05/06/21
Message Overview:
Message Description:
THE TRIM AIR LOOP is detected shorted to ground by the IASC-2 during the system self-test. Check the loop and wiring isolation to ground.
Possible Causes:
- Trim Loop Sensor 1 (MT61)
- Trim Loop Sensor 2 (MT36)
- Trim Loop Sensor 3 (MT69)
- Trim Loop Sensor 4 (MT124)
- Associated Wiring
Troubleshooting Tips:
Advisory Wire/Service Bulletin: None
Forum Articles/Infoservice/Newsletter: None
Bleed Loop Routing and Visual Aid Guide
Troubleshooting Guide for Bleed Loops
NOTE: If the bleed loop resistance value is within limits try cleaning the bleed loop connections per SPM 51-25-00-110-801 and then reapply lubrication to the connections per SPM 20-20-00. Lubrication part number is called out in each bleed loop installation procedure. Run the aircraft after the cleaning and applying lubing procedure and see if the BLEED LOOP FAULT clears.
Quick Links:
| Operational Test of the Bleed-Air Leak-Detection and Warning System | AMM 36-21-00-710-801 |
| Initiation Test of the Bleed-Air Leak-Detection Loops | AMM 36-21-00-740-801 |
| Resistance Check of the Bleed-Air Leak-Detection Loops | AMM 36-21-00-760-801 |
| Bleed-Air Leak-Detection System - Component Location | AMM 36-21-00-992-801 |
| Removal of the Trim-Air-Supply Leak-Detection Element | AMM 36-21-09-000-801 |
| Installation of the Trim-Air-Supply Leak-Detection Element | AMM 36-21-09-400-801 |
| Removal of the Aft Trim-Air Leak-Detection-Element | AMM 36-21-09-000-802 |
| Installation of the Aft Trim-Air Leak-Detection-Element | AMM 36-21-09-400-802 |
| Removal of the Forward Trim-Air Leak-Detection-Element | AMM 36-21-09-000-803 |
| Installation of the Forward Trim-Air Leak-Detection-Element | AMM 36-21-09-400-803 |
| Removal of the Baggage-Compartment Leak-Detection Element | AMM 36-21-09-000-804 |
| Installation of the Baggage-Compartment Leak-Detection Element | AMM 36-21-09-400-804 |
| Fenwal Safety Systems | CMM 26-14-47 |
| Solvent Cleaning | SPM 51-25-00-110-801 |
| Wiring - Maintenance Practices - ALL | SPM 20-12-00-02 |
| Wire Repair - Maintenance Practices - ALL | SPM 20-12-10-02 |
| Electrical Connectors - Maintenance Practice - ALL | SPM 20-20-00-02 |
Troubleshooting Recommendations:
- Gain access to IASC #2 located behind the lavatory sink just forward of the water pallet.
- Disconnect connector A56P4 from the IASC and check resistance between pins 9 and 15. Resistance should be approximately 32.3 ohms.
- Isolate a system short through standard practices by isolating the four* different loops and wiring from the IASC. Repair faulty wire segment or replace loop as required.
* Only the first loop is identified, the IASC and MDC can not identify which section of a multiple loop segment is at fault during a short or open condition and will only list the first loop of the segment.
Trim Air Loop Information
Trim Air Loop Schematic
NOTE: Measure Insulation resistance and line resistance of the detection loops as per the Leak Detection Elements Troubleshooting Sheet. Make sure line resistance & insulation values are within the specified limits.
NOTE: Often while troubleshooting this discrepancy the CAS is not usually present. However what you may find is higher than normal resistance such as 10 ohms or higher above nominal. It is not high enough to trip a bleed loop fault CAS message but high enough to assist in troubleshooting. When you get the resistance value down to nominal values then it is safe to say you've corrected the discrepancy.
