The noise of older aircraft can be reduced in two principal ways: retrofitting the aircraft with a quiet propulsion system, and changing the flight operational procedures used in flying the aircraft. The former approach has already proved to be expensive, time consuming, and difficult to implement even though low-noise propulsion system technology exists. The latter method seems to hold promise of being less expensive and easier to implement. One operational technique which might reduce the noise beneath the landing approach path is the decelerating approach. This technique requires intercepting the 3 deg approach path at a relatively high speed with the aircraft in the cruise configuration, then reducing the thrust to idle and allowing the aircraft to decelerate along the 3 deg approach path. As the appropriate airspeed is achieved, the landing flaps and landing gear are deployed for a normal flare and landing. Because the engines, which are the predominant noise source on landing approach, are at idle thrust, a significant reduction in the noise beneath the approach path should be realized

Putman, T. W.

English

NASA Technical Reports Server

NASA TM X-56020
FLIGHT EXPERIENCE WITH THE DECELERATING NOISE ABATEMENT APPROACH
Terrill W. Putnam
(NASA-TK-K-56 2C) FLIGHT EXPERIENCE WITH 
N74-12720
THE DECELERATING NOISE ABATEMENT APPROACH
(NASA) -8- p HC $3,C0 CSCL 20A Unclas
/1 G3/12 2k953
January 1974
NASA Flight Research Center
Edwards, California 93523
https://ntrs.nasa.gov/search.jsp?R=19740004607 2020-03-23T12:27:50+00:00Z
FLIGHT EXPERIENCE WITH THE DECELERATING
NOISE ABATEMENT APPROACH
Terrill W. Putnam
Flight Research Center
INTRODUCTION
Major advances in developing low-noise aircraft propulsion systems have been
made in recent years. Some of these low-noise engines are being used on the new
wide-body jets, such as the Boeing 747, McDonnell Douglas DC-10, and Lock-
heed L-1011. It has taken many years and huge sums of money to develop these
relatively quiet aircraft engines, and further advances will require more time and
money. However, the wide-body jets account for only a small portion of the take-
offs and landings at our airports today; so, in order to lessen the airport noise prob-
lem, methods of reducing the noise of the older aircraft must also be found.
The noise of older aircraft can be reduced in two principal ways: retrofitting
the aircraft with a quiet propulsion system, and changing the flight operational pro-
cedures used in flying the aircraft. The former approach has already proved to be
expensive, time consuming, and difficult to implement even though low-noise pro-
pulsion system technology exists. The latter method seems to hold promise of being
less expensive and easier to implement.
One operational technique which might reduce the noise beneath the landing
approach path is the decelerating approach. This technique requires intercepting
the 30 approach path at a relatively high speed with the aircraft in the cruise config-
uration, then reducing the thrust to idle and allowing the aircraft to decelerate
along the 30 approach path. As the appropriate airspeed is achieved, the landing
flaps and landing gear are deployed for a normal flare and landing. Because the
engines, which are the predominant noise source on landing approach, are at idle
thrust, a significant reduction in the noise beneath the approach path should be
realized.
A series of standard and decelerating landing approaches was flown at the
Flight Research Center with a small jet transport aircraft, and measurements were
made of the noise on the ground beneath the flightpath. This paper compares the
noise levels for the different approach techniques and shows time histories of the
approach profiles and rates of deceleration.
J
SYMBOLS
t.
D duration correction, 10 log 1
EPNL effective perceived noise level, EPNdB
PNLM maximum perceived noise level
t i  duration time for ith flight (see p. 5)
VASI visual approach slope indicator
TEST AIRCRAFT
The Lockheed Jetstar aircraft (fig. 1) used in these tests is a four-engined,
medium-range, jet transport. In the standard configuration the aircraft accommo-
dates 8 to 10 passengers in its pressurized cabin, and has a crew consisting of a
pilot and copilot. The JetStar is powered by Pratt & Whitney JT 12A-6 turbojet
engines. The maximum takeoff weight is 18,562 kilograms.
