QNH at level TA

[xfeed]

Hello everyone,

In the London TMA the transition altitude is 6,000'. Most of the departures out of EGLL have you climbing to TA and you are not to climb above this until instructed by ATC. My question is this: Do you remain at QNH, since you have not been cleared above the TA or do you select STD 1013 when nearing/reaching 6,000'? My main confusion about this is that at some pressure settings you could be several hundred feet higher than 6,000' once you set the altimeter to STD. I also know different airlines have different SOPs regarding when to set STD.

I appreciate any information you all may have.

Cheers,

xfeed

[Capt Claret]

Surely if cleared to TA one must remain on QNH, for the very reason you state. To select STD Pressure could result in a large altimetry error.

Operating in the tropics, I've seen folk select standard pressure early when in the vicinity of cyclonic conditions, and not seem to notice that they can be winding the altimeter up 500' or so! :eek:

[rodthesod]

Xfeed,

Hi, I agree with Capt Claret. Normally you change to 1013.2 when cleared to a FL above Transition and that change normally occurs when close to TA - depends on your Company SOP. CAA used to advocate changing in climb immediately when cleared to FL, but several years ago reverted to 'within 2000ft of TA' iirc. Check out the following thread for more info:
Changing altimeter setting (http://www.pprune.org/forums/showthread.php?t=230685&highlight=changing+altimeter)

rts

[issi noho]

If you look at the definition of TA it is the altitude at or below which your vertical position of an aircraft is controlled by reference to altitude.

So if cleared to TA you remain QNH; once cleared above you should set std. You may set std as soon as the clearance to a FL is received (some company SOP's may differ but the law says you may). In decent you again can set qnh as soon as a clearance to an altitude is received. If ATC require FL passing reports you can delay changing your subscale.

[xfeed]

Thanks everyone for your very clear answers. Makes perfect sense to me.

Cheers!