Would a steeper descent help to reduce noise?


Would a steeper descent help to reduce noise?

The next carrot the aviation sector is dangling, is a steeper descent during the approach of airports, this is an increase of the glide path. This slope is normally fixed by ICAO at 3° (52.5 meter per kilometer for the decrease in altitude).

Increasing it to 3.1° (54 m/km) would mean aircraft would fly 36 meters higher at 20 km from touchdown. Everyone will agree this is not significant. Moreover the gain in noise would be 0.3 dB since noise decreases with the square of the distance between the source and the observer. Figures for 3.2° – as suggested for Heathrow – instead of 3° are 56 m/km and 70 meter at 20 km. A decrease of less than 0.6 dB in noise!

The gain for 3.5° would be 176 meters at 20 km. Hence a gain of 1.45 dB. This is going in the right direction but cannot be really perceived as a difference by the human ear. This value is theoretical and is valid in the absence of sound reverberation on the ground (buildings, etc.). The aviation lobby is presenting it as a ‘major improvement’… Let’s remain objective!

With 5° there would be a difference of 700 meters at 20 km distance. This would convert in a gain of 4.5 dB: this is significant to the human ear.

Engine noise versus aerodynamic noise

But there is a potential major drawback with an important increase in slope: at 3° aircraft loose speed in a natural way before landing (with the help of landing gears and wing flaps). This may not be the case at 3.2° and will not be the case at 3.5°: pilots will then need to use other means to reduce the speed (more flaps, airbrakes), which will increase the aerodynamic noise! It’s unsure whether less noise will be produced altogether.

The true noise generated by an aircraft depends on several factors (as engine noise and its power, airframe noise and speed) and the relative predominance of each noise source is depending on the aircraft type and  its engine. Airframe noise generated by modern aircraft is often higher than engine noise during landing, whereas  this could be the opposite for old aircraft. And by the way: Airbus is not authorising 3.5° descent for their 330s and 340s. Although they are slowly disappearing, Boeing does the same for their 744.

The real impact of a dedicated noise abatement landing procedure is linked with the detailed composition of the aircraft landing on the airport. A noise comparison between several landing procedures could be made only through a noise impact survey. Those noise impact surveys are possible and can be made with existing acoustic software. So please examine it, experts!

Posted by Wouter Looman  Posted on 25 Jan 
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