On-board 4D guidance systems, which predict and control the touchdown time of an aircraft to an accuracy of a few seconds throughout the descent, were developed and demonstrated in several flight test programs. However, in addition to refinements of the on board system, two important issues still need to be considered. First, in order to make effective use of these on-board systems, it is necessary to understand and develop the interactions of the airborne and air traffic control (ATC) system in the proposed advanced environment. Unless the total system is understood, the advanced on-board system may prove unusable from an ATC standpoint. Second, in planning for a future system in which all aircraft are 4D equipped, it is necessary to confront the transition situation in which some percentage of traffic must still be handled by conventional means. In terms of 4D, this means that some traffic must still be given radar vectors and speed clearances (that is, be spaced by conventional distance separation techniques), while the 4D-equipped aircraft need to be issued time assignments. These apparent differences are reconciled and efficient ATC operation is developed

Erzberger, H.

Lee, H. Q.

Obrien, P. J.

Tobias, L.

English

NASA Technical Reports Server

MIXING 4D-EQUIPPED AND UNEQUIPPED AIRCRAFT 
IN THE TERMINAL AREA 
L. Tobias, H. Erzberger, and H. Q. Lee 
NASA Ames Research Center 
Moffett Field, CA 94035 
and 
P. J. O'Brien 
FAA'Technical Center 
Atlantic City, NJ 
First Annual NASA Aircraft Controls Workshop 
NASA Langley Research Center 
Hampton, Virginia 
October 25-27, 1983 
171 
https://ntrs.nasa.gov/search.jsp?R=19840012509 2020-03-22T08:14:14+00:00Z
I 111 , .- , . . . . .-.-.. _- -- 
INTRODUCTION 
On-board 4D guidance systems, which can predict and control the touchdown time of 
an aircraft to an accuracy of a few seconds throughout the descent;have been 
developed and demonstrated in several flight test programs. However, in addition 
to refinements of the on board system, two important issues still need to be 
considered. First, in order to make effective use of these on-board systems, it is 
necessary to understand and develop the interactions of the airborne and air 
traffic control (ATC) system in the proposed advanced environment. Unless the 
total system is understood, the advanced on-board system may prove unusable from an 
ATC standpoint. Second, in planning for a future system in which all aircraft are 
4D equipped, it is necessary to confront the transition situation in which some 
percentage of traffic must still be handled by conventional means. In terms of 4D, 
this means that some traffic must still be given radar vectors and speed clearances 
(that is, be spaced by conventional distance separation techniques), while the 4D- 
equipped aircraft need to be issued time assignments. How to reconcile these 
apparent differences and develop an efficient ATC operation is the subject of this 
paper. 
MIXING 4D EQUIPPED AND UNEQUIPPED AIRCRAFT 
IN THE TERMINAL AREA 
1 v- \ -- -- _--- I--- - 
172 
OBJECTIVES 
The objectives of this study are to develop efficient algorithms and operational 
procedures for time scheduling a mix of 4D-equipped and unequipped aircraft in the 
terminal area, and, using the NASA Ames real-time air traffic control (ATC) 
simulation facility, to evaluate the system operation under various mix conditions. 
. DEVELOP CANDIDATE OPERATIONAL PROCEDURES AND TIME- 
SCHEDULING ALGORITHMS FOR CONTROLLING A MIX OF 4D- 
EQUIPPED AND UNEQUIPPED AIRCRAFT IN THE TERMINAL AREA 
l EVALUATE THE SYSTEM OPERATION UNDER VARIOUS MIX 
CONDITIONS 
173 
I , , , , _. , _ . . _ . . . - __._.. -..-~- 
OPERATIONAL PROCEDURES 
The basic operational procedure is as follows: the ATC computer generates 
time assignments for all aircraft as they enter the greater terminal area. 
For the 4D-equipped aircraft, the controller assigns the aircraft a route and a 
touchdown time. The 4D-equipped aircraft generates and flies the 4D route. The 
controller was instructed not to alter this assigned time unless necessary for 
