What Is This Switch In Avionic Bay ?

2 March 2021, 09:54

Avionic bay is a section under the cockpits of aircraft where system boxes, circuit breakers and batteries are located. Some deactivation / reactivation and troubleshooting procedures are performed from this section.

Apart from circuit breakers and boxes, there is also a switch in this compartment. Let’s take a look at the function of switch.


Due to some failures and maintenance operations, aircraft may need to fly with the landing gear down. To explain this with a few examples; with the covid-19 pandemic that has been going on since 2019, many companies have storaged some of the aircraft in their fleet.

According to the storage procedure, landing gear retraction/extension and freefall extension tests need to be performed in return-to-service applications of an aircraft that has been storaged for more than 12 weeks (It may be different depending on the aircraft types.).

CREDIT: www.businesstraveller.com

Flight With Landing Gear Down

Because of an airport that does not have enough equipment (Jacks, external hydraulic power device) to perform these tests, storaged aircraft fly to a better equipped airport with the landing gear down and the landing gear doors closed.

This procedure is applied since it cannot be determined whether there is a failure in the landing gear because the test cannot be performed. Also some failures that prevent the landing gear operation are examples for this situation. (eg turn-buckle link breaking)

According to the flight with landing gear down procedure in ATA Chapter 5 Special flights” tab, these ALTERNATE VMO/MMO SWITCH (Boeing) or L / G DOWN VMO / MMO SELECTION (Airbus) are set to ON position in E&E compartment.


ADIRU provides attitude,heading and navigation data for aircraft systems by sensing from pitot tubes, static ports, total air temperature probes and angle of attack sensors.

FOR B737

When the VMO/MMO Switch is set to ON position, the ADIRU adjust to Vmo/Mmo curve to compansate for a flight with the landing gear down.

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