part of the eye that controls the amount of light entering by varying the size of the pupil.
3. Pupil: The dark area in the center of the iris; its size changes rapidly.
4. Lens: Its shape is adjusted by muscles—a process called accommodation. The lens thickens to focus on near objects and flattens for distant points.
5. Retina: Located on the rear wall of the eyeball, it contains two types of light-sensitive cells: rods and cones.

Visual Impairments and Refractive Errors

A blind spot occurs at the point where the optic nerves enter the retina between the rods and cones; this is typically only noticeable when viewing stimuli that appear very fleetingly.

Common Refractive Errors

• Hypermetropia (Farsightedness): Caused by a shorter eyeball where the image forms behind the retina, resulting in blurred vision for close objects.
• Myopia (Nearsightedness): Caused by a longer eyeball where the image forms in front of the retina, resulting in blurred vision for distant objects.
• Cataracts: Clouding of the lens, usually associated with aging.
• Astigmatism: A misshapen cornea causing objects to appear irregularly shaped.
• Color Deficiency: Usually hereditary.
• Night Myopia: Related to the differing frequency of colors that prevail at night, as red and orange predominate during the day.