I have to agree with @easik that we should generally remove inflation from the equation as inflation is a matter of Profit/Loss, not appreciation/depreciation. What you are describing is indeed true but nobody really looks at their stock, 401k or housing investments and calculates and starts calculating the inflation-adjusted value for the money they put in before calculating their return on investment (neither does the IRS for that matter), they just look at the "value" and see it went up.
If you want to include inflation than you also need to start including things like the cost of money (i.e. the interest rate on the loan you likely took out to buy the plane), opportunity costs (i.e. what the money could have earned had you placed it into a "smarter" investment), maintenance/sunk costs (e.g. every month you sit on a plane that isn't flying but is in annual is money lost towards the annual inspection and storage costs, while every hour you put on the plane before selling it is decreasing its value, unless you plan to overhaul the engine before listing it), etc. The list goes on pretty much ad nausem with regards to the various things you "could" consider in weighing your analysis of whether owning a plane was ultimately profitable for you.
The other problem with including inflation is that the market varies widely, especially over the "short" term. Just look at housing leading up to 2008 and housing since 2008 or college tuitions or computers. I'd even argue cars are in line with inflation only because of the features added to them.
So yeah, the fact your $20,000 doesn't buy as much today as it did in 1970 is an important profit/loss consideration but it does not change the question of whether something appreciated or depreciated in value.
