14 CFR §133.19

1. (a)The applicant must have the exclusive use of at least one rotorcraft that—
   1. (1)Was type certificated under, and meets the requirements of, part 27 or 29 of this chapter (but not necessarily with external-load-carrying attaching means installed) or of § 21.25 of this chapter for the special purpose of rotorcraft external-load operations;
   2. (2)Complies with the certification provisions in subpart D of this part that apply to the rotorcraft-load combinations for which authorization is requested; and
   3. (3)Has a valid standard or restricted category airworthiness certificate.
2. (b)For the purposes of paragraph (a) of this section, a person has exclusive use of a rotorcraft if he has the sole possession, control, and use of it for flight, as owner, or has a written agreement (including arrangements for the performance of required maintenance) giving him that possession, control, and use for at least six consecutive months.