“Set standard”
 

There is a school of thought that one should set QNH during descent as soon as one is given an altitude not at transition. A number of airlines follow this procedure.

Set standard when passing transition altitude. Imagine You are departing passing 2000 ft the ATC says "Climb FL180" (no constraints) You will set 18000 ft and call "18000" blue. When passing the TA 18000 will become FL180. Now if You find some weirdo who says "Oh but ATC said to climb FL180 and the PFD says 18000" tell him there is no need to panic, to keep on breathing, and when passing TA You will call for set standard.

CORRRRECT !!

Not true, below the transition alt. entered in the perf. page the FMGS will reference altitude regardless of what is set in the baro ref. window. You can set STD whenever you like, as long as SRS is still engaged (ie. you haven't pulled open climb) the aircraft will accelerate at the correct time.

Or you could follow whatever the company SOP says, which in every airline I've worked for has been to set STD as soon as cleared to a FL.

Of course, it goes without saying.

I am assuming the reason why this question is asked is because of the relation Setting STD has to the FMA's on the PFD. To state the obvious when flying NAPD you remain in MAN FLX SRS until you have climbed through the NAPD1 altitude you have selected in the Perf page of the FMGC.

There will be some cases with a low QNH that if you were to Set STD whilst still in the NAPD 1 phase, thus climbing via SRS, that setting Standard would cause the aircraft to go from an altitude of 2,700 to FL30 straight away, which is only a QNH change of roughly 10 hPA.

If you selected 3,000 as your ACC altitude in the Perf Page to comply with NAPD 1 then by setting STD, in this case, you will have trigged the aircraft to enter CLIMB mode earlier slightly early, and thus not completing NAPD1, beginning the reduction in nose attitude and the increase in speed.

Talking about TA : "The paragraph 1.1.2.1.3 PANS-OPS, Aircraft Operations, Volume I, states that the height above the aerodrome of the transition altitude shall be as low as possible but normally not less than 900 m (3000ft)."
Does anybody have any reference of airports with TAs lower than 3000 ft AGL ? I can't seem to recall any.

Learn something new every day. I wonder what the logic is. 3000ft is pretty low, and usually quite busy.

Having everybody flying Flight Levels ensure the proper vertical separation without the need to constantly update the local QNH on the altimeter. AFAIK there is a project for a common TA in Europe like in the US but I don't know at what stage the project is.

Don't forget practicality and reality. In the UK, for example, the TA is usually around 6000ft. If you've been cleared (passing 2000ft) to FL80, why wait to set STD? Is it more likely that ATC will suddenly re-clear you to A6000ft, or is it more likely that you'd forget to do it (distraction, whatever) and never make it to FL80. If ATC re-clear you lower, that is their mistake and they should reasonably expect you to take a few seconds (reasonably more than a few) to reconfigure the FCU for the new clearance.

WRT low transition altitudes, going into AMS, you will get cleared down to FL45 and maybe even lower. It's no big deal, you just fly it. On a procedural arrival/approach, more attention must be paid to the TA and it's significance should make up part of your brief, other than that, it isn't a big issue.

Thanks sonicbum.

I must be missing something though. Why not just use the tools available? The altimeter setting will flash when you get to TA/TL. If the perf page is properly set up, why go through the mental gymnastics that some companies impose? Set your altimeter when passing TA/TL, and if you forget, the system will remind you.

Sounds simple enough. What am I missing here?

Check Airman

Sure, that's an option - for those aircraft with that feature, but not all have it and you have to cater for everyone, surely?

I go along with my (ex)operator and agree with the philosophy of changing setting when first cleared to a FL/Altitude when climbing/descending.

In 99+% of occasions the clearance will not change, so to change when cleared makes sense and to deal with the 1% as the exception.

As I said in the post above you will not trigger climb mode by setting STD before the ACC altitude, the FMGS will reference QNH altitude below the TA that has been entered in the take-off perf. page.

The OP asked about the A320, so I figured it was safe to make that assumption. In any event, you're right. Do current generation Boeings (apart from the 767) not have a feature that prompts you to change altimeter settings?