Helmic regulator

The Helmic Regulator is a component found on the control panel of a Type 40 TARDIS.

This device's principle function is to control the accuracy of travel through the Time vortex. If incorrectly handled, the helmic regulator can greatly distort the landing position of the TARDIS.

When Harry Sullivan took his first journey in the Doctor's TARDIS, which was supposed to be to Earth's moon, he inadvertently altered the flight path through both space and time by manipulating the helmic regulator. This caused the TARDIS to materialise aboard Nerva station several thousand years into the future. (DW: The Ark in Space)