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Night Noise Quota Count

21st October, 2005

A briefing was prepared for MPs by the AEF on the ‘Night Noise Quota Count Scheme’. The briefing was prepared in connection with the Civil Aviation Bill in Oct 05, but is of general relevance to the night flights issue.



The Night Noise Quota Scheme professes to be a regime that will encourage the uptake of quieter aircraft but its numerous shortcomings in fact allow far more planes to fly at night, while maintaining the same supposed ‘noise climate’.

While these planes may indeed be marginally quieter, it is the number of noise events, rather than a token reduction of a few decibels, that causes the misery of sleep deprivation to residents living under flight paths. It is essential, therefore, that the cap on numbers of movements at night is retained.

Although the Bill [Civil Aviation Bill] as currently worded merely enables the Secretary of State to set a limit based on noise rather than movements, given the strong business lobby for a quota-only system AEF suspects it will only be a matter of time before the movements limit is abolished altogether.


The Scheme rates all aircraft types according to their respective noisiness of landing and and take-off using a measure called EPNdB ‘effective perceived noise’ in decibels. Band of EPNdB are assigned a Quota Count (QC) rating, this being done on an exponential scale.

For each reduction of 3 in EPNdB  the QC is halved:
EPNdB over 101.9 is QC/16
EPNdB 99 – 101.9 is QC/8
EPNdB 96 – 98.9 is QC/4
EPNdB 93 – 95-9 is QC/2
EPNdB 90 – 92.9 is QC/1
EPNdB less than 90 is QC/0.5
EPNdB less than 87 is exempt (ie QC of zero).

A limit is placed on the total number of QC points per 6 month season (how these are assigned per night is at the discretion of the airport operator). Thus under a pure quota count system, if planes rated at 96 EPNdB were replaced with planes rated at 95 EPNdB, twice as many could be flown during the restricted period.

The environmental objective is to keep within a given ‘average noise’ limit for the whole night, measured in Leq. Leq stands for Level equivalent and is calculated by adding together the noise energy of all the noise events across a given time period and then taking the continuous level (ie. it irons out the peaks and troughs).

 An extreme case will illustrate the way Leq works. One concorde on departure had equivalent noise energy to 120 Boeing 757s – so one [Boeing 757] plane every 2 minutes for 4 hours, produced the same Leq as 2 mins of concorde followed by 3 hrs 58 mins of silence.

There is no official noise index for showing night noise in the UK (although Leq is officially recognised during the day period between 0700 and 2300). However, the Government believes that producing ‘noise maps’ for airports at night using Leq contours is an adequate way of expressing aircraft noise, and has produced maps for the London airports in its recent consultation on the night noise regime.

 This method is an inadequate as a way of assessing the impact of a small number of noisy events distributed over a long and otherwise tranquil period. This is explicity stated by the World Health Organisation in their guidelines for noise levels:

Where there are no clear reasons for using other measures, it is recommended that LAeq,T be used to evaluate more-or-less continuous environmental noises. However, when there are distinct events to the noise, as with aircraft or railway noise, measures of individual events such as the maximum noise level (LA Max) or the weighted sound exposure level (SEL) should also be obtained in addition to LAeq,T.” )[NB: ‘LAeq,T’ is simply a fuller description of ‘Leq’ – the ‘A’ indicating the weighting scale used and T specifying the time period] (WHO Guidelines for Community Noise, Executive Summary, p2.)

As planes get marginally quieter many more will be allowed to fly at night under a pure quota count scheme. But it is the frequency of noise events that can ruin a night’s sleep. If I am woken up by all noise events over 90 dB, I will not be pleased to hear twice as many, even if they are 92 dB rather than 95 dB. Hence it is essential that a numbers limit on night movements is retained.

Other problems with the QC system:

It is misleading to equate a 3dB reduction with a halving of ‘annoyance’, even for the individual event. EPNdB is a measure of ‘noise energy’ and it is by no means certain that a halving of noise energy results in a halving of noise heard by the human ear, despite the name.

Research over many years has show that halving the noise energy, ie reducing the noise level 3dB, by no means halves the perceived noise. The ear detects it only as a slight reduction. For noise to sound half as loud, the noise level must be reduced by about 10dB.

It is because the perceived loudness is not proportional to noise energy that the ‘logarithmic’ scale of decibels was introduced into the science of acoustics.

QCs are assigned according to certified rather than actual measured noise. There is evidence that actual practices are often noisier – sometimes one whole QC band noisier. DfT applies a reduction on arrival noise by 9 EPNdB. This has some justification given the way noise is certified, but it fails to account for the different quality of noise and the different set of people affected by departures. It has the effect of artificially lowering the QC of arrivals – and most of the movements at night are arrivals.

A fuller explanation, with reference to the most recent change in quotas at Heathrow, Gatwick and Stansted is given on the DfT web site. While this is informative and factually correct, it does not (of course) explain the flaws inherent in  the system.