If they take 75% of your time and pay 75% of your basic in return, and
pay in full for all the flighthours delivered
where's the catch?
They save 25 % of your basic salary and in winter they don't need you. In line with other part time offers I have seen.
You could make a (good) case that you should be restricted to 75 % of 900 block hours in a year, or your work days will be very exhausting and not in line with a healthy family life. But then they'll probably think you get to save the cake and eat it too.