Adverse Elevator Effect in
Airplane Landing Flare
Nihad E. Daidzic, Ph.D., Sc.D., ATP/CFII
Associate Professor of Aviation
Adjunct Associate Professor of Mechanical Engineering
WATS 2010
April 27-29, 2010, Orlando, FL, U.S.A.
Day: 2
Conference Stream: WATS Session: 5.3
Adverse Elevator Effect in
Airplane Landing Flare
• It is my immense pleasure and honor to be at the WATS
2010 conference. I would like to thank all participants for
coming to this session and my fellow speakers in this and in
the other sessions.
• Let me also thank Halldale Media for organizing such a
wonderful conference. In particular, I would like to thank Mr.
Andy Smith, President of Halldale Media Group, Mr. Chris
Lehman, WATS conference chair and editor of CAT, and our
session moderator.
Adverse Elevator Effect in
Airplane Landing Flare
Last year my WATS2009 presentation focused on landings on
contaminated runways. This year, I am presenting initial results
of mathematical modeling and simulation on how to possibly
improve airplane landing touchdown control (and accuracy).
Disclaimer:
I am by no means telling you how to do land an
airplane!!! Do not practice this flare and touchdown
maneuver, I am about to tell you, alone or at home!!!
Adverse Elevator Effect in
Airplane Landing Flare
But I will offer an idea and alternative to “regular” or
“normal” landings where the pilot is letting airplane,
more or less, asymptotically approach the runway
surface (grease it).
The problem with the “traditional” flare technique is
that it consumes a lot of runway, often causing
overruns and/or severe stresses on the brakes,
structure, tires, runways surface, etc.
We have heard Capt. John Bent, yesterday talking about
the shear number of overruns that occurred in this year
alone (GA and corporate fleet not even included).
Adverse Elevator Effect in
Airplane Landing Flare
In almost every runway overrun accident I investigated,
landing long was a contributing (and significant) factor.
Often the problem was with coming high and fast over
threshold (and of course slick surface), but often also
extended float, ballooning, and bounce contributed to
overrun.
So, that got me thinking what can be done to have
better control of touchdowns. The topic here focuses
only on the last 5 seconds before touchdown (flare). It
is not a magic maneuver that will “convert” bad
approach end up in good landing.
Adverse Elevator Effect in
Airplane Landing Flare
OBJECTIVE
• To better understand the airplane dynamics and pilot-airplane
interaction in the last 5 seconds, or so, of the landing flare and
touchdown.
• To devise a new touchdown technique to achieve more
accurate and consistent touchdowns during flare maneuver
utilizing the Adverse Elevator Effect (AEE) (or reverse
elevator response).
• To advocate the use of Direct Lift Control (DLC) for more
accurate and consistent touchdowns for large transport category
airplanes (especially jumbo and super-jumbo).
Adverse Elevator Effect in
Airplane Landing Flare
INTRODUCTION
• Recent DOT/FAA/AR-07/7 Report (2007) “A Study of Normal Operational
Landing Performance on Subsonic, Civil, Narrow-Body Jet Aircraft During
Instrument Landing System Approaches” gave some statistics on day-today narrow-body transport-category airplane landings.
• Some of the results involving A319/320/321 family and B737-400 found
out that 50% (median) of touchdowns (main gear) occur beyond 1500 ft
(460 m) (from runway threshold) and about 10% beyond 1900 ft (580
m) following LOC/GS approach.
• More than 25% of the landings were 10+ knots faster at TCH than
appropriate.
• Auto-lands were more accurate than manual landings
Adverse Elevator Effect in
Airplane Landing Flare
INTRODUCTION (cont)
• More than 25% of airplanes (B737) needed more than 5 seconds to lower
the nose gear (about 1100+ feet “penalty”). However there is no information
on simultaneously landing long and lowering the nose slowly.
• None of the 50,000 landings recorded (40,764 accepted for statistical
analysis) resulted in overrun or accident. (So why bother?)
• A320’s initiated flare at 30 ft, while B737-400’s typically at 40 ft on
average. B737-400 had inferior landing performance (landing longer). Some
touchdowns beyond 3000 ft were recorded (extended float).
Adverse Elevator Effect in
Airplane Landing Flare
INTRODUCTION (cont)
• The SD (67%) of landing scatter appear to be about ±500 ft. Study did
not address this directly. This means that still 33% of landings have
scatter larger than 500 ft.
• Despite all industry efforts runway overruns and veer-offs continue to
occur persistently and stubbornly.
• The statistics for the wide-body landing long or overrunning the
runway end is even worse.
• In my opinion, insufficient attention is given to the control of touchdown
accuracy and landing flare in the training of new as well as re-training of
certified professional (airline) pilots.
Adverse Elevator Effect in
Airplane Landing Flare
MOTIVATION
• Improved landing techniques (utilizing AEE and DLC) would minimize
scatter of touchdown points, reduce long landings (and subsequently hard
braking), improve touchdown control, and reduce overrun accidents.
• LAHSO operations will become safer, more consistent, and more readily
accepted.
• Simultaneously, significant savings in runway and airplane maintenance
(brakes, reverse thrust, airframe, etc.) and passenger discomfort during
ground landing phase (high negative acceleration and vibrations) could be
achieved.
Adverse Elevator Effect in
Airplane Landing Flare
 L FL 
L min 
ROD
2  g 
h FL
 APP
APP
2
APP
  n FL
2

