The main landing gear (MLG) overheat detection system is a single-loop overheat detection system. The loop is connected to the FIREX control unit (FIREX-CU). Each main wheel well (MWW) has one overheat detection element connected in series with each other and the FIREX-CU. The loop is electrically connected to the FIREX-CU. This makes the loop single-fault tolerant against open circuits. The detection element is attached to the aircraft with clamps. The ends of the sensing element have two terminal lugs that are attached to a bracket. Also, the bracket is grounded to the aircraft structure by bonding jumpers.

The MLG overheat detection system has an interface with the SYSTEMS TEST control panel. The main landing gear (MLG) overheat detection system has a main landing gear (MLG) sensing element.

The MLG overheat detection elements are in a circle near the main landing gear brakes at the top of the MLG wheel well. The routing of the two terminal lug ends is made through a hole in the wheel well for the protection of the wire connections. It senses the heat from a fire on the tire or on the brakes.

The alarm trip temperature (169 °C [336.2 °F]) is given when approximately 100 °C (212.0 °F) is added to the maximum temperature of operation (80 °C [176.0 °F]) to which the sensing elements are exposed. The 100 °C (212.0 °F) value decreases the risk of incorrect or irregular alarms from unusually high temperatures.

The alarm temperature value is in relation to the sensing element length, core material and the properties of other elements in the loop. The alarm condition goes out when the temperature decreases to <147 °C (296.6 °F).

The FIREX-CU receives 28 VDC power from the L ESS BUS and R ESS BUS for detection and monitoring functions. The FIREX-CU discrete signals and ARINC 429 bus signals interface with the data concentrator unit (DCU) for alarm and status reporting functions.

The FIREX-CU continuously monitors the resistance to ground of the MLG loop. In overheat conditions, the loop resistance decreases and supplies a loop overheat indication at the FIREX-CU. The FIREX-CU includes short-circuit discriminator circuits to identify the difference between loop overheat and loop failures.

The FIREX-CU monitors the MLG overheat detection system in single-loop logic mode. In this mode, only one fire signal from one of the MLG elements (left or right) is necessary to give an overheat alarm. If MWW overheat conditions are met on the loop, the FIREX-CU transmits the applicable overheat warning to the EICAS through the ARINC 429 and hardwired discrete. Faults on the detector loops are identified by the FIREX-CU with indications supplied to the EICAS through the data concentrator unit (DCU).

The GEAR BAY OVHT warning message will show when an overheat condition has been sensed in the main wheel well (MWW).

The GEAR BAY DET FAIL caution message will show when the MWW overheat detection has failed.

The EICAS messages that follow are related to the MLG overheat-detection system:

A test of the fire detection system can be done with the rotary test switch on the SYSTEMS TEST control-panel set to the FIRE DET position. When the test switch is momentarily pushed (approximately 1 second), it starts a test of the entire fire, overheat and smoke detection system.