When is 1013 not 1013??

[Pheasant Plucker]

Answer: when it is 1008mb or 1009mb:hmm:

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:eek:

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??

[Gulfstreamaviator]

Would this actually be a GPE, or if as stated within 200ft, be assumed to be an acceptable level.

The intentional or unintentional wrong setting of an altimeter is not acceptable.

How would this affect TCAS.???

glf

[763 jock]

From experience, passing NAT traffic shows on TCAS as separated by whole thousands of feet. On the 767 we have three altimeters, the two mains are set to 1013 and the standby is usually a few hundred feet off even when that is set to 1013. Some operators may set regional/destination QNH on the standby, according to their respective SOP.

I'm not sure what data you are receiving, but crews are not deliberately flying the NAT with anything other that 1013 set. BTW, the Strategic Lateral Offset Procedure is not "unofficial".

[10W]

I'm not sure what data you are receiving, but crews are not deliberately flying the NAT with anything other that 1013 set. BTW, the Strategic Lateral Offset Procedure is not "unofficial".

It is Mode S data, which is downlinking the Barometric Pressure Setting from the aircraft.

The problem seems to have been identifed as concerning Fokker and Airbus aircraft. For these aircraft, once the aircraft is above the Transition Level, the value being sent is the last selected pressure setting used before the Transition Level was passed. In some cases, depending on the pilots altimeter setting procedure, this might be the airfield QNH and thus it will stick at this value until the aircraft descends below the Transition Level again.

A quick look at the radar a few moments ago confirms this. Airbuses have a variety of settings, whereas Boeings, Embraers, and others all have 1013 displayed.

[Pheasant Plucker]

Ok...didn't mean to imply that the 'SLOP' was unofficial!

Thanks 10W, thought I'd uncovered a conspiracy:)

[763 jock]

Just to add to the confusion, it could also depend what transponder is selected and which autopilot is in use by the crew. My knowledge is a bit sketchy, but I believe the No1 Transponder usually sends LHS Air Data and No2 sends RHS Air Data.

When will we be able to see what you lot on the ground are up to?:cool:

[sycamore]

I thought the Txpdr alt was fixed at a datum of 1013, from a separate source ,and independent of whatever the crew altimeters were set to..

[763 jock]

Sycamore.

I think you are right.

But these guys can also now see what our sub-scale is set to. I assume they can only see the sub-scale for the ADC that is linked to the selected XPDR. On my type, L ADC talks to the No1 XPDR and R ADC talks to the No2.

[Admiral346]

In the Airbus, you push (if I remember correctly) the Alt Set knob to switch to 1013.
When switching back to QNH, you pull and it'll display the last QNH set (usually the one from your Origin). Then you'd turn it to the new QNH.

On the CRJ, you have a "push for STD" function also, however there is no "pull", you simply turn from 1013 to set the new local QNH.

So without knowing for sure, it seems Airbus "remembers" the QNH, or has 2 Alt settings, one STD, one QNH running through it's guts.
And the (much simpler) CRJ just has one, which happens to be 1013 when flying levels.

Nic

[wiggy]

Admiral

FWIW the later Boeings have a similar selection for STD vs. QNH. It might well be that Pheasant Plucker is seeing the "old" QNH on his gizmo.

In 20+ years of flying the tracks I've never heard of an unofficial vertical offset procedure.

[411A]

I thought the Txpdr alt was fixed at a datum of 1013, from a separate source ,and independent of whatever the crew altimeters were set to..
It most certainly is on the Boeing, Grumman, and Lockheed aircraft that I've flown...and has been for a very long time.
Dunno about Airboos.

[de facto]

Standard is 29.92 not 1013.:E

[L337]

standard is 29.92 or 1013.2

[Slasher]

1013.25 heck o' pascals

[Admiral346]

I thought the Txpdr alt was fixed at a datum of 1013, from a separate source ,and independent of whatever the crew altimeters were set to..

I think some confusion is coming up.

Of course, any aircraft will send the alt signal (mode C) in relation to standart alt setting.

Here we are talking about mode S, and what a controler can see when interrogating the setting. That obviously differs on some aircraft from the 1013.

Nic

[Admiral346]

In 20+ years of flying the tracks I've never heard of an unofficial vertical offset procedure.

There is none, and I would believe it were dangerous to knowingly reduce seperation.

The only time you do not fly your level is in case of emergency, offsetting 30nm and flying some level +-500 ft.

Nic

[EXWOK]

Ummmm.............30NM??

You sure about that?

(I'm presuming you're talking about NAT procedures).

[IvorBiggun]

15 miles these days.

Do people read updated manuals?

And at BA absolutely no way any sort of altimeter "offset".

1013 set on the mains and 1013 on the standby above MSA and Transition (on our fleet anyway), and reset to QNH during approach briefing.