Note:

In certain airports, engine throttle cutback (i.e., a lower power setting) is required to reduce the noise level after reaching the altitude shown. At cutback, an aircraft should maintain at least a 4% climb gradient. In the event of an engine failure, it should be able to maintain altitude.

Make linear interpolations for in-between aircraft masses. For takeoff, make linear interpolations for in-between mass.

The arithmetic sum of noise levels at the three noise measuring position is referred to as the “cumulative noise level”; and the difference between this level and the arithmetic sum of the noise limits allowed at each measuring point is referred to as the “cumulative noise level margin.”

The maximum noise requirements in EPNdB from ICAO, Annexure 16, Volume I, Chapter 3, are listed in Table 15.1 and plotted in Figure 15.3.

Approach

This is for any number of engines (use linear interpolations for in-between masses).

Sideline

This is for any number of engines (use linear interpolations for in-between masses).

Figure 15.3. ICAO Annexure 16 (Chapter 3) noise requirements

Figure 15.4. Typical noise footprint (≈10 km long) of aircraft showing engine cutback profile

A typical footprint of the noise profile around a runway is shown in Figure 15.4. The engine cutback area is shown with the reestablished rated thrust for an enroute climb. Residential developments should avoid the noise-footprint areas.

Stage IV requirements for new type designs from January 1, 2006 are as follows:

a cumulative margin of 10 EPNDB relative to Stage 3

a minimum sum of 2 EPNDB at any 2 conditions

no trades allowed

The airframe also produces a significant amount of noise, especially when an aircraft is in a “dirty” configuration (e.g., flaps, slats, and undercarriage deployed). Figure 15.5 shows the sources of noise emanating from the airframe. The entire wetted surface of an aircraft – more so by the flow interference at the junction of two bodies (e.g., at the wing–body junction) – produces some degree of noise based on the structure of the turbulent flow causing pressure pulses that are audible to the human ear. The noise is aggravated when the undercarriage, flaps, and slats are deployed, creating a considerable vortex flow and unsteady aerodynamics; the fluctuation frequencies appear as noise. In the conceptual design phase, care must be taken to minimize gaps, provide fillets at the two-body junction, make streamlined struts, and so forth. Noise increases as speed increases. Care must be taken to eliminate acoustic fatigue in structures and to design them to be damage-tolerant; material selection is important.

Figure 15.5. Typical sources of noise emanating from an airframe

Figure 15.6. Relative noise distributions from various aircraft and engine sources

Typical noise levels from various sources are shown in Figure 15.6 at both takeoff and landing. Aircraft engines contribute the most noise, which is reduced at landing when the engine power is set low and the jet efflux noise is reduced substantially. There is more noise emanating from the airframe at landing due to higher flap and slat settings, and the aircraft altitude is lower at the measuring point than at the takeoff measuring point. Because the addition of noise level is in a logarithmic scale, the total noise contribution during takeoff and landing is almost at the same level.

The power plant constitutes the nacelle and is the main sources of noise at takeoff when an aircraft is running at maximum power. All of the gas turbine components generate noise: fan blades, compressor blades, combustion chamber walls, and turbine blades. With an increase in the BPR, the noise level decreases because a low exhaust velocity reduces the shearing action with ambient air. The difference in noise between an AB turbojet and a high-BPR turbofan can be as high as 30 to 40 EPNdB. Figure 15.7 shows that in subsonic-flight speed, noise radiation moves ahead of an aircraft.

To reduce noise levels, engine and nacelle designers must address the sources of noise, as shown in Table 15.2. The goal is to minimize radiated and vibrational noise. Candidate areas in engine design are the fan, compressor, and turbine-blade; gaps in rotating components; and, to an extent, the combustion chamber. Engines are bought-out items for aircraft manufacturers, which must make compromises between engine cost and engine performance in selecting what is available on the market. Aircraft and engine designers communicate constantly to make the best choice without compromising safety.

Figure 15.7. In-flight turbofan noiseradiation profile