CHAPTER 5
UTILITY SYSTEMS
The S-3 aircraft bleed air leak detection system
The utility systems of an aircraft provide an
(fig. 5-1) consists of four loops of bleed air leak sensing
additional measure of flight safety, pilot comfort and
elements located close to the auxiliary power unit
convenience, and contribute to the overall mission
(APU) duct, bleed air ducts, and an engine start port
capability of the aircraft. Many aircraft have utility
leak detector located near the engine ground start port.
systems that rely on a bleed air system to function.
Sensing elements are mounted between ducts and the
aircraft structure because bleed air temperature is
AUXILIARY BLEED AIR
sufficiently high to cause structural damage. Support
SYSTEMS
clamps with quick-release fasteners are used to mount
LEARNING OBJECTIVE: Describe the
the sensing elements, which are protected by bushings.
auxiliary bleed air system.
The bleed air leak detector control is located in the
lower section of the left load center. This system is a
An aircraft's auxiliary bleed air system furnishes
fire-detection type of system that responds to heat.
supply air for air-conditioning and pressurization
High temperature causes a chemical reaction in the
systems, as well as for electronic cooling, windshield
sensing element, which provides a ground for the
washing, anti-icing, and anti-gravity (g) systems. These
warning circuit that turns on an indicator light on the
bleed air systems also pressurize fuel tanks, hydraulic
annunciator panel. A test circuit is activated by the
reservoirs, and radar wave-guides on several types of
BLEED AIR LEAK DETECT switch on the eject panel
aircraft.
located on the eyebrow panel.
The air for these systems is tapped off downstream
of the air-conditioning turbine before any cooling takes
COMPONENTS
place, or at various points within the air-conditioning
system. Bleed air for these systems can range up to
The bleed air leak detection system consists of a
400F at pressures of up to 125 pounds per square inch
bleed air leak detector control, sensing elements, and an
(psi). Because each type of aircraft has a somewhat
engine start port leak detector.
different approach in system design, temperatures, and
Bleed Air Leak Detector Control
pressures, the system and components in this manual
will be representative of types found throughout the
The detector control is located in the lower section
Navy. Under no circumstances should this manual be
of the left load center. The detector control contains two
regarded as the final source of technical data to perform
modules with four electrical circuits. Each circuit has a
aircraft maintenance. For the most up-to-date
test function and a control function. The test function
information, refer to the maintenance instruction
verifies all sensing element loops, and the control
manual (MIM) for the system concerned.
function turns on a warning indicator light on the
Q5-1. Name four aircraft systems that use auxiliary
annunciator panel.
bleed air.
Bleed Air Leak Sensing Elements
Q5-2. An auxiliary bleed air system can reach up to
what pressure?
The sensing elements are metal tubes with center
conductors isolated from the tubes by a solid chemical.
BLEED AIR LEAK DETECTION
The sensing elements are mounted between the bleed
SYSTEM
air ducts and areas to be protected from very high
temperatures that would develop if a duct were
LEARNING OBJECTIVE: Recognize the
punctured or ruptured. The sensing elements are
operating principles and components for a
mounted within 2 to 5 inches of the duct.
bleed air leak detection system.
5-1
