Aircraft altitude, ground velocity, and altitude rate signals are input to a computer which, using a unique control law, generates a pitch control surface command signal suitable for guiding an aircraft on its flare path to a specified runway touchdown point despite varying wind conditions

Hansen, Rolf

Lambregts, Antonius A.

NASA Technical Reports Server

United States Patent 1191 [111 4,354,237 
Lambregts et al. [45] Oct. 12, 1982 
R#Jf 
METHOD AND APPARATUS FOR 
PRODUCING AN AIRCRAFJT FLARE PATH 
CONTROL SIGNAL 
Inventors: Antonius A. Lambregts, Renton; Rolf 
Hansen, Bellevue, both of Wash. 
Assignee: The Boeing Company, Seattle, Wash. 
Appl. No.: 162,451 
Filed: Jun. 24, 1980 
Int. Cl.3 .......................... G06G 7/78; G05D 1/12 
U.S. CI. .................................. 364/428; 73/178 T; 
244/183; 244/187; 364/429 
Field of Search ....................... 364/428, 429, 430; 
340/26; 318/583; 244/183, 186, 187; 73/178 T 
References Cited 
U.S. PATENT DOCUMENTS 
-48 
3,169,730 2/1965 Gaylor et al. ....................... 244/187 
3,265,333 8/1966 Montooth ........................... 244/187 
3,295,796 1/1967 Gaylor ................................ 244/187 
3,310,259 3/1967 Swift et al. ..................... 244/187 X 
3,333,795 8/1967 Hattendorf et al. ................ 244/187 
3,437,292 8/1969 Greene ................................ 244/187 
3,463,422 SA969 Watson ........................... 244/187 X 
-c 
3,489,378 1/1970 Watson et al. ...................... 244/187 
3,601,339 SA971 Watson ................................ 244/187 
3,604,908 9/1971 Loome et al. ....................... 364/430 
3,618,002 11/1971 Stinson ............................ 244/183 X 
3,626,163 12/1971 Dommasch ......................... 364/429 
3,652,835 3/1972 Devlin et al. ....................... 364/429 
3,752,967 8/1973 Victor ................................. 364/427 
3,976,267 SA976 Meyer et al. ................... 364/430 X 
4,006,871 2/1977 Simpson .......................... 364/428 X 
4,093,158 6/1978 Clews et el. ........................ 244/182 
4,106,731 SA978 Bliss ................................ 364/428 X 
4,141,522 2/1979 Lambregts .......................... 244/186 . 
Primary Examiner-Jerry Smith 
Attorney, Agent, or Firm-James P. Hamley; Bernard A. 
Donahue 
VI ABSTRACT 
Aircraft altitude, ground velocity, and altitude rate 
signals are input to a computer which, using a unique 
control law, generates a pitch control surface command 
signal suitable for guiding an aircraft on its flare path to 
a specified runway touchdown point despite varying 
wind conditions. 
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2 Claims, 4 Drawing Figures 
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L FLARE 
?/TCH 
COMMAND 
FLARE 
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https://ntrs.nasa.gov/search.jsp?R=20080004214 2019-08-30T02:17:02+00:00Z
U.S. Patent oct. 12, 1982 Sheet 1 of 2 4,354,237 
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U.S. Patent m. 12, 1982 Sheet 2 of 2 4,354,237 
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4.354,237 
1 
METHOD AND APPARATUS FOR PRODUCING 
AN AIRCRAFT FLARE PATH CONTROL SIGNAL 
BACKGROUND OF T H E  INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract No. NAS1- 
14880 and is subject to the provisions of Section 305 of 
the National Aeronautics and Space Act of 1948, Public 
Law 85-568 (72 Stat. 435; 42 USC 2457). 
The present invention pertains to the aircraft guid- 
ance art and, more particularly, to a system for control- 
ling aircraft flight during landing flare. 
A critical portion of aircraft landing trajectory is 
commonly known as flare-out or flare. Flare is that 
portion of the landing trajectory between the fixed 
angle glideslope and aircraft runway touchdown. Thus, 
it is desirable, particularly for commercial aircraft, that 
the flare profile depart smoothly from the fixed angle 
glideslope approach providing a smooth transition to 
runway rollout. 
In commercial aircraft, attempts have been made at 
generating landing flare commands to be used either as 
a flight director to the pilot or for automatic, i.e. autopi- 
lot, landing. The problem with these systems, however, 
is that groundspeed variations affect touchdown disper- 
sion. That is, since aircraft groundspeed is not taken into 
account, the actual touchdown point of the aircraft on 
the runway can vary considerably depending on the 
groundspeed. This is undesirable both for safety reasons 
and because reduced touchdown dispersion is essential 
to effective runway utilization and continued aircraft 
operation under adverse weather conditions. 
Another problem with existing flare control systems 
is that the flare height from which flare is initiated may 
be affected by aircraft groundspeed. This has an adverse 
effect on the ability of the pilot to monitor the system 
for proper operation and may place the start of the flare 
ahead of the runway threshold where terrain features 
may make it difficult to use radio altimeters. 
SUMMARY OF THE INVENTION 
It is an object of the present invention, therefore, to 
provide apparatus for generating, and a method of pro- 