safety reasons. The unequipped aircraft must still be controlled by radar vectors. 
However, the controllers can use the position of the 4D aircraft to achieve the 
time assignments for the unequipped aircraft. 
ATC COMPUTER GENERATES TIME ASSIGNMENTS 
. 40 EQUIPPED: . CONTROLLER ASSIGNS TOUCHDOWN TIME 
l AIRCRAFT GENERATES AND FLIES 4D ROUTE 
l ASSIGNED TIME NOT ALTERED 
. UNEQUIPPED: . CONTROLLER ISSUES RADAR VECTORS 
. CONTROLLER USES 40 AIRCRAFT POSITiONS 
TO ACHIEVE TIMES FOR UNEQUIPPED 
174 
ON-BOARD SYSTF3 
A complete on-board 4D guidance system is a complex entity involving interaction 
between numerous guidance, control, and navigation subsystems in an aircraft. The 
integrated collection of these subsystems augmented with special algorithms to 
provide fuel-efficient time control essentially constitutes the 4D flight manage- 
ment system of an equipped aircraft. The basic steps in the trajectory synthesis 
are shown below. For a number of years, NASA has designed and flight tested 
research systems incorporating various types of time control methods for both STOL 
and conventional aircraft. These tests have demonstrated the ability to predict and 
control arrival time accurately under varied operational conditions, achieving 
arrival time accuracies of f10 sec. 
. AIRCRAFT SYNTHESIZES TRAJECTORY 
1. HORIZONTAL PROFILE: TURNS AND STRAIGHT LINES 
2. VERTICAL PROFILE: LEVEL FLIGHT AND CONSTANT DESCENT ANGLE 
SEGMENTS 
3. AIRSPEED PROFILE: CONSTANT CAS AND DECELERATION SEGMENTS 
l ARRIVAL TIME ACCURACIES OF +lO set ACHIEVABLE 
. CONTROLLER CAN VECTOR AIRCRAFT; THEN ASSIGN NEW TIME VIA CAPTURE 
175 
CAPTURE TRAJECTORIES 
A 4D-equipped aircraft which has been vectored off its 4D route can be assigned a 
revised time and a waypoint to capture the 4D route. This figure shows two aircraft 
positions P1 and P2. A capture trajectory is shown by a dotted line from position 
P1 to the capture waypoint 3. If the touchdown time associated with this 
trajectory is too early, the aircraft continues to fly according to its last vector 
clearance until it reaches position P2, where the pilot captures the 4D route via 
the trajectory shown. 
1 
FIXED TRAJECTORY 
CAPTURED TRAJECTORY 
SELECTED BY PILOT 
I 
RUNWAY 
PREDICTED CAPTURE 
TRAJECTORIES 
WAYPOINT 
Al RCRAFT 
POSITION 
176 
TIME-SCHEDULING ALGORITHMS 
The 4D-equipped aircraft have the capability of meeting a touchdown-time assignment 
to an accuracy of a few seconds. It is now desired to use this capability to 
formulate efficient operational procedures for.the time scheduling of all aircraft 
in the terminal area. This will be developed in three parts: (1) determine the 
minimum time separation conditions given the minimum distance separations; (2) 
determine the interarrival time separations for two consecutive.aircraft to be used 
in aircraft scheduling; and (3) develop a scheduling algorithm for assigning 
landing times. 
l TRANSLATION OF DISTANCE SEPARATIONS TO TIME 
l TIME SEPARATIONS AT TOUCHDOWN 
. INTERACTIVE SCHEDULING ALGORITHMS 
177 
TRANSLATION OF DISTANCE SEPARATIONS TO TIME 
The minimum separation distance rules depend on aircraft weight category and are 
summarized in this figure. These distances can be converted to minimum separation 
times using speed profile data. The result is the matrix T, where each element is 
the minimum separation time at touchdown so that at no time when aircraft are along 
a common path is the separation distance rule violated. 
SMALL 
1st 
TO LARGE 
LAND 
HEAVY 
MINIMUM DISTANCE SEPARATION 
TRAILING A/C 
SMALL LARGE HEAL 
I 6 43 5 3 4 3 
IY 
I 
SPEED PROFILES ALONG COMMON PATH 
COMMON PATH LENGTH: 5 n.mi. 
160- 
LARGE 
0 
E Y 
6 130 f 
w 
MINIMUM t11 t12 t13 
TIME SEPARATION T = 
I I 
‘21 t22 f23 
MATRIX t31 f32 t33 
178 
TIME SEPARATIONS AT TOUCHDOWN 
It is assumed that, if two consecutive aircraft are 4D-equipped, the interarrival 
times given by T can be used for scheduling purposes. However, unequipped aircraft 
will need additional time buffers to prevent separation distance violations. If 
