Clarrification

[jau]

Hiya,
Looking through my air law book, a bit confused about altimeter setting proceedures, would someone mind just setting me straight?

Above 3000ft on a VFR flight you can set the standard setting of 1013, and follow the quadrangle rule. What happens though, when passing under, say, LTMA who's base is 4500ft. Would you have to reset the altimeter to regional to avoid infringing the airspace? Do VFR flights above 3000 just use regional QNH, changing the setting when crossing an ASR or do they normaly use standard?

Thanks for the help.

[bookworm]

The base of the London TMA is defined by the London QNH (not the regional QNH). The transition altitude within the extent of the London TMA is 6000 ft, so you should definitely fly on the London QNH when below it.

[Warped Factor]

Or you can fly on the nearest appropriate airfield's QNH, eg Luton.

Just don't use the regional, being the lowest forecast QNH for the region it may differ significantly from the actual and you could end up flying above the base of CAS rather than below it.

WF.

[jau]

Thanks for that guys!

[bookworm]

Or you can fly on the nearest appropriate airfield's QNH, eg Luton.

I've never thought hard about LTMA airports and altimeter setting. Do Luton, Essex, Gatwick etc. approach control use a common (presumably Heathrow) QNH, switching inbounds to the local QNH at some point of the approach? It's only a millibar or so, I would imagine, but there must be differences.

[Warped Factor]

The London (Heathrow) QNH is the one input in to the radar processors and is the one that drives what level readouts are seen on the radar displays for traffic operating below the TA.

However all the various approach units use their own QNH for traffic they are working at altitudes in their areas of responsibility.

The MATS Pt 2 states that any differences in the various QNHs used are deemed to be insignificant.

WF.

[dpm]

The base of the London TMA is defined by the London QNH (not the regional QNH). The transition altitude within the extent of the London TMA is 6000 ft, so you should definitely fly on the London QNH when below it.

Sorry, I know I should know this, but how do I find the transition altitude when it differs from the usual 3000ft? The fact that the TMA base is stated as QNH is obviously a giveaway, but is there anything else? (I don't have a map in front of me so apologies is this is a dumb question.)

Cheers, David.

[Warped Factor]

dpm,

All is made clear in the AIP, see ENR 1-7-1.

WF.