ducing an aircraft flare path control signal which mini- 
mizes touchdown dispersion due to groundspeed varia- 
tions. 
It is a further object of the invention to provide the 
above described apparatus for, and method of generat- 
ing an aircraft flare path control signal wherein the 
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altitude at which flaie is initiated < independent of 
groundspeed. 
Briefly, according to the invention, apparatus for 
producing an aircraft flare path control signal includes 
a means for producing a signal h representative of air- 55 
craft altitude. Also, suitable means produces a signal 
VG representive of aircraft groundspeed and further 
means produces a signal h representative of aircraft 
altitude rate. A computer produces a pitch control sur- 
face command signal 6, according to the relationship: 60 
&=K& + v d ( K h / K i ) /  VG REFj(h + h d }  
where Ki, Kh, V c ~ E F a n d  heare constants selected for 
a given system configuration. 65 
A method for producing an aircraft flare path control 
signal comprises the first step of producing signal h 
representative of aircraft altitude. Second, a signal VGis 
2 
produced representative of aircraft groundspeed. Third, 
a signal h is produced, representative of aircraft altitude 
rate. Finally, a pitch control surface command signal 
6, is computed according to the relationship: 
6,=Ki{h+ v d ( K h / K d / v G  RE.d(h+hB)) 
where Kh, Kh, v ~ ~ ~ ~ a n d  hgare co stants selected for 
a given system configuration. 
BRIEF DESCRIPTION O F  T H E  DRAWINGS 
FIG. 1 is a plot of aircraft altitude h versus altitude 
rate h for the prior art flare-out control system; 
FIG. 2 is a plot of aircraft altitude h versus runway 
distance X and shows the touchdown dispersion of the 
prior art system from its glideslope intercept point; 
FIG. 3 is a plot of aircraft altitude h versus altitude 
rate h for an aircraft following a control signal accord- 
ing to the instant invention; and 
FIG. 4 is a block diagram illustrating the preferred 
embodiment of a system for generating a control signal 
according to the instant invention. 
DETAILED DESCRIPTION OF T H E  
PREFERRED EMBODIMENT OF THE 
INVENTION 
In one type of flare control law widely used today, 
the aircraft sinkrate is proportional to altitude. Thus, 
excluding damping terms, which are a part of any flare 
equation, the basic exponential flare control law is of 
the form: 
6, ,=Kih+Kh(h+hd (1) 
where 
&=pitch control surface command 
h=altitude rate, 
h=altitude, 
Ki, Kh, hg=constants selected 
for a particular system configuration. 
For the condition of a zero pitch control command 
i.e., the aircraft's servo system reduces the pitch com- 
mand to zero, the equation takes the following form: 
K i i  +Kh(h + h E ) = O .  (2) 
FIG. 1 is a graph of this equation, showing altitude on 
the vertical axis versus altitude rate, on the horizontal 
axis. Exemplary values of the various constants are 
selected, i.e. the value of Kh/& is assumed to be 0.205 
and the value of hB is assumed to be 12. 
Assuming constant control law parameters, the soh- 
tion for the above differential equation is 
h =  - rh&-r /T-hB 
. .  
h=-h&-r/T (3) 
where 
T = K/;/Kh 
h,=initial sinkrate= -VG tan f i  
VG= groundspeed 
fi=glidepath angle. 
The expression for the flare height from which flare 
where, 
must be initiated to satisfy the above equation is 
bo= VG r tan P-hB. ( 5 )  
4,354,237 
3 4 
is produced in a manner discussed herein below, to bc 
FIG. 1 illustrates this flare height ho for values of 110, coupled to the aircraft's flight dynamics. 
125 and 140 groundspeed knots. Thus, since the flare The pitch control command signal computed as 
height ho varies as a function of groundspeed VG(ab0Ut follows. The altitude signal h is summed with a constant 
six feet for a 15 knot variation in VG) this has an adverse 5 signal hi, supplied from block 30. Thus, the output from 
effect on the ability of the pilot to monitor the system summer 14 is of the form h+hi,. This signal is applied as 
for proper operation and may also place the start of the one input to a multiplier 32. 
flare ahead of the runway threshold where terrain fea- The groundspeed block 40 produces an output signal 
tures may make it difficult to use radio altimeters. VG corresponding to aircraft groundspeed. Such 
FIG. 2 is a plot of aircraft altitude h versus runway 10 groundspeed signals VG are commonly provided on 
position X and illustrates the distance X T D  between the commercial aircraft and, as such, will not be discussed 
aircraft actual touchdown point and the glideslope in- here. 
tercept point with the runway. It is assumed that the The groundspeed signal VG is multiplied by a factor 
aircraft is on a glideslope angle P and that flare is initi- l / V ~ R ~ r i n  block 42. The signal 1Nc R,pis a constant 
ated at the altitude h,, which is approximately 42 feet. I5 value depending upon a particular system configura- 
tion. The signal out of block 42 i s  then multiplied by a 
gain factor Kh/Kj, in block 44. Both Kh and Ki, are 
constants selected for a particular system configuration. 
