i can only say what i know from real life: things should depreciate - in terms of accounting - linearly. meaning:
if the lifespan for the purpose of tax deduction is 25 years, the plane costs 25M, then after the first year, the planes worth should be 25M/25*(25-1)=24.
at year 25, the value should be 25M/25*(25-25)=0. this is not considering checks.
the Dchecks would probably have to be deducted over 8 years. meaning:
after 8 years, the plane is worth 25M/25*(25-8)=17M, you add 1M for the Dcheck, value is 18M now.
after 10 years, the value is 25M/25*(25-10)+1M/8*(8-2)=15.75M
but i am rather sure, that AWS is not using accounting standards, as this would mean that prices would have to be logged for all the major checks and the value of the plane would have to be divided for depreciation into the value of maintenance and the value of the plane itself.