A NEW METHOD TO CONFIRM CATEGORY I11 AUTOLAND PERFORMANCE

by Harold K. Cheney*

ABSTRACT

and Canh T. Pham

Douglas Aircraft Company McDonnell Douglas Corporation

Long Beach, California

Flight testing and certification of Category 111 autoland systems require the measurement of touchdown positions for numerous land- ings. XILS, a new computerized flight-test procedure, has been developed to efficiently calculate an aircraft's position during an instrument landing system (ILS) landing and rollout. This method uses glide slope and localizer deviations, radio altitude, inertial reference system (IRS) ground speed, and ILS geometry. It provides data of the longitudinal distance from the glide slope transmitter and lateral distance from the localizer beam's centerline. The input data needed to make the calculations are recorded on the test aircraft's data tape. The test method described has been successfully used to confirm Part 25 Category 111 autoland performance, and is faster and less expensive for obtaining autoland performance data than previ- ous flight-test position-tracking procedures. This paper presents the equations and validation methods used by Douglas Aircraft Com- pany to establish the procedure for autoland flight testing.

INTRODUCTION

This paper describes a procedure for obtaining aircraft longitudinal and lateral touchdown position for instrument landing system (ILS) flight tests. The procedure, developed to analyze the results from the numerous demonstration landings required to validate Category 111 autoland performance, is Douglas Aircraft Company's XILS com- puter program. XILS uses the ILS deviations, radio altitude, and inertial reference system (IRS) ground speed data to calculate posi- tion information. I t has significantly reduced the time and expense of obtaining touchdown dispersion data. The procedure was demon- strated during certification of the Sperry Digital Flight Guidance Computer for the MD-80 and MD-87 aircraft.

AUTOLAND TESTING

Figure 1 presents a simplified depiction of the ILS system currently used at Category I1 and 111 airport facilities. The system uses two radio beams to guide approaching aircraft to the runway: a glide slope signal for vertical guidance and a localizer beam for lateral guidance along the runway centerline. The prescribed touchdown zone, or box, is described in FAA Advisory Circular No. 20-57A (Reference 1) .

Simulator tests are used to confirm that the autoland system will satisfactorily meet FAA requirements for the full range of weight, center of gravity, wind, field altitude, and temperature conditions

that it will encounter, while flight tests are used to demonstrate the system's performance under characteristic flight conditions at typical airports. To be certified, the system's flight test data must correlate with the much larger data base obtained through simulations. Toward this end, the XILS program is designed to produce the touchdown position and lateral deviations of individual autoland flight tests.

* ,.

Figure 1. Instrument Landing System (ILS) . ..*

SPACE POSITION TRACKING

To analyze autoland performance tests requires a system that measures the aircraft's position during the test. In the past, this position-tracking data has been provided by a wide variety of systems, from observers on the side of a runway to sophisticated laser tracking systems. A common method has been to use an airborne belly camera that photographs an approaching runway and uses the runway lights as position references.

The XILS program uses the ILS beams as the reference system, and calculations made throughout the approach and rollout provide data for analysis. The aircraft reference position is defined as the point midway between the bottom of the uncompressed main landing gear wheels when the struts are extended.

For autoland tests and demonstrations, two items of information are required: the touchdown location and the lateral distance from the runway centerline during the landing roll. For touchdown, both lon- gitudinal and lateral locations are required. The longitudinal distance along the runway is the distance from the glide slope transmitter. The operations for calculating this location are summarized in Figure 2 and discussed in the next section. The lateral distance from the

*Tc\l and Cert~ficalion; A lAA A\\oc~ate Fellow.

Copyright @ American Institute of Aeronautics and Astronautics. Inc., 1988. All rights reserved.

centerline depends on longitudinal location and is discussed in the section "Lateral Calculations."

PITCH ATTITUDE HEIGHT OF GIs RECEIVER

AIRCRAFT ANTENNA

GEOMETRY r GLlM SLOPE

DISTANCE TO THRESHOLD

I ANOLE

DISTANCE FROM TIME AT

THRESHOLD TO THRESHOLD

ILS OIS ANTENNA

DISTANCE FROM

ILS OIJ ANTENNA LONGITUDINAL TOUCHDOWN

TOUCHDOWN POSITION

Figure 2. Longitudinal Distance from Glide Slope Transmitter

LONGITUDINAL CALCULATIONS

A primary XILS parameter, "LONG" (the longitudinal position of the aircraft's landing gear relative to the runway threshold) is calculated using two equations. The first one, used during the landing approach, is employed primarily to define the time the aircraft crosses the runway threshold. The second equation is used to calculate the aircraft's distance from the threshold. The calculation of LONG approaching the threshold consists of two elements: "HIGH" and "ANGGS."

