Wrong message?
I have to say I'm not too suprised that at this stage the councillors have voted the extension down. It is an easy scapegoat for them to present to voters that that are acting to tackle climate change (but please don't say anything about SCC and the port...) by refusing it. I remain reasonably confident that on a whole council or even government level there is a good chance of the extension being approved.
I do feel the airport have quite got the right tune in how they have prevented a few of their ideas though.
'Larger aircraft' - I feel they should emphasise this as a redistribution of what type of traffic the airport will see as the extension will not really open up the possibility of any 'larger' aircraft than currently operate (maybe a 321N would just about work for PMI length routes, but seems very unlikely?). This could instead be presented as 'we expect the airport to go from a traffic mix of 60x 80 seat prop + 10x 120 seat jet + 2x 180 seat jet to something like 40x 80 seat prop + 15x 120 seat jet + 10x 180 seat jet to demonstrate how they can see growth through slightly less flights but on slightly larger aircraft.
Regarding noise, the prospect of less flights could be promoted more clearly, and also comparisons between aircraft. I'd be interested to see just how much noisier a 320 CEO/NEO would be compared to a Q400 of E195, as I suspect there may not be much difference. Unless the contrary is true then it feels like this is a good way to reassure those concerned about noise and the prospect of huge aircraft suddenly being commonplace at the airport.
On an environmental side, I'd be interested to see the comparison between the impact the airport has on the local air quality and that of the port. I expect you could remove the airport entirely and Southampton would still have air quality issues in the centre of town. A prominent argument on environmental grounds is that NOx emissions in the worst case may triple (can't remember the figure but it's out there). That is obviously not good but it needs some context to show how that impacts the overall air quality and NOx emissions in Eastleigh and Southampton. Not much value in protesting about one big looking number if it is orders of magnitude below the overall levels of pollution in the area, and therefore making very little difference in the grand scheme of things.
It might be that all of these details are in the information the airport has provided, but even as an interested outside I've not found them, so it feels to me as though they have not presented the finer details of the case strongly enough. The emotive 'approve the extension to save the airport' is a good headline, with some logic behind it (i.e. airport breaking even at 1.2m passengers), but I think the finer details have not been emphasised enough and this has benefitted the against case.