Educational familiarization only. This original TechOpsBase lesson was developed from privately supplied legacy A350 training and MSG-3 material. It does not reproduce manufacturer pages, proprietary diagrams, maintenance task steps, numerical maintenance limits, dispatch criteria or controlled maintenance data. Current approved aircraft data, correct effectivity, operator procedures and authorization govern all aircraft work.
Resource profile
- Aircraft: Airbus A350 family
- ATA chapter: 36 - Pneumatic
- Resource: A350 ATA 36 Leak Detection, Automatic Isolation and Localization
- Audience: Enthusiasts, students, junior technicians and professionals
- Level: Intermediate-to-advanced
- Status: Draft pending technical review
Learning objectives
- Explain the subsystem architecture and purpose.
- Identify the major components and interfaces.
- Trace command, pneumatic flow and feedback.
- Recognize protection and failure patterns.
- Apply evidence-based maintenance reasoning.
1. Purpose and operational value
OHDS not only detects excessive local heat; it coordinates isolation attempts and gives maintenance a usable fault location.
2. Architecture and system flow
Detection logic considers loop availability and overheat status to avoid both missed detection and unnecessary isolation.
The first isolation attempt closes the valves closest to the expected leak boundary.
If the zone remains hot, a second upstream isolation attempt removes additional pressure sources.
If automatic isolation cannot secure the zone, the crew is instructed to perform manual isolation.
3. Major components and functions
Leak-confirmation logic
Combines loop condition into a valid overheat decision. It must receive the correct pneumatic or electronic input, perform the expected function and return a believable output or status. A fault can result from electrical power, data communication, a blocked or leaking pressure/sense line, pneumatic contamination, mechanical friction, incorrect configuration, a failed sensor or an upstream/downstream system request. Useful evidence includes loop availability, agreement and event sequence. A maintenance conclusion is strongest when command, feedback and physical pneumatic effect agree.
First isolation request
Closes the primary valves intended to depressurize the affected segment. It must receive the correct pneumatic or electronic input, perform the expected function and return a believable output or status. A fault can result from electrical power, data communication, a blocked or leaking pressure/sense line, pneumatic contamination, mechanical friction, incorrect configuration, a failed sensor or an upstream/downstream system request. Useful evidence includes request, valve feedback and pressure decay. A maintenance conclusion is strongest when command, feedback and physical pneumatic effect agree.
Second isolation request
Uses additional upstream valves if the first boundary fails. It must receive the correct pneumatic or electronic input, perform the expected function and return a believable output or status. A fault can result from electrical power, data communication, a blocked or leaking pressure/sense line, pneumatic contamination, mechanical friction, incorrect configuration, a failed sensor or an upstream/downstream system request. Useful evidence includes continued overheat, alternate command and wider system impact. A maintenance conclusion is strongest when command, feedback and physical pneumatic effect agree.
Crew manual action
Provides a final isolation path when automatic attempts are unsuccessful. It must receive the correct pneumatic or electronic input, perform the expected function and return a believable output or status. A fault can result from electrical power, data communication, a blocked or leaking pressure/sense line, pneumatic contamination, mechanical friction, incorrect configuration, a failed sensor or an upstream/downstream system request. Useful evidence includes warning, selected controls and resulting pressure. A maintenance conclusion is strongest when command, feedback and physical pneumatic effect agree.
CMS location data
Identifies the leak zone for targeted access and troubleshooting. It must receive the correct pneumatic or electronic input, perform the expected function and return a believable output or status. A fault can result from electrical power, data communication, a blocked or leaking pressure/sense line, pneumatic contamination, mechanical friction, incorrect configuration, a failed sensor or an upstream/downstream system request. Useful evidence includes zone code, event time, access reference and related messages. A maintenance conclusion is strongest when command, feedback and physical pneumatic effect agree.
4. Normal operating sequence
1. Detect
A valid overheat condition is recognized.
2. First isolate
The closest source/section valves close.
3. Reassess
OHDS checks whether the hot condition clears.
4. Second isolate
Additional upstream valves close if required.
5. Manual/maintenance
Crew action or targeted maintenance follows if isolation remains unsuccessful.
5. Control, monitoring and protection
A reset changes the control state but does not repair a leaking duct or damaged sensing loop.
A successful first isolation narrows the likely leak section; an unsuccessful first attempt is valuable location evidence.
The visible warning can be the final result of several automatic valve commands, so the message sequence must be preserved.
6. Failure modes and maintenance reasoning
- Leak remains after first isolation: Leak outside first boundary, failed valve or continued heat soak.
- Leak remains after second isolation: Failed isolation, alternate source or severe residual heat.
- Warning clears immediately after reset: Intermittent loop/control issue or temporarily cooled real leak.
- Wrong location: Loop wiring/allocation, installation or data-mapping issue.
- Valve closes without true leak: Erroneous sensing or control request.
7. Interfaces with other systems
- PADS and EBAS isolation valves.
- Wing ice protection and air-conditioning isolation.
- FWS/CDS.
- CMS/TSM.
- Structural access and inspection.
8. Practical maintenance scenarios
First closure removes pressure and heat falls
Use the isolated segment as the primary inspection zone.
Both loops healthy but heat persists after source closure
Consider heat soak while confirming full depressurization.
CMS points to one panel area
Use the approved troubleshooting table and inspect duct, joints, insulation and sensing-element installation.
9. Technician takeaways
- Isolation sequence is diagnostic evidence.
- Do not reset before preserving leak-location data.
- Residual heat and continuing pressure must be separated.
- Manual isolation is the last protection layer, not normal operation.
Maintenance boundary
This resource explains system architecture, normal operation, indication and maintenance reasoning. It intentionally excludes removal/installation procedures, wiring-pin checks, valve rigging, leak-test limits, servicing limits, maintenance intervals, software part numbers, dispatch decisions and release-to-service criteria.
Review prompts
- Which pneumatic source should be available in this configuration?
- Which valve or controller establishes the expected flow path?
- Which pressure, temperature or position feedback proves the command was achieved?
- Is the symptom local to one side, one source, one user or the complete manifold?
- Could a user-system demand or isolation command explain the observed state?
- Is the fault in the physical air path, the pneumatic control path or the electronic command path?
- What evidence must be preserved before reset, source change or manual override?
- What hot-air, pressure, moving-equipment and structural-protection boundary applies?
