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how is arifcaft noise measured?
- effective perceived noise levels
- noise and number index (NNI)
- leq 57dB
effective perceived noise levels
EPNL is an estimate of the relative loudness of a particular type of aircraft during take off, overflight and landing
noise and number index
- noise and number index (NNI) which combines the number of flights and the noise levels of aircraft above 80dB as it presumed that aircraft below 80dB do not cause serious annoyance
- it has been replaced with the Leq 57dB
Leq 57dB
- is an average sound level over a particular period
- for UK airports, an average of 57dB between 7am and 11pm in the summer is used
- levels above this are considering annoying
- areas with noise above this level will experience 'significant community annoyance'
methods of reducing noise pollution - military aircraft
- major urban areas are avoided, unless close to the airfield
- in the UK there are 13 major avoidance areas that include conurbations, civil airports and certain key industrial and medical sites
- flight paths are varied
- the UK is divided into 20 Low Flying Areas (minimum flying height 250ft) of which three are Tactical Training Areas (minimum flying height 100ft)
- compensation may be paid for damage caused to property or harm to livestock
- flights may avoid sensitive ecological sites e.g seabird cliff breeding colonies
civil aircraft
- airliners that cruise at altitudes high enough that little or no sound reaches the ground, so noise nuisance is usually limited to the area around airports where aircraft are close to the ground
- many airports are located closet o urban areas to reduce travel times which increases the risk of noise causing annoyance in urban areas
what is the level of noise nuisance affected by?
- by the timing and number of flights each day
- the sound level produced by each aircraft
airport design and location
- location away from major population centres
- taxi areas away from residential areas
- engine test areas
- acoustic insulation
- land use restrictions
- noise deflection / absorption
- multiple landing runways
location away from major population centres
new airport development are located in areas with a low population density to minimise the number of people affected
taxi areas away from residential areas
- aircraft taxiing before take off can increase noise levels for nearby residents
- the layout of the airport can separate taxi areas and residential areas
engine test areas
areas where engines are tested on the ground at high thrust are located away from residential area, are only used during the day and are surrounded by acoustic screens
acoustic insulation
buildings affected by aircraft noise may have free acoustic insulation provided e.g double glazing
land use restrictions
there may be restrictions on land uses near airports e.g allowing industry but not housing developments
noise deflection / absorption
baffle mounds and acoustic barriers can deflect or absorb noise around an airport
multiple landing runways
- at a busy airport with a single runway for landing, aircraft may have to use noisy reverse thrusters to slow down so they can get off the runway before the next aircraft arrives
- having multiple runways allows more time for slowing down, so the wheel brakes can be used which are quieter than reverse thrusters
aircraft engine design
- high bypass-ratio engines
- chevron nozzles
- engine hush kits
- engine acoustic liners
high bypass-ratio engines
- early jet engines had a single tube around the jet with the turbulent exhaust gases providing thrust but also creating a lot of noise
- modern jet engines have a second tub and a turbofan on front of the jet that forces 'bypass air' around the inner cowling
- the bypass air smooths the flow of the exhaust air and reduces noise levels
- the bypass ratio is the ratio of bypass air: jet exhaust gases
- the higher the ratio, the quieter the engine
chevron nozzles
- these are used on high bypass engines to make them even quieter
- a serrated trailing edge on the outer cowling mixes the bypass air with the surrounding air more smoothly
engine hush kits
these work like chevron nozzles and can be fitted to low bypass ratio engines to make them quieter
engine acoustic liners
acoustic liners are used inside the outer cowling and around the inner cowling to absorb noise
aircraft body design
- blended wing aircraft
- aerodynamics
- lighter aircraft
blended wing aircraft
in this design the engines can be located on top of the aircraft so the body acts as a barrier reducing the noise that reaches the ground
aerodynamics
- fairings on the undercarriage reduce turbulence around the wheels and leg struts
- fairings covering wing flap hinges reduce turbulence
- riveted construction creates surface indentations which creates turbulence and noise
- welded panels create less turbulence
lighter aircraft
- using composite materials e.g carbon fibre, makes aircraft lighter so less engine thrust is needed
- e.g the Airbus A380 uses lightweight composite materials for 25% of its structure
- the newer A350 uses 53% composite materials
airport operation
- take off angle
- flight path planning
- constant descent angle
- night flight restrictions
take off angle
- aircrafts are noisier when taking off then landing because the engines are running at higher power and the take off angle is steeper
- which allows the aircraft to climb rapidly to an altitude where it cannot be heard on the ground
flight path planning
flight path routes are planned to avoid densely populated areas whenever possible
constant descent angle
- having a constant descent angle (usually 3) for the final descent before landing makes the engine noise less intrusive
- there are no periods of high thrust as there are with a stepped descent
- a steeper descent angle allows aircraft to stay at greater altitudes for longer, so ground level noise is further reduced
night flight restrictions
most airports near large residential areas restrict the number of night flights especially take offs
control of noisier aircraft
- noise limits
- charges for noisier aircrafts
- restricted flight times
- quota count system
- control of supersonic flights
noise limits
- older noisiest 'chapter 2' aircraft were banned from most major airports in 2002 when 'chapter 3' limits were introduced (15dB reduction in maximum noise)
- all new aircrafts since 2006 must meet 'chapter 4' noise limits (10dB lower than chapter 3)
- new regulations being introduced from 2017 will make noise regulations stricter with a further 7dB reduction
charges for noisier aircrafts
- aircrafts pay charges for using airports and at many airpots the charge is based on noise levels
- at heathrow, the noisiest aircraft pays 10x as much as similar sized quiet aircraft
- night flights cost nearly 3x as much as day flights
- there are fines for breaking flight noise limits
- fines in 2015 were £500 per dB above the limit for day and £4000 for night flights
restricted flight times
- airports near high population density areas often restrict or ban night flights
- take offs are controlled more than landings as they are noisier
quota count system
- airports may encourage airlines to operate quieter aircraft by restricting flight times for noiser aircraft or by using a point system
- used at Heathrow and has also been adapted for use at other airports
- aircraft types are allocated points depending on their noise levels
- each airline is allocated a certain number of points
- the total point score of the actual flights must not exceed their allocation
- it is easier to keep within the point total if new purchases are of quieter aircraft
control of supersonic flights
- no longer any supersonic civil airliners
- concorde ceased commercial flights in 2003
- concorde was banned from almost all airports and was not allowed to fly supersonic over most countries
