Early transition to QNH vs TCAS

[Chilli Monster]

Originally Posted by Alexban

during descent ,it's not so important what pressure you've selected

No, it's not - until you HIT the mountain when you've got low pressure met conditions

Basic Altimetry 101!

[alexban]

chilli ,low pressure...how low? in a mountain area..you are enroute,cleared to descent to alt X which has a minimum 2000' obstacle clearance,am I right?
And, the moment you'll go bellow actual cleared altitude ,forgetting to change from STD to QNH, the ATC will advise you about that (their computer correct automatically for the QNH)..of course,you'll say now that in a remote,3rd world country....Then I'll say you have an EGPWS onboard..
Better to do all your checks correctly,and follow SOP ,and you'll be ok.
Basic CRM..

[Chilli Monster]

chilli ,low pressure...how low? in a mountain area..you are enroute,cleared to descent to alt X which has a minimum 2000' obstacle clearance,am I right?

Yes, but why erode that 2000ft safety margin by incorrect Altimeter Setting Procedures. As to how low - 2000 / 30 = 166mb - = QNH of 847mb. OK, it's never going to get that low, however, if we take a more realistic figure of 975mb (which have been possible) then your 2000ft above terrain now becomes 860ft.

If the terrain is NOT mountainous and the MSA is based on 1000ft obstacle clearance then this scenario could actually put you 140ft below the level of the obstacle.

And, the moment you'll go bellow actual cleared altitude ,forgetting to change from STD to QNH, the ATC will advise you about that (their computer correct automatically for the QNH)..of course,you'll say now that in a remote,3rd world country....

Not even third world country. Mode 'C' is not an instantaneous readout. By the time the data is interrogated and transmitted back to the radar screen you may have desended another 200ft.

Better to do all your checks correctly,and follow SOP ,and you'll be ok

True, SOP being (for the above reasons) that QNH is set as soon as you are cleared to an altitude. May I remind you of your answer which triggered this:

during descent ,it's not so important what pressure you've selected, most important it's when you level off.

Which is NOT SOP for many operators, nor is it SOP for many countries Altimeter Setting Regulations, and is the total opposite of "best practice" for the reasons I have explained.

Now do you see why your reasoning is flawed?

[alexban]

chilli, regarding your quote ,I was answering the topic starter fearing about TCAS conflicts during descent ,this is why I've said 'it's not important during descent ".