Heathrow claims a steeper angle of 3.2 degrees instead of 3 degrees reduces noise because aircraft are higher in the sky for longer.
Aircraft noise activists said the new angle would make no difference to noise levels for people living locally.
Frankfurt Airport where planes already land at 3.2 degrees said the move to reduce noise worked "very well".
The trial follows an Airports Commission report backing a proposed third runway at Heathrow, which stated the plan should come with severe restrictions to reduce noise effects.
Heathrow's head of sustainability and environment, Elizabeth Hegarty, said the 3.2 angle meant "the aircraft will be further away from the ground for a lot longer, so people on the ground will experience less noise".
"This is one of many steps we are taking to improve the noise environment around the airport," she said.
The trial is optional, but the airport claims a large number of airlines is expected to take part.
Murray Barter, of activist group Communities Against Increased Aircraft Noise (CAIAN), said the trial was nothing more than a "tick box exercise by the airline industry to show the government they are actually doing something to mitigate noise on the ground".
He said: "Other things make a much bigger difference to aircraft noise such as wind direction, atmospheric conditions [and] type of aircraft."
Ms Hegarty said Heathrow was "about 15% quieter than most other big European airports".
Frankfurt Airport senior executive manager Steffan Mauel said its introduction of steeper approach angles "has been working very well" to reduce noise for people living nearby.
He said: "We can really recommend this increased flight slope."
The trial has been approved by the Civil Aviation Authority and is planned to run until 16 March 2016.
If results are positive, Heathrow would be the only UK airport to introduce steeper approaches as a means of noise reduction.
Planes landing at Heathrow are to trial a steeper approach path in a bid by the airport to reduce noise.