121.5 "On Guard"

[rotatrim]

Listening to 121.5, I sometimes hear aircraft from the same company calling each other using the phrase "Callsign ABC Callsign XYZ on guard - how do you read" or something similar. What's the purpose of that?

[Crótalo]

It's possible that the aircraft being contacted has lost communications with ATC, and ATC has asked another aircraft to attempt contact on 121.5 (aka "Guard") which the lost-comm aircraft may be monitoring separately.

Using another aircraft (instead of ATC) can be helpful due to the line-of-sight limitations of VHF.

I don't know if you were specifically asking about the "on guard" part of the call, but when transmitting on 121.5, that phrase (or something similar, such as "on 121.5") is used to let others know that a) you're intentionally transmitting on that frequency, and b) to separate it from the sound of routine transmissions on "regular" frequencies (since many aircraft listen to two freqs at once -- their regular ATC comm freq, plus the emergency/guard freq.)

[Global Pilot]

I would agree. In my limited use of HF, when we were at low level and ATC/Radio had difficulty contacting us, 121.5 was used by other aircraft to relay information. We always maintained a listening watch on 121.5. The "on guard" call was to draw your attention to that freq as we may have been listening out on more than one.

[FlapsOne]

I think the answer to the question is:

VHF 121.5, UHF 243.0 and HF2828 (I think) are referred to as GUARD.

[Astra driver]

In the US the emergency frequency, 121.5 is commonly refered to as the "Guard" frequency, since we are required by the F.A.R.'s to maintain a listening watch on this freqency whenever possible.

It is quite common for ATC to loose contact with an aircraft on the asssigned freq due to range limits of the ground transmitters before the controller is able to hand the aircraft off to the next sector. When this happens ATC will usually ask another aircraft in the vicinity to relay a message usually via the assigned freq first and if that fails, via 121.5. When making such a transmission you would normally state "Calling on guard" somewhere in the transmission so that the recipient knows which freq to reply on.
Once communications have been established corrrect radio procedures dictate that you then switch to a discrete frequency so as not to tie up the emergency frequency.