the probability density function of an unequipped aircraft meeting an assigned time 
via controller vectoring is known (this can be determined in the specific 
experimental context), then time buffers can be determined to keep the probability 
of separation distance violation belay a desired level. These time buffers result 
in a revised time separation matrix T described below. 
BUFFERS ADDED TO PREVENT MINIMUM SEPARATION VIOLATIONS 
FOR TWO CONSECUTIVE AIRCRAFT AT TOUCHDOWN: 
IF BOTH EQUIPPED, T’ = (t’ij) = (tij + aa) 
IF ONE EQUIPPED, T” = (t”ij) = (tij +Sb) 
IF BOTH UNEQUIPPED, T”’ = (tij’) = (tij +a,) 
WHEREoG a,< $,< 6, 
179 
' INTERACTIVE SCHEDULING ALGORITHMS 
The previous discussion established the time.separation matrix at touchdown shown 
as a function of:weight category, and whether or not aircraft are 4D equipped. It 
is assumed that the feeder fix time for each aircraft is known. Based on this time 
and on the desired time to traverse the route, a desired touchdown time for each 
aircraft can be determined. This information can be used to generate an initial 
time schedule, as described in reference 1. However, in addition to setting up an 
initial schedule, algorithms are required to revise the schedule. Missed 
approaches need to be accommodated.. Also, the controllers may need to change the 
aircraft arrival rate. It may be that they also are required to block out specific 
time periods from the computer schedule to accommodate a missed approach or a 
priority landing. These are important aspects of the complete scheduling problem. 
WITH TIME SEPARATION CONSTRAINTS CAN NOW GENERATE SCHEDULE 
. ESTABLISH TOUCHDOWN ORDER 
. PROVIDE FOR REVISIONS 
- CHANGE ARRIVAL RATE 
- MISSED APPROACHES 
- EMERGENCIES 
180 
EVALUATION OF SYSTEM OPERATION UNDER VARIOUS MIX CONDITIONS 
The candidate operational procedures and time schedule algorithms previously 
described were used in a real-time ATC simulation study of operations under various 
mix conditions. 
. SIMULATION FACILITY 
. SCENARIO AND TEST CONDITIONS 
. RESULTS 
181 
SIMULATION FACILITY 
The simulation was conducted using the NASA Ames ATC Simulation Facility shown in 
this figure. It includes two air traffic controller positions, each having its own 
color computer graphics display. In this study, one was designated arrival control 
and the other, final control. The controllers each communicate with one or two 
keyboard pilots. Each keyboard pilot can control up to 10 computer-generated 
aircraft simultaneously. The clearance vocabulary includes standard heading, 
speed, and altitude clearances as well as special clearances for 4D-equipped 
aircraft. This figure also depicts piloted simulators. Previous studies have 
utilized one or two piloted simulators which were connected by voice and data link 
to the ATC Simulation Facility; however, in this study, no piloted simulator was 
used. 
DATA LINK TO FAA 
TECHNICAL CENTER 
ATC SIMULATION 
FACILITY 
KEYBOARD PILOT, GROUND CONTROL 
STATIONS STATIONS 
f 
4 
r x 
TEST AIRCRAFT SIMPLE 
FLOWN AT PILOTED 
CROWS LANDING SIMULATORS 
MOVING & FIXED BASE 
PILOTED 
AIRCRAFT SIMULATORS 
182 
APPROACH CONTROLLER DISPLAY 
The route structure and runway configuration investigated are shown in this figure. 
Two routes, Ellis, from the north, and Sates, from the south, are high-altitude 
routes flown by large or heavy jet transport-type aircraft. Aircraft on these 
routes fly profile descent procedures, but may or may not be 4D equipped. Hence, 
there is a mix of 4D-equipped and unequipped aircraft of the same speed class along 
the same route. In addition, low-speed aircraft were considered which flew the 
Deerpark route from the east, but shared a 5 n. mi. common path length and used the 
same runway as the jet traffic. The Deerpark traffic was unequipped, and always 
constituted 25% of the traffic mix. To assist the controller in integrating the 
4D-equipped and unequipped traffic, a flight data table (FDT) was provided to the 
left of the route structure. The information supplied includes aircraft type, 
