Believe this is in need of been posted on here again, hope Cyrano doesn't mind but this is in response to my question in regards to an airline operating a route that was dropped by larger aircraft.
It doesn't really work like that, Nick. There's this thing called revenue management.
So for example on the Q400, maybe (I'm making up these numbers) the first 20 seats were sold at £40 one-way, the next 20 at £60, the next 20 at £80 and so on. If there were 30 passengers a flight on average, the majority of them would in all probability have paid the lower fares.
On a 31-seater D328, things will be different, of course because the aircraft is smaller but also because the cost per seat is higher, so the average fare needs to be higher. So maybe there'll only be 5 seats at £40, then 5 at £60, then 10 at £80. The average fare across 30 passengers on a 31-seater aircraft will be a lot more than the average fare across 30 passengers on a 78-seater. And unless airline passengers are insensitive to price a higher average price means lower average demand.
In other words, if you had an average of 30 passengers a flight on a 78-seater and you switch to a 31 seater, you won't have an average of 30 passengers, because the fares on offer will be higher so the demand will be lower.
(And even more basically, even if the fares were magically the same, you still wouldn't have an average of 30 passengers. That figure is an average across different passenger loads, for example if five flights carried 10, 30, 25, 50, and 35 passengers the average is 30. If the highest possible number of passengers in a flight is 31 rather than 78, your average is going to be a lot smaller.)
And finally: you make the entirely wrong assumption that a full or nearly-full flight means a profitable flight.
So hope this explains why Edinburgh or Belfast hasn't been taken up at southend