The bleed-air leak-detection system senses leaks from the bleed-air ducts that are installed in different locations of the aircraft. The bleed-air leak-detection system has two integrated air system (IAS) controllers and elements that sense bleed-air leaks.

The names of the different areas to which the bleed-air leak-detection system gives protection are identified as loops.

The elements that sense bleed-air leaks are known as loop elements. The name of the circuit for each loop element is also related to the name of the loops. The loop elements are installed adjacent to the ducts and components of the intermediate and high-pressure bleed-air system.

A bleed-air leak will cause a message and indication to show on the engine indication and crew alerting system (EICAS). This tells the flight crew that there is a leak from a bleed-air duct and shows where the leak occurs. This helps the flight crew to isolate the bleed-air leak and make the correct decisions. The data from the bleed-air leak is also recorded by the maintenance diagnostic computer (MDC).

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Each loop element has a concentric outer conductor, a layer of eutectic salt compound and an inner conductor. The loop elements are connected to the IAS controllers through the routing of the usual aircraft harnesses. The loop elements are electrically connected in series and the concentric outer conductor is connected to the aircraft ground. The layer of eutectic salt compound insulates the outer conductor from the inner conductor.

When hot bleed-air gets on the loop element, the eutectic salt compound becomes conductive and lets the outer conductor make contact with the inner conductor. The IAS controller senses that the outer conductor is grounded and supplies the inner conductor with alternating current for its resistance measurement. The IAS controller measures this resistance and shows the correct message on the EICAS.

If the temperature in the area of the wing anti-ice ducts is more than 460 °F (237.80 °C), the loop element in that area senses a bleed-air leak. In all other areas of the aircraft, a loop element senses a bleed-air leak when the temperature is more than 255 °F (123.90 °C).

The loop elements are installed with clips and the electrical connectors that are installed at the ends are installed with flange and nut assemblies. The clips are installed 8 in (20.30 cm) from each other. Usually, the loop element supports are attached to the structure. If it is not possible to install the loop elements to the structure, P-clamps attach them directly to the bleed-air ducts.

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The left and right-hand bleed air system ducting is monitored through a single 109 inch loop from one IAS controller. The left system duct is monitored by IASC #1 with one loop, the right bleed air system including the APU and external air connection is monitored by the IASC #2 and three loops 403 inches long.

Left Bleed Loop (Single)

The left bleed loop is installed in the aft equipment compartment. The routing of the left bleed loop is from the left air-turbine-starter duct to flow-control valve (FCV) No. 1. The routing of the left bleed loop also includes the cross-bleed valve of the ACS system. The left bleed loop senses leaks on the left side of the intermediate-pressure bleed-air system.

Right Bleed Loop (Single)

The right bleed loop is installed in the aft equipment compartment. The routing of the right bleed loop is from the auxiliary power unit (APU) discharge duct to FCV No. 2. The routing of the right bleed loop also includes the high-pressure ground connection, the right air-turbine starter and the cross-bleed valve of the ACS system. The right bleed loop senses leaks on the right side of the intermediate-pressure bleed-air system.

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Left Pylon Loop (Dual)

The left pylon loop is installed in the aft equipment compartment. For more leak detection, the left pylon loop has two loop elements. The routing of the left pylon loop is from the high-pressure port of the left engine to the left wing anti-ice valve (WAIV).The routing of the left pylon loop also includes the left high-pressure valve (HPV). The left pylon loop senses leaks in the left high-pressure bleed-air system.

Right Pylon Loop (Dual)

The right pylon loop is installed in the aft equipment compartment. For more leak detection, the right pylon loop has two loop elements. The routing of the right pylon loop is from the high-pressure port of the right engine to the right WAIV. The routing of the right pylon loop also includes the right HPV. The right pylon loop senses leaks in the right high-pressure bleed-air system.

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The trim air loop is installed in the aft equipment compartment below the baggage compartment. The routing of the trim air loop is from the upper bleed duct to the upper precooler inlet-duct. The routing of the trim air loop is also from the pack outlet duct to the flight compartment trim duct. It also includes the mid-trim discharge duct, forward-trim discharge duct and trim split duct. The trim air loop senses leaks in the trim air supply of the temperature control and indication system.