route, scheduled touchdown time, and anticipated delay. The main test variable was 
the mix of traffic. Three mix cases were run: 25, 50, and 75% 4D equipped. 
13 : 27 : 12 
ID TYPE FIT 
Rl 4 SA 37oll 00 
I1 I44 SA 3824 00 
Jl U EL 4305 -09 
Tl 4 SA 4429 00 
A2 HU EL 4715 05 
E2 U SA 
M2 HU EL 
----_ 
DEERPARK 
G2 H4 SA 5247 2549 
ELLIS AND SATES: JET A/C 
4D EQUIPPED OR UNEOUIPPED 
DEERPARK: LOW SPEED A/C 
MIX CONDITIONS: 
0, 25, 50 75% 4D EQUIPPED 
183 
EXPERIMENT CONDITIONS 
In this study, a saturated arrival traffic flow was used. It is assumed that 
instrument flight rule (IFR) conditions prevail, and that all aircraft use runway 
4R; furthermore, no departures, winds, or navigation errors are simulated. For 
purposes of this study, it was assumed that all aircraft depart the feeder fix at 
their scheduled departure times. Magnitude departure errors that can be 
tolerated as well as the means to provide ground computer assists to nullify 
departure errors are main issues addressed by current research. 
. SATURATED ARRIVAL TRAFFIC FLOW 
l NO WINDS, NO NAVIGATION ERRORS 
l ALL AIRCRAFT DEPART AT SCHEDULED TIM& 
184 
CONTROLLER EVALUATIONS 
Three research air traffic controllers from the FAA.Technical Center participated 
in this study. Controllers were asked to compare operations under the traffic mix 
conditions. The 25% equipped case was rated the condition with the heaviest 
workload. The main difficulty seemed to be that the controllers were establishing 
distance spacing of most of the traffic,, and they felt that by not altering the 
flight path of the 4D-equipped aircraft, they were occasionally.losing some slot 
time. They were, however, quite pleased with the 50% 4D-equipped case, which 
allowed for easy handling of the unequipped aircraft. The 75% 4D-equipped case was 
rated most orderly by all the controllers, but when this many aircraft were 4D- 
equipped (the only unequipped aircraft were the Deerpark arrivals, which always 
constituted 25% of the traffic sample), there was "basically nothing to do." The 
controllers were asked if there was any difficulty in handling the mix of speed 
classes, the slow traffic on Deerpark and the'jet traffic on Ellis and Sates. They 
indicated that spacing behind the low-speed aircraft was sometimes a problem, since 
they had to allow for a large initiai separation along the common path length. The 
controllers indicated that the time order information displayed on the flight data 
table was useful; however, the touchdown time and delay information was not used. 
. COMPARISON OF MIX CONDITIONS 
. CONTROLLING THE MIX 
. USE OF DISPLAYED TIME DATA 
185 
AVERAGE NUMBER OF CLEARANCES 
This figure provides the average number of clearances/aircraft. It can be seen that 
as more aircraft are 4D ,equipped, the average number of clearances per aircraft 
decreases. This is fairly obvious in the experiment context described, since 4D- 
equipped aircraft were not vectored. They were assigned a touchdown time which was 
not altered in most cases. 
% EQUIPPED 
0 
25 
50 
75 
AVERAGE NUMBER OF 
CLEARANCES/AIRCRAFT 
5.2 
4.5 
2.7 
2.4 
186 
EFFECT OF 4D ON LOW-SPEED TRAFFIC 
The previous figure shows the decrease inthe average number of controller 
clearances as a greater percentage of aircraft are 4D equipped. A major concern 
is: does the average number of clearances for the.unequipped aircraft increase as 
the percentage of equipped aircraft increases? The answer to that question is 
provided in the figure below, which gives the average number of clearance/aircraft 
for the Deerpark route-only. Recall that the Deerpark traffic was always 25% of the 
traffic sample, and that all Deerpark is unequipped aircraft. This figure indicates 
that the average number of clearances given to the Deerpark unequipped aircraft is 
the same, independent of the mix condition. Also shown is the average time in the 
system (in minutes) for the Deerpark traffic, which is also seen to be independent 
of the mix condition. 
LOW-SPEED (DEER PARK) TRAFFIC IS 25% OF ALL 
TRAFFIC IN EACH TEST CONDITION 
