I don't think many people live in downtown Carlisle. The airport's catchment will be the whole of northern Cumbria and parts of Scotland and Northumberland so the difference in trannsport to the station or airport is probably limited, provided you travel by your own car.
Exactly, there's no advantage between airport and train station, so why wouldn't you just take the train if going to London? For most parts of Annan and Northumberland, they are on the rail line to Carlisle too - so connecting onto a london train becomes easier than going to the airport.
However, there seems to be a lot of focus in here about the Southend flights. Don't forget there is Dublin & Belfast too. I read one of the early planning permission applications, which included a study on feasibility of the Southend and Dublin routes.
It suggested that Southend would never be viable, and would always need subsidies to operate, while Dublin was expected to prove profitable after 3 years.
Heathrow 3rd runway has been approved, and as part of the approval there is a requirement to launch a certain number of new domestic destinations. FlyBE have released a proposed route map of what they'd like to consider as part of their support for the runway - Carlisle was amongst the proposed routes.
A route to Heathrow with onward connections would certainly be viable.
Building a terminal at Carlisle also opens up other possibilities. What about flights to the Isle of Man on a small 10 seater aircraft? perhaps even scenic tours of the Lakes & Hadrians wall. Occasional passenger charter flights to elsewhere in the UK, again on 10 seaters. A lot of Northern football teams tend to use charters when playing teams in the deep south - could Carlisle make use of the airport when playing Plymouth, Exeter or elsewhere?
We have the airport there, and if passenger facilities can be offered at little to no cost, then we may as well do so. It wouldnt take a great deal of flights to cover the costs of them.
Pax flights without a subsidy would bankrupt the airport, like it did to Blackpool. Low cost airlines would pay no landing or navigation charges, and would want paying for each passenger that got off at Carlisle.
Larger aircraft would require extra fire, security cover etc, that means greater costs for the airport operator. Freight would be the only real option.
As others have stated, aircraft size is limited to ATR42 size, so not a great issue. Cargo flights also require fire coverage, so thats a mute point. Security cover will be minimal, immigration will be provided by staff from other airports on an ad-hoc bases when needed, and any other jobs can be covered by Stobart Rail staff if needed.
The terminal is being built and operated as cheaply as possible, which will allow them to see just how much demand there is with minimal risk. It can always be expanded if the demand is there, or maintained at little to no cost if the demand is limited.
But at least there will be a facility there if someone wants to use it.