My father had the "perpetual" award on TWA.

It went like this:

Triple mile bonus on all tickets and these were the days when miles accrued on award ticket (irrespective of class of service).

He purchased LAX-LHR and earned approx 33,000 miles (11K times 3), which he redeemed for another LAX-LHR ticket (they only required 30K for RT US/Europe then) that also earned 33K miles, which he redeemed for another LAX-LHR ticket that earned 33K miles, etc.

Small wonder TWA went under - (Imagine what could be earned today with mileage runs if this was still around).