The Environmental Research and Consultancy Department (ERCD) of the Civil Aviation Authority (CAA) estimates the noise exposures around Gatwick on behalf of the DfT.
The magnitude and extent of the aircraft noise around these airports are depicted on maps by contours of constant aircraft noise index (Leq) values.
Leq is a measure used to express the average sound level (typically expressed in dBA) over a given period of time.
The CAA have now promised to use the better Lden (Level Day Evening Night) measurement which is similar to Leq but puts a 5 decibel penalty to noise in the evening and a 10 decibel penalty to noise at night.
Despite this we have issues with this measure since the average sound level is not an accurate measurement of the noise issue. Moreover it is the difference between the sound level without planes and the sound level with planes, and the frequency at which this occurs, that causes the problem.
The most recent restrictions to night flying at Gatwick were introduced in October 2006 and consist of four main elements as follows:
- A maximum number of flights during each season (11,200 in summer and 3,250 in winter) between 11.30pm and 6.00am;
- A supplementary quota system to encourage the use of quieter aircraft. Aircraft types are given a Quota Count (QC) value, according to how much noise they make on landing and take-off. These are: QC 0.25, QC0.5, QC1, QC2, QC4, QC8 and QC16, which is the noisiest category. Aircraft movements score QC values against a maximum allowable Quota for each season (6,700 for summer and 2,300 for winter) between 11:30pm and 6am, "the night quota period";
- Aircraft of QC4 cannot be scheduled to operate between 11.30pm and 6.00am;
- Aircraft of QC16 and QC8 cannot be scheduled to operate between 11pm and 7am.