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)

The Helmic Regulator also appears in the Tenth Doctor's TARDIS in the form of a bicycle pump. He mentions it as one of the last things he needs to do before taking off (atleast in this specific flight), stating he needs to fire it up when he narrates what he is 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 sallowing the universe by 'venting the Thermo Buffer', 'flooring the Helmic Regulator' and frying the 'Zyton Crystals' which caused the TARDIS to blow up, the resulting supernova and black hole canceled each other out leaving universal matter constant and unaffected. (DW: Time Crash)