Human factors.  A multidisciplinary field encompassing the behavioral and social sciences, engineering, and physiology, to consider the variables that influence individual and crew performance for the purpose of optimizing human performance and reducing errors. 

Hung start. In gas turbine engines, a condition of normal light off but with rpm remaining at some low value rather than increasing to the normal idle rpm. This is often the result of insufficient power to the engine from the starter. In the event of a hung start, the engine should be shut down. 

Hydroplaning. A condition that exists when landing on a surface with standing water deeper than the tread depth of the tires. When the brakes are applied, there is a possibility that the brake will lock up and the tire will ride on the surface of the water, much like a water ski. When the tires are hydroplaning, directional control and braking action are virtually impossible. An effective anti-skid system can minimize the effects of hydroplaning. 

Hypemic hypoxia. A type of hypoxia that is a result of oxygen deficiency in the blood, rather than a lack of inhaled oxygen. It can be caused by a variety of factors. Hypemic means “not enough blood.” 

Hyperventilation. Occurs when an individual is experiencing emotional stress, fright, or pain, and the breathing rate and depth increase, although the carbon dioxide level in the blood is already at a reduced level. The result is an excessive loss of carbon dioxide from the body, which can lead to unconsciousness due to the respiratory system’s overriding mechanism to regain control of breathing. 

Hypoxia. A state of oxygen deficiency in the body sufficient to impair functions of the brain and other organs. 

Hypoxic hypoxia. This type of hypoxia is a result of insufficient oxygen available to the lungs. A decrease of oxygen molecules at sufficient pressure can lead to hypoxic hypoxia. 

## I 

IAF.  See initial approach fix. 

IAP.  See instrument approach procedures. 

IAS. See indicated airspeed. 

ICAO.  See International Civil Aviation Organization. 

Ident.  Air Traffic Control request for a pilot to push the button on the transponder to identify return on the controller’s scope. 

IFR.  See instrument flight rules. 

ILS.  See instrument landing system. 

ILS categories.  Categories of instrument approach procedures allowed at airports equipped with the following types of instrument landing systems: 

ILS Category I:  Provides for approach to a height above touchdown of not less than 200 feet, and with runway visual range of not less than 1,800 feet. 

ILS Category II:  Provides for approach to a height above touchdown of not less than 100 feet and with runway visual range of not less than 1,200 feet. 

ILS Category IIIA:  Provides for approach without a decision height minimum and with runway visual range of not less than 700 feet. 

ILS Category IIIB:  Provides for approach without a decision height minimum and with runway visual range of not less than 150 feet. 

ILS Category IIIC:  Provides for approach without a decision height minimum and without runway visual range minimum. 

IMC.  See instrument meteorological conditions. 

Inclinometer. An instrument consisting of a curved glass tube, housing a glass ball, and damped with a fluid similar to kerosene. It may be used to indicate inclination, as a level, or, as used in the turn indicators, to show the relationship between gravity and centrifugal force in a turn. 

Indicated airspeed (IAS). Shown on the dial of the instrument airspeed indicator on an aircraft. Indicated airspeed (IAS) is the airspeed indicator reading uncorrected for instrument, position, and other errors. Indicated airspeed means the speed of an aircraft as shown on its pitot static airspeed indicator calibrated to reflect standard atmosphere adiabatic compressible flow at sea level uncorrected for airspeed system errors. Calibrated airspeed (CAS) is IAS corrected for instrument errors, position error (due to incorrect pressure at the static port) and installation errors. 

Indicated altitude. The altitude read directly from the altimeter (uncorrected) when it is set to the current altimeter setting. 

