The reason domestic air travel has been falling is principally down to three factors:
1) the increase in frequency, capacity and/or reduction in journey time for the rail links up against almost all of the major air flows in the country (London to Manchester / Leeds / Newcastle / Edinburgh / Glasgow).
2) the increase in costs of domestic air travel due to air passenger duty.
3) the increase in effective domestic air journey time due to increased air security post the 2006 ‘liquids’ bomb attempt.
HS2 will further enhance (1). But don’t be under the impression that domestic air traffic has been reducing naturally without ‘pressure’ from other factors.
Slight embarrassment - I saw an EZY flight STN-MAN on google flights, but evidently I hadn’t specified non-stop
It was via Belfast.
The important part of all of this is that HS2 will capture a decent share of London -Scotland central belt traffic, and with trains stopping at Manchester there is the potential to kill off the handful of Manchester - LHR flights (in combination with Manchester becoming more of a long haul airport itself). However it will absolutely not kill off domestic aviation.
I would add:
4) The high cost of slots at British airports when they could be more productively used for other services. Domestic air travel outside London is almost entirely confined to the smaller regional airports which have cheaper/vacant landing slots.
E.g. If I wanted to Fly Manchester to Bristol I would generally be routed via Dublin or even Amsterdam!
In not suggesting that HS2 will kill of domestic flights nor am I suggesting that other factors aren't impacting on the amount of air travel.
The point I was making was that you'd only need to impact on a few routes in the London/elsewhere market to better the 1 million passengers expected to switch form air travel to HS2, those routes are the ones which would likely most benefit from HS2.
Outside of London the following also happen which could also be impacted by HS2:
Birmingham/Central Belt ~500,000
Manchester/Southampton ~220,000
Newcastle/Southampton ~117,000
Birmingham/Central Belt journey times are likely to fall from 4 hours to around 3:20.
Manchester/Southampton journey times are likely to fall from about 4:15 to around 2:45.
Newcastle/Southampton journey times are likely to fall from 5:30 to around 4 hours.
(Note journey times from Southampton using HS2 have been calculated by adding current journey times to Paddington to HS2 journey times from Euston, as such there's likely to be some margin of error with actual times when services start, however are likely to be a reasonable benchmark comparison).
Also both the Southampton routes are likely to see an increase in frequency making them more attractive in that regard as well.
Now clearly that's only 8 million out of 23 million passengers which currently use domestic flights. However airlines like to want to be quite efficient and so it may not require much of a drop for routes to be cut.
Manchester/Southampton could see a noticeable drop given that is likely to be sub 3 hours after HS2. Such a drop would likely result in fewer flights which would then in turn result in yet fewer air passengers.
A future scheme to speed up Crewe to Central Belt times would likely see further drops in air passengers.
The other big factor to consider is that 15% of people make up 70% of air travel in any given year. This is likely to mean that relatively few people are making up a large proportion of those air passengers. You only need a fairly small number of people to stop flying (due to environmental concerns, retirement, death, etc.) and it could have a significant impact on air travel.
Chances are that as companies become more environmentally friendly, and with less of a time difference due to HS2 and more of a viable option with Skype, that they could well look at putting in place significant flight restrictions.
For instance if you're a company which sees most of its CO2 emissions from transport. If you've got a few people flying fairly regularly between offices and HS2 means that the average time saving is 30 minutes of you fly then you may well accept the "lost" time if it reduces your CO2 emissions. Especially if you reduce the numbers of trips made by using Skype, which gains you a lot of time back.
There's also going to be a generation who will be very keen to be green, they are probably just about to start to be getting into significant roles within companies just as those who are less favourable towards rail travel are starting to be retiring in significant numbers. Their voices are likely to be applying pressure to reduce business air travel.
If businesses turn away from air travel, as there's a viable alternative in the form of HS2, then it's not that unlikely that those 8 million passenger movements could likely fall by 25%, with 50% being a possibility.
However if there's taxes applied to the CO2 emissions generated by companies and their activities then I'd expect that fall to be much, much higher even if the value was fairly low. It would certainly make the likes of BA do things like code share with TOC's and Heathrow to remove the support that they give to regional routes.
The difficult domestic routes to deal with are those to Northern Ireland. Which do make up a lot of the remaining 15 million domestic air passengers makes up about 6 million, with a significant amount (somewhere around 2 million) of the remainder being from those going to the Channel Islands.
Whilst neither of these are likely to be fixed by rail any time soon (and may actually go up in the short term as people move away from long distance holiday travel) if we can get from 23 million down to 19 million (a 50% fall due to HS2 on routes which it impacts on, even if some of that fall is down to other factors on other routes) that's going to make some significant inroads to our domestic air travel CO2 emissions.