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AC NO: 20-578 

DATE: 12 Jan 71 

ADVISORY 

CIRCULAR 

DEPARTMENT OF TRANSPORTATION 

FEDERAL AVlATlON ADMINISTRATION 

SUBJECT: AUTOMATIC LANDING SYSTEMS (ALS) 

1. 

2. 

* 3. 

4. 

PURPOSE. This circular sets forth an acceptable means of compliance 

but not the only means for the installation approval of automatic 

landing systems in transport category aircraft which may be used 

initially in Category II operations. Approval of these aircraft for 

use under such conditions will permit the accumulation of data for 

systems which may be approved for Category IIIa in the future. 

REFERRIYCES. Federal Aviation Regulations 21.21, 25.1301, 25.1309, 

25.1581, and 121.579; ICAO Annex 10. 

CANCELLATION. AC No. 20-57, dated 29 January 1968, is cancelled. * 

DISCUSSION. 

a. Applicants are obtaining approval of aircraft equipped with systems for use during Category II operations. Automatic landing 

capability is included in, or planned for, some of these aircraft. 

Manufacturers and operators have indicated a desire to utilize 

that capability under visual landing conditions, when such aircraft are found satisfactory for making automatic landings during 

normal operations. 

b. The accumulation of service experience in an operational environment and the development of confidence in such systems are considered essential parts of the evolutionary process in aircraft 

design associated with further reduction in weather minimums for 

future operations. 

c. Aircraft which demonstrate in service use capability of meeting 

touchdown limits, as defined in paragraph 5b, may be expected to 

comply with touchdown dispersion criteria for operation under 

Category IIIa conditions, according to best information available 

to FAA at this time. 

Initiated by: FS- 130 
AC 20-57A 

5. AUTOMATIC LANDING SYSTEM CRITERIA. 

12 Jan 71 

a. General, ALS evaluation should be based on the use of ILS approach 

facilities which meet the following performance on a two-sigma 

basis (95.5% probability of occurrence in a normal distribution): 

Localizer (to touchdown) - Course Alignment Accuracy f 10' 

- Bends & 5ua 

- Airborne Receiver 

Centering error - + 5ua 1 

Other characteristics as recommended in ICAO Annex 10 dated 

April 1968 for Category II. 

Glide Slope - Airborne Receiver Centering Error - -i- 1Oua 1 

Other characteristics as recommended in ICAO Annex-10 dated 

April 1968 for Category II. 

Monitor - Automatic monitoring system as recommended in ICAO 

Annex 10 dated April 1968 for Category II. * 

Computer analysis of system performance should be based on these 

ground facility characteristics. Analysis of results of in-flight 

demonstration should include compensation for beam deviations (such 

as subtraction of beam deviations beyond 10 feet and treatment of 

the remainder on a probability basis with other variables) when 

these are found to be in excess of the permitted tolerances. 

b. Aircraft Touchdown Limits. The automatic landing system should be 

demonstrated co achieve the accuracy limits listed below, considering the full range of c.g. limits and landing weights. 

(1) Environmental conditions to be considered in the determination 

of dispersion limits are: Headwinds up to 25 knots; tailwinds 

up to 10 knots; crosswinds up to 15 knots; moderate turbulence, 

wind shear of 8 knots per 100 feet from 200 feet to touchdown. 

(2) Confirmation of compliance with such limits may be demonstrated 

by a combination of: 

(a) An analysis considering reasonable combinations of environmental conditions noted in (1) above, using wind model 

described in Appendix 1. 

(b) Flight demonstrations as appropriate to verify results of 

* the analysis. 

' Or the equivalent when a receiver with a high level output is used. * 

d’ - 

Page 2 Par 5 
* 

12 Jan 71 AC ZO-57A 

* 

(3) Distribution of touchdowns about a nominal point on a two-sigma 

basis should be established with reference to the desired airplane/system characteristics, as follows: 

(a) Longitudinal dispersion about the nominal point of the 

main landing gear touchdown should not exceed 1500 feet 

total, but need not be symmetrical about the nominal 

point. 

