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I was idly reading SI2017/02 as recently published by the CAA when I spotted that is notification that SERA is being implemented in CAP493 (aka MATS Part 1) with effect from 24th April 2017. In particular, the wording of Section 2, Chapter 1, para 6A.2 reads:

SI2017/02 wrote:6A.2 Except for helicopters using a “Police”, “Helimed” or “Rescue” callsign, ATC shall not issue any further VFR clearances to aircraft wishing to operate in accordance with VFR to or from an aerodrome, or enter the aerodrome traffic zone, or aerodrome traffic circuit, of an aerodrome within Class D airspace when the official meteorological report at that aerodrome indicates, by day or night, a ground visibility less than 5 km and / or a cloud ceiling less than 1500 ft (SERA.5005(b)(1)&(2)).
Note 2: UK General Exemption ORS4 No. 1195 enables the pilot in command of an aircraft to transit Class D airspace in accordance with VFR by day, remaining clear of cloud with surface in sight and an indicated airspeed of 140 kt or less, with a flight visibility of 5 km or for helicopters, a flight visibility of 1500 m.

Note that ORS4 No. 1195 only provides an exemption for aircraft wishing to transit the Class D airspace outside of the ATZ and circuit pattern under SERA5005(a). It does NOT provide an exemption to aircraft wishing to take-off, land or fly within an ATZ under SERA5005(b)!

ORS4 No. 1195 wrote:1. The Civil Aviation Authority [snip] exempts any aircraft being flown within the UK at or below 3,000 feet above mean sea level and within Class D airspace from the requirements of SERA.5001 (VMC visibility and distance from cloud minima) Table S5-1 and SERA.5005(a) (visual flight rules) [snip] when it is flying in accordance with the conditions in paragraph 2.
Explanatory Notes:
1) This exemption does not include exemption from the requirements of SERA.5005(b) (Visual Flight Rules).

SERA wrote:SERA.5005 Visual Flight Rules (pages 40-41)
(a) Except when operating as a special VFR flight, VFR flights shall be conducted so that the aircraft is flown in conditions of visibility and distance from clouds equal to or greater than those specified in Table S5-1.
(b) Except when a special VFR clearance is obtained from an air traffic control unit, VFR flights shall not take off or land at an aerodrome within a control zone, or enter the aerodrome traffic zone or aerodrome traffic circuit when the reported meteorological conditions at that aerodrome are below the following minima:
(1) the ceiling is less than 450 m (1 500 ft); or
(2) the ground visibility is less than 5 km.

If I've read this correctly, it means that with effect from 24th April 2017 (i.e. about 5 weeks away), airfields whose ATZ intersects Class D airspace such as Redhill, Denham, White Waltham, Derby, Gamston, Netherthorpe, Sandtoft, Barton, and probably others I've not spotted will be unusable for take-off or landing if the cloudbase is below 1500' or the visibility on the ground is less the 5km without a Special VFR clearance, and that SVFR clearance can only come from the Class-D ATC, not the tower at the field you want to actually land at.

Can someone please tell me how I've misread this?
Is there a formal Sera definition of the word within? Quite a few dictionaries seem to say "enclosed by the boundaries of something". So an atz like WW would not be within Heathrow zone by that definition as the atz itself is not enclosed by it.
I'm not sure I follow you, Paul. Last time I checked, all runways were inside the ATZ...

If I'm reading correctly (and that's the question I originally asked), under CAP493 as of 24th April 2017 it will be illegal to fly VFR with a cloudbase under 1,500' in Class D airspace for the purposes of taking off, landing or flying a circuit (transits are OK). Let's assume we're at White Waltham where the eastern half of the ATZ is actually inside the London CTR which is Class-D airspace (AD 2-EGLL-1 section 2.17). This means that the final approach for runways 21, 25 and 29 are all inside the London CTR, so you wouldn't be allowed to fly a VFR approach with a cloudbase below 1500' - SVFR might be an option if Heathrow clear it since it's their Class-D that you're flying through. You could make an approach to runways 03, 07 and 11, however a go-around would leave you flying "in the aerodrome traffic circuit" in VFR in the CTR which is also a no-no. If that wasn't enough, AD 2-EGLM-1 section 2.17 states that the whole of the Waltham ATZ is Class-D anyway, so the rules may apply to all runways anyway. I think this would mean that there is no runway at White Waltham available to a VFR flight when the cloudbase is below 1500' - but I'm desperately hoping someone can prove me wrong.