People living under Heathrow Airport’s flight path have been sent leaflets about changes to rules that allow the use of both runways simultaneously.
The runways are restricted to take-off or landing and switch each afternoon, except in an emergency, to give residents respite from the noise.
As part of a trial starting next month the threshold for triggering the emergency system will be lowered.
Airport operator BAA said it would allow it to minimise disruption.
A BAA spokesperson said leaflets had been sent to 150,000 homes that would be affected by the trials.
Under the changes, dual use can be deployed when a plane faces a 10-minute wait to land or take off and if 30% of all flights are delayed by more than 15 minutes.
The average time delay for flights at the west London airport is currently 12 minutes.
The first trial finishes on 29 February while the second will run from 1 July 2012 until 30 September - coinciding with the London Olympics.
John Stewart, chairman of the HACAN Clearskies campaign, said it was a major concern for people living in west London.
– from the BBC website - 2 October 2011