Hmm.
The Charlotte Airport website stresses that it's up to the airline to arrange assistance, rather than the airport authorities:
http://www.charmeck.org/Departments/Airport/Pa ssenger+Services/Accessibility+Resources+and+S pecial+Needs.htm
Even so, it might be worth phoning the airport, shortly before departure, to emphasize your needs:
http://www.charmeck.org/Departments/Airport/Co ntact+Us/home.htm
(Prefix those numbers with 001 from the UK).
I'd also advise you to email US Airways shortly before departure, to remind them of your requirements:
http://www.usairways.com/awa/content/contact/c ustomer_relationsform.aspx
Then hassle the US Airways check-in staff to pass the message through to Charlotte.
When I ran a railway station we got loads of 'passenger assistance' bookings. With the best will in the world, it was sometimes possible to overlook a booking when things got really busy. It was, however, far harder to forget bookings when you'd been phoned three times by the company's customer services department, then been phoned by the station staff who were putting the passenger onto the train, phoned again by the relative who was seeing the person onto the train, phoned yet again by the conductor of the train and finally phoned by the passenger himself!
Keep hassling and eventually, with a bit of luck, the message should get through! ;-)
Chris