Learning hub · Human Performance · Lesson 2

Illusions and spatial disorientation

Your senses evolved for walking on the ground, not for flying. Without a clear horizon they give you false information that feels completely real. This lesson explains why, which illusions to expect, and the one cure that works.

About 12 minutes, plus the quiz. Part of Human Performance. It builds on Hypoxia, fatigue and knowing your limits, which introduces the idea.

1. How you sense motion

Balance depends on three inputs that your brain blends together: vision, the vestibular system in the inner ear, and the postural sense (pressure and stretch signals from your skin, joints and muscles, the “seat of the pants”). On the ground the three agree. In flight they often do not, and vision is the only one of the three that is reliable. Take it away, in cloud or on a dark night, and the other two start to mislead you.

The vestibular system has two parts. The semicircular canals are three fluid-filled loops, one for each axis (roll, pitch and yaw). They sense a change of rotation. The otolith organs sense linear acceleration and the direction of gravity. They cannot tell the difference between gravity and a push forwards: both just feel like a force on your head.

The weakness of the canals is that the fluid catches up. In a steady turn it soon moves at the same speed as the canal wall, and the signal stops. A turn that starts slowly never produces a signal in the first place. The FAA handbook gives a detection threshold of about 2° per second: slower rotation than that is not felt at all. (A standard rate turn is 3° per second, so it is felt at the start, and then the feeling fades.) Spatial disorientation is the name for the result: you no longer know the true attitude, position or motion of the aeroplane.

2. The common illusions in cloud or at night

• The leans. A slow turn, below the threshold, goes unnoticed. When you level the wings you feel as if you are banking the other way, and you may lean your body to match. It is the most common illusion.
• Graveyard spiral. In a long, steady, coordinated turn the sensation of turning fades. The aeroplane starts to lose height. Your inner ear says “level and descending”, so you pull back. That tightens the turn and makes the descent faster. The airspeed rises, the altimeter unwinds, and the sensation tells you nothing is wrong.
• Somatogravic illusion. A rapid acceleration, for example a take-off in the dark, pushes your head back in the same way as pitching the nose up would. You feel a steep climb and may push the nose down, towards the ground. A sudden reduction of power can do the opposite and tempt you to pull up into a stall.
• Coriolis illusion. If you are in a long turn and then tilt your head, for example to pick up a dropped chart, the fluid in more than one canal is disturbed and you get a strong feeling of tumbling. Keep head movements small when you are on instruments.
• Inversion and elevator illusions. A sudden change from a climb to level flight can make you feel you are tumbling backwards. An updraft can feel like a climb, and a downdraft like a descent. Each tempts you to push or pull the wrong way.

Two illusions are purely visual. Autokinesis: a single light stared at in the dark seems to move, and you may try to follow it. False horizon: sloping cloud, a coastline of lights or a pattern of ground lights looks like the horizon, so you tilt the aeroplane to match it. Look at the real instruments, and keep scanning.

3. Visual illusions on the approach

Even when you can see, the picture can lie. These illusions mostly appear on a final approach, because you judge height by the shape of the runway. They are described in the FAA handbook; the effect is the same in Europe.

What you see	What it looks like	The risk
Narrow runway	You are higher than you are	You fly a low approach and land short
Wide runway	You are lower than you are	You flare high and land hard, or float
Up-sloping runway or terrain	You are higher than you are	A low approach
Down-sloping runway	You are lower than you are	A high approach
Haze	The runway seems farther away	You descend too early and fly low
Very clear air	The runway seems closer	A high approach, then an overshoot

The black hole approach happens at night over water or dark, empty land, with the runway lights as the only lights. With no reference you feel too high, and tend to fly a lower approach than you want. Lights in a straight line, such as a road or a train, can also be mistaken for a runway. Defences: know the aerodrome, use the PAPI or other glide path aid if there is one, cross-check your altimeter against the published height for your distance, and fly a stabilised approach. If it does not look or feel right, go around.

4. What to do

You cannot switch the illusions off. Every pilot gets them, and they do not mean you are a poor pilot. Training makes you recognise them and respond correctly. The rules are short.

1. Believe the instruments. The attitude indicator, the altimeter, the airspeed indicator and the turn indicator do not have feelings. When the instruments and your body disagree, the instruments are right. Pitot-static and gyro faults are possible, so cross-check: see pitot-static and gyro instruments.
2. Level the wings first, then the pitch. Make small, smooth corrections. Large pulls feed the illusion.
3. Keep head movement small and avoid fumbling in the cockpit while on instruments.
4. Get out of the situation. If you are a VFR pilot without instrument training and you have flown into cloud, keep the wings level and make only gentle, smooth changes by instruments, tell ATC (a PAN PAN is appropriate, see the IFR phraseology lesson), and ask for help to reach clear air. Your instructor will teach the recovery your school uses.
5. Avoid the situation. Do not take off into low cloud or at night over unlit country if you have no instrument training. Illness, medication, alcohol, fatigue and mild hypoxia all increase the risk.

This is a training summary, not instrument-flying training. A VFR pilot who is not trained to fly by instruments can lose control of the aeroplane quickly in cloud. Make the decision early, ahead of the weather. Real flights use official data and the flight manual, and your instructor must teach the recovery.

5. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

1. You are in a gentle, unnoticed turn at 1.5° per second. How long does it take to change heading by 90°, and would your inner ear tell you? (The threshold is about 2° per second.) Then compare with a standard rate turn.

   Show the answer

   1. Time = angle ÷ rate = 90 ÷ 1.5 = 60 seconds.
   2. 1.5° per second is below the roughly 2° per second threshold, so you feel nothing, and the heading changes by a quarter circle without any sensation.
   3. A standard rate turn is 3° per second, so 90° takes 30 seconds. You feel the start, but the sensation then fades as the fluid catches up.
   4. Conclusion: a slow drift in cloud is the most dangerous kind, because your body cannot warn you. Only the instruments can.

2. You are on a final approach at night at an unfamiliar aerodrome with a narrow runway. The runway looks high in the windscreen, as if you are well above the glide path. Your altimeter shows 400 ft lower than the published height for your distance. Which is wrong, your eyes or the altimeter, and what do you do?

   Show the answer

   1. A narrow runway makes you feel higher than you are. This is the runway width illusion.
   2. The altimeter can be wrong only for a known cause, such as a wrong QNH setting or a wrong chart height, so check those. If both are right, believe the altimeter over the picture.
   3. Check the QNH setting and the published height, and use the PAPI if there is one.
   4. If the instruments say you are low, correct to the proper path, and if the approach is not stable, go around. Do not trust the picture over the data.

3. You take off at night from a dark runway with a rapid acceleration. Soon after rotating you feel as if the aeroplane is climbing very steeply, yet the airspeed is rising quickly and the vertical speed is only 300 ft per minute. What is happening, and what should you do?

   Show the answer

   1. A fast acceleration pushes your head back, which the otoliths read as nose-up. This is the somatogravic illusion.
   2. The instruments show a modest climb and rising speed, so the feeling is false.
   3. The wrong response is to push the nose down because it feels too steep. That could fly the aeroplane into the ground.
   4. Look at the attitude indicator and hold a normal climb attitude, check the airspeed, and let the feeling pass. Brief the instrument transition before a night departure.

6. Watch

A short explanation of the leans and how a pilot becomes disoriented, which goes with section 2. Check what it says against your instructor’s advice.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

More to watch

A second video on the visual illusions that affect approach and landing, which goes with section 3. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.

Open on YouTube if the player does not load. Not made by VectorEFB.

7. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

Next: Decisions, stress, alcohol and medication. Back to the learning hub.