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hydrostatic testing (gas cylinder test). A type of test required for cylinders used to carry compressed
gases. The cylinder is filled with water under a pressure higher than that of the gas it is to carry, and this
pressure is held for a specified length of time. Cylinders are hydrostatically tested rather than being tested
with compressed air because, if the cylinder should fail, the incompressible water will not cause an
explosion, as would occur if a cylinder filled with compressed air should fail.
When oxygen cylinders are hydrostatically tested for installation in an aircraft, they are filled with
water and pressurized to 5/3 of their working pressure. Standard-weight cylinders (DOT 3AA) must be
hydrostatically tested every five years, and lightweight cylinders (DOT 3HT) must be tested every three
years.
hydroxide. A chemical compound consisting of a metal, usually a base, or a non-metal, usually an acid,
and a negative hydroxyl ion (OH). Hydroxides are found in both organic and inorganic chemicals. Some of
the more familiar hydroxides are sodium hydroxide (NaOH) and potassium hydroxide (KOH).
hygrograph (meteorology). The record produced by a continuous-recording hygrometer.
hygrometer. An instrument used to measure the amount of moisture in the air to determine its humidity.
hygroscopic material. A material that absorbs moisture from the air. Silica gel is a hygroscopic material
used to protect delicate equipment from damage caused by moisture in the air. When bags of silica gel are
placed in the equipment, any moisture in the air is absorbed by the silica gel, and the air is kept dry.
hyperbola (mathematical curve). A plane curve traced by a point that moves in such a way that the
difference of its distance from two fixed points, called foci, remains constant.
hyperbolic navigation system. A system of electronic navigation in which a master and a remotely located
slave station transmit a signal simultaneously. The difference between the time the signals from the two
stations are received in an aircraft is carefully measured and plotted on a chart of hyperbolic curves. This
information gives a line of position along which the aircraft is located.
An electronic computer determines the position by plotting lines of position from two sets of
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stations. The aircraft is located at the latitude and longitude the lines cross.
hyper engines. A series of experimental high-performance aircraft engines built with U.S. Army and Navy
financing in the early 1930s. The goal of these engines, which had been unattainable up to that time, was to
develop one horsepower for each cubic inch of piston displacement.
hypersonic flight. Flight at air speeds greater than five times the local speed of sound, more than Mach
5.0.
hyperventilation. A breathing problem caused by too little carbon dioxide in the lungs. Hyperventilation
causes a person to breathe too rapidly, and this results in dizziness and may even lead to unconsciousness.
hypotenuse (mathematics). The side of a right triangle opposite the right angle.
hypoxia. A physiological condition in which a person is deprived of the needed oxygen. The effects of
hypoxia normally disappear as soon as the person is able to breathe air that contains sufficient oxygen.
hysteresis. A condition in which a physical action lags behind its cause. A spring is often slow to return to
its original length when the load is removed. This slow return is caused by friction within the material of
which the spring is made. This internal friction causes hysteresis.
hysteresis error (instrument indication error). An instrument error that causes a difference in the
readings of an instrument when the indicated values are increasing, compared with the readings at the same
point when the values are decreasing. Hysteresis error is caused by internal friction in the instrument
mechanism.
hysteresis loop (magnetic materials). A curve showing the way flux density in a magnetic material varies
with the magnetizing force applied to it. The shape of a hysteresis loop gives an indication of the magnetic
characteristics of the material.
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IA (Inspection Authorization). An authorization issued by the Federal Aviation Administration to certain
licensed and experienced aviation maintenance technicians that allows them to perform annual and
progressive inspections. This authorization also allows them to approve aircraft for return to service after
certain types of major repairs and major alterations.
IAF (initial approach fix) (air traffic control). The fixes depicted on the instrument approach procedure
　
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