 TD
  1  2
 APP

L FL 
2

ROD APP

 2  g 
  n FL
APP

h FL
 APP
  L FL
Figure 1: Idealized flare maneuver. (Not to scale! Angles are highly exaggerated!)
Adverse Elevator Effect in
Airplane Landing Flare
v FL   FL
2
h FL 
2
2  g  ( n FL  1)

v

 FL   1  w 
v TAS 


 
2
APP

  TD 
2
ROD
2
APP
 ROD
2  g   n FL
v FLGS  v FL   FL
2
TD

ROD
2
APP
2  g   n FL
 n FL  n FL  1
Adverse Elevator Effect in
Airplane Landing Flare
Shallow approaches and gentle flare
increases landing distance (well, we
knew that already!)
Adverse Elevator Effect in
Airplane Landing Flare
RODAPP [fpm]
Touchdown @ 120 fpm
Main gear Flare Height
for sustained n=1.1
(pitch rate of 0.8 deg/s)
1000
42.12 ft (42.73)
900
34.00 ft (34.62)
800
26.74 ft (27.35)
700
20.32 ft (20.94)
600
14.77 ft (15.38)
500
10.07 ft (10.68)
Flare height as a function of approach ROD for 120 fpm touchdown
ROD. Only 0.62 feet less than for 0 fpm TCHDN (in parenthesis)!
Note: Neglected pitch response time, AEE, and other factors!
Adverse Elevator Effect in
Airplane Landing Flare
Flare load factor “n” as a
fraction of “g”
RODAPP =700 fpm
RODTD =100 fpm
Flare height [ft]
1.69 (Stall for vREF)
3
1.25
8.3
1.2
10.4
1.15
13.8
1.10
20.7
1.05
41.5
Flare height as a function of “g”-loading in steady pull-up maneuver for given
approach and touchdown ROD (AEE and other effects not accounted for).
Adverse Elevator Effect in
Airplane Landing Flare
“Gentle” flare maneuver significantly
increases flare height and touchdown
point!
But smooth and gentle is what captain
(and passengers) like!
Adverse Elevator Effect in
Airplane Landing Flare
Traditional Flight Controls are Wrong?
• There are many “adverse” effects in flying airplanes:
adverse-yaw (initial yaw in the wrong direction),
adverse-aileron (initial climb instead of roll), etc.
• Now we also have Adverse Elevator Effect (AEE).
• Traditional flight controls/surfaces are all
wrong. Initial reaction is almost always opposite of
the desired.
Adverse Elevator Effect in
Airplane Landing Flare
So what is Adverse Elevator Effect (AEE)?
Adverse Elevator Effect –
The Last Five Seconds
Figure 2: What
is Adverse
Elevator
Effect (AEE)?
Why is this
airplane not
lifting-off.
Used with
permission from
Airliners.net. (not for
commercial use)
Author/Photographer: Andrés
Contador AirTeamImages
Adverse Elevator Effect in
Airplane Landing Flare
Adverse Elevator Effect
• Adverse elevator effect (AEE) or negative tail-lift
causes the airplane to initially accelerate vertically in
the opposite direction of desired (reverse response).
• In the flare/round-out maneuver (pull-up) the
airplane will initially accelerate downward, increase
sink rate, and go below the glidepath, before the
main wing start producing enough positive flight
loads and rounding/curving of the vertical trajectory.
Adverse Elevator Effect in
Airplane Landing Flare
Aeronautics/Aerospace Engineers knew well about the reverse
elevator response and looked for the ways to minimize and/or
eliminate it (not possible with tailplane surfaces).
