The helmic regulator was a component of 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 change the landing position of the TARDIS.
When Harry Sullivan took his first journey in the Doctor's TARDIS, it was supposed to be to Earth's moon. He inadvertently altered the flight path through space and time by handling the helmic regulator. This caused the TARDIS to materialise aboard Nerva Station several thousand years into the future. (TV: The Ark in Space)
The helmic regulator also appeared 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 one 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 anomaliser, and turning off the handbrake. (TV: Smith and Jones)
When the Fifth and Tenth Doctors' TARDISes crashed into one another, the Tenth Doctor prevented a black hole from swallowing the universe by venting the thermo-buffer, flooring the helmic regulator and frying the zeiton crystals. This caused the TARDIS to create a supernova. The supernova and black hole cancelled each other out, leaving universal matter constant and unaffected. This averted the universe's destruction. (TV: Time Crash)
While phoning Amy and Rory about his adventures without them, the Eleventh Doctor mentioned how he should be returning to them "any day", but the TARDIS' helmic regulator was playing up. When he finally reached them, he blamed the helmic regulator for arriving too early in their timestream and promptly left. (WC: Pond Life)