an old twin will be relatively expensive to run because
- you
have 2 engines and some ~80% of the 2nd engine is used to pull along that
engine, plus the now substantially larger aircraft (longer wings are needed
because the engines take up a fair bit of them, for example)
- most twins
are very old and an old plane will tend to have a significant appetite for
airframe parts (true for old singles too of course but a twin has more bits on
it)
Neither of those reasons are true.
Two smaller engines mounted on the wings are less efficient in weight, drag and specific fuel consumption than one larger one, no doubt. But it's nothing like 80% of the 2nd engine. That's ridiculous. Take a basic airframe available in twin and single versions. A 2x200hp Seneca vs 300hp Saratoga. The Seneca is faster. It is not substantially larger, it is about the same size. For that matter look at the Tecnam 2x100hp vs. 160-200hp singles.
I have never understood your theory that an old plane is expensive to maintain because of airframe parts. I have owned 2 old twins and the airframe parts cost has been negligible. I don't believe a properly maintained old twin cost more to maintain today than it did 20 years ago. However, they do cost a lot to maintain because they are complex and labour-intensive. There's more to go wrong and even trivial things can take a few hours labour before anything has happened. What is hugely expensive is taking a badly maintained old twin and fixing it. The other mistake is to base maintenance cost expectations on what the purchase price of the airplane is. The old twins have depreciated a lot in real terms but that hasn't changed the maintenance cost from the time in the 70s when they were the equivalent of $1-2m corporate airplanes.
Oooo, though I'm generally a fan of some "legacy" types, NOT a Seneca II, or
a number of other Pipers of that era. I have had first hand experience with very
expensive repairs required for corrosion damage, because Piper would not support
that type, with the required replacement parts. I quote the Piper Tech Rep:
"Sir, that's a 40 year old airplane, we have not seen it in 40 years, and we
don't want it in the air
There are thousands of Senecas flying about giving dependable service. In my 10 years of ownership I never had a part problem. And at least Piper didn't succumb to an equivalent of the SID debacle which has cost twin Cessna owners $100k each in some cases - ask any Australian C310 owner...
I have yet to hear of any consistent set of anecodotes about parts cost and availability for any old aircraft. They always contradict themselves. One person says
"the factory was struck by a giant meteorite in 1971 and vaporised. The company then went bust and all the drawings were burned. Parts sourcing is thus a nightmare for this Type"
Then someone replies
"Nonsense. The owners' association has a list of PMA suppliers and every single part can be sourced just as quickly and cost-effectively as for a type still in production"