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directional; the strength of the signal it picks up changes with the direction between the antenna and the
station. But the signal strength is the same if the station is in front of the antenna or behind it.
The signals picked up by both the loop antenna and the sense antenna are fed into the same
equipment, and as the two signals mix, they form a pattern that allows the ADF system to measure the
exact direction between the loop antenna and the station being received. Using the sense antenna eliminates
the problem of 180° ambiguity. It allows the system to distinguish whether the station is in front of the
antenna or behind it.
sensible heat. Heat energy added to a material that causes it to change its temperature, but not its physical
state. The other kind of heat is latent heat, which causes a material to change its physical state but not its
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 514
temperature.
When a pan of water is put on a hot stove, sensible heat enters it until the water reaches a
temperature of 100°C (under standard conditions). At this temperature, the water begins to boil, or change
its physical state from liquid to gas without increasing its temperature. Heat added to the water after it
begins to boil is called latent heat.
sensitive altimeter. A form of multipointer pneumatic altimeter with an adjustable barometric scale that
allows the reference pressure to be set to any desired level. Sensitive altimeters have a high degree of
mechanical amplification in their movement. The small amount of movement of the expanding bellows
inside the instrument is multiplied enough to rotate one of the pointers completely around the dial for a
pressure change equivalent to only one thousand feet of altitude. This is approximately one inch of
mercury.
sensitive relay (electrical component). An electromagnetically operated switch whose operating coil
requires very little current to close the switch contacts. Sensitive relays are used in photoelectric and
thermocouple circuits. The very small current produced by a photocell or thermocouple operates the relay,
and the relay contacts control enough current to operate other equipment.
Sensitive relays are being replaced today with solid-state devices that do the same job, but have no
moving parts.
sensitivity (radio receiver specification). A measure of the signal strength needed to produce
distortion-free output. The output must not have any excessive noise (unwanted electrical signal) in it, and
the output must be electrically identical to the input, except for its higher voltage. A radio receiver with a
high sensitivity can produce a usable output from a very weak, or faint, input signal.
sensitivity time control (radar technology). A radar circuit designed to correct for range attenuation so
the echo intensity on the scope is proportional to reflectivity of the target regardless of range.
sensor (automated equipment). The component in a piece of automated equipment affected by some
physical condition. The sensor sends signals into the equipment so it will operate in the correct manner. An
automatic cowl flap actuation system has a temperature sensor affected by the cylinder head temperature.
This sensor sends signals to the cowl flap motor. If the cylinder head temperature is too low, the motor
closes the cowl flaps, and if the temperature is too high, the motor opens them.
A photoelectric cell is a form of sensor. It is affected by the amount of light or by the color of the
light.
separation (air traffic control). The spacing of aircraft to achieve their safe and orderly movement in
flight and while landing and taking off.
sequence valve (fluid power system component). A type of flow control valve that allows one system to
fully operate before another system begins its operation. Sequence valves are used in aircraft hydraulic
systems to cause the wheel well doors to open fully before the landing gear is released from its uplocks.
sequential logic devices. Digital memory devices that have several inputs and several outputs. The
combination of bits at the outputs is determined by the bits on the inputs and the bits that were previously
on the inputs.
serial operation (computer operation). Data transfer by moving bits sequentially, one at a time, rather
than simultaneously, as is done by parallel operation. Series operation is much slower than parallel
operation, but it requires far less complex circuitry.
series circuit. A type of circuit in either an electrical or a fluid power system in which all of the
components are connected in such a way that they provide only one path from the source of current or fluid
flow, through all of the components, and back to the source.
　
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