Pilots (in general) knew very little, or nothing, about the reverse
elevator response, but used the AEE maneuver for a long time
not quite understanding why it works.
Adverse Elevator Effect in
Airplane Landing Flare
Adverse Elevator Effect
• Although the reverse response of the elevator (AEE)
was know for quite some time and was used to
understand the airplane’s longitudinal control and
dynamics, incredibly in all these years it was never
associated with actual landing performance.
• The AEE was neglected as insignificant in cruise
flight. But in landings, the understanding of AEE is
crucial in accurate touchdowns.
Adverse Elevator Effect in
Airplane Landing Flare
Adverse Elevator Effect
Incredibly, to the best of our knowledge, no SOP, FAA
material, professional pilot literature, training
manuals, etc. ever mention AEE!
Adverse Elevator Effect in
Airplane Landing Flare
AAE – Handling/Flying Qualities
• The size of the Airplane is roughly limited by the
square/cube law. (if we assume technology and materials do not change)
• The handling qualities are limited to the cube/fifthpower law. Aerodynamic or driving moments are
proportional to the third power of length-scale, while
the inertial (resisting) moments are proportional to
the fifth power.
• The bigger the airplane the more sluggish it is!!!
Adverse Elevator Effect in
Airplane Landing Flare
AAE – Handling Qualities
• Due to AEE on the Space Shuttle elevons, the
orbiter dips 25 feet before it regains the altitude (or
slope in descent) after 2.2 seconds have elapsed
(unsatisfactory handling quality in pitch). That is why
Shuttle orbiter needs 15,000 foot runway and has
poor longitudinal control. (NASA-TM-80186)
Adverse Elevator Effect in
Airplane Landing Flare
Figure 3: Aircraft in flare (round-out). An additional downward force is created on the
elevator to pitch the airplane up and subsequently create more lift on the main wing.
However, generation of the additional main-wing lift takes time. (not to scale).
Adverse Elevator Effect in
Airplane Landing Flare
Figure 4: What is “really” happening in flare (Not to scale! Angles are highly
exaggerated).
Adverse Elevator Effect in
Airplane Landing Flare
Adverse Elevator Effect
• Adverse elevator effect can (and does) lead
to hard-landings, bounce, ballooning and large
dispersion of touchdown points.
Adverse Elevator Effect in
Airplane Landing Flare
Adverse Elevator Effect
Did you ever notice/sense that feeling of accelerated
sink when you pull stick/yoke in flare (I did)?
Or did you notice climb after you bounce and push
the stick/yoke forward (should never be done unless
stalling, but then it is irrelevant).
If you did, then you just became one of many
“victims” of AEE.
Adverse Elevator Effect in
Airplane Landing Flare
Aircraft response to step (Heaviside) upward elevator input and
pure-pitching motion. Longitudinal stability model for flat earth and
fixed glidepath.
Pitch stiffness and damping, structure elasticity, etc., are neglected
I yy 
d
W
d
g

