Complex noise certification for light aircraft
Even small propeller-driven aircraft must comply with specified noise limits as part of their certification. This does not only apply to completely new developments. Changes to existing types can also make a reassessment necessary, for example if the engine or propeller is modified.
This is particularly complex with propeller aircraft, because the engine, propeller, rotational speed, power and flight condition jointly determine the noise generation. A new engine-propeller combination therefore cannot simply be assessed on the basis of theoretical data.
Defined measurement flights are carried out for the certification. In the process, both the flight path of the aircraft and the meteorological conditions and the measurement system must be precisely documented.
What ENCODE takes over
ENCODE stands for „EASA's Noise Compliance Demonstration Evaluator“. The Excel tool provided by the EASA is intended for the evaluation of noise measurements in accordance with Chapter 10 of ICAO Annex 16, Volume I.
After entering or importing the measurement data, the programme first checks whether the individual measurement runs meet the required conditions.
These include, among others:
- Altitude and flight path
- overflight speed
- Wind conditions
- Temperature and humidity
- Number of usable measurement runs
- further ambient conditions relevant to the measurement
Measurements that lie outside the permitted limits can thus be detected early.
Measured values must be corrected
Initially, the microphone on the ground measures only the sound pressure actually registered during the respective flight. However, this raw value is not sufficient for certification.
Measurement conditions inevitably vary slightly from flight to flight. ENCODE therefore takes the necessary corrections into account and normalises the measurements to standardised conditions.
For example, factors such as the actual distance of the aircraft from the measuring point, atmospheric influences and engine and propeller parameters are taken into account.
Subsequently, the tool calculates the noise level relevant for certification, including the statistical confidence interval.
Fewer errors in data analysis
It is precisely these calculations that have, until now, accounted for a significant proportion of the work following the actual measurement flights.
The corresponding ICAO regulations are detailed and require numerous corrections and plausibility checks. Furthermore, if calculations are carried out individually in custom spreadsheets, there is the risk of differing interpretations or simple input errors.
ENCODE establishes a standardised process here. Manufacturers and EASA can build on the same calculation methodology, which can reduce the need for clarifications during the certification process and speed up the assessment.
No new noise limits
It is important to note that, with ENCODE, EASA is not introducing any new requirements for small aircraft.
The tool mirrors the existing requirements of ICAO Annex 16 and the associated technical procedures. It therefore neither changes the permitted noise limits nor the fundamental nature of the prescribed measurement flights.
The formal noise assessment report is still required. The results generated using ENCODE can be submitted to EASA together with the other supporting documentation.
The tool therefore simplifies the process of demonstrating compliance, but does not replace the actual certification process.
Especially interesting for light propeller aircraft
ENCODE is primarily aimed at manufacturers and development companies of light propeller-driven aeroplanes that are subject to the relevant ICAO noise regulations. This includes numerous single- and multi-engine models in general aviation.
Especially smaller manufacturers could benefit. For them, the effort involved in obtaining approval is particularly high in relation to their often low production volumes. Standardised and freely available tools can therefore help noticeably to reduce development and certification costs.
This can also become relevant when modifying existing aircraft. New propellers, for example, are frequently developed to improve performance, efficiency or cabin comfort. At the same time, however, a different propeller alters the aircraft's noise emissions and may therefore necessitate a reassessment of noise levels.
Importance for quieter aircraft
Noise remains one of the most important issues for the acceptance of general aviation, particularly at smaller airfields.
Technical improvements to propellers and engines can significantly reduce noise levels. Modern propeller geometries, lower blade tip speeds and optimised operating speeds are among the most important adjustments.
A simplified certification process can therefore also indirectly facilitate the introduction of quieter technology. If changes can be evaluated and documented more easily, at least part of the regulatory burden is reduced.
And what about microlights?
A comparable solution would also be interesting for microlights.
However, their noise certificates do not automatically follow the same approval procedures as for EASA-certified aircraft. In Germany, separate national requirements apply to microlights.
ENCODE is therefore not initially a general tool for microlight noise certification. Whether parts of the methodology can also be used or suitably adapted for airsports equipment in the future remains to be seen.
Conclusion
With ENCODE, EASA is digitising and standardising what has hitherto been a relatively labour-intensive part of the noise certification process for light propeller-driven aeroplanes.
Survey flights and certification documentation remain essential. However, the subsequent checking, correction and calculation of the data can be carried out in a much more structured manner using the free tool.
For manufacturers, this means less manual evaluation and a lower susceptibility to errors. Particularly in the case of smaller aircraft projects and modifications to existing models, ENCODE can thus make a practical contribution to making certification procedures more efficient.
Source references:
Aviation magazine