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The pack loop is installed in the aft equipment compartment. The pack loop has two loop elements. The routing of the pack loop is from the lower bleed duct of the ozone converter and FCV No. 1 to the pack inlet duct. The pack loop also includes the lower precooler inlet duct. The pack loop senses leaks from the air supply to the ACS.

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Left Anti-ice Fuselage Loop

The left anti-ice fuselage loop is installed in the aft equipment compartment, the left underwing and the overwing fairings. The routing of the left anti-ice loop is from the left HPV to the wing root duct. The routing of the left anti-ice loop also includes a part of the wing cross-bleed valve (CBV). For more leak detection, the left anti-ice fuselage loop has two loop elements. The left anti-ice fuselage loop senses leaks from the duct of the left HPV to the wing root duct.

Right Anti-ice Fuselage Loop

The right anti-ice fuselage loop is installed in the aft equipment compartment, the right underwing and the overwing fairings. The routing of the right anti-ice fuselage loop is from the right HPV to the wing root duct. The routing of the right anti-ice loop also includes a part of the wing CBV. For more leak detection, the right anti-ice fuselage loop has two loop elements. The right anti-ice fuselage loop senses leaks from the duct of the right HPV to the wing root duct.

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The wing anti-ice loop is installed in the left and right leading edge of each wing. The routing of the wing anti-ice loop is from the left and right wing root to the left and right wingtip. For more leak detection, the wing anti-ice loop has two loop elements. The wing anti-ice loop senses leaks from the piccolo ducts and their related slip joints which are installed from the wing roots to the wingtips.

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There are two integrated air system controllers, IAS No. 1 and IAS No. 2 used in the bleed-air leak detection system. The two IAS controllers are the same and divided into two channels. The two channels are channel A and channel B. The two IAS controllers only use channel B for the bleed-air leak-detection system.

The two IAS controllers continuously monitor the resistances of the loop elements. If there is a bleed-air leak, there will be a change in the related loop elements resistance. The related IAS controller measures the change of resistance and sends fault data through an ARINC 429 data bus to the MDC and EICAS. The IAS controllers cause a red warning message to show on the EICAS. At the same time, the environmental control system (ECS) or ANTI-ICE synoptic page shows the area of the bleed-air leak.

With this data, the flight crew can make the correct selection on the applicable control panel to isolate the bleed-air leak. The flight crew will use the AIR COND/BLEED or ANTI-ICE control panel (center pedestal) to isolate the bleed-air leak. The IAS controllers also have an event location function. The event location function of the IAS controllers records bleed-air leaks that occur in flight. On the ground, maintenance persons can get this data with the MDC of the maintenance diagnostic system (MDS). This bleed-air leak data will help find and repair the problem.

Each loop element has two conductors. One of the conductors is the outer conductor and the other one is the inner conductor. Each of these conductors has one input and one output to each IAS controller. Thus, there is a total of four electrical interfaces with each IAS controller.

The resistance of the inner conductor of each loop element is calibrated. When there is a bleed-air leak, the IAS controller measures the resistance between the end of the loop element and the area of the leak. The IAS controller uses the ratio of resistances with and without a bleed-air leak to find the approximate location of the leak.

There is an insulation material between the inner and outer conductors of each bleed loop element. The insulation material is salt which has almost no resistance. When there is a leak in the duct, the hot air will hit the bleed leak element. The hot air will melt the salt and the salt becomes an electrical conductor. This changes the electrical current in the inner and outer conductors of the bleed leak element. The IAS controller reads the resistance values and calculates an approximate location of a leak.

If one of the two conductors breaks in flight, the bleed-air leak detection system stays serviceable. This is because the IAS controllers measure the resistances of the inputs and outputs of the two conductors. But if the two conductors break in flight, the related area will be unserviceable because the bleed-air leak detection system cannot sense two broken conductors. The subsequent power-up built-in test (PBIT) of the IAS controllers will find the unserviceable loop element.

Wing Anti-ice Valve Auto-Close

The IAS controllers will automatically close the two WAIVs when there is a bleed-air leak in the wing ducts downstream of the HPVs. This isolates the area, stops the leak and causes a WING ANTI-ICE LEAK warning message to show on the EICAS. The flight crew will use the wing-related functions of the ANTI-ICE control panel to close the HPV for the left or right engine. This will find the area of the leak. This check from the flight crew is necessary to make sure that the bleed-air leak is between the HPV and the WAIV.

