AC 20-191 Appendix A Term D efinition
• Threshold The height of the straight line extension of the glide path above the Crossing runway at the threshold.
Height (TCH)
• Threshold The theoretical height above the runway threshold at which the Crossing aircraft’s glideslope antenna would be if the aircraft maintained the Height (TCH) trajectory established by the mean ILS glideslope or MLS glidepath.
(Pilots Handbook of Aeronautical Knowledge)
• Touchdown The first 3000 ft. of usable runway for landing, unless otherwise Zone specified by the FAA, or other applicable ICAO or State authority
(e.g., for Short Take Off and Landing (STOL) aircraft, or in accordance with a Special Federal Aviation Regulation (SFAR)).
• Wet Runway A runway surface that is neither dry nor contaminated but has visible dampness, moisture or water less than 1/8 inch in depth or both. Wet is considered to be any condition “not clear and dry” on any part of the useable area of the runway (useable area does not include edges, sides, melting of ice or snow banks at edges or sides, area beyond the advertised plowed and sanded surface, overruns, etc.).
A-15
AC 20-191 Appendix B APPENDIX B. WIND MODEL FOR APPROACH AND LANDING SIMULATION B.1 Wind Models.
This appendix describes three types of wind models, namely wind model A, B and C. In carrying out the performance analysis, the applicant may use one of the models of wind, turbulence and wind shear. Wind model analysis is required to obtain approval of systems related to CAT III, and the FAA recommends wind model analysis to obtain approval of systems related to Category II. GAST D also must implement a wind model analysis.
B.2 Wind Model A.
B.2.1 Mean Wind.
The mean wind is the steady state wind measured at landing. This mean wind is composed of a downwind component (headwind and tailwind) and a crosswind component. The cumulative probability distributions for these components are provided in Figure B-1 (downwind) and Figure B-2 (crosswind). Alternatively, the mean wind can be defined with magnitude and direction. The cumulative probability for the mean wind magnitude is provided in Figure B-3, and the histogram of the mean wind direction is provided in Figure B-4. The mean wind is measured at a reference altitude of 20 ft. AGL. The models of the wind shear and turbulence given in the following sections assume this reference altitude of 20 ft. AGL is used.
B.2.2 Wind Shear.
The wind shear component is that portion that affects the air mass moving along the ground (i.e., ground friction). The magnitude of the shear is defined by the following expression:
h+ 0.15V = 0.20407 V ln wref 20 0.15 where Vwref is the mean wind speed measured at h ft. and V 20 is the mean wind speed (ft/sec) at 20 ft. AGL.
B.2.3 Turbulence.
The turbulence spectra are of the Von Karman form.
B.2.3.1. Vertical Component of Turbulence.
The vertical component of turbulence has a spectrum of the form defined by the following equation:
B-1
AC 20-191 Appendix B L σ2 ( 2w1+ 2.67 1.339L Ω) Ω w w ) wΦ ( = ( ) 112 62π1+ 1.339L Ω w where:
= spectral density in (ft./sec)2 Φ w σw = root mean square (rms) turbulence magnitude in ft/sec = 0.1061V20(ft/sec).
Where V 20 i s expressed in ft/sec.
Lw = scale length = h (for h < 1000 ft) Ω = spatial frequency in radians/ft = ω/VT ω = temporal frequency in radians/sec, and VT = aircraft speed in ft./sec.
B.2.3.2. Horizontal Component of Turbulence:
The horizontal component of turbulence consists of a longitudinal component (in the direction of the mean wind) and lateral component (see Figure B-5). The longitudinal and lateral components have spectra of the form defined by the following equations:
B.2.3.2.1 Longitudinal Component:
( ) L σ2Φu Ω = ( u u π 1+(1.399ΩLu) 2 ) 56 B.2.3.2.2 Lateral Component:
L 2v σv (1+ 2.67(1.339L Ω)2 ) Φv(Ω) v = 2 ( +( 11 π 1 1 .339L 2 ) 6 vΩ) B-2
AC 20-191 Appendix B B.2.3.3. Turbulence Intensities and Scale Lengths.
The RMS turbulence intensities are defined as below σw = 0.1061V 20 (ft/sec)
• When h≥1,000ft. σ = σ = σ u v w
• When h<1,000ft.
0.4 σ =σ =σ u v w 0.177+ 0.000823h
• When h≤0ft.