HIGH is the height of the aircraft glide slope (G/S) receiver antenna above the ground. It is obtained from radio altitude, pitch attitude, and aircraft orientation. Radio altitude, or ALT, provides the basic measurement of the aircraft's height and is calibrated to indicate the height of the main landing gear tires at a reference pitch attitude. The equations used to calculate HIGH are presented in Figure 3.

GLlDE SLOPE ANTENNA,

HIGH = ALT + (LGS - LALT) x SIN (PITCH) + VGS x COS(PITCH) - .

Figure 3. Height of Aircraft Glide Slope Antenna

ANGGS, as shown in Figure 4, is the angle between a line extending from the aircraft glide slope antenna to the intersection of the glide slope null line with the runway and a horizontal plane.

NULL LINE

RUNWAY - . 9

UHF GLlDE SLOPE

GLlDE SLOPE ANTENNA

AA I ANGGS I

ANGGS = GSA + GSDEV x GSBIlSO

.. .. . Figure 4. Angle to Glide Slope Antenna " * -

LONG is then calculated as shown in Figure 5 and is a negative value before reaching the runway threshold. Because of variations in ter- rain, the values of LONG are not correct until theaircraft is above the level area approaching the runway. The computer calculates LONG at 0.2-second intervals. The purpose of this first calculation is to determine the time the aircraft wheels are at the threshold.

ILS GLIDE SLOPE THRESHOLD

ANTENNA GLlDE SLOPE

LONG pi SIGN FOR LONG

LONG = GSD - HIGHKAN (ANGGS) - LGS ., 0 . . .- Figure 5. Longitudinal Distance to Threshold

The second set of equations is used after the aircraft crosses the threshold. The values of LONG then become positive. This is a more accurate distance calculation that is obtained by integrating ground speed obtained from IRS. The equations for this operation are:

LONG (I) =

where

LONG (I) =

LONG (1-1) =

SPD - -

VCV - -

0.3376 - -

LONG (1-1) + 0.3376 X (SPD + VCV),

Distance of landing gear from threshold for current point

Distance of landing gear from threshold for previous point

IRS ground speed (in knots)

Correction for IRS speed (in knots)

0.2-second interval x knots to feet/second conversion

VCV is the value required to adjust the indicated IRS ground speed to the correct value. If the landing were monitored until the aircraft stopped rolling, VCV would be the delta speed that adjusts the indi- cated speed at that time to zero. During Douglas Aircraft Company tests, the speed indicated by the anti-skid braking system is the reference used to obtain the VCV value; it is an approximation suitable for the accuracy that autoland touchdowns require.

To provide the final distance values, the following equation is used:

DIST = LONG - GSD,

where

DIST = Distance from glide slope transmitter

LONG = Distance from threshold

GSD = Distance from threshold to ILS glide slope antenna

The DIST value for touchdown is obtained using touchdown time and the data array of DIST versus time.

LATERAL CALCULATIONS

"LAT," the lateral distance of the aircraft centerline at the main landing gear from the centerline of the ILS localizer beam, is deter-

mined using the indicated lateral deviation and longitudinal distance from the ILS localizer antenna. Figure 6 summarizes the LAT calculation operations.

ILS LANDINO DISPERSION 0212W7 SHIP NUMBER: 1333 ENOINEER: CTP

FLIGHT NUMBER: 0017 CARD: W2

I I ANGLO I 1 1 LOCALIZER

" W a , .a.c.-

Figure 6. Lateral Distance from Localizer Null Line

DEVIATION

BEAM WIDTH

"ANGLO" - the angle between two lines extending from the localizer antenna, one down the runway centerline and the other to the aircraft localizer antenna - is calculated using the localizer indi- cated deviation, as shown in Figure 7.

- ANOLE A/C LOCALIZER

ANTENNA TOILS LOCALIZER BEAM

CENTERLINE

Figure 9. Typical XILS Plot

VALIDATION OF TOUCHDOWN POSITION

LONG

DISTANCE TOIFROM

THRESHOLD

DISTANCE LOCALIZER

ANTENNA FROM THRESHOLD

TOUCHDOWN

The accuracy of the XILS procedure was confirmed by a variety of checks during the program's development. Because minor ILS beam variations exist at some airports and occur at a given airport for dif- ferent landings, XILS and external tracking system results are not identical. A typical comparison of the statistical results from a group of landings at various airports follows.

