Helmic regulator

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

This device's principle function was to control the accuracy of travel through the Time vortex. If incorrectly handled, the helmic regulator could 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)

The Helmic Regulator also appears in the Tenth Doctor's TARDIS in the form of a bicycle pump. He mentioned it as one of the last things he needed to do before taking off (at least in this specific flight), stating he needed to fire it up when he narrated what he was doing to Martha Jones, along with closing down the gravitic anomalyser, and turning off the handbrake. (DW: Smith and Jones)

When the Fifth and Tenth Doctors' TARDISes crashed into one another, the Tenth Doctor was able to prevent a black hole from swallowing the universe by 'venting the Thermo Buffer', 'flooring the Helmic Regulator' and frying the 'Zeiton Crystals' which caused the TARDIS to create a supernova. The supernova and black hole subsequently cancelled each other out, leaving universal matter constant and unaffected, and preventing the destruction of the universe. (DW: Time Crash)