AVG. TIME IN SYSTEM, AVG. # OF CLEARANCES 
MIX minsec PER Al RCRAFT 
0 19:16 6.9 
25 18:56 6.5 
50 19:05 6.2 
75 19:05 6.4 
187 
LOSS OF 4D SCHEDULING 
There was a desire to examine how traffic handling is disrupted if a breakdown of 
the 4D scheduling computer occurs. To investigate this, during a 75% 4D-equipped 
run, the FDT was removed from the screen so that the controllers no longer had a 
display of schedule times and order for aircraft in their sector. Furthermore, all 
feeder-fix departures from then on would not have any 4D time assignment, and would 
have to be vectored. The map display which showed aircraft positions was not 
removed. Initially, there was no change. The 4D-equipped aircraft already in the 
control sector could still be left alone since they would continue to follow their 
previously assigned 4D route. This is in contrast to a totally ground-based 4D 
system in which the ground system generates clearances for every aircraft; when 
that type of system fails, all aircraft are affected in a short time. The only 
difficulty experienced with the system tested was that after the failure occurred, 
controllers continued to allow traffic to depart the feeder fixes at the higher 
arrival rate for the 75% equipped case, rather than to adjust to the baseline 
vector arrival rate. If the flow-rate adjustment for new feeder-fix departures is 
made when the failure occurs, then it seems clear that the use of the on-board 4D 
system provides a safe transition to the standard vector mode. 
OBJECTIVE: DETERMINE EFFECTS OF ATC COMPUTER OUTAGE 
ACTION: DURING A 75% 4D RUN, FLIGHT DATA TABLE REMOVED 
OBSERVATION: NO INITIAL CHANGE (BUSY PERIOD). SOME PROBLEMS WITH 
HIGH ARRIVAL RATE OF NEW ARRIVALS 
CONCLUSIONS: ONBOARD 4D PROVIDES SAFE TRANSITION TO VECTOR MODE. 
NEED TO ESTABLISH AS PART OF PROCEDURE AN IMMEDIATE 
CHANGE OF FLOW RATE FOR NEW DEPARTURES 
188 
CONCLUSIONS 
Algorithms were developed to obtain an initial time schedule and to provide for 
revisions for a mix of 4D-equipped and unequipped aircraft in the terminal area. 
These algorithms were used to develop a candidate set of operational procedures for 
mixing 4D-equipped and unequipped jet aircraft along the same route, and for mixing 
different speed classes along merging routes. A basic rule established was not to 
alter the 4D equipped aircraft once they were assigned a landing time. This 
procedure resulted in the controllers learning to use the 4D aircraft positions to 
effectively vector the unequipped aircraft to their assigned landing slot. 
However, procedures were also demonstrated to vector the equipped aircraft and to 
reassign touchdown times. In addition, it was shown that a loss of the ground 
based 41, system results in a smooth transition to vector operations. Controller 
evaluations indicated that the 25%-equipped case was the most difficult to handle. 
Nevertheless , quantitative data actually showed a decrease in the number of 
controller clearances with respect to the 0% 4D-equipped case. Controllers felt 
that the procedure of not altering the 4D-equipped aircraft when so few were 
equipped was workable, but that it was a more complex task. Nevertheless, fuel was 
saved even in this case, compared to 0% 4D-equipped aircraft. The controller 
workload as measured by the average number of clearances per aircraft decreased as the 
percentage of 4D-equipped aircraft increased. Moreover, this average decrease was 
not accomplished at the expense of the unequipped aircraft. The number of 
clearances for the unequipped aircraft as well as the time delays was independent 
of mix condition. 
l DEVELOPED SCHEDULING ALGORITHMS AND OPERATIONAL 
PROCEDURES 
l ALL MIX CONDITIONS EFFECTIVELY CONTROLLED 
. REDUCED CLEARANCES AS PERCENTAGE 4D INCREASED 
. UNEQUIPPED NOT PENALIZED BY 4D 
189 
REFERENCE 
1. Tobias, L.: Time Scheduling of a Mix of 4D Equipped and Unequipped Aircraft, 
NASA TM-84327, 1983. 
190 


Mixing 4D-Equipped and Unequipped Aircraft in the Terminal Area

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