OPTIMIZED PROFILE DESCENT
Optimized Profile Descent (OPD) is an arrival procedure that is designed to allow aircraft to use idle engine power. These procedures are supposed to be less noisy than procedures that require the use of speed brakes, flaps, slats or thrust, all of which generate a great deal of noise.
There are two problems with the promise of Optimized Profile Descent procedures. First, they are designed to fly over a narrow corridor, without dispersion. Second, they are optimized to model conditions that the real world does not always obey. As wind speed and temperature vary, the OPD procedure remains constant. The automated Flight Management Systems of airplanes flying that procedure will adapt as required, which can only add noise, perhaps a lot of it, as they deploy speed brakes, flaps, slats or jet thrust to compensate for the real-world conditions.