Answer: when it is 1008mb or 1009mb
Middle of the night, bored, checking out the latest gizmo on our radar display. Through the wonders of Mode 'S' we can now find out the barometric pressure that you have set in the cockpit
The eastbound oceanics were just beginning to trickle across the screen, probably a dozen that I could see, well spread out. Thought I'd see if our new toy worked. Tried the first couple, everything in order, 1013mb set. Tried another: 1008mb. Ok.. Tried yet another (same company, European): 1008mb again. Both showing level at FL370. Had a look at one more (different company, USA): 1009mb!! - though showing level at FL350.
I thought this strange - but then it dawned on me!
Is this an unofficial procedure that some crews adopt flying across the Atlantic; similar to a lateral offset, but in the vertical plain?? If so, is it used to provide a bit of randomness to their flight level, but one that doesn't upset TCAS or us controllers, by showing the aircraft level at it's cleared flight level when, in reality, it is actually within + or - 200ft of that level?
This probably isn't a big secret in the pilot community if I'm correct, but it's not something that I've come across or read about in my 20+ years as a controller. It would seem to me to be more than coincidence as 25% of the traffic that I could see didn't have 1013mb set, though I went home shortly after so didn't have time to check further aircraft.
Am I correct??
Or, of course, is it just an altimeter setting error??