In determining the appropriate treatment of leased aircraft for ratemaking purposes, it is the Board's policy to recognize actual rental expenses. In unusual circumstances where the leased aircraft value (determined on a constructive depreciated basis) in relation to net book value of owned aircraft operated by the same air carrier is significantly in excess of the ratio for the aggregate of the domestic trunklines and local service carriers (computed on the same basis), a reasonable profit element may be added which shall reflect the additional risks of operations with the leased aircraft, to the extent that such risks are not compensated by the return on investment. Such profit element would be determined by applying the standard rate of return, less 6 percentage points, to the value of the leased aircraft, on a constructive depreciated basis, to the extent the ratio of such value to depreciated cost of owned aircraft plus the value of leased aircraft exceeds the average for the domestic air carriers. Rental cost plus allowable profit, if any, will not be recognized in amounts exceeding depreciation plus return on investment computed as if the aircraft had been purchased by the carrier.