The total distance is expressed analytically as 
Xj-D=(hB/tan p)- V p [ l n  ( h e / V ~  T tan p)+ 11. (4) 
Thus, the output from block 44 i s  a signal having a 
Thus, it is evident, assuming a constant hB, r and /3, 20 value 
that, as with flare heinht altitude h,, the dispersion dis- 
The basic controlling equation then becomes Kh{i+  v G [ ( K h / K h / v G  REF](h+hB)). (9 
Ki,[h + (1 /d (h  + ho)l =o (') 
This signal is, then, the desired improved control law 
and inserting the value for 1 / ~ ,  this becomes signal. 
In all aircraft flight guidance control systems, there 
(9) are certain control law damping signals which are used 
to dampen short and long term oscillations. These are 
This control law is shown graphically in FIG. 3. well understood by those skilled in this art and, as such, 
Plots are made for three different groundspeeds, as will not be discussed here. Hence, the control law 
before, 110, 125 and 140 knots. It is apparent from this 45 damping terms are summed with the control signal in a 
graph that the flare height h,is no longer dependent on summer circuit 52 before being passed as the output 
groundspeed VG. pitch flare command signal 
FIG. 4 is a block diagram illustrating the preferred It should be understood that the pitch flare command 
embodiment of the apparatus for producing the aircraft signal may be used in the aircraft either as an input to 
flare path control signal in accordance with this newly 50 the flight director to indicate to the pilot a desired air- 
derived control law. craft pitch angle or it may be coupled directly to the 
Here, an altitude block 10 produces an output signal aircraft's autopilot for automatic control of aircraft 
h representative of aircraft altitude. A suitable means flare-out. 
for producing the altitude signal h is commonly avail- In summary, apparatus for, and the method of pro- 
able in commercial aircraft and, as such, will not be 55 ducing an improved aircraft flare path control signal 
discussed here. has been described. This signal is adapted to control 
The altitude signal h is passed to a pair of summers 12, aircraft flare-out such that the landing flare distance 
14. Also passed to an input of summer 12 is the output remains constant despite conditions of varying air- 
from block 16. Block 16 produces a fixed signal corre- speeds or groundspeeds. In addition, the flare height h, 
sponding to the altitude height for flare initiation h,. As 60 remains constant despite groundspeed variations. 
discussed above, in a system according to the instant While a preferred embodiment of the invention has 
invention the flare initiation altitude h, is constant re- been described in detail, it should be apparent that many 
gardless of groundspeed variations. The summer 12 acts modifications and variations thereto are possible, all of 
to subtract the signal ha from the signal h such that its which fall within the true spirit and scope of the inven- 
output is negative if and only if the aircraft altitude has 65 tion. 
dropped below the flare initiation altitude. Upon this 
occurrence, a logic circuit 18 activates a switch 20 
which causes the pitch flare command signal ti, which 
40 
Ki{k + v G [ ( K h / K d /  VG REF](h 4- hB))=O. 
We claim: 
1. Apparatus for producing an aircraft flare path 
control signal comprising: 
4,354,237 
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means for producing a signal h representative of air- 
craft altitude; 
means for producing a signal VG representative of 
aircraft groundspeed; 
means for producing a signal h representative of air- 
craft altitude rate; 
computer means for producing a pitch control sur- 
face command signal S,, according to the relation- 
ship: 
2. A method for Producing an aircraft flare Path con- 
(a) producing a signal h representative of aircraft 
(b) producing a signal VG representative of aircraft 
(c) producing a signal h representative of aircraft 
(d) computing a pitch control surface command sig- 
trol signal comprising the steps of: 
altitude; 
groundspeed; 
altitude rate; 
nal Se, according to the relationship: 
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Gec=KA{h+ VG[(K~/KI; ) /VG REF](h+hB)l 
6ec=Ki{h + v ~ [ ( K h / K i ) /  VG REd(h  + h d )  
where Kh, Kh, VG REF and hg are constants se- 
lected for a given system configuration; and 
control means for actuating the aircraft control sur- 
faces to track said command signal 8, whereby 
aircraft landing flare distance is substantially unaf- 
where Kh, Kh, VG REF and hg are constants se- 
lected for a given system configuration; and 
(e) actuating the aircraft control surfaces to track said 
command signal Se, whereby aircraft landing flare 
distance is substantially unaffected by the aircraft’s 
relative groundspeed. 
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* * * * *  fected by the aircraft’s relative groundspeed. 2 0  
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45 
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Method and apparatus for producing an aircraft flare path control signal

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