The IAS controllers cannot automatically stop a leak in the area between the HPV and the WAIV. When the leak is no longer sensed, the WING ANTI-ICE LEAK warning message changes to L(R) WING ANTI-ICE FAIL caution message.

A bleed-air leak can occur in the pylon which is upstream of the HPVs. In this case, a L (R) PYLON LEAK warning message will show on the EICAS. Then the flight crew will have to decrease engine power and if the leak continues, do a shutdown of the related engine.

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The L (R) BLEED LEAK warning message shows if there is a short circuit in the loop element of the left (right) bleed loop.

The PACK LEAK warning message shows when the conditions that follow occur:

• An IAS controller senses a hot air leak from the bleed-air ducts that go to the pack of the ACS
• There is a bleed-air leak from the ducts that are installed from the left FCV to the pre-cooler cross-over valve and to the pack inlet.

The L (R) PYLON BLEED LEAK warning message shows when an IAS controller senses a hot air leak from the HPV or bleed-air ducts of the left (right) pylon.

The TRIM AIR LEAK warning message shows when the conditions that follow occur:

• There is a short circuit in the loop element of the trim air loop
• There is a bleed-air leak from the ducts that are installed from the right FCV to the pre-cooler cross-over valve and to the hot air check-valve (HACKV).

The WING ANTI-ICE LEAK warning message shows when the conditions that follow occur:

• There is a bleed-air leak at the outlet of the left or right HPV
• There is a bleed-air leak at the outlet of the left or right WAIV
• There is a bleed-air leak from the damaged body of the left or right WAIV
• There is a bleed-air leak from the damaged body of the wing CBV
• There is a bleed-air leak from the ducts that are installed from the WAIV to the wing root on the left or right side
• There is a bleed-air leak from the wing anti-ice ducts between the HPV and the WAIV on the left or right side
• An IAS controller senses a hot air leak from the bleed-air ducts (piccolo ducts) in the leading-edge area of the wings.

The L BLEED LOOP FAIL caution message shows when the conditions that follow occur:

• The loop element of the left or right bleed loop is disconnected
• The trim loop is disconnected
• The auto power supply and supervisor of IAS controller No. 1, channel B are not serviceable
• The CPU of IAS controller No. 1, channel B is not serviceable
• The ARINC 429 interface of IAS controller No. 1, channel B is not serviceable
• The bleed-air leak detection function of IAS controller No. 1, channel B is not serviceable
• The IAS controller No. 1, channel B cannot get bleed-air leak data from a loop element of the left bleed loop
• The bleed-air leak detection function of IAS controller No. 2, channel B is not serviceable
• The IAS controller No. 2, channel B cannot get bleed-air leak data from a loop element of the left bleed loop.

The R BLEED LOOP FAIL caution message shows when the conditions that follow occur:

• The auto power supply and supervisor of IAS controller No. 2, channel B are not serviceable
• The CPU of IAS controller No. 2, channel B is not serviceable
• The IAS controller No. 2, channel B cannot get analog data

The PACK LOOP FAIL caution message shows when the loop elements of the pack loop are disconnected.

The L (R) PYLON LOOP FAIL advisory message shows when the loop elements in the left (right) pylon loop are broken and cause an open-circuit.

The TRIM AIR LOOP FAIL caution message shows when the conditions that follow occur:

• The loop element of the trim air loop is disconnected
• The auto power supply and supervisor of IAS controller No. 2, channel B are not serviceable
• The central processor unit (CPU) of IAS controller No. 2, channel B is not serviceable
• The central processor unit (CPU) of IAS controller No. 2, channel B is not serviceable
• The IAS controller No. 2, channel B cannot get analog data
• The bleed-air leak detection function of IAS controller No. 2, channel B is not serviceable
• The IAS controller No. 2, channel B cannot get bleed-air leak data from a loop element of the trim air loop.

The WING A/ICE LOOP FAIL advisory message shows when the two channels of the wing anti-ice loop are unserviceable.

The L (R) WING ANTI-ICE FAIL caution message will show when a low temperature condition is sensed by the left (right) leading-edge temperature sensor.

