The pretty basic Q I would ask is: who actually needs a 5km runway?
Anyone who operates heavy long-haul equipment in hot-and-high conditions. See e.g. Upington in South Africa.
So, why Paris?
This may be helpful https://en.wikipedia.org/wiki/List_of_longest_runways
Perhaps due to the Concorde? Quite heavy for the higher speeds required for take-off/landing.
So, why Paris?
Same reason as Heathrow, with 300 extra meters accounting for Paris summers being on average warmer than London summers.
Same reason as Madrid adding 200something m on top of Paris.
Numerical example – a 747-400 at MTOW will require 3’400m for takeoff at sea level, ISA+18C.
So if you plan to welcome aircraft in that weight class, you need to make sure your runway meets the performance requirements of the type, working with either factory data or company performance manuals.
Ah, but all these little runways require short field technique ! For a real runway look no further than KEDW, Edwards Air Force Base north of L.A.:
Rwy 2-20: 21119 ft. Although AFAIK this is the Space Shuttle rwy on the dry lake bed. Longest ‘normal’ rwy there is around 15k ft.
On a more serious not – as @T28 says. This also explains why many dirt strips in southern Africa are seriously long. Hot ’n High wreaks havoc with your performance.
I had a trial flight in microlights in Luqa in a hot summer day, the instructor suggested using the long 13kft runway to avoid a long taxi, I think he was not sure about WnB on those flex wings 
@c172driver
I spared mentioning KEDW due to the 11km runway being a bit sandy ;). According to wikip the shuttle landed there frequently.
It would be a good area for local bimbles. You could takeoff, cruise and land again, all while over a runway ;)
Peter wrote:
The pretty basic Q I would ask is: who actually needs a 5km runway?
Denver is a mile high city (haha) and pays trinute to the US standard ops fact that heavy jets land on short runways and takeoff on long ones (which, like so many other aviation parameters in the US, makes a lot of sense).
