Noise, according to the International Civil Aviation Organization (ICAO), is considered the main factor in the negative impact of aviation technology on the population in the vicinity of airports. Therefore, it is not surprising that ICAO systematically tightens requirements for aircraft noise on the ground. Aircraft manufacturers creating new aircraft have to pre-lay down such characteristics of their aircraft that could meet international requirements for decades to come. The new domestic short-medium-haul aircraft MC-21-300 was no exception here. Its certification flight tests, including noise on the ground, are currently underway at the Flight Research Institute named after M. M. Gromov in Zhukovsky near Moscow.

Sayings and applications

There is a Russian proverb - "Much noise, but little use". It can be said that aircraft manufacturers literally understand it, trying to reduce the noise characteristics of their aircraft on the ground as much as possible.

The consolidated statement of continuing ICAO policies and practices in the field of environmental protection, adopted by the executive committee of this organization following the 40th session of the ICAO assembly in October 2019, states in particular: “In the area of ​​reducing noise and emissions from international civil aviation , significant progress has been made. For example, due to significant technological advances, the noise level of aircraft currently produced has been reduced by 75% ... compared to aircraft in the 1960s.”

Back in 1971, the ICAO Council adopted Annex 16 to the Convention on International Civil Aviation. The appendix was called "On Environmental Protection". In its first volume, the requirements for the terrain noise produced by aircraft of different classes were set out. This application is valid to this day. True, for almost half a century of its existence, the document has been repeatedly corrected: the requirements for aircraft, including in the area of ​​the level of noise they produce, have been regularly tightened. The requirements are followed today by all members of this international organization, and there are already 193 of them: 192 UN member states (only Liechtenstein, which does not have an international airport, is not a member of ICAO) and plus the Cook Islands (a self-governing island state formation in the southern part of the Pacific ocean, non-UN member and in free association with New Zealand).

There have been cases where older aircraft models that did not meet the revised ICAO noise standards had to be taken out of service. For example, Russian airlines faced this in 2002, when old Soviet-built aircraft, such as Tu-134, Tu-154M, Tu-154B, Il-86 and some others, were no longer allowed to fly over the territory of the European Union. Their noise characteristics on the ground were higher than the new ICAO standards. So, another well-known Russian proverb is also very true for aviation, the beginning of which is placed in the title of this note: "The quieter you drive, the further you will be." Only “quieter” is not “slower” here, but “quieter” - that is, “less noisy”.

Another 7 decibels

The source of the noise emitted by the aircraft is primarily its propulsion system. “Of course, the main source of noise on an aircraft is the engine, - says Tatyana Lesnykh, 1st category design engineer at Irkut Corporation. - Since we have received certificates for the noise level for the engine, it would be possible to certify the aircraft with them right away. But the noise on the ground is also influenced by other elements of the aircraft - for example, the airframe, landing gear, wing mechanization."

As the ICAO reported in the materials of the same 40th session, "the creation of trusting relationships with the population is increasingly seen as a rational practice in solving the problem of negative reactions to aircraft noise." But, of course, the organization proposes to deal with the noise first of all to the creators of aircraft, which they successfully do. “In 2017, the noise restrictions of Chapter 14 of Appendix 16 of ICAO came into force, - says the head of the sector, head of the Aviishum testing laboratory of the Flight Research Institute named after M. M. Gromov Andrey Nakvasin. - In addition, the total - for three measurement points - noise levels on the terrain of heavy civil aircraft are limited by 7 decibels. From 2021, these restrictions will apply to all types of civil aircraft. ”

All new civil aircraft are allowed to operate with positive results of certification flight tests for noise on the ground. These tests determine the compliance of noise levels with the requirements of the regulations for take-off, climb and approach.

Achievements and improvements

“Aircraft and aircraft engine manufacturers are actively working to reduce aircraft noise, and major advances in airframe and propulsion (engine and nacelle) design, combined with improvements in aircraft performance, are further helping to reduce noise levels.

Aircraft manufacturers continue to work to reduce aircraft noise from all significant sources, in particular jet noise, fan noise and airframe noise ...