2
  
dt
2
2
 h 
dt
2
 I yy      M t   L t  u t   l t
1
2



 h   v  S
g
2
  t      t 
W

  t   0
 C L


  t    L t  u t 
 

 h  w
Adverse Elevator Effect in
Airplane Landing Flare
C L 
P 
C L

g   Lt
W
 h t  
K R
t P
4
R 
v
2
K 
t
2
2
t
2
2
g
2
W
 Lt  lt
K
b
I yy
24
  t      t   R
Q 
1
Q S C L
v
K R 3

 h t  
t  Pt
6
  t     t   R t
K R 2


 h t  
t P
2
 t     t   R
Adverse Elevator Effect in
Airplane Landing Flare
 h t  
K R
t P
4
24
t
2
0

t h  0 
2
t  h  0 
t h  0
 2 
12 P
K R
t  h 0 
3
 3 . 46  
t h  0
6

t  h  0
I yy
 
Q  S  C L  l t
 1 . 41  
2
For B747-100
W  636 , 000 lb
S  5 ,500 ft
I yy  3 . 31  10 slug - ft
7
 
I yy
Q  S  C L  l t
 0 . 41 sec
2
2
b  196 ft
l t  95 ft
t  h  0  1 . 42 sec
c  28 ft
Q  65 . 7 lb/ft
2
t  h  0  0 . 82 sec
AR  7
C L  5 . 7
t  h 0  0 . 46 sec
Adverse Elevator Effect in
Airplane Landing Flare
Figure 5: Height, vertical speed, and vertical acceleration increments for B747100 for free-flight longitudinal dynamics. Step elevator up input.
Adverse Elevator Effect in
Airplane Landing Flare
Figure 6: Height, vertical speed, and vertical acceleration, increments for B747100 for free-flight longitudinal dynamics. Linear/Ramp elevator up input.
Adverse Elevator Effect in
Airplane Landing Flare
Push-over (negative load) close to ground
dK
K y2
y

dt
W
g
F

y
i
 ROD
I yy
dq
dt
q1  
d
dt
 dK
y

g
K y1
2
 ROD
W
    Lt   t
1
vy2
t2
 dv
y
v y1

 F

 ROD  
g   Lt
t   P t
W

 dq 
 M t
0
M t
dt
t1
q1
   M t    Lt   lt
y
I yy
t
 M t
 dt 
t
I yy
0
t  Rt
I yy
 q t 


