Aviation Physiology and Human Factors

What does flight do to your body?

Body map

Four groups: thin air, pressure change, fooled senses, and forces and toxins. Tap + to open.

• The body in flight four kinds of threat What altitude, pressure change, acceleration and toxins do to the body, and how the senses can be fooled in flight.
  • Thin air less oxygen pressure As height increases, air pressure and the oxygen available both fall: half of sea-level oxygen at 18,000 ft and a quarter at 36,000 ft.
    • Dalton: oxygen pressure falls Oxygen stays about 21% of the air, but its partial pressure falls with height.
    • Hypoxia 4 types, see the flow Lack of oxygen to the brain. Mild cases cause inattentiveness, poor judgment and uncoordinated movement; serious oxygen lack can kill within minutes.
    • Hyperventilation CO2 washed out, not hypoxia Over-breathing; dizziness and tingling look like hypoxia.
    • Time of useful consciousness halved by rapid decompression How long you can still function effectively after the oxygen supply fails. It becomes very short at high altitude, and rapid decompression can halve it.
    • Cabin altitude 6,000 to 8,000 ft Pressurisation keeps the cabin at an artificial low altitude: about 6,000 ft when flying at 30,000 ft, with a maximum of 8,000 ft.
  • Pressure change gas expands, bubbles form Changing pressure makes trapped gas expand or compress and lets dissolved nitrogen form bubbles. The gas laws explain these effects on the body.
    • Boyle: trapped gas expands ears, sinuses, gut At constant temperature, gas volume is inversely proportional to the surrounding pressure: trapped gas expands in a climb and compresses in a descent.
    • Henry: decompression sickness nitrogen bubbles The gas dissolved in a liquid is proportional to the pressure above it. A rapid fall in pressure lets nitrogen leave the blood as bubbles.
      • Bends joint pain Nitrogen bubbles in the joints cause pain like rheumatism (the bends).
      • Chokes chest burning, coughing Bubbles in the respiratory system cause shortness of breath with a burning, gnawing, piercing pain.
      • Creeps itching under the skin Nitrogen bubbles under the skin feel like something crawling.
      • Staggers nerves, vision, paralysis Bubbles affecting the brain cause loss of mental functions and of control of movement.
    • Charles: chilling in explosive decompression At constant volume, gas pressure is directly proportional to temperature. This explains the temperature changes that come with rapid decompression.
    • Graham: gas exchange high to low concentration Gases move from higher to lower concentration until equal. This is how oxygen passes from the lungs into the blood and on to the tissues.
    • Wait before flying Diving and giving blood both make flying risky for a time: wait 24 hours after diving and 48 hours after giving blood.
      • After scuba diving 24 h, dive of 30 ft or more After diving with compressed air to 30 ft or more, do not fly within 24 hours, and certainly not the same day.
      • After giving blood 48 h Giving blood disturbs the circulation for several days; pilots should wait at least 48 hours before flying.
  • Fooled senses trust the instruments Your eyes and balance organs can fool you. The only cure for disorientation is to rely on solid reference points on the ground or on your flight instruments.
    • Night vision rods, look off centre Full dark adaptation needs about 30 minutes. It is lost within seconds of viewing a bright light and impaired above 5,000 ft cabin altitude, by carbon monoxide and smoking.
    • Empty field myopia eyes focus at 1 to 2 m In a featureless sky the eyes rest at short focus.
    • Semicircular canals rotation Inner-ear balance organs that detect angular acceleration. In steady rotation the fluid stops moving, so they can no longer sense the turn.
      • The leans feel banked the other way Slow roll below the threshold, then an abrupt level-off.
      • Coriolis tumbling Moving the head in a long turn.
      • Graveyard spin recovery feels like a spin The canals adapt to a long spin; the pilot re-enters it.
    • Otolith organs linear motion Balance organs (utricle and saccule) that detect linear acceleration. They cannot tell a pitch change from an acceleration.
      • Takeoff somatogravic feel nose-up, push down Forward acceleration.
      • Elevator feel nose-up, trim down Upward acceleration.
    • Visual illusions on approach Visual illusions are especially dangerous because pilots normally trust their eyes most. Many occur on approach and at night.
      • Black hole feel too high, fly too low Dark ground before a lit runway.
      • Narrow runway feel too high, fly too low A pilot used to standard-width runways judges he is too high on an unfamiliar narrow runway and rounds out too low.
      • Upslope runway feel too high, pitch down A runway or ground sloping up makes you feel too high, so you fly a flatter, lower approach.
      • Autokinesis a still light seems to move Staring at one light in the dark.
  • Forces and toxins blood and oxygen cut off Acceleration forces and substances such as carbon monoxide and alcohol affect the body and lower its tolerance in flight.
    • +Gz blood drains to the feet Increased positive G drains blood towards the feet, bringing tunnel vision, greyout and possibly blackout.
      • Tunnel vision the first sign Increased G brings weight gain, poor mobility, displaced organs, then tunnel vision.
      • Greyout comes before blackout Vision greys as blood drains from the head.
      • Possible blackout tilt the seat back A tilted-back seat reduces the chance of blackout.
    • −Gz redout: blood to the head Blood surges to the head: the face flushes and the eyes bulge. Return to a normal flying attitude.
    • Carbon monoxide picked up over 200 × more readily Binds haemoglobin far more strongly than oxygen: anaemic hypoxia.
    • Alcohol cleared 10 to 15 mg/100 ml/h Alcohol slows the brain, clouds judgment and makes you less able to cope with low oxygen. Leave 24 hours between your last drink and take-off.

