Air Departure Tax reduction in Scotland: noise impact assessment

3. Approach To The Impact Assessment

3.1.1. This section sets out the rationale for the levels of noise that were chosen as the criteria for assessing the impact of aircraft noise in this report.

3.1.2. The UK Government formally published the Aviation Policy Framework^[17] (APF) in March 2013, defining what it believes is a balanced approach to securing the benefits of aviation. Its stated objective is that the aviation industry needs to grow to benefit the UK economy while respecting the quality of life of people affected by aviation activity. 

3.1.3. The overall objective of the APF is to: ‘…limit and where possible reduce the number of people in the UK significantly affected by aircraft noise’. 

3.1.4. Changes in aircraft movements as a result of the proposed reduction in ADT need to be assessed against the policy set out within the APF. Key noise criteria relating to the onset of annoyance and when properties become eligible for compensation are set out in the APF as follows:

• 57 dB L_Aeq,16h – considered to be the average level of aircraft noise marking the onset of significant community annoyance.
• 63 dB L_Aeq,16h – the level above which airport operators are expected to offer acoustic insulation to noise-sensitive buildings.
• 69 dB L_Aeq,16h – the level above which airport operators are expected to offer assistance with the costs of moving^[18].

3.1.5. The L_Aeq,16h noise metric was adopted by the UK Government in 1990 and is commonly used in the UK to describe the average daytime noise levels of aircraft. The concept of assessment criteria for aviation noise was expanded on during the appraisal for increasing UK airport capacity in the Appraisal Framework Consultation^[19] (AFC) document.  The document recommends the use of the L_Aeq,16h and L_night for assessing aircraft noise impacts.

3.1.6. In 2002 the European Commission published Directive 2002/49/EC^[20], which established the L_den as a common environmental noise indicator for the European Union. Consequently, all noise mapping undertaken for the END is required to present contours using the L_den noise metric.

3.1.7. Studies undertaken by the Civil Aviation Authority (CAA) show that the L_Aeq,16h and L_den^[21] noise metrics show a strong degree of correlation at larger UK airports with the L_den approximately 1.5 dB higher than the corresponding L_Aeq,16h. Consequently, it was considered that there would be no substantial benefit to model both the L_Aeq,16h and L_den noise metrics for the purposes of this report and, in line with UK aviation policy, the L_Aeq,16h has been used to represent daytime aircraft noise levels. 

3.1.8. The following range of contours that should be used to assess potential aviation noise impacts are identified in the AFC document:

• L_Aeq,16h – average summer’s day: 54 dB and above in 3 dB increments; and
• L_night – average summer’s night: 48 dB and above in 3 dB increments.

3.1.9. The 57 dB L_Aeq,16h was identified as an indicator of community annoyance based on the findings of the 1982 Aircraft Noise Index Study (ANIS)^[22].Since the publication of these criteria listed above, the Civil Aviation Authority’s (CAA) Survey of Noise Attitudes^[23] identified that the sensitivity of people to aircraft noise had increased. The study found that the same percentage of people annoyed by aircraft noise in the 1982 ANIS study at 57 dB L_Aeq,16h now occurs at approximately 54 dB L_Aeq,16h. To account for the increased sensitivity to noise, the range of criteria L_Aeq,T stated in the AFC document has been expanded to include the 51 dB L_Aeq,16h noise contour and the 45 dB L_night noise contour. Consequently, the range of noise contours considered in this assessment are as follows:

• L_Aeq,16h – average summer’s day: 51 and above in 3 dB increments; and
• L_night – average summer’s night: 45 and above in 3 dB increments.

3.1.10. This approach is in line with the developing UK aviation strategy (which is due to be published in the middle of 2019) as discussed in the Consultation Response on UK Airspace Policy^[24] that identifies 51 dB L_Aeq,16h and 45 dB L_night as the onset of adverse levels of aircraft noise. As such, the discussion of the noise prediction results that account for the proposed ADT reduction scenarios has been undertaken with reference to area of land and population covered by the 51 dB L_Aeq,16h and the 45 dB L_night noise contours.

3.1.11. The assessment of aircraft noise has traditionally been undertaken through the area of land covered by noise contours. However, the Airport Commission introduced the concept of assessing population affected by aircraft noise as being more relevant to the assessment of aircraft noise impacts. Consequently, the assessment has been undertaken with reference to both the area of land and the population affected by changes in aircraft noise.