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?