J430.
The metro utilises a gravity feed crossflow system, primarily for use during single-engine operations. There is no specificed procedure for balancing the fuel inflight in the flight manual and the practice of "sideslipping" was common in Eagle, Air Nelson and Airwork (even if management are not prepared to admit it) albeit somewhat unorthodox. It is not fitted with a crossfeed system as is known in more modren aircraft.
A problem that would occur is that the fuel caps, and the seals around the base of the cap, would become loose with age and wear - the result being to create a slight venturi effect when inflight. If this venturi effect were to occur on the "high side" (i.e the tank with the higher imbalance) then there was the possibility of the fuel feeding into the wrong tank creating an even greater imbalance.
This was explained by an engineer when we defected an aircraft during a similar procedure as we were were unable to "crossfeed" correctly with anything less than an initial 20 degree angle of bank on the aircraft.