How can you signal a mechanic outside the airplane?

Using the MECH button on the CALLS panel is the correct method to signal a mechanic outside the airplane because this function is specifically designed to alert ground personnel. When pressed, it activates an external horn, providing an audible signal to the mechanic or ground crew, ensuring they are aware of the communication request. This dedicated feature is intended for clear communication between the flight crew and ground staff in various operational scenarios.

The other methods listed do not provide the same level of effectiveness for signaling a mechanic outside the airplane. The PA button on the ACP, while useful for making announcements within the cabin, does not transfer communication outside the aircraft. Similarly, using the MECH transmission key without the accompanying external signaling function does not ensure that the mechanic will receive the message. Selecting the MECH transmission key and then pressing the PA button combines two different functions that are not designed to work together for outside communication, limiting their effectiveness for this purpose.