INSTRUMENTATION AND DATA REDUCTION
The placement of the microphone used in these tests is shown in the diagram
of figure 2. The microphone was mounted on a tripod 1.22 meters high along one
edge of runway 15 on Rogers Dry Lake. The lakebed has a smooth, hardpacked,
sandy-clay surface capable of supporting very large aircraft.
A condenser-type microphone with a 2.54-centimeter diameter was used. It
was oriented with the diaphragm parallel to the ground surface.
The data signal from the microphone was driven through a cable by an amplifier
to an instrument van. Electrical power for the microphone and line-driving ampli-
fier was supplied by batteries through an inverter. The cable from the microphone
was terminated at the van with a line-isolation transformer, and the signal was
routed to an amplifier and recorded on an instrumentation-type magnetic tape re-
corder. The frequency response of the microphone and recording system was flat
within +1 decibel over the frequency range of 50 hertz to 10,000 hertz. The time of
day provided by a time-code receiver was also recorded so that the acoustic data
could be correlated with the aircraft position. The microphone system was cali-
brated with a piston-phone in the field before and after the tests.
The data were reduced by using a computer-controlled, real-time, one-third-
octave-band analyzer. The data were scaled, frequency corrections were applied
if necessary, and overall sound pressure and perceived noise levels were calculated.
2
The time constant used for data reduction was 1 second. The data were corrected
to standard-day conditions; however, no correction for ground reflection was made.
TEST PROCEDURES AND CONDITIONS
It was planned to fly the aircraft on a 30 glide slope approach at several different
rates of deceleration. In addition a single-segment, 60 glide slope approach was to
be flown to compare the noise levels between the standard 30 approach and the 60
approach. The primary constraints on the 30 glide slope tests were that the air-
craft had to be at normal handbook approach speed and over the microphone posi-
tion at FAR, Part 36 conditions (ref. 1), that is, an altitude of 112.8 meters and an
approach glide slope angle of 30. The rate of deceleration of the aircraft for the
different engine power settings and aircraft configurations was not known, thus
preliminary flights were made on a trial-and-error basis to determine the initial
speed and altitude required for the aircraft to be in the landing configuration and
at landing speed over the noise measurement point. On the basis of these data,
intercept altitudes and speeds for the different approaches were plotted on tracking
radar display maps. These maps and information were then used by an air control-
ler in the Flight Research Center control room to determine the initial aircraft posi-
tion and speed. After intercepting the glide slope, the pilot maintained the slope
by using the visual approach slope indicator (VASI) light located on the runway.
Backup glide slope information was supplied by the air controller from the tracking
radar.
Table 1 summarizes the aircraft operating schedule during the landing approach
tests. Flights 1 and 2 were made by intercepting the glide slope at the reference
landing speed with the aircraft in the landing configuration. The engine thrust was
then adjusted to maintain the glide slope and the reference speed. The aircraft
continued down the glide slope until it was approximately 500 meters to 1000 meters
beyond the microphones, at which time the aircraft configuration and engine power
were changed to allow the aircraft to go around for another approach.
The decelerating approaches, flights 3 and 4, were made by intercepting the
glide slope at the predetermined altitude and speed with the aircraft in the cruise
configuration. On the basis of the aircraft speed, the air controller notified the
pilot to retard the throttles just before the glide slope was intercepted. Then the
aircraft was allowed to decelerate until the maximum speed for takeoff flap extension
was reached, at which time the takeoff flaps were extended. When the maximum
speed for gear and landing flap extension was reached, the landing gear and flaps
were lowered. The approach was continued so that the aircraft speed over the noise
measuring point was approximately the same as on flights 1 and 2. After passing
the noise measuring point, the aircraft made a go-around without actually landing.
Figure 3 shows the wind conditions between the ground and the maximum test
altitude approximately 4 hours before the flight tests. These data were not available
at a time closer to the flights; however, since no significant changes in the weather
occurred during this period, these data are believed to be representative of the
winds during the flight tests.
