Transponder code question.

[M.Mouse]

If an aircraft squawking an ATC assigned code suffers, for example, a decompression and makes a Mayday call before commencing a rapid descent I find that many airlines' policy appears to be to routinely change the squawk to 7700.

I clearly remember from years ago that this was not the preferred procedure unless specifically requested to do so by ATC.

Has that policy changed or is my memory wrong?

What is the current ATC expectation and if possible can someone supply a definitive reference?

I am generally considering European airspace but would be interested in policy elsewhere in the world.

[Sam Asama]

Generally speaking it`s better to switch to 7700 (workload permitting), as it makes clear to ALL controllers (not just the one you`re talking to) that there is an emergency in that sector.

If you can picture what a busy ATC sector or facility can be like, this fact alone means other controllers or sectors will be less apt to bother the concerned controller with other items that can wait. Additionally, if the controller did not hear your `Mayday`or `declaring an emergency`for any reason (other traffic talking over you perhaps) he or she will certainly see the immediate and obvious 7700.

The controller will let you know if and when the 7700 is not longer necessary for ATC purposes.

[jackieofalltrades]

I was under the impression now that the vast majority of ATC systems would now 'hold' and display the previous code along with the emergency 75/76/77 codes?

Essentially, yes they do. The system remembers the ident of the aircraft before it switched to the 75/76/7700 code and will still display it. However, there is now the added bonus for controllers in that the emergency aircraft will show through all filters and thus those working other sectors will immediately know you're dealing with such.

[The Many Tentacles]

To second what Jackie says, at London en-route and terminal any emergency code will cut through any height filters selected on the radar which makes it much easier to coordinate with any controller underneath your sector

[Talkdownman]

If there is doubt then there is no doubt, squawk special purpose codes.

[M.Mouse]

Thank you for the informative replies.

Clearly the response of the software to a special purpose code makes the situation much clearer and obviously desireable.

[BwatchGRUNT]

squawk emergency, whatever that may be 77/76/75 ASAP - simple as that in the UK at least