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 Overheat Detection System Architecture and Zones
- 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 detects abnormal heating adjacent to pneumatic ducts before escaping bleed air damages structure, systems or fuel-tank boundaries.
2. Architecture and system flow
Hot-air duct zones are monitored by two independently powered and monitored sensing loops.
Each BOMU contains an overheat-detection card for one loop, while interlinks allow each monitor to consider the other loop's health.
Hosted OHDS applications provide gateways to other aircraft systems; the dedicated OHDCs perform the core detection and isolation logic.
Zone segmentation supports both automatic response and maintenance localization.
3. Major components and functions
Sensing elements
Change electrical characteristics when exposed to abnormal heat. 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 continuity, insulation, local heating, installation and zone identity. A maintenance conclusion is strongest when command, feedback and physical pneumatic effect agree.
Loop A
Provides one independent detection path. 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 BOMU/OHDC channel, continuity and fault status. A maintenance conclusion is strongest when command, feedback and physical pneumatic effect agree.
Loop B
Provides the second independent path. 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 independent monitor, continuity and comparison. A maintenance conclusion is strongest when command, feedback and physical pneumatic effect agree.
OHDC
Consolidates loop condition and initiates isolation/alert logic. 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 input, interlink, request output and fault memory. A maintenance conclusion is strongest when command, feedback and physical pneumatic effect agree.
OHDS application
Transfers detection data through the IMA network. 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 CPIOM/AFDX status and external-system communication. A maintenance conclusion is strongest when command, feedback and physical pneumatic effect agree.
4. Normal operating sequence
1. Normal monitoring
Both loops continuously monitor their assigned zones.
2. One loop unavailable
The remaining valid loop supports detection according to system logic.
3. Overheat detected
The monitors validate the zone condition and send isolation/warning requests.
4. Location recorded
Maintenance data identifies the affected segment or access zone.
5. Control, monitoring and protection
A dual-loop architecture reduces false isolation while preserving detection after one loop failure.
Loop faults and true overheat events are different maintenance conditions and must not be treated as interchangeable.
Optional aircraft installations can add detection zones; effectivity must be confirmed before interpreting a missing or unexpected loop.
6. Failure modes and maintenance reasoning
- One loop failed: Sensing element, connector, wiring or OHDC channel.
- Both loops report overheat: Probable real heat/leak condition.
- One loop reports overheat, other loop failed: System uses the remaining valid input according to protection logic.
- Zone unavailable: Common wiring/installation damage or both monitoring paths.
- Intermittent loop fault: Connector, chafing, moisture, thermal movement or sensing-element degradation.
7. Interfaces with other systems
- BOMU power and communication.
- BAS/PADS isolation valves.
- FWS/CDS.
- CMS leak-location data.
- Wing, pylon and fuselage structure.
8. Practical maintenance scenarios
Single-loop fault without heat evidence
Inspect the electrical loop and installation rather than isolating a good pneumatic source unnecessarily.
Repeated overheat at one structural location
Investigate duct joint, insulation and airflow evidence.
Optional cargo-zone message on one aircraft only
Confirm installed effectivity and loop allocation.
9. Technician takeaways
- Loop fault is not the same as duct leak.
- Zone identity is critical maintenance data.
- Both loops and their monitors are independent.
- Effectivity can change the installed detection map.
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?
