Salute Muffles
In 'Controls', you need to assign a key to 'Rise Gear manually', and another key to 'Lower gear manually'.
You press the manually rise gear key a number of times until you see the red light go on for gear up.
When lowering, again, you need to press the manually lower gear key, until the green light for gear down goes on.
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Cheers RAF74 Buzzsaw