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Data is collected in the aircraft by digital sensors and transmitted to the ground facilities. Replies
from the ground may be printed out so the appropriate flight crewmember can have a hard copy of the
response.
ACC (active clearance control). A system for controlling the clearance between tips of the compressor or
turbine blades and the case of high-performance turbofan engines.
When the engine is operating at maximum power, the blade tip clearance should be minimum, and
the ACC system sprays cool fan-discharge air over the outside of the engine case. This cool air causes the
case to shrink enough to decrease the tip clearance. For flight conditions not requiring such close clearance,
the cooling air is turned off, and the case expands to its normal dimensions.
Control of the ACC system is done by the FADEC, or Full-Authority Digital Electronic Control.
accelerate. To increase the speed of an object, or make it move faster.
accelerated-life test. A form of operational test of a system or component in which unusual conditions are
used to cause a premature failure.
An accelerated-life test is used to locate weak points and predict the service life the system or
component will likely have under normal operating conditions. The test conditions used in an
accelerated-life test are much more severe than will ever be encountered in normal operation.
accelerate-stop distance (aircraft performance). The length of runway needed for an aircraft to
accelerate to a specified speed, and then, in case of engine failure, be able to stop on the runway.
accelerating agent. A component or substance used to hasten a chemical action or change.
accelerating pump (carburetor component). A small pump in a carburetor used to produce a
momentarily rich fuel-air mixture to the engine when the throttle is suddenly opened.
The fuel supplied by the accelerating pump prevents the hesitation that would otherwise occur
between the time the engine stops operating on the idle metering system and the time there is enough air
flowing through the carburetor for it to supply fuel through the main metering system.
acceleration. The amount the velocity of an object, measured in feet per second, is increased by a force
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 6
during each second it is acted upon by that force. Acceleration is normally expressed in terms of feet per
second, per second (fps2).
acceleration caused by gravity. The acceleration of a freely falling body caused by the pull of gravity.
Acceleration caused by gravity is expressed as the rate of increase of velocity over a given unit of
time. This rate, in a vacuum, near sea level at a location of 40° north latitude is 32.2 feet, or 9.8 meters, per
second, per second. This acceleration decreases with an increase in altitude until it becomes zero outside of
the earth’s gravitational field.
acceleration check (gas turbine engine maintenance check). A maintenance check of a gas turbine
engine in which the time required for the engine to accelerate from idle RPM to its rated-power RPM is
compared with the time specified for this acceleration by the engine manufacturer.
acceleration control unit. See ACU.
acceleration error (magnetic compass error). An error in the indication of a magnetic compass that
shows up when the aircraft accelerates or decelerates while flying on an easterly or westerly heading.
The float in an aircraft magnetic compass is unbalanced to compensate for the downward pull of
the vertical component of the earth’s magnetic field (dip error), and the inertia caused by a change in speed
acts on this unbalanced condition.
When the aircraft accelerates on an easterly or westerly heading, the compass indicates that the
aircraft is turning to the north, and when it decelerates on either of these headings, the compass indicates
that the aircraft is turning to the south.
acceleration switch. A switch in a piece of airborne electronic equipment actuated by an abnormal
acceleration.
Emergency Locator Transmitters (ELTs) have an acceleration switch that causes them to begin
transmitting if the aircraft crashes and subjects the ELT to an abnormally high longitudinal acceleration.
Acceleration switches are also called inertia switches.
acceleration well (carburetor component). An enlarged annulus around the discharge nozzle of some
float-type carburetors. The acceleration well fills with fuel when the engine is idling, and when the throttle
is suddenly opened, this additional fuel discharges into the engine through the main discharge nozzle.
accelerator (plastic resin component). A substance added to a catalyzed resin to shorten the time needed
for the resin to cure.
　
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