 d
0
t
  
 q t dt
0

M t
2 I yy
t  R
2
t
2
2
Adverse Elevator Effect in
Airplane Landing Flare
Push-over (negative load) close to ground
• In the case of B747-100, creating upward tail lift of
128,000 lbf (n=0.8) by pushing-over for 500 ms will
decrease instantaneous ROD by 195 fpm. At the
same time the pitch deck angle will decrease “only”
2.6 degrees and the de-rotation moment will exist
upon touchdown for faster lowering of the nose gear.
Adverse Elevator Effect in
Airplane Landing Flare
Adverse Elevator Effect
• Adverse elevator effect can be, in my opinion, used
beneficially by pilots for better control and more
accurate touchdowns and shorter flare distances on
EXISTING airplanes. Pull-up is initiated at the height
with more “g” (say 1.2-1.25) and then control column
is pushed briefly 1-2 seconds before touchdown
(TTD) or at 3-5 feet height. This will also enable faster
lowering of the nose gear (de-rotation started), more
stall protection in push, and provide more than
sufficient clearance for the nose gear.
Adverse Elevator Effect in
Airplane Landing Flare
Due to AEE it seems as if the airplane only pitches up,
decreasing ROD, while staying on the vertical glidepath.
TDN is not far from the extended straight vertical slope
Figure 7: The “new” landing technique (Not to scale! Angles are highly exaggerated).
Adverse Elevator Effect in
Airplane Landing Flare
Adverse Elevator Effect
It is erroneously believed in the pilot community
that smooth touchdowns obtained by push-over
maneuver are due to the rotation of the main gear
away from the runway (main gear behind airplane
CG). However, the vertical speed of CG would not
be affected by this pitch rotation alone. It was the
AEE that was always responsible for subsequent
smooth landings, but (many) pilots didn’t know it.
Adverse Elevator Effect in
Airplane Landing Flare
Direct Lift Control
DLC (Direct Lift Control) provides ± 0.1g vertical
acceleration (with minimal adverse pitch changes)
for better glide-path control without delays and
reverse elevator effect reduced. Also DLC minimizes
the risk of bounce and ballooning and provides
better TCHDN control (e.g., L-1011).
Gliders use it ALL the time on landing.
Adverse Elevator Effect in
Airplane Landing Flare
Adverse Elevator Effect (coming soon)
• Daidzic, N.E. (2010) "The Last Five seconds - Adverse Elevator
Effect in Airplane Landing Flare", Professional Pilot, April 2010,
submitted.
• Daidzic, N.E., Peterson, T.L. (2010) "Improving Touchdown
Control by Utilizing Adverse Elevator Effect", International
Journal of Applied Aviation Studies (IJAAS), in preparation.
Adverse Elevator Effect in
Airplane Landing Flare
CONCLUSIONS I
 Precise touchdown control is essential in daily airplane operations and especially so
on contaminated runways and/or during LAHSO operations .
 Adverse Elevator Effect can be used beneficially by pilots. It has been used by trialand error, but with little understanding.
 The credit to smooth touchdowns has been incorrectly given to the rotation of the
main landing gear assembly during push-over (behind CG).
 Averse Elevator Effect , if not understood properly, may (and does every day so)
cause hard touchdowns, bounce, ballooning, float, etc.
 Flare height should be adjusted for existing approach ROD or GS, rather than always
using the same gear-height (say 30 ft).
Adverse Elevator Effect in
Airplane Landing Flare
CONCLUSIONS II
 Better touchdown control and shorter landings can be achieved by initiating flare
somewhat lower than normal height, by pulling more “g’s” and then after achieving about
7o nose-up attitude and descending through 2-5 feet above runway “jerk” the control
forward for about 0.5 seconds. The push-over maneuver will reduce ROD by additional
160-200 fpm touching down close to runway glideslope intercept. The momentum will
already exist to lower the nose and better stall protection exists in push-over , say, 0.8 g.
 It is better to use quick pulses (jerks) on the control column than smooth pushing and
pulling. Airplane responds faster to step and pulse inputs. Timing and good judgment and
measurement of flare height is crucial.
 We believe that by using the AEE to our advantage, touchdown/landing distances can be
reduced by 500-1000 feet on average. This is especially important as it occurs in the highspeed landing portion where every second counts.
Adverse Elevator Effect in
Airplane Landing Flare
CONCLUSIONS III
 If you pull-up to much, you will balloon. If not enough pull the airplane could touch down
hard.
 Therefore, it is better to have a two-step flare: 1) Pull, then 2) Push.
 If you bounce it is a bad idea to push. You’ll climb and then suddenly accelerate
downward (well we knew this too, but perhaps not why).
 You don’t push to “put” the airplane on the runway as some might think. If you want to
“put” it down when close to runway, then pull to plant the tires onto runway. However, a bad
bounce can occur!
 “Pumping” the stick/yoke might not do anything due to dead time in pitch response.
 In my opinion, more training should be given to landing and touchdown control.
Adverse Elevator Effect in
Airplane Landing Flare
FUTURE WORK
 Proper flare and touchdown control training of (new and seasoned) pilots with the help
of accurate radio-altimeters or visually can be implemented and learned.
 We (at MSU) are working in developing the training curriculum for different types of
transport-category airplanes that could be implemented in respective airline training. Pilots
CAN learn to utilize AEE to their advantage.
 We plan to do more complex simulations for various airplane types in ground effect and
determine the sweet spot (when to pull and when to push).
 We would like to perform piloted ground-simulator study. Initial “experiments” have
confirmed that AEE indeed can be used for our advantage. Need FAA and aviation/airline
training community help!
 We want to observe and understand worst case scenarios with this “new” landing
technique. What can go wrong and what are the consequences if any?
Adverse Elevator Effect in
Airplane Landing Flare
"Una volta che hai gustato il volo, camminerai sulla terra con gli occhi
rivolti sempre in alto, perché là sei stato, e là agogni a tornare“
"Once you experience flight, you will walk on the earth with your eyes
looking up to the sky, because there you have been, and there
you wish to go back“. (Leonardo da Vinci (1452-1519))
THANK YOU!
Questions?