(b) Lateral dispersion of the aircraft centerline at the main 

landing gear should not exceed 27 feet either side of the 

centerline of the runway. 

(4) A suitable computer analysis should show that under realistic 

environmental conditions including the wind model described 

in Appendix 1, the touchdown performance will be such that 

landing outside of the prescribed dispersion area will be 

improbable. 

(a) Longitudinal touchdowns between a point at least 200 feet 

beyond the threshold and that point down the runway at 

which the pilot is in a position to see at least 4 bars 

(on 100' centers) of the 3000 foot touchdown zone lights. 

(b) Lateral touchdowns with the outboard landing gear no 

closer than five feet from the lateral limits of a 150 

foot runway. * 

c. Automatic landing system malfunction should not: 

(1) 

(2) 

(3) 

Cause significant displacement of the aircraft from its 

approach path, including altitude loss,, 

Upon system disconnection, involve any out of trim condition 

not easily controlled by the pilot. 

Cause any action of the flight control system that is not 

readily apparent to the pilot, either by control movement or 

advisory display. 

d. Means should be provided to inform the pilot continuously of the 

mode of operation of the automatic landing system. Indication of 

system malfunction should be conspicuous and unmistakable. Positive indication should be provided that the flare has (or alternatively has not) been initiated at the minimum normal flare 

engage heights. * 

Par 5 Page 3 
AC 20-578 12 Jan 71 

6. AIRPLANE FLIGHT MANUAL. Information pertinent to the operation of the 

automatic landing system and aircraft limitations should be included 

in the airplane 

Acting Director, 

Page 4 Par 6 
12 Jan 71 AC 20-578 

Appendix 1 

* APPENDIX 1. WIND MODEL FOR APPROACH SIMULATIONS 

The cumulative probability of mean windspeed is as shown on page 2 of 

this appendix. 

Turbulence is made up of three components, longitudinal, vertical and 

lateral (see page 3 of this appendix). 

The turbulence rms level is directly proportional to the mean windspeed. 

The effect of turbulence is a function of the aerodynamic characteristics 

and dimensions of the airplane. 

The power spectrum of turbulence is defined by: 

w= frequency rad/sec 

CT = rms turbulence intensity 

L = scale length feet 

v = airplane velocity feet per second 

Longitudinal Turbulence 

L = 600 feet 

cu = .15 mean headwind or tailwind (knots) 

Vertical Turbulence 

L = 30 feet 

6w = 1.5 knots 

The effect of vertical turbulence is small and a constant level 

is satisfactory. 

Lateral Turbulence 

L = 600 feet 

5 = .15 mean crosswind (knots) 

Page 1 
AC 20-5711 

Appendix 1 

12 Jan 71 

WIND SPEED-KNOT 

-99.3 

-99.9 

m-99 

SO 

-0 

-0 

-0 

50 

-0 

-0 

LO 

10. 

\ 

1.t 

Page 2 
12 Jan 71 AC 20-57A 

Appendix 1 

* Longitudinal Turbulence 

Gust 

Model 

r White 

Noise 

Generator TLo&+l 

- Span --Airspeed 

Averaging Simulation 

TLong. 

is &: where L is Longitudinal Scale Length = 600 ft 

v V = Approach Speed (ft/sec) 

Vertical Turbulence 

Gust 

Model 

r r 

White ___c 1 - Area - Incidence 

Noise TVert.S+l Averaging Simulation I 

is &: where L is Vertical Scale Length, approx 30 ft 

L TVert. V V = Approach Speed (ft/sec) 

Lateral Turbulence 

TLat, 

is &: where L is Lateral Scale Length = 600 ft 

V V = Approach Speed (ft/sec) 

T represents time constant "tau." 

S represents the Laplace operator cc U where t denotes a real variable 

and S a complex variable: 

2 Cflt,l ;s -&*‘f (t)dt 

0 

Page 3 
DEPARTMENT OF TRANSPORTATION 

FEDERAL AVIATION ADMINISTRATIOS 

Washington, I1.C. 205!M 

Official Business 

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