3
The surface temperature during the tests was 21.10 C, and the relative
humidity was 35 percent.
RESULTS AND DISCUSSION
Aircraft Profiles
The elevation, velocity, and deceleration profiles for the landing approaches
are presented in figure 4. These data were obtained from the tracking radar; thus
the velocity plotted is with respect to the ground, and the accelerations were ob-
tained by differentiating the ground speed with respect to time.
The elevation and velocity profiles for the constant-speed, 60 approach (flight 1)
are shown in figure 4(a). The aircraft ground speed was approximately constant
at 79 meters per second, and the aircraft passed over the microphone position at
an altitude of 229 meters, which is about twice that specified in FAR, Part 36 for
noise certification of aircraft. This means that the aircraft noise level would be6 decibels less than the 30 glide slope noise level because of the greater distance ofthe aircraft from the measuring station.
The elevation and velocity profiles for the standard 30 approach (flight 2) are
shown in figure 4(b). The velocity decreased somewhat on this approach, from82 meters per second to 76 meters per second, even though it was supposed toremain constant. The altitude over the noise measuring position was 120 meters,
which was approximately as planned.
Figure 4(c) presents the elevation, velocity, and deceleration profiles for the30 glide slope approach at idle power (flight 3). The planned altitude of the air-craft over the noise measuring point was 120 meters; however, the velocity de-
creased to about 67 meters per second. As shown in figure 4(c), the aircraftinitially decelerated rather slowly. When the landing gear and landing flaps were
extended at an altitude of 1500 meters to 2000 meters before the microphone location
was reached, the deceleration increased about 2 meters per second squared as the
aircraft passed over the noise measuring position.
It should be noted that it is difficult to obtain a desired velocity precisely at apoint on the flightpath unless specialized information is displayed to the pilot. Infact, this particular approach was the fourth attempted on the day of the tests. Thethree previous attempts were aborted because the aircraft decelerated to an un-
acceptable speed before passing over the noise measuring point. Each attempted
approach was initiated at a lower altitude until the profile shown in figure 4 (c) wasflown. It is believed that the previously established initial altitude and velocity
conditions did not permit the aircraft to pass over the noise measuring point at theprescribed conditions because on the day the initial conditions were established
the tailwind component was larger than on the test day. Thus it appears that anyguidance information given to the pilot for a decelerating approach should includewind effects.
The decelerating approach on flight 4 resulted in the elevation, velocity, anddeceleration profiles shown in figure 4(d). The engines were throttled to provide
4
about one-half the thrust required for a standard 30 approach. Over the noise meas-
uring station, the aircraft altitude was approximately 120 meters and the velocity
was 69 meters per second. The deceleration profile was significantly different for
flight 4 than for the other flights for several reasons. The deceleration was initially
rather slow until an altitude profile correction was made about 4000 meters from the
microphone position; this caused the deceleration to decrease to zero. With the ex-
tension of the landing gear and the landing flaps, the deceleration did not decrease
as rapidly as on flight 3 because of the thrust generated by the engines. However,
over the noise measuring station, the deceleration was about the same on flights 3
and 4-approximately 2 meters per second squared.
Acoustic Results
Time histories of the perceived noise levels measured during the landing
approaches are shown in figure 5. To calculate an effective perceived noise level,
a duration time, t i , was defined as the time interval during which the perceived
noise level was within 10 decibels of the maximum perceived noise level. These
times are shown in figure 5 for the landing approaches. The duration correction
was then calculated by using the equation
t.
1
D = 10 log 5
This correction, which accounts for the effects of the noise time history and duration,
was used to calculate the effective perceived noise level with the following equation:
EPNL = PNLM + D
It should be noted that for spectra with no discrete tones, as in this study, the per-
ceived noise level equals the tone-corrected perceived noise level.
