Vertigo
Beware of false sensations. Your inner ear will give you feelings that are overpowering. With low time under the hood you must avoid attempting to extend maneuvers beyond just holding a heading or a standard rate bank. Any more may exceed your skill capacity for aircraft control in IFR conditions. Any tenseness will cause the inexperienced to over-control. If you have learned to use trim well as a VFR student you may be able to trim successfully for hands-off IFR flight. An aircraft trimmed for hands-off in pitch can be flown in roll (heading) with just rudder input. Descents are controlled by slow power reductions, only.

May cause nystagmus (trembling of the eyes) which makes reading of instruments impossible. Rare but can occur in extremes of weather or flight conditions. Other types of disorientation are illusions such as caused by runway/cloud sizes, shapes, or slope.

Unrecognized spatial disorientation is caused by a combination of a focused attention, distraction and instrument fixation. These most often occur in companion with loss of situational awareness due to excessive work load.

Recognized spatial disorientation is when the pilot is aware of his disorientation. Being aware means that the pilot should be able make his recovery with power and attitude corrections. Pilots have, over the radio, acknowledged their vertigo and inability to overcome it prior to crashing. Spatial disorientation is considered incapacitating when the attitude of the aircraft is so unusual that pilot is unable to determine the cause, organizing the instrument information, and making control decisions.

Vertigo is the #1 cause of Air Force fatal accidents. Vision is the pre-installed vertigo preventative. A moments glance out-the-window is all it takes. This will overcome any sensations from other sources. However, without vision, the organs of balance in the inner ear take over. The semicircular canals approximate the three axes. They contain a fluid that stimulates our senses of angular acceleration in these axes. Our sense of uprightness is done by the otolith organs. Tiny stones affect hair sensors in reaction to "gravity". Otoliths sense linear accelerations, not angular accelerations, and regardless of the direction interpret such accelerations as gravity. In our muscles and joints we have sensors that give additional information about push or pull. Unless one or all of these sensors are confirmed by vision we are on our way to vertigo.

The simulation used for vertigo such as the Barany chair, the yardstick on the nose, etc. in no way prepare the pilot for the real occurrence of vertigo. Unlimited motion in all three axes as well as planetary motion for acceleration are required. Even with these in place it is additionally necessary to create a work overload. The instructor should expose the student to simulations that closely approximate the real thing. That is except for safety of altitude, configuration and airspeed. To train the recovery the same forces and psychological pressures must be recreated over and over by in-flight simulations.

Illusions
What ever the illusion, it will take a few moments for you to get reoriented. If the illusion occurs in trimmed flight the problem is not as severe as it might be if it occurs during maneuvers as it usually does. Spatial disorientation caused by loss of horizon, change in power, banking, or other acceleration forces can lead the pilot to believe that the aircraft is doing something that it is not. These forces cause the pilot's vestibular and proprioceptive system to give the somatogravic illusion.

A pilots first reaction will be to over-control in a direction exactly the opposite to what should be done. The only safe procedure during the onset of any illusion is to increase your instrument scan rate.

The brain is capable of separating out the conflict of information between the eye and the inner ear when the eyes are looking outside the cockpit. A conflict between the sensory inputs of the body can be overwhelmed when the natural horizon disappears. When the conflict of information comes with the eyes viewing relatively unfamiliar instruments inside the cockpit, the brain can become confused and then give improper information. This is the origin of an illusion. You must trust your instruments in IFR conditions. (See IFR material)

Natural Illusions
Rain
Rain gives the illusion of being higher than you actually are. Combined with haze, you will fly lower approaches during rain.
Visibility
Low visibility can cause the illusion that things are further away. Entering a fog during descent will give an illusion of pitching up.
Wind
An updraft can cause such vertical sensation as to cause the pilot to put the nose of the aircraft down. He senses a climb where the attitude of the aircraft has been level. Avoid the weather conditions that can cause illusions. Believe your instruments. Get an instrument rating.
Health
Illness, medicine, alcohol, fatigue, or hypoxia will make susceptibility to an illusion more likely. It takes very little to cause disorientation

Night Illusions
Runway and approach light illusions will always be a problem if you are in an unfamiliar area. For this reason it is always desirable to make a daylight familiarization flight to an airport before a first time night arrival. Night has its own illusions that are covered in the night flight lessons. The distance of lights is greatly affected by the relative clearness and haze existing. A region of no lights such as might exist off the end of a runway toward the ocean can cause disorientation because of IFR illusions. The best solution is to go on instruments until established inland at altitude.

Variable Visibility
If, while on approach, you should suddenly face reduced visibility you will get an illusion of a sudden pitch up in aircraft attitude. Failing to recognize this illusion will lead to an instinctive and abrupt descent in the approach flight path.

Your ability to determine distance is greatly affected by haze. It is not unusual to call a distance at over twice the actual distance. The sudden appearance of bright lights during an a night approach through haze will create the illusion that the airport is much closer than previously realized. This results in a high approach.
Low
If there is no VASI or VAPI for vertical guidance if you get too low the runway lights will begin to disappear. A steep approach is always better at night. An arrival at an airport with and approach lighting system (ALS) tends to be lower and at a shallower angle than otherwise. Low approach results when runway is approached over dark area. Don't use landing light until close to ground. The visual cues used for a normal night landing seem much the same as you get with a rapid increase in sink rate.
Inverted
If there is a strong crosswind and you are crabbing to the runway instead of slipping you will get the illusion of being inverted. If the airport is well lighted in a surrounding dark area you will have an illusion of being higher than you actually are. Again, a steep approach has much advantage. 
High
Rain on the windshield will give the illusion of being higher than you are. If you mistake roadways for runways. It will give illusion of being quite close when lights are bright. High approach results.
Rising
If you are low and pitch the nose up as a correction or through the use of flaps, the illusion will indicate that you are rising. Any reduction of power will cause you to land short. 
Diving
At night, banking into or away from a line of lights will give the illusion that a dive or a climb is occurring. The same dive or climb illusion can happen by a change in aircraft pitch occurs while flying toward a light. A 10 degree bank with only the approach lights visible can cause an illusion that the lights are sloping from above.
Distance 
Lights that appear dim, as seen through haze, will be reported as more distant than they are. Example: I once reported an airport as in sight from twelve miles when the tower had me on radar at only five miles.

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