14 CFR §382.35

1. (a)As a carrier, you must not require passengers with a disability to sign a release or waiver of liability in order to receive transportation or to receive services or accommodations for a disability.
2. (b)You must not require passengers with a disability to sign waivers of liability for damage to or loss of wheelchairs or other assistive devices, or for the loss of, death of, or injury to service animals. Carriers may note pre-existing damage to an assistive device to the same extent that carriers do this with respect to other checked baggage.