As shown in figure 5, the shortest duration time measured was on the standard
30 approach (flight 2), and the longest duration time was on the 60 approach
(flight 1). The longer duration time was the penalty for a 60 glide slope approach.
However, the peak perceived noise level was reduced significantly. The idle-power
decelerating approach on flight 3 also had a relatively long duration time compared
to that of the standard approach; whereas, the partial-power approach showed little
increase in duration time but a large reduction in peak perceived noise level.
The sound pressure level spectra at the time of maximum perceived noise level
for each landing approach are shown in figure 6. All the spectra are characterized
by prominent dips at 80 hertz and 200 hertz. The dips are caused by destructive
interference between the incident sound wave and the wave reflected by the ground
surface.
To properly evaluate the results of this study, they must be compared with a
standard such as the approach noise criterion in FAR, Part 36 (ref. 1). This
5
comparison is shown in figure 7. The approach noise criterion for the JetStar air-
craft at its maximum takeoff weight is 102 EPNdB. The noise level at the approach
noise measuring point for a standard 30 approach is 106.8 EPNdB. The approach
noise measured under a single-segment 60 approach is 98.6 EPNdB, or a reduction
of approximately 8 EPNdB from the standard approach noise level. As previously
mentioned, 6 decibels of this noise reduction are attributed to the fact that the air-
craft altitude over the noise measuring point was twice as great for the 60 approach
as for the 30 approach. The remaining 2-decibel reduction is a result of the de-
creased thrust required on the 60 approach. The approach noise is reduced further
on the idle-power 30 approach. The noise measured on this approach is 90.7 EPNdB,
which is a reduction of 16.1 EPNdB from the noise generated on a standard approach.
The crosshatched area in figure 7 represents the probable level of nonpropul-
sive aerodynamic noise discussed in reference 2. Unpublished results of power-off
landings at the Flight Research Center with the JetStar aircraft confirm that the non-
propulsive noise level of the aircraft in a landing configuration is about 90.7 EPNdB
under these test conditions. Thus it appears that 16.1 EPNdB is the maximum noise
reduction achievable for the JetStar on landing approach.
The noise level of 97.7 EPNdB measured beneath the landing approach path for
the partial-power 30 approach is also a significant reduction in approach noise. Thusit seems that a partial-power decelerating approach can be just as effective in re-ducing approach noise as a 60 approach.
CONCLUDING REMARKS
A preliminary flight-test program evaluated the potential noise reduction that
could be achieved through the use of a decelerating landing approach. The noise,
measured at the FAR, Part 36 noise measurement station for approaches, was com-pared with the noise for a standard 30 glide slope approach and a 60 approach. Thefollowing observations can be made:
(1) Reductions in approach noise of as much as 16.1 EPNdB for the JetStar air-
craft can be achieved through the use of the decelerating approach. For this air-
craft the lower limiting noise level on approach with idle thrust is the nonpropulsive
airframe noise.
(2) Decelerating approaches cannot be flown consistently without some onboardguidance system that accounts for the effects of wind on the deceleration of the air-
craft.
(3) Decelerating approaches at power levels between standard approach power
and idle power can result in significant reductions in approach noise levels.
Flight Research Center,
National Aeronautics and Space Administration,
Edwards, Calif., Jan. 4, 1974.
6
REFERENCES
1. Anon.: Federal Aviation Regulations. Part 36-Noise Standards: Aircraft Type
Certification, Appendix B. FAA, Dec. 1969.
2. Blumenthal, V. L.; Streckenbach, J. M.; and Tate, R. B.: Aircraft Environ-
mental Problems. AIAA Paper No. 73-5, Jan. 1973.
