The MLG overheat-detection system is a single loop system, with one loop in each MLG wheel well. Each loop has one sensing element. The control unit monitors the resistance of each loop. If it decreases below the preset value for that zone, the control unit causes the applicable indications in the flight compartment.

The function of the main landing gear (MLG) overheat-detection system is to give a warning in the flight compartment if the MLG wheel well is too hot.

The MLG bay overheat detection system does not use dual loops of fire sensing elements. There is a single loop sensing element in each main wheel well, connected independently to the FIDEEX through airplane wiring harness.

An overheat sensed at either sensing element results in the (common) wheel well overheat warnings being triggered. The warnings are similar to those of the other systems. The trip point of the sensing element in each wheel well is 177 °C (350 °F). The resistance between the inner wire of the sensing element and the outer casing decreases as the temperature increases.

When the resistance is at a preset value the control unit sends a signal to the data acquisition unit (DAU). The DAU causes the engine indicating and crew alerting system (EICAS) to show the warning, MLG BAY OVHT. There is also an aural (voice message) warning.

A short in one of the wheel well sensing elements renders it inoperative. That sensing element alone is isolated by the FIDEEX, and a CAS advisory (cyan) message reading MLG BAY OVHT FAULT is displayed. The other sensing element is still maintained in the detection circuit and is capable of providing a warning concerning the associated wheel well only.

If both sensing elements are shorted, the amber CAS caution message reading MLG BAY OVHT FAIL will be displayed, and the MLG detection system will be inoperative.

An open sensing element circuit is not expected to affect the detection capability of the system. Depending on various factors, continued flight with an MLG BAY FAULT or FAIL message is permitted (with certain restrictions) as per the MMEL.

The following common points must be observed during maintenance/troubleshooting with respect to the engine, APU and wheel well fire detection systems:

• Particular care should be taken during inspection of firewire loops to ensure that the insulating medium does not seep out of the wire, nor should foreign matter be allowed to contaminate the medium, since the trip points of the elements can become downgraded in either of these conditions 
  
  
• The element at each location is specially tailored to maintain a specific resistance value to ensure that its trip point is within acceptable limits. Therefore, replacement wires must match the specification for the particular location where it is installed 
  
  
• The NVM of the FIDEEX CU records all significant faults for future retrieval by the airplane's CAIMS system, and applicable maintenance action as prescribed in the AMM must be undertaken as specified, within time limits imposed

Caution: 
In order to preclude the failure of firewires in service, it is important to handle all detector elements with extreme care and to clamp them properly to prevent their vibration or chafing with other components.

The various warnings and the associated detection systems can be function-tested using either one of the two EMS CDUs (Electrical Management System Control Display Units) in the cockpit, by first selecting FIRE PROTECTION System and then TEST.

Note that the sensor elements are not physically heated to verify their operation during this test. Instead, a resistance value (equivalent to the trip value of the detectors) is used by the control unit to simulate a "trip" of the loops. However, a check for continuity in the core wires, and for an absence of shorts in the elements, along with an integrity check of the entire circuits is performed by the FIDEEX CU during this test.

Serviceability of each system is confirmed if the related CAS and aural warnings, as well as the red lamps in the three fire handles, are activated during the test.