1
0.4 σ =σ =σ u v w 0.177 The turbulence scales are defined as below
• When h≥1,000ft. L = L = L = 1,000 u v w
• When h<1,000ft L = h w 1
1.2 L = L = h u v ( 0.177+ 0.000823h)
• When h≤0ft. L = L = L = 0 u v w B-3
AC 20-191 Appendix B Figure B-1 Headwind-Tailwind Probability of Exceedance, %
99.99 99.9 99 90 80 70 60 50 40 30 20 0.01 30 TAILWIND SPEED .KT (WIND FROM 180 DEG) 10 HEADWIND SPEED .KT (WIND FROM O DEG) -300.01 0.1 10 20 30 40 50 60 70 80 90 99 99.9 99.99 Probability of not Exceeding, %
HEADWIND-TAILWIND DESCRIPTION B-4
AC 20-191 Appendix B Figure B-2 Crosswind CROSSWIND SPEED.KT COUNTERCLOCKWISE FROM 0 DEG 99.99 99.9 99 90 Probability of Exceedance, % 80 70 60 50 40 30 10 0.1 0.01 30
0.1 20 30 40 50 60 70 80 99 99.9 99.99 _ Probability of Exceedance, %
CROSSWIND DESCRIPTION B-5
AC 20-191 Appendix B Figure B-3 Annual Percent Probability of Mean Wind Speed Equaling or Exceeding Given Values PERCENT PROBABILITY OF EXCEEDANCE 20
0.2
o. O 5 10 15 20 25 30 35 40 Wind Speed Tw kt
ANNUAL PERCENT PROBABILITY OF MEAN WIND SPEED
EQUALING OR EXCEEDING GIVEN VALUES 9-&
AC 20-191 Appendix B Figure B-4 Wind Direction Relative to Runway Heading Wind Direc tion Histog ra m Wind Direction Rela tive to Runwa y Hea ding P e r c e n t 0 2 4 6 8 10 12 180190200210220230240250260270280290300310320330340350 0 10 20 30 40 50 60 70 80 90 100110120130140150160170180
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Wind fr om the L eft Wind from the R ight B-7
AC 20-191 Appendix B Figure B-5 Selected Description for Variances of Horizontal Turbulence Components.
— f[0.177+0.823hh ¡] ‡ h≤h¡
1.0 mani
1.2
1.0
0.8 h/h j 0.6
0.2
1.0 1.2 1.6 2.0 B-8
AC 20-191 Appendix B Figure B-6 Selected Integral Scale Description.
Lw =1 d,h = hi h,h<hi LWoU h h < hj Lu ={ Ow I 10.177 + 0.823 h/t ] 1.2 d nzhi Ly=Lu hj= Altitude above which turbulence is isotropic
1.2 Г
1.0
0.8 h/h¡
0.6
0.4 L=L
0.2
0.2 0.4 0.6 0.8 1.0 Lw/h¡›Lu/h¡= Lv/h¡
SELECTED INTEGRAL SCALE DESCRIPTION В-9
AC 20-191 Appendix B B.3 Wind Model C.
This wind model is a harmonized derivative of Wind Model A incorporating the changes defined in JAA paper AWOG 904a. The changes are a result of experience from pilot-in-the-loop simulator tests for Category III HUD certification, where the wind shear and turbulence intensities defined in JAA paper AWOG 904a were found to be more representative of nominal wind conditions for Category III approaches and compatible with motion-based, pilot-in-the-loop simulators. Wind Model C is therefore more appropriate for use in Category III HUD certification programs; however, this does not preclude its use for other types of landing system.Wind Shear (Vwref).
B.3.2 The Model A equation for the Wind Shear component:
h+ 0.15V = 0.20407 V ln wref 20 0.15 is replaced for Wind Model C with:
h+ 0.046Vwref = 0.165 V20 ln 0.046 B.3.3 Root-Mean-Square (RMS) Turbulence Scale (σw).
The Model A equation for the RMS Turbulence Scale: σw = 0.1061V20 (ft/sec) is replaced for Wind Model C with: σw = 0.0625V20 (ft/sec) B.3.4 Turbulence Scale Factor (Lu).
The Model A establishes three values for Turbulence Scale Factor Lu depending on the altitude h, but for Model C, Lu = 600 feet, for all values of h B.4 Wind Model B.
B.4.1 Mean Wind.
It may be assumed that the cumulative probability of reported mean wind speed at landing and the crosswind component of that wind are as shown in Figure B-7.
Normally, the mean wind that is reported to the pilot is measured at a height that may be between 20 ft.(6m) and 33 ft. (10m) above the runway. The models of wind shear and turbulence given in the following paragraphs assume this reference height.
B.4.2 Wind Shear.
Wind shear should be included in each simulated approach and landing unless its effect can be accounted for separately. The magnitude of the shear should be defined by the expression:
u = 0.43 U log10 (z) + 0.57 U, for z ≥ 0.05 m u≅ 0, for z < 0.05m where z is the height in meters u is the mean wind speed at height z (meters) B-10
AC 20-191 Appendix B U is the mean wind speed at 10m (33 ft.).
B.4.3 Abnormal Wind Shear.
The effect of wind shears exceeding those described above should be investigated using known severe wind shear data.
B.4.4 Turbulence.
B.4.4.1. Horizontal Component of Turbulence.
It may be assumed that the longitudinal component (in the direction of mean wind) and lateral component of turbulence may each be represented by a Gaussian process having a spectrum of the form:
( ) 2σ2 LΦ Ω = •π 1+Ω 2 L2 where Φ(Ω) = a spectral density in (meters/sec)2 per (radian/meter).
σ = root mean square (rms) turbulence intensity = 0.15 U L = scale length = 183m (600 ft.)
Ω = spatial frequency in radians/meter.
B.4.4.2. Vertical Component of Turbulence.
It may be assumed that the vertical component of turbulence has a spectrum of the form defined by the equation above for the horizontal component with the following values:
σ = 1.5 knots with L = 9.2m (30 ft.), or alternatively σ = 0.09 U with L = 4.6m (15 ft.) when z < 9.2m (30 ft.), and L = 0.5 z when 9.2 < z < 305m (30 < z < 1000 ft.)
B-11