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For civil acft like B737-800 or B777-200, where is the location of transponder?

I'm asking about the "responder", not the cockpit device.

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For civil acft like B737-800 or B777-200, where is the location of transponder?

Electronic equipment is distributed at several locations known as electrical compartments, avionics bays, or other similar names, where elements are interconnected and cooled. The different pieces of equipment (line replaceable units / LRU in engineering parlance) are then connected to the cockpit, or to other locations, using the avionics network.

LRU are placed in standard racks. In general these compartments can only be accessed while on the ground through a door at the bottom of the fuselage.

On the B737 NG (-600/-700/-800/-900) there are several racks at the level of the cargo bays, a bit aft of the nose wheel gear:

enter image description here

Source: B737 NG, General Familiarization Manual.

Boeing uses ATC to refer to the transponder. There are two ATC electronic units, both in rack E1. Layout of rack E1:

enter image description here

Source: B737 NG, General Familiarization Manual.

Rack E1 on a B737-800 (without the top shelf):

enter image description here

Source: Airliners.net, photo by Petr Volek

A transponder looks like this (Collins TPR-901):

enter image description here

Source: Collins Aerospace.

There are two connectors on the back panel for the top and bottom antennas (ATC antennas location), and two connectors for the network. This model can be seen on this other avionics bay, for a B737-700 (sorry for the terrible quality):

enter image description here

Source: digi.no.


More: How are electronic systems locked and connected in the electronic bay?

Youtube:

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    $\begingroup$ Suggest adding the location of the transponder antennae. Usually on the bottom of the aircraft? $\endgroup$
    – Terry
    Commented Jul 29, 2023 at 21:53
  • $\begingroup$ @Terry: Good suggestion, thanks. I added a link to an existing answer. $\endgroup$
    – mins
    Commented Jul 29, 2023 at 23:24

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