Vortex Generators & Noise Reduction
Milton Request Signals That The FAA Should Establish A Position On This “Retrofit”
The Milton, MA board voted to send letters to airlines operating A-320 aircraft requesting the Boston carriers to retrofit older model Airbus aircraft with a device known as a “vortex generator” to reduce noise during descent. A request by an individual community to airlines to install equipment to reduce noise is not a new development. Those, who can remember the transitions from Stage I to Stage II and then to Stage III, will be weary of an uncoordinated noise policy which is likely to result as Milton’s and other communities follow this example.
A vortex generator is a noise-reducing component, a simple 5cm piece of sheet metal mounted upstream of the vents has shown to solve the problem. The early production A-320s create a distinctive high-pitched noise – similar to the sound created when blowing over the mouth of a bottle. When air passes over circular pressure equalization vents for the fuel tanks located under each wing, just before landing gear and flaps are deployed for landing, the opening “whistles.”
Although the aircraft is fully compliant with 14 CFR Part 36 (and its EASA equivalent), some European carriers (British Airways, EasyJet and Lufthansa) are adapting their planes with the vortex generator, which can reduce the overall noise level of the aircraft when landing by up to four decibels at distances of between 10 and 17 kilometers away from the airport. The devices are standard equipment in newer Airbus aircraft.
Between 1969 and 2005, the FAA and Congress the (Aviation Safety and Noise Abatement Act of 1979) established noise standards with increasing noise reduction standards (Stages I-IV) for operation and production. As these rules became more exacting, some airports attempted to establish rules to require that the quieter aircraft must only land on their runways. [Oddly, some airports tried to accommodate the noisier models to keep the traffic flowing through their facilities.]
One method for complying with these incremental thresholds was installing hushkits. They were devices, called multilobe exhaust mixers, which are installed on the rear of the engine. This equipment mixed the exhaust gases of the jet core with the surrounding air and the small amount of bypass air available. Similar systems were also employed on many modern turbofan engines as standard equipment to further reduce noise.
Most kits also made further modifications to the exhaust with acoustically treated tailpipes, revised inlet nacelles and guide vanes, all of which reduce forward propagating high-pitched noise caused by the small, high-speed fan.
Without a clear national policy, thirty years ago, individual communities tried to set local standards either to reject or accept the hushkitted airplanes. The Milton request, not mandatory YET, signals that the FAA should establish a position on this “retrofit.” If not airport lawyers, airline counsel and the FAA attorneys will repeat the late 1970s- early 1980s battles in courtrooms.