"mnm Alt 3500"

[gingernut]

I'm planning a vfr flight around the Manchester Zone.

I'm a little puzzled by the annotation "DAVENTRY CTA A FL45+ (MNM ALT 3500) on my quarter mil. map (N53 07 W001 50).

What exactly does the "MNM ALT 3500" mean ?

Excuse my ignorance, can't seem to find any clues in the map key, or my text books.

[NinjaBill]

Minimum Altitude.

If the local pressure is very low, then its possible that FL45 (flight levels are FL x 100ft on a pressure setting of 1013) could be lower than 3500ft on the local QNH.

It is saying that if the pressure is so low that the altitude of FL45 is lower than 3500ft, than the base of the airspace will not start until 3500ft. You will probably find that this is over a number of high obstructions (hills?) to enable an aircraft to maintain clear of the airspace if those conditions exist.

[gingernut]

thanks Ninja. Does that mean I have to remain below 3500ft altitude, to remain clear of controlled airspace ?

[NinjaBill]

You must remain below FL45, or below 3500ft, which ever is higher. If the local QNH is > 1013, than you can fly above 4500 ft altititude, as long as you remain below FL45. IF you have 2 altimeters in your aircraft, set one to the QNH, to give your altitude, and the other to 1013, and ensure that you do not climb above 4500ft on the altimeter set to 1013.

[Gertrude the Wombat]

Does that mean I have to remain below 3500ft altitude, to remain clear of controlled airspace ?

No, you have to remain below whichever is the higher of 3500ft or FL45 on that particular day (which will most often be FL45).

[gingernut]

cheers chaps, makes sense now