LATERAL DISTANCE FROM LOCALIZER

LATERAL TOUCHDOWN

POSlTlON

LOCALIZER ANTENNA

TIME

AIRCRAFT LOCALIZER

Standard Mean Deviation (Feet) (Feet)

-3..,L

Figure 7. Angle to Localizer Beam Centerline ""**

Longitudinal Distance

XILS Belly Camera

As shown in Figure 8, ANGLO becomes part of the LAT equation, whose accuracy depends on the quality of the localizer beam during the test.

Lateral Distance

XILS Belly Camera

This is good overall agreement when individual landings differed by as much as 100 feet in longitudinal distance and 10 feet in lateral distance. XILS provides an evaluation of autoland performance relative to the ILS beams as they existed during the test.

ILS LOCALIZER ANTFNNA

AIRCRAFT AIRCRAFT LANDINO LOCALIZER

OEAF ,ANTENNA

A comparison of touchdown locations obtained by XILS and belly cameras for landings at a typical airport is shown in Figure 10. For the test airport, the two systems' results differed by up to 60 feet.

LAT = (LONG - LOCD - LLOC) x TAN (ANGLO)

LONGITUDINAL TID LOCATION (TYPICAL) 800

0 Figure 8. Lateral Distance from Localizer Beam Centerline

For the LONG and LAT calculations, detail equations not included in this paper include minor adjustments for heading and roll attitude.

XILS LONG FROM GIs TRANSMITTER

OUTPUT

As shown in Figure 9, the XILS program presents its primary outputs - the touchdown values for DIST versus LAT - as a printed plot. The program can produce similar plots of time from touchdown ver- sus lateral distance and radio altitude versus distance.

BELLY CAMERA LONG FROM GIS TRANSMITTER .w-7,

Figure 10. Correlation of XILS and Belly Camera

LATERAL DISTANCE VALIDATION

The validation of lateral distance (LAT) during landing rollout is accomplished using IRS data. For the IRS tracking procedure, the distances along and from a reference line are calculated. Distance values are obtained by iteration of the instantaneous velocities relative to the reference line. The component velocities are obtained using the following two equations:

VLONG =

VLAT =

where

VLONG =

VLAT =

VN/S =

VE/W =

TRD =

VN/S x cos (TRD) + VE/W x sin (TRD)

VE/W x cos (TRD) + VN/S x sin (TRD),

Longitudinal velocity along the reference direction

Lateral velocity perpendicular to the reference line

North component of IRS indicated velocity

East component of IRS indicated velocity

Reference direction

For the initial IRS calculation, the reference direction (TRD) is loaded as the runway heading. Normal, slow variations of the indicated IRS velocities cause the mean indicated reference direction to differ from the runway heading. Normally, this would be rectified by correcting the component velocities (Reference 2). This is not practical for autoland tests, but a similar adjustment can be made by changing TRD. Figure 11 depicts typical results of this type of calculation for verifying the XILS lateral distance. The XILS line is representative of the localizer information that is available to help the autopilot control the aircraft's direction. The agreement is not exact, but it is within 10 feet for two completely different sources of lateral position informa- tion. The oscillations of lateral distance in the early part of the run are due to variations in the localizer beam. The IRS tracking procedure has been confirmed by laser tracking.

LONOlTUDlNAL DISTANCE (FT)

LATERAL DISTANCE (FT) -o,-

rnwm

Figure 11. Correlation of XILS and IRS Tracking Data

7000

O a m -

5000-

4mo

3000-

2000

1WO

- o

-2000

PROGRAM INPUT INFORMATION

-

-

-

OIS XMTR

- CENTERLINE OF LOC. BEAM

I

The information required to operate the program is provided by the following computer program records, or run cards:

- l W - 50 0 50

2. Airport constants

3. Calculation times

4. Test data parameters

5. Airplane constants

Card 1 is used to define information for a group of runs. It provides the following items:

Aircraft type

Airport

Date

Aircraft serial number

Flightnumber

Initials of analysis engineer

Card 2 is usually not required because the information for most air- ports used in autoland tests is loaded in the program software. The airport designation on Card 1 selects which set of airport values are to be used. The autoland test requires the following information:

ILS glide slope angle (GSA)

Glide slope half angle width for 150 millivolts (GSB)

Distance from threshold to ILS glide slope antenna (GSD)

