PACK LOOP FAIL
(Caution)

The amber 'PACK LOOP FAIL' caution CAS message indicates possibly one or both bleed leak detection loops, MT73 and MT141 in the pack inlet ducting have failed during the power up self-test.

• The IASC system has detected problems with both bleed air leak loops in the pack inlet. Through ARINC bus to the DCU the EICAS has displayed the amber PACK LOOP FAIL message.

• Pack Duct Leaking
• Bleed Loop Routing
• One or both Pack Bleed Loops SHORTED/OPEN (MT73) (MT141)

Advisory Wire/Service Bulletin:

• AW300-30-0354 - Potential Piccolo Tube Damage When Replacing a Bleed Loop Element

Forum Articles/Infoservice/Newsletter: None

Troubleshooting Guide for Bleed Loops

Abbreviation Guide:

• The ducting has a insulation cover that will contain the air leaking from the ducting routing it to a insulation vent hole.
• 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.
• There are two loops along each line, IASC 1 channel B monitors Pack Loop A. IASC 2 channel B monitors Pack Loop B.
• The intermediate pressure system loop element senses a bleed-air leak when the temperature is more than 255°F (123.90°C).

If no fault is shown or the system is intermittent, do the following steps:

1. Go into the system diagnostics to the IASC #1 channel B (Pack A Loop) or IASC #2 channel B (Pack B Loop) to display the IASC Channel B menu. The IASC LRU test page will provide pages to perform a leak detection system test and check past event locations. The test will check all of the loops the IASC is monitoring and pass or fail them. This test may fail if there is a lot of action going on in the IASC, retesting may pass the failed loops. This test is disabled with the wing heat system on. Perform the test of the bleed air leak detection loops IAW AMM 36-21-00-710-801.
2. Perform Initiation Test of the Bleed-Air Leak-Detection Loops. This test will identify if the respective loop has a short or open. Per AMM 36-21-00-740-801.
3. If a loop fails, or a fault has been logged into the MDC, use the steps below to further isolate the condition.
4. Interrogate the MDC as follows:

   1. On the cursor control panel, ensure the toggle switch is to the right and select Anti-Ice, ECS and Fuel buttons simultaneously. The MDC main menu should now be displayed on Display Unit #3.
   2. Using the joystick, select Current Faults (if the fault is active) or Aircraft History to research stored faults. From Aircraft History you can then select Fault Message History and locate the fault. Going into advanced diagnostics will provide an equation id for the fault identified.
   3. Select the MDC fault or equation from the drop down below to provide additional information to assist in troubleshooting.
      Select an MDC Fault Code/Message:B3-007407 - SHORT CIRCUIT [PACK LOOP A]B3-007408 - OPEN CIRCUIT [PACK LOOP A]B3-008031 - SHORT CIRCUIT [PACK LOOP B]B3-008032 - OPEN CIRCUIT [PACK LOOP B]
   4. Determine which loop is posting the B3, then use a Tegam meter to check capacitance and inductance between the inner and outer conductor of the corresponding loop. Perform the impedance check per the Leak Detection How To.
   5. Pack channel A bleed loop information.
   6. Pack channel B bleed loop information.
   7. The test will identify if the pack loop has a short or open (greater than 26Ω). Perform resistance checks on the applicable loop per AMM 36-21-00-760-801.
      NOTE: Before replacing a loop always look at the loop for routing, ensure loop is not too close to the plumbing or the bleed hole on a valve. Review AMM 36-21-00-992-801 for component location. See individual loops for specific routing information.
   8. If the one or both of the pack LDEs do not meet the resistance checks then replace the LDE.
      

      LOOP	VALUE (OHMS)
      Pack	26Ω