Human Factors 1

Breathing

• All cells of human body require oxygen (O₂) to function

• Body stores very little O₂, continuous breathing required to oxygenate blood

• Cells remove O₂ from blood “burn” along with fuel to produce energy, return Carbon Dioxide (CO₂) to blood

• Lungs exchange CO₂ from blood incoming O₂

• Level of CO₂ in blood controls breathing rate

Circulation

• Heart pumps blood to lungs to oxygenate, then around body, back to lungs to remove CO₂

• Heart rate is dependent on number of factors

  • Age

  • Physical fitness

  • Stress

Ears

• Primarily used for hearing

• Ear drum creates a sealed air cavity in the middle ear

• Eustachian tube connects middle ear to back of throat

  • Normally closes

  • Opens to equalise pressure between middle and outer ear

Vestibular system

• Inner-ear function

  • Maintains balance and spatial awareness

• Made up of

  • Semicircular canals - detect rotational motion

  • Otoliths - detect linear acceleration

Eyes

• Cornea focuses light on retina via pupil (aperture) and lens

• Retina has 2 types of cells

  • Rods: high sensitivity, but monochrome

  • Cones: low sensitivity, detect colour

• Centre of eye (fovea) has only cones, periphery has a mixture (mostly rods)

• Optic nerve transmits signals to brain but also creates a blind spot

Hypoxia

• Not enough oxygen getting to cells, mainly the brain

• Caused by

  • Not enough oxygen in the air being breathed in

  • Ineffective exchange of carbon dioxide with oxygen

    • Hyperventilation

    • Carbon monoxide poisoning

  • Lack of blood flow to the brain

    • “g”-effects

  • Common symptoms

  • Impaired judgement

  • Confusion

  • Headache

  • Unconsciousness

Hypoxia at altitude

• Percentage of oxygen in the air remains approximately constant throughout the troposphere

• Air press reduces with altitude which reduces partial pressure of oxygen in the lungs

  • P_O2 = 0.21*P_air

  • Sea level: 213hPa

• As partial pressure drops less oxygen is transferred into the blood

• A healthy person will have no issues below 10,000ft

  • Above this supplementary oxygen is required

Hyperventilation

• Breathing deeper and more rapidly than required

• Caused by

  • Stress

  • Anxiety

• Symptoms

  • Dizziness

  • Tingling sensation in extremities

  • Vision impairment

  • Unconsciousness

• Treatment - rebreathe exhaled air

“g”-effects

• Heart pumps blood to the brain against force of gravity

• Upward acceleration increases apparent weight of body

  • “g”-force

• Blood appears “heavier” to the heart, making it harder to pump

  • Pools in legs, reducing supply to brain

• Symptoms

  • Greying of vision

  • Tunnel vision

  • Unconsciousness: g -LOC

• Treatment: reduce g-load

Decompression sickness

• Nitrogen bubbles escaping from blood due to low air pressure

  • “The bends”

• Symptoms

  • Headache

  • Nausea

  • Unconsciousness

• Usually not an issue below 18,000ft

• However, diving underwater increases nitrogen pressure in blood increasing likelihood of DCS

  • Do not fly within 12 hours of any diving activity

  • Increase to 24 hours if diving below 10m

Illness

• Any illness is likely to impair your ability to fly safely

• Cold/flu can block Eustachian tubes leading to barotrauma

• Any heart or lung issues increase susceptibility to hypoxia and g-effects

• Any stomach problems can become rapidly debilitating in flight

  • Most common cause of pilot incapacitation

• Generally, if you’re taking any medication you shouldn’t fly without first consulting an Aeromedical Examiner

Carbon monoxide poisoning

• Aircraft heaters typically utilise waste heat from the exhaust system

• Cracks in exhausts can leak carbon monoxide into the cabin

• Carbon monoxide displaces oxygen in the blood leading to hypoxia

• Additional symptom: bright pink skin colour

• If suspected shut off heater and ventilate cabin

Alcohol

• Alcohol both impairs judgement and increases susceptibility to other conditions

• Flying blood alcohol limit is 200mg/L

  • This is 4 times stricter than UK drink driving limit

  • Effectively zero in practice

• Traditional advice was "Eight hours bottle to throttle”

  • This is the absolute minimum

Smoking

• Smoking reduces lung capacity increasing susceptibility to hypoxia, “g”-effects and carbon monoxide poisoning

• Moderate smoking can mimic the effects associated with an altitude of 6,000ft

  • Thereby reducing tolerance to higher altitudes

• Usually banned anywhere on an airfield as well as in the sircraft