Why not use engine master switch? 

The engine master switch was not used to stop Cathay Pacific flight 780 during the emergency landing on April 13, 2010, because of the following critical reasons:

1. Engine Control Malfunction: The Airbus A330-300 experienced a dual engine failure where both engines were stuck at idle power due to contamination in the fuel system. This significantly limited the crew’s ability to control the engine thrust.
2. Maintaining Some Thrust: Even though the engines were not responding normally, they were still producing some idle thrust. Turning off the engines with the master switch would have resulted in a complete loss of all engine power, which would have been catastrophic as it would have left the aircraft without any thrust and reliant solely on gliding.
3. Hydraulic and Electrical Power: The engines provide hydraulic and electrical power to the aircraft. Shutting down the engines would have led to a loss of these critical systems, which are essential for controlling the aircraft, especially during an emergency.
4. Control and Landing: The pilots needed to maintain as much control as possible to manage the emergency landing. With the engines providing at least some power, they were able to control the descent and approach to Hong Kong International Airport more effectively.
5. Procedure Adherence: The pilots followed the emergency procedures as outlined by the aircraft manufacturer and the airline. These procedures did not include shutting down the engines using the master switch under such circumstances.

Overall, the decision to keep the engines running, despite their limited functionality, was based on maintaining the aircraft’s control and ensuring a safer landing with the available resources.