ANTI-ICING SYSTEMS
Engine Start Port Leak Detector
LEARNING OBJECTIVE: Recognize the
The leak detector is mounted inside the right main
operating principles and components for an
landing gear door. The leak detector is a heat-sensitive
anti-icing system.
element that completes a ground circuit when subjected
to temperatures in excess of 225F. Its purpose is to
On days when there is visible moisture in the air,
detect leakage from the engine ground start port in the
ice can form on aircraft leading edge surfaces at
event of a check valve failure. The leak detector is
altitudes where freezing temperatures start. Water
wired in parallel with the loop 2 (2 BL LEAK) sensing
droplets in the air can be super-cooled to below freezing
elements.
without actually turning into ice unless they are
disturbed in some manner. This unusual occurrence is
OPERATION
partly due to the surface tension of water droplet not
allowing the droplet to expand and freeze. However,
The bleed air leak detection system is powered by a
when aircraft surfaces disturb these droplets, they
single-phase, 400-hertz (Hz), 115-volts alternating
immediately turn to ice on the aircraft surfaces. The ice
current (Vac) power from the essential ac bus. The test
may have a glazed or rime appearance. Glazed ice is
circuit uses 28 volts direct current (Vdc) power from
smooth and hard to detect visually. Rime ice is rough
the essential bus. When the bleed air leak detect switch
and easily noticed.
on the elect panel is held in the TEST position, 28 Vdc
Frost is formed as a result of water vapor being
is applied to the test relays. This completes the
turned directly into a solid. Frost can form on aircraft
transformer circuit through the sensing elements to
surfaces in two ways. First, it can accumulate on
ground. The transformer conducts and applies a signal
aircraft parked in the open and overnight when the
to a transistor circuit, which closes control relays. A
temperature drops below freezing and proper humidity
ground circuit is completed through the control relay
conditions exist. Second, it can form on aircraft
contacts to turn on the 1 BL LEAK, 2 BL LEAK, CAB
surfaces, caused by flying at very cold altitudes and
BL LEAK, and APU BL LEAK indicator lights on the
descending rapidly into warm, moist air. In this case,
annunciator panel. When the momentary BLEED AIR
frost deposits will result before the structure warms up
LEAK DETECT switch is released to the OFF position,
because of the marked cooling of air adjacent to the
the annunciator panel indicator lights go off. In normal
cold skin. Ice or frost forming on aircraft creates two
operation, when the temperature of a section or a short
basic hazards:
segment of the sensing element exceeds 255F, the
The resulting malformation of airfoil, which
chemical in the tube conducts electric current, which
completes a transformer circuit to ground similar to the
could decrease the amount of life; and
test relay. The indicator light on the annunciator panel
The additional weight and unequal formation of
will come on to indicate the loop is overheating. This
the ice, which could cause unbalancing of the
system will function even if there is an open (break) in
aircraft, making it hard to control.
the loop. During test position, the open element will
prevent the indicator light from turning on. The
Enough ice to cause an unsafe flight condition can
individual loops will function separately or
form in a very short period of time; thus some method
simultaneously if a leak is located in an area common to
of ice prevention or removal is necessary.
two loops. This will allow the pilot to take action to
An anti-icing system is designed to prevent ice
minimize damage due to bleed air leakage.
from forming on the aircraft. A deicing system is
Q5-3. Describe an S-3 aircraft bleed air leak
designed to remove ice after it has formed. An example
detection system.
of this is the deice boot system for the E-2 aircraft. An
aircraft anti-ice system removes ice from propellers and
Q5-4. During normal operation, describe what
the leading edges of wings and stabilizers. These
happens when the temperature in the sensing
systems may use electrical heaters, hot air, or a
element exceeds 225F.
combination of both to remove the ice formation. As an
Q5-5. What are the major components of a bleed air
AME, you are primarily concerned with hot air as a
leak detection system?
method to remove the formation of ice on wings and
stabilizers. The P-3 wing anti-icing system is used as an
Q5-6. How many modules make up the bleed air
example in this chapter to describe a hot-air system.
leak detector control?
5-3