Along with the development of noise abatement technologies, it is critical to consider the broader context of a balanced approach to aircraft noise management, which includes land use planning and management, noise abatement operating procedures and operational constraints, in order to address noise in the most cost-effective manner. ” a consolidated statement of continuing ICAO policies and practices in the field of environmental protection (adopted in October 2019 by the organization's executive committee following the 40th session of the ICAO assembly).

Noise will measure "Aviashum"

Certification flight tests in various areas, including tests to determine the level of noise on the ground, are carried out at the Flight Research Institute named after M. M. Gromov since 1973. Currently, certification tests of the MC-21-300 aircraft are carried out at its acoustic measuring base by the Aviashum testing laboratory of the Flight Research Institute. The laboratory was accredited by the Federal Air Transport Agency.

The uniqueness of the practical experience and technical capabilities of the Flight Research Institute named after M. M. Gromov allows to carry out such work at the highest professional level. “During the flight, many parameters are monitored using acoustic, meteorological, trajectory and onboard systems, - says Andrey Nakvasin. - The results of all measurements are promptly processed and analyzed after the completion of each flight."

In the grass the Grasshopper sat

What is the acoustic measuring base of the Flight Research Institute named after M. M. Gromov? First of all, these are acoustic measuring systems equipped with satellite synchronizing channels and microphones, which are installed at measuring positions with an accurate geodetic reference to the airfield runway. “To measure the noise levels of the MC-21-300 aircraft, sets of domestic equipment“ EKOFIZIKA GPS / RTA ”are used, which provides registration of sound pressure in the linear operating range of 23-127 decibels with a frequency of 48 kilohertz, - says Andrey Nakvasin. - Due to the presence of satellite signal registration channels in this equipment, the synchronization accuracy of acoustic measurements at all measuring positions is 1/48000 of a second." The signal from the microphones is transmitted to the spectrum analyzers, from where the measurement results are fed to the experimental data processing system.

ICAO standards, in addition to the requirements for the characteristics and methods of installation of measuring equipment, also impose requirements on the measurement conditions, in particular, on the levels of background noise from extraneous sources. In the area of ​​acoustic measurements during the test flight, a silence mode is introduced using an air traffic control system. Depending on the air situation, it either restricts the flights of foreign aircraft within a radius of 10 km from the control point of the aerodrome, or adjusts the time of the MC-21-300 approach to the measuring base in such a way as to exclude the influence of extraneous noise on the measurement results.

But the seemingly unexpected "living sources of interference" can also affect the measurements. “In summer, a lot of different animals live in the fields around the airfield. Even grasshoppers that sit on the microphone windscreens can interfere with measurements. Therefore, when testing at the measuring point, there is a specialist who controls the absence of insects on the microphones,” - explains Andrey Nakvasin with a smile.

However, ICAO Appendix 16 certification testing requirements are not limited to background noise levels. The flight trajectories of the test aircraft, its flight configuration and engine operating modes are taken into account. The meteorological situation is also important both at the surface of the earth and at the flight altitude of the aircraft.

Meticulous piloting

The MC-21-300 noise test program provides for a significant number of aircraft flights, in each of which a series of passes over the microphones is performed at different altitudes in take-off and landing configurations under different engine operating conditions.

“In 2018, we carried out preliminary tests of the MC-21-300 aircraft for noise on the ground, - recalls Andrey Nakvasin. - The results obtained at that stage showed that the characteristics of the MC-21-300 noise have a significant margin in relation to the current standards.” However, two years ago, the tests were carried out at a take-off weight significantly less than declared for certification. The design of the aircraft also differed significantly from the standard one. With such differences, it is impossible to accurately calculate the change in acoustic characteristics. Therefore, certification of the aircraft required a new test program in Zhukovsky, which will allow obtaining the noise characteristics on the terrain of an aircraft of a typical design.

ICAO standards limit the levels of corrections that, when processing test results, are introduced to take into account deviations from the specified trajectories and speeds of flight over the acoustic measuring base. Therefore, the main requirement for the flight crew when conducting noise tests is to accurately maintain all modes and ensure minimum deviations from the original trajectories and speeds. In this case, the values ​​of the corrections to the measured noise levels will be minimal.

