15.3 Noise Emissions

design philosophy. Automation and microprocessor-based data management have advanced to a wider operational capability without a corresponding increase in a pilot’s workload. In this way, readers can see an aircraft as a system and subsystems.

15.2.6 Military Aircraft Survivability Issues

Military aircraft design for combat survivability has been considered for some time, primarily as a consequence of potential damage occurring in combat. Crew-ejection capability is an integral part of survivability. In the last two decades, stealth technology – as a good survival measure – has advanced by minimizing an aircraft’s signature. Electronic defense and countermeasures are other ways to thwart retaliation and increase survivability.

15.2.7 Emerging Scenarios

New topics are gaining importance and must be considered by the next generation of engineers. The emerging scenarios affecting civil aviation result from acts of terrorism in recent years. Aircraft design must include damage limitation from an explosion in the cargo compartments and containment of terrorist activities within the cabin. Also, damage incurred from runway debris demands a new perspective on an old problem (i.e., the Concorde crash case). With greatly increased numbers of passengers crossing international boundaries, general health care and contagious infections are becoming important issues.

15.3 Noise Emissions

Noise is produced by pressure pulses in air generated from any vibrating source. The pulsating energy is transmitted through the air and is heard within the audible frequency range (i.e., 20 to 20,000 Hz). The intensity and frequency of pulsation determine the physical limits of human tolerance. In certain conditions, acoustic (i.e., noise) vibrations can affect an aircraft structure. Noise is perceived as environmental pollution.

The intensity of sound energy can be measured by the sound pressure level (SPL); the threshold of hearing value is 20µPa. The response of human hearing can be approximated by a logarithmic scale. The advantage of using a logarithmic scale for noise measurement is to compress the SPL range extending to well over a million times. The unit of noise measurement is a decibel, abbreviated to dB, and is based on a logarithmic scale. One “Bel” is a tenfold increase in the SPL; that is, 1 Bel = log1010, 2 Bel = log10100, and so on. A reading of 0.1 Bel is a dB, which is antilog100.1 = 1.258 times the increase in the SPL (i.e., intensity). A twofold increase in the SPL is log102 = 0.301 Bel, or 3.01 dB.

Technology required a meaningful scale suitable to human hearing. The units of noise continued to progress in line with technology demands. First was the “A-weighted” scale, expressed in dB(A), that could be read directly from calibrated instruments (i.e., sound meters). Noise is more a matter of human reaction to hearing than just a mechanical measurement of a physical property. Therefore, it was believed that human annoyance is a better measure than mere loudness. This

Figure 15.1. Perceived noise level (PNL) expressed in PNdB

resulted in the “perceived-noise” scale expressed in PNdB, which was labeled as the associated “perceived noise level (PNL),” shown in Figure 15.1 from various origins.

Aircraft in motion presented a special situation with the duration of noise emanating from an approaching aircraft passing overhead and continuing to radiate rearward after passing. Therefore, for aircraft applications, it was necessary to introduce a time-averaged noise – that is, the effective perceived noise level (EPNL), expressed in EPNdB.

In the 1960s, litigation from damages caused by aircraft noise caused the government regulatory agencies to reduce noise and impose EPNdB limits for various aircraft classes. Many airports have a nighttime curfew for noise abatement and control, with additional fees being charged for using the airfield at night. Through research and engineering, significant noise reduction has been achieved despite the increase in engine sizes that produces several times more thrust.

The United States was first to impose noise certification standards for aircraft operating within that country. The U.S. airworthiness requirements on noise are governed by FAR Part 36. An aircraft MTOM of more than 12,500 lb must comply with FAR Part 36. The procedure was immediately followed by the international agency governed by ICAO (see Annexure 16, Volume I). The differences between the two standards are minor, and there has been an attempt to combine the two into one uniform standard. Readers may refer to FAR Part 35 and ICAO Annexure 16 for further details.

Because existing larger aircraft caused the noise problem, the FAA introduced regulations for its abatement in stages; older aircraft required modifications within

Figure 15.2. Noise measurement points at takeoff and landing

a specified period to remain in operation. In 1977, the FAA introduced noise-level standards in three tiers, as follows:

Stage I: Intended for older aircraft already flying and soon to be phased out (e.g., the B707 and DC8). These are the noisiest aircraft but least penalized because they are soon to be grounded.

Stage II: Intended for recently manufactured aircraft that have a longer lifespan (e.g., the B737 and DC9). These aircraft are noisy but must be modified to a quieter standard than Stage I. If they are to continue operating, then further modifications are necessary to bring the noise level to the Stage III standard.

Stage III: Intended for new designs with the quietest standards.

Stage IV: Further increased stringency was applied for new aircraft certification during 2006.

ICAO standards are in Annexure 16, Volume I, in Chapters 2 through 10, with each chapter addressing different aircraft classes. This book is concerned with Chapters 3 and 10, which are basically intended for new aircraft (i.e., first flight of a jet aircraft after October 6, 1977, and a propeller-driven aircraft after November 17, 1988).

To certify an aircraft’s airworthiness, there are three measuring points in an airport vicinity to ensure that the neighborhood is within the specified noise limits. Figure 15.2 shows the distances involved in locating the measuring points, which are as follows:

1.Takeoff reference point: 6,500 m (3.5 nm) from the brake release (i.e., starting) point and at an altitude given in Table 15.1.

2.Approach reference point: 2,000 m (1.08 nm) before the touchdown point, which should be within 300 m of the runway threshold line and maintained at least at a 3-deg glide slope with an aircraft at least at a 120-m altitude.

3.Sideline reference point: 450 m (0.25 nm) from the runway centerline. At the sideline, several measuring points are located along the runway. It is measured on both sides of the runway.