The Science of Aircraft Noise
Contrary to popular perception, aircraft noise is not solely produced by an airplane's engine(s). Instead, it is a complex combination of primary and secondary sources which all contribute to the noise emitted by an aircraft in flight.
As a plane flies, it pushes through the air, disturbing the smooth fluid flow of air molecules much like a boat creates a wake moving through the water. This wake is the result of the interaction of the air molecules with the aircraft's fuselage, wings, tail, and engines which creates friction. The air friction generates turbulent airflow and drag, which manifests itself as noise.
When planes land they extend their landing gear and flaps to slow down and prepare for landing, this action increases the air friction and dramatically increases the turbulent airflow around the airplane, which produces additional noise.
Jet engine noise is created by the sound of the engine's moving parts and by the sound of air being expelled at high speed. Most of the engine noise comes from the exhaust or jet behind the engine as the high-speed air from the combustion process mixes with the low-speed airmass the aircraft is flying through. Modern jet aircraft with high bypass engines also have a distinctive buzz saw tone which is usually most audible during takeoff. This noise is caused by the interaction of the airflow with the fan blades at the front of the engine.
The level of noise generated varies according to aircraft size and type and can differ even for identical aircraft depending on factors such as weather conditions. Aircraft engines do not produce as much lift in hot weather when the air is less dense, and this can cause planes to fly over the community at a lower altitude during climb out.
Other factors that influence noise include the weight of the aircraft, including passengers, baggage, cargo, and the amount of fuel onboard. The direction a plane is flying relative to your location also affects the transmission of noise, and how loud you perceive a particular aircraft to be.
Aircraft Noise Certification Information
All fixed-wing aircraft operated in United States airspace are required by law to meet specific noise certification standards.
The FAA in cooperation with other regulatory bodies around the world working through the International Civil Aviation Organization (ICAO) has established noise thresholds that all civil aircraft must meet in order to operate in the U.S.
Under current regulations the operation of all non-military Stage I and II aircraft are prohibited in U.S. Airspace. Stage III, IV and V aircraft are currently allowed to operate in the U.S. without restrictions.
The most current FAA Stage 5 aircraft noise standard went into effect on December 31, 2017, and applies to all newly certified aircraft built after this date with a Maximum certified Takeoff Weight (MTOW) greater than or equal to 121,254 lb. Newly certified aircraft with an MTOW of less than 121,254 lb. must be compliant by December 31, 2020.
The new Stage 5 noise standard requires a cumulative noise reduction of 7 (EPNdB) Effective Perceived Noise level in decibels below the Stage 4 noise standard. The previous Stage 4 noise standard required a cumulative reduction of 10 EPNdB below the Stage 3 noise standard.
To establish the certified noise level of a new aircraft type, during the airplane's initial type certification flight test program the plane is flown over specialized microphones placed at fixed distances from the runway. As shown in the graphic below, noise measurements are recorded at three specific locations to capture takeoff noise, landing noise, and sideline noise. The aircraft is flown at its certified maximum takeoff weight for the Takeoff and Sideline measurements and its maximum landing weight for the Approach measurement with flaps and landing gear deployed.
The maximum noise level permitted for a particular aircraft type to meet a specific noise stage, whether it be III, IV, or V is not a fixed number. Larger, heavier widebody aircraft are allowed to make more noise than smaller, lighter narrowbody aircraft and still be compliant with a particular noise stage.
The maximum noise level permitted per aircraft type is also not fixed across the three measurements points. Generally speaking, the maximum noise level allowed is highest for the Approach measurement, with Sideline being the next highest and Takeoff being the lowest.
The approach phase of flight is when the aircraft is lowest to the ground as it descends on the final approach to the runway and takes into account the added airframe noise from the extended flaps, landing gear, and high engine power settings. The sideline measurement records the noise of the aircraft on its takeoff roll and very initial climb away from the runway. The Takeoff measurement is captured under the departure path of the plane when it is climbing away from the airport with the landing gear retracted, and while the flaps are in the process of being raised. The airplane is the greatest distance away from the ground at this measurement point relative to the Approach and Sideline measurements which helps explains why the maximum noise level allowed is lowest at this point.
As shown in the graphic below to derive the noise margin below the Stage IV limit, the measured noise level at each of the three measurement points (Approach, Sideline, and Takeoff) is subtracted from the maximum noise limit allowed for a particular aircraft at each measurement point. This process establishes the aircraft type's margin below the Stage IV limit at each measurement point. To calculate the aircraft's cumulative margin below Stage IV, the individual margins for all three points are added together.