Takeaway: Thin air starves you of oxygen, falling pressure expands and releases gas, senses can lie, and G and toxins cut oxygen delivery.

How high can you go on air alone?

Altitude zones

Read from the ground up: each band needs more help to keep oxygen reaching the body.

• Total space equivalent complete vacuum Needs spacecraft life support.
• Partial space equivalent sealed cabin or pressure suit The Armstrong line, where body fluids boil, is at 63,000 ft.
• Oxygen under pressure 100% O2, positive pressure Forces oxygen into the lungs against the low outside pressure.
• 100% oxygen pure oxygen needed Keeps sea-level oxygen pressure in the lungs.
• Oxygen and air mix diluter-demand Supplementary oxygen mixed with air.
• Physiological zone air is enough The body works normally on air.

Memory hook: Air, mix, pure, pressure, suit: one more step every band.

Takeaway: Air up to about 10,000 ft, then an oxygen mix, 100% oxygen, oxygen under pressure, and above 50,000 ft a sealed cabin or suit.

What changes as you climb?

Climbing

The air keeps 21% oxygen, but pressure falls. Read from the ground up.

This lesson is part of the Exam-Ready plan. See plans

What does each gas law look like in real life?

Gas laws

The air mix, then five laws you can see in everyday things. Tap each number.

This lesson is part of the Exam-Ready plan. See plans

How do lungs, heart and blood keep you going?

Body systems

Follow the oxygen: lungs, blood, heart, and the brain that runs them. Tap each number.

This lesson is part of the Exam-Ready plan. See plans

Which kind of hypoxia is it?

Which hypoxia?

Read each case and press the stamp that applies.

This lesson is part of the Core plan. See plans

Hypoxia, hyperventilation or CO poisoning?

Hypoxia or not?

Dizzy and tingling at the controls? Answer in order.

This lesson is part of the Exam-Ready plan. See plans

How long can you stay useful without oxygen?

TUC

Bar length is time, on a stretched scale. Blue is a normal climb, red a rapid decompression.

This lesson is part of the Core plan. See plans

What do you do after a rapid decompression?

Decompression drill

Bang, mist, cold. Fly the four steps in order.

This lesson is part of the Core plan. See plans

How long to wait, how many G, how high?

Body in flight

Three views: waiting times, G in a turn and the thinning air.

This lesson is part of the Exam-Ready plan. See plans

Why look slightly to one side at night?

The eye

Light enters through the cornea and lens and lands on the retina at the back.

This lesson is part of the Core plan. See plans

How does the eye work and fail?

Eye at work

Light passes the cornea and lens to the retina. Tap each number.

This lesson is part of the Exam-Ready plan. See plans

How does the ear hear and balance?

The ear

Sound comes in from the left; balance sits deep inside. Tap each number.

This lesson is part of the Exam-Ready plan. See plans

Which illusion is fooling you?

Illusions

Each card is one false feeling. Tap a card to see what the pilot does wrong.

This lesson is part of the Exam-Ready plan. See plans

How many g in a turn?

G in a turn

Load factor in a level turn and beyond. Tap a bar.

This lesson is part of the Exam-Ready plan. See plans

What keeps a pilot fit to fly?

Fit to fly

Five areas of fitness. Tap + to open each.

This lesson is part of the Exam-Ready plan. See plans

What does the cabin do to you?

Cabin and you

Humidity, heat, vibration and light. Find a row, read across.

This lesson is part of the Exam-Ready plan. See plans

Numbers to know

Numbers

The figures examiners like, in one list.

This lesson is part of the Core plan. See plans

Pairs that catch students out

Don't mix up

Read both sides, then the line that tells them apart.

This lesson is part of the Core plan. See plans

Test yourself

Ten questions from this chapter.

This lesson is part of the Core plan. See plans