7
TABLE 1.- SUMMARY OF AIRCRAFT OPERATING SCHEDULE DURING LANDING APPROACHES
Calibrated airspeed at
Flight Glide slope, Intercept Intercept calibrated Caibrated airspeed at Engine speed,
number deg altitude, m airspeed, m/sec m/sec percent
1 6 975 74.6 74.6 76
2 3 640 74.6 74.6 84
3 3 884 118.3 69.4 Idle
4 3 579 102.9 69.4 69
-
IM9 97- I
Figure 1. Test aircraft. E-21742 : ; : ::: ;: ;:
[=I> DIRECTIONA OF FLIGHT
LAK'EBED RUWWAY /5
YM I C ROPHO VE
LOCAT/ON VASt LIGHT
RECORDING
STATIO/
Figure 2. Schematic diagram of microphone location in relation to the approach ground track and
VASI light.
1000
750
ALTITJDF tA
500
250
-15 -10 -5 0 5 10 15 0 5 10 15
TA IL WIND, MSEC CROSSVW/V' FROMI RIGHT
t/ 5 EC
Figure 3. Wind conditions between the ground and the maximum test altitude
approximately 4 hours before the flight tests.
11
S20
/00
/OO
VELOCIT
/-SEC 9
70
601
800
600
ALTITUDE, m
qO0
200
0 i i
0 2000 -o000 6000 8000 /qooo
DISTANCE FROM MIC.?OPHOE, /1
(a) Elevation and velocity profiles for 60 approach (flight 1).
Figure 4. Aircraft elevation, velocity, and acceleration profiles during landing
approaches.
12
/20
VELOCITY,
70r
800
600i
SII
0 2000 'Y000 60oco 8000 ,000
DISTANCE PR/OM MICROPHONE ,M
(b) Elevation and velocity profiles for standard 30 approach (flight 2).
Figure 4. Continued.
13
--.1
ACCELERATION,
M/SECa
-2
-3
120
I/0
/00
VELOC /TY,
/IEC
90
70
o60
8oo
600
ALTITUDE, M
200
o 2ooo0 ooo 6000 8000 /0,000
DISTANVCE FROM MICROPHONE, t'
(c) Elevation, velocity, and acceleration profiles for idle-power 30 approach (flight 3).
Figure 4. Continued.
14
-I
AccELERATN/ON
M/SECa
-2
-3
120
/00
VELOCITY)
M/S EC
80
70
60
800
600
ALTITUDE, A
oo00
200
O 2000 L/o00 6000 O8000 1,000
DISTANCE FROM MICROPHONE, M
(d) Elevation, velocity, and acceleration profiles for partial-power 30 approach(flight 4).
Figure 4. Concluded.
15
120
DURATIoN TIMES
, = , 7 seC
1:= 5' SEC
StO 78 SEC
5. 8 SEC
/---- STANVDAkD 30 APPRoACH .(FLIGHT a)/00
PERCEIVeO NOISE
LEVE , pNJ8 6' APPROACH (FLIGHT I )
90 _PARTIAL-POWER
30APPROACH (FLI6HT /)
IDL E-POWER
0 _ 30 APPRoACH (FLIGHT 3)
70 I I I
0 5 /0 15 20
RELATIVE TIME, SEC
Figure 5. Time histories of perceived noise levels during the landing approaches.
100
90
SOUND PRESSPRE
LEVEL IdB
70
60
O 60 APPRoACH
0 STANDARD 3APPROACl
o50 O IbL-POwER 3" APPROACH
A PART/AL- POWER 30 APPROACH
0 I I I I I I LI I IA
50 200 2000 /lo,ooo
FREQUENCY) HR
Figure 6. Noise spectra at the time of maximum perceived noise level for the landing approaches.
20 
6
0 STANDARD 30 APPFRO4cH
0 IDLF-Pow'ER 3 0 APPRoAtCH
A PaRTIAL-POWER 3' APPROACH
I10
EPNL)
EPNVdJ
/ oo _ (RPE I)
REFERElEA, ? 2
80_ i 
_ I
5 so0 00 x /.:.
M1AXIMUM TAKEOFF WEIGHT, KG
Figure 7. Comparison of present results with FAR, Part 36 criterion for approach
noise.
18


Flight experience with the decelerating noise abatement approach

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