Localizer half-angle width for 15 millivolts (LOCB)

Distance from threshold to ILS localizer antenna (LOCD)

True heading of runway (for LAT sideslip correction) (TRUH)*

Card 3 is required for every calculation and provides:

Run start time

Run stop time

Filter value for smoothing lateral distance values

Flight card number (to identify the test)

Correction value for the indicated IRS ground speed (VCV)

Touchdown time

Card 4 provides an override of the test data parameters to be used from the flight tape if they are different from those specified in the program software. The aircraft data parameters required for the calculation include:

Localizer millivolt deviation (LOCDEV)

Glide slope millivolt deviation (GSDEV)

Radio altitude indicated height (ALT)

Aircraft pitch attitude (PITCH)

Indicated IRS ground speed (SPD)

True heading (HED, for LAT sideslip correction)*

Roll attitude (ROLL, for LAT roll attitude correction)*

Card 5 establishes the ability to provide the aircraft constants. Nor- mally, these are included in the software and obtained by the aircraft identification on Card 1. The following constants are required:

1. Header and control information *Minor corrections; equations are not included in this paper.

Height from bottom of aircraft's main landing gear tire to glide slope antenna (VGS)

Height from localizer antenna to landing gear (VLOC)

Distance from landing gear to glide slope antenna (LGS)

Distance from localizer antenna to landing gear (LLOC)

Distance from radio antenna to landing gear (LALT)

SUMMARY

A position-tracking method suitable for determining autoland per- formance has been developed by using the following calculation steps and test data:

Time at the threshold is determined using ILS glide slope devia- tion and radio altitude.

Distance along the runway is determined using time at the threshold and IRS ground speed.

Lateral distance from the localizer beam centerline is determined using distance along the runway and ILS localizer deviation.

Touchdown position is defined by the distance information and touchdown time.

The XILS program provides the required autoland performance data at less cost than previous position-tracking procedures. The lower cost stems from recording all of the required test data on the aircraft data system, making it unnecessary to merge position data from another source.

ALT

ANGGS

ANGLO

G/S

GS A

GSB

GSDEV

GSD

HIGH

ILS

IRS

GLOSSARY

Radio altitude indication of main gear wheel height.

Angle between a line from the aircraft glide slope antenna to the ILS glide slope antenna location.

Angle of a line from aircraft localizer antenna to the ILS antenna to the localizer beam centerline.

Glide slope.

Airport ILS glide slope angle.

Glide slope half-angle width for 150 millivolts.

Millivolt deviation, measured by glide slope receiver.

Distance from threshold to G/S transmitter antenna.

Altitude of aircraft glide slope antenna above ground.

Instrument Landing System.

Inertial Reference System (test instrumentation).

LALT -

LAT -

LGS -

LLOC

LOCB

LOCD

LOCDEV

LONG

MV

SPD

TRD

VCV

VE/W

VGS

VLAT

VLONG

VN/S

XILS

Distance from radio antenna to landing gear.

Aircraft lateral distance from localizer beam centerline.

Distance from main landing gear to aircraft glide slope antenna.

Distance from aircraft localizer antenna to main gear.

Localizer half-angle widfh for 150 millivolts.

Distance of localizer transmitter antenna from threshold.

Millivolt deviation measured by localizer receiver.

Distance of MLG wheels from runway threshold.

Millivolt (ILS receiver indication of deviation from reference line).

IRS ground speed (knots).

Track reference direction for IRS.

Correction for IRS ground speed.

East component of IRS velocity (in knots).

Vertical distance of main landing gear to aircraft glide slope antenna.

Lateral velocity perpendicular to IRS reference line.

Longitudinal velocity along IRS reference direction.

North component of IRS velocity (in knots).

Douglas Aircraft Company's landing dispersion computer program.

ACKNOWLEDGMENTS

The basic idea for the XILS program was conceived by Mr. I. Sandoval of Douglas Aircraft Company's Test and Certification department. The detail equation development and initial checkout was accomplished by Mr. Milt Jones of Arizona State University. The final software to make the program operational was prepared and is being maintained by Mr. Douglas Bell of Douglas Test and Certification.

REFERENCES

1. FAA Advisory Circular No. 20-57A for Automatic Landing Systems, January 12, 1971.

2. Cheney, Harold K., Takeoff Performance Using Onboard Instrumentation, Douglas Aircraft Company, paper DP 7339, presented to the Society of Flight Test Engineers Annual Sym- posium, Newport Beach, California, August 1983.