After the next flight MC-21-300, which included a whole series of passes over the acoustic base of the Flight Research Institute named after M. M. Gromov in various modes, the aircraft commander in that flight, test pilot of the Yakovlev Design Bureau Andrei Voropaev, says: “The main problem of these flights is precisely keeping the trajectory and keeping the speed accurately. When performing certification flights to determine noise on the ground on these aircraft, it is necessary, without changing the engine operating mode, to maintain accurate speed and maintain an accurate trajectory. That is, leaving 20 meters to the left or to the right is already considered a failure. Therefore, we use all possible means to ensure that the regimes are valid ”.

According to Voropaev, another difficulty of such test flights is that each of them provides for a large number of modes. “Today, for example, the flight lasted 4 hours, - explains the test pilot. - And in each approach the speed keeping accuracy was plus or minus 3 knots. The height of the passage above the control point at the end of the strip where the microphones are located was 120 meters plus or minus 20. Therefore, it is necessary to make the correct calculation from a distance of 6 kilometres from what moment, depending on weather conditions, to start descending. And at the same time, do not change the operating mode of the engine: in accordance with the task, it must always remain in a specific mode. This is the main difficulty - such scrupulous piloting."

Therefore, in all such flights, all crew members participate in the control of the aircraft. As Andrey Vopropaev says, when making a pass, the right pilot (co-pilot) precisely sets the engine operating mode, and the left pilot (aircraft commander) keeps the trajectory precisely. In addition, the navigator - the third member of the crew participating in these test flights - constantly gives hints about the beginning of the next mode, about adjusting the trajectory, and so on.

Real-time solutions

Another great helper in noise testing is the Flight Experiment Control Group. During each flight, trajectory measurements are carried out in real time, transmitted via a telemetric communication channel, on the basis of which the experiment is controlled.

Tests using telemetry began at the Flight Research Institute named after M. M. Gromov back in the 1980s. Before that, data processing and analysis of results could only be carried out after the end of the flight. All data were on paper in the form of graphs and tables. Engineers on the ground analyzed them. Only after that, the leading test engineers decided what to do next, made up the task for the next flight - what experiments to conduct, in what modes.

The advent of the ability to transmit telemetry information revolutionised flight testing. Now it has become possible to observe in real time how the aircraft systems behave, receive all test data, immediately analyze them and immediately make decisions about the further course of the tests - to repeat some mode, or it is no longer necessary, or to somehow change it ... And all this in real time when the plane is in the air. In total, since the beginning of the use of telemetry data transmission at the Flight Research Institute named after M. M. Gromov has already completed more than 8 thousand such test flights.

In the laboratory of the institute, where telemetry information is received and processed, the entire current situation of the test flight is displayed on several screens. “On the first screen we have performance characteristics, aircraft configuration, - says the head of sector 94 of the laboratory of NIO-9 Flight Research Institute named after M. M. Gromov Valentina Dossieva. - The next screen displays the status of the aircraft control systems. It features the same dashboard as on board the aircraft. On the next screen is a map showing the location of the aircraft and its flight path. The next one shows the air situation, thanks to which we can see all the aircraft that are currently in the air in the airfield area. The next screen shows how the noise mode runs. It reflects the permissible corridor and the real trajectory of the aircraft, the position of the aircraft at the moment in this corridor, as well as the data on engine speed ”.

To confirm compliance with ICAO standards, the aircraft must fulfil the so-called matrix of modes. This sometimes requires several hundred passes over the microphones. “During one test flight, the aircraft, as a rule, performs more than 20 modes of approach or takeoff with strictly specified requirements for speed, engine speed and flight trajectory, - says Tatiana Lesnykh. - The head of the tests conducts radio communication and informs the crew after each mode, what are the comments and what needs to be corrected. For example, change the engine speed, speed, trajectory."

Severe meteorological restrictions

An important component of noise certification flights is their meteorological support. “The test method strictly limits the area of ​​permissible meteorological parameters - wind speed, cloud base, atmospheric precipitation, the nature of changes in temperature and air humidity along the height,” - explains Andrey Nakvasin.