The BLEED LOOP FAULT advisory message shows when the conditions that follow occur:

• The loop elements of the left or right pylon loop are disconnected
• There is a short circuit in the loop elements of the left or right pylon loop
• When one of the two loop elements of the pack loop at the pack inlet duct is not connected
• When there is a short circuit in one of the two loop elements of the pack loop at the pack inlet duct
• The loop elements from the left pylon to the left wing are disconnected
• There is a short circuit in the loop elements from the left pylon to the left wing
• The loop elements for the left or right wing anti-ice loop are disconnected
• There is a short circuit in the loop elements of the left or right wing anti-ice loop
• There is a short circuit in the loop element of the trim air loop
• The loop elements from the right pylon to the right wing are disconnected
• There is a short circuit in the loop elements from the right pylon to the right wing
• The auto power supply and supervisor of IAS controller No. 1, channel B are not serviceable
• The CPU of IAS controller No. 1, channel B is not serviceable
• The ARINC 429 interface of IAS controller No. 1, channel B is not serviceable
• The bleed-air leak detection function of IAS controller No. 1, channel B is not serviceable
• The IAS controller No. 1, channel B cannot get bleed-air leak data from a loop element of the left bleed loop
• The auto power supply and supervisor of IAS controller No. 2, channel B are not serviceable
• The CPU of IAS controller No. 2, channel B is not serviceable
• The IAS controller No. 2, channel B cannot get analog data
• The bleed-air leak detection function of IAS controller No. 2, channel B is not serviceable
• The IAS controller No. 2, channel B cannot get bleed-air leak data from a loop element of the left bleed loop
• The IAS controller No. 2, channel B cannot get bleed-air leak data from a loop element of the trim air loop

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The bleed-air leak-detection system has interfaces with the systems/components that follow:

• Integrated Air System (IAS) Controller
• Air Conditioning System (ACS)
• Temperature Control and Indication
• Anti-Ice System
• Wing Anti-Ice System
• Engine Indication and Crew Alerting System (EICAS)
• Intermediate-Pressure Bleed-Air System
• High-Pressure Bleed-Air System
• Maintenance Diagnostic Computer (MDC)

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Power-Up Built-In Test

When electrical power is first applied to the aircraft after each landing, the two IAS controllers do a power-up built-in test (PBIT) The PBIT makes sure that there is continuity in the inner conductor of each loop element.

If the PBIT finds the inner conductor of a loop element broken, the loop element is unserviceable. If the inner conductor of a loop element is broken in flight, the loop element stays serviceable until there are two areas that are broken.

In flight, the IAS controllers cannot sense two broken areas in a loop element. The loop element that was unserviceable in flight only shows in the subsequent PBIT when the aircraft is on the ground.

One Loop Element System

If the loop element in a one loop element system is not serviceable, the PBIT causes the related IAS controller to show an amber caution message on the EICAS. In a one loop element system, it is only the one loop element that does the bleed air leak detection.

Two Loop Element System

If the two loop elements in a two loop element system are serviceable, the PBIT uses an AND logic. For a successful PBIT, the two loop elements must be serviceable. This AND logic prevents incorrect warnings because the two loop elements must sense the bleed air leak together to make the warning correct.

If one of the two loop elements in a two-loop element system is not serviceable, the PBIT uses an OR logic. The PBIT causes the related IAS controller to show an amber caution message on the EICAS. The OR logic only lets the serviceable loop element do the bleed air leak detection. This OR logic gives more dispatch reliability.

Continuous Built-In Test

In flight and on the ground, the two IAS controllers continuously do a continuous built-in test (CBIT) The CBIT examines the bleed air leak detection functions of channel B in the two IAS controllers. The CBIT also examines the internal components of the two IAS controllers.

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The two IAS controllers of the bleed-air leak-detection system continuously monitor the resistances of the loop elements.

When there is a bleed-air leak, the resistance of the related loop element decreases and sensed by the related IAS controller. The two IAS controllers also continuously monitor the function of channel B and the IAS controller's internal components.

In the environmental control system, the status of the valves is shown on the ECS synoptic page. In the wing anti-ice system, the status of the valves is shown on the ANTI-ICE synoptic page. The valves have a white contour with flow lines that move and change color when the valves open or close. The ducts show as flow lines and the flow line color is usually the same as the adjacent flow line.

The flow line's colors are as follows:

• The color red shows an unserviceable status.
• The color amber shows a caution.
• The color white shows a usual status with no airflow.
• The color green shows a usual status with airflow.
• The color magenta shows an invalid or unknown status.

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