(2)
Aircraft Requirements. The certificate holder may not conduct any operation under Part 135 unless each aircraft used in its Part 135 operations is:

(a)
Owned by the certificate holder and remains, without interruption in the certificate holder’s legal and actual possession (directly or through the certificate holder's employees and agents) during all of its Part 135 flights; or

(b)
Leased by the certificate holder or otherwise in the legal custody of the certificate holder and remains in the certificate holder's exclusive possession or custody during all of its Part 135 flights.

(c)
For each aircraft which the certificate holder uses under these operations specifications, the aircraft owner or other lessee of the aircraft may operate the aircraft under Part 91, under the control and responsibility (including potential liability for an unsafe operation) of the owner or other lessee, as long as the following condition is met:

The certificate holder ensures that the maintenance of the aircraft continues to adhere to the certificate holder’s maintenance program at all times or, when the aircraft is returned to the certificate holder but before the aircraft is operated under Part 135 again by the certificate holder, that aircraft undergoes an appropriate airworthiness conformity validation check.

(3)
Exclusive Aircraft Use Requirements for Part 135 Operations. At least one aircraft that meets the requirements for at least one kind of operation authorized in the certificate holder’s operations specifications must remain in the certificate holder’s exclusive legal possession and actual possession (directly or through the certificate holder's employees and agents) as specified in Section 135.25.  This

U.S. Department of Transportation
Federal Aviation Operations Specifications Administration
aircraft cannot be listed on any other Part 119 certificate holder’s operations specification during the term of the exclusive use lease.