At the surface of the earth, these data are continuously recorded by the specialized meteorological station of the Flight Research Institute named after M. M. Gromov. Its sensors are installed on a 10 m high mast. With the help of the meteorological station, the specialists of the Aviashum testing laboratory measure meteorological parameters and transmit them to the head of the tests. It assesses the compliance of weather conditions with ICAO requirements and decides on the feasibility of performing a test flight under these conditions. During the flight, the head of the test evaluates the validity of the test modes by the wind speed near the ground, and, if necessary, suggests the crew to repeat the non-valid modes.

The ICAO requirements for atmospheric conditions during certification tests are no less stringent than the requirements for maintaining the flight path. Engineer of testing laboratory Aviashum of the Flight Research Institute named after M. M. Gromov Mikhail Danilov remembers them by heart: “Meteorological parameters should not exceed the following limits: no precipitation, the temperature should not be higher than +35 ° С and not lower than -10 ° С.The average wind component should not exceed 6.2 m/s, and the maximum wind speed should not be more than 7.7 m/s. The average crosswind component should not exceed 3.6 m/s, the maximum - not exceed 5.6 m/s. Humidity should not be higher than 95% and not lower than 20%."

Sounding helicopter

Twice during the test flight, the aircraft goes into the holding area for 15 minutes. Such breaks are necessary to measure atmospheric parameters up to the MC-21-300 flight height above the microphones. This task is performed by the flying laboratory of aerological sounding based on the Mi-8AMT helicopter. “The meteorological limitations of ICAO Appendix 16 do not allow performing test flights under certain atmospheric phenomena, for example, during temperature inversion, - explains Andrey Nakvasin. - This is a positive gradient of temperature change with height. A probe helicopter with aerological equipment on board can determine the presence of a temperature inversion 15 minutes before the start of a test flight of the aircraft. If the inversion is present, the flight will be postponed to a later period, when after warming up the surface of the earth, the temperature inversion will be destroyed. "

Flying laboratory Mi-8AMT is equipped with an aerological probe mounted on an external sling. The length of the suspension is 12 meters, which ensures the removal of the equipment outside the zone of influence of the main rotor at low flight speeds. The Mi-8AMT is also equipped with the KBTI-M onboard trajectory measurement complex, which allows meteorological data to be linked with high accuracy to the flight path and receive operational information about the state of the atmosphere in real time.

ICAO requirements prescribe to perform weather sounding no earlier than half an hour before the first test mode for noise on the ground. Further meteorological soundings follow one after another with a period of 45-60 minutes, in order to be able to control the change in parameters during the flight of the aircraft. Therefore, as a rule, to ensure one flight of the MC-21-300 aircraft in terms of noise, the helicopter performs 4 meteorological soundings: one - before the aircraft's flight, during the flight - two more, after the end of the flight, the final meteorological sounding is performed.

Results for certification

Based on the data obtained from the results of measurements using a ground meteorological station and an upper-air probe, the measured noise levels are corrected for the difference between the actual atmospheric conditions and the initial ones. The air space between the microphones and the noise source is broken up into layers by the aircraft. Taking into account the temperature and humidity of the air in each layer ultimately affects the noise levels obtained from the results of these tests.

Complex algorithms are used to process and analyze the results of noise tests on the ground. A high-tech system of integrated data processing displays in real time for each point of the aircraft trajectory the values ​​of all measured parameters: the current noise level at control points on the ground, atmospheric characteristics, engine operating mode and others. Based on this information, baseline noise levels are calculated.

Experts from independent certification centres are present at the tests and supervise together with employees of the Irkut corporation and the Flight Research Institute named after M. M. Gromov for the execution of regimes along the given trajectories. These experts, monitoring compliance with all requirements for trajectories, speeds, operating modes of engines and on-board systems, confirm the validity of the data obtained during the tests. In future, they will also confirm the test reporting documentation. Then the report will be approved by the certification centres and submitted to the aviation authorities of the Russian Federation - to the Federal Air Transport Agency to confirm compliance with ICAO standards. As a result, the test results of the aircraft will be used for certification in accordance with Russian standards and for preparation for certification in Europe.