Operational use of Primary and Secondary Channel
 

Hello, I am new to this forum, I am an engineer working for a project to support VHF comms for aircrafts. I would like to clarify the operational use of primary and secondary VHF channels. I understand that the primary channel is used for ATC, what is the usage of the secondary? Only as a backup?
Thanks to anyone willing to answer

In my company, and most I’ve flown in, 121.5 unless operationally required to be on another frequency. This would normally only be for Parallel runway Approaches, calls to the company/ground handlers, and in Australia, we have some airports without ATC, so then we use the 2nd VHF for the local traffic frequency (CTAF).

Hope that helps

I use mine to tune into ATIS broadcasts or to contact a second enroute airfield where necessary. I’ve also used it to contact handling agents.

I would think most others do similar.

VHF 1 is usually used for "primary" communications with ATC. On many aircraft, the VHF 1 box is kept alive - for example by battery - when the aircraft electrical system is compromised. So it makes sense to use VHF 1 for primary ATC comms rather than having to switch boxes and frequencies in the event of electrical failure.

VHF 2 is used as a back-up for VHF 1 in the event of failure, and also for "house keeping" functions such as talking to handling agents, obtaining weather, talking to company operations, and in some cases - for example Gibralter - contacting the destination ATC to obtain approach clearance, while VHF 1 is tuned to an en-route ATC agency.

Thank you Denti, (below). I forgot to mention monitoring guard on VHF 2 Oops!

Pretty much the same around the industry i guess.

VHF 1 is for ATC, VHF 2 usually kept on guard listening watch, and can be used operationally for weather, ops and the like if those are not reachable via datalink. VHF 3 is on datalink and not usually used for voice comm.

Also in many parts of the world box 2 is needed for inflight broadcast procedures and negotiating entry at FIR boundaries, so ATC functions in addition to monitoring guard and house keeping stuff.