Copyright © 2000 by B.A. Marks, M.J. Heller, R.S. Traven, E.O. Etz



## Improving Single Engine Controllability **With A Fl**  **y -** **By -** **Wire Flight Control Sy** **stem** Mr. Bret Marks, Carrier Suitability Engineer - The Boeing Company Mr. Mike Heller, Flight Control System Engineer - The Boeing Company Mr. Ricardo Traven, Test Pilot - The Boeing Company Lt Erik Etz, Test Pilot - United States Navy

**Ab stract**  For conventional multi -engine jet aircraft, single engine minimum control airspeed (Vmc) has traditionally been attributed to the ability of the rudder to overcome the yawing moment produced by the asymmetric thrust associated with the loss of an engine. Consequently, Vmc evaluations have historically focused on thrust configuration (distance between engines, engine splay angle, thrust magnitude) and rudder effectiveness. H owever, i n establishing Vmc for the Boeing F/A- 18E/F Super Hornet, a critical connection to lateral control inputs was also revealed. The minimum airspeed at wh ich the aircraft was controllable following a simulated dynamic engine failure was heavily dependent on the aggressiveness of the pilot in the lateral axis. I n addition, by automatically commanding full rudder to oppose the thrust- induced yawing moment generated by the operating engine, the fly- by-wire flight control system minimized the dependency of Vmc o n pilot delays in applying full rudder. The dependency on lateral stick was accentuated during Vmc flight demonstration testing when an unexpected departure occurred at low altitude, nearly resulting in the loss of the aircraft. The departure was attri buted to loss of d irectional control resulting from excessive lateral stick deflection used in attempting to maintain a wings level condition. This event initiated a study to identify candidate control law modifications to minimize the potentially harsh consequence of large lateral stick inputs following a dynamic engine failure. The result is a revision to the flight control software that virtually eliminates lateral stick- induced yaw departures below Vmc, significantly increasing the margin of

safety in a failed engine, catapult launch scenario. T his paper describes the mechanics of

s ingle engine controllability, flight test results from a severe loss-of-control event, the philosophy, implementation and flight demonstrated results of flight control system revisions designed to improve Vmc characteristics, and the applicability of these efforts to any multi-engine, fly- by-wire aircraft. Photograph Courtesy of D.S. GroenCopyright © 2000 by B.A. Marks, M.J. Heller, R.S. Traven, E.O. Etz 2 **Introduction** Designed to satisfy the U.S. Navy’s need for increased mission and payload flexibility, the F/A- 18E/F, Fi gure 1, is an evolutionary upgrade of the combat proven night strike F/A- 18C/D. T he Super Hornet offers greater range and endurance, increased payload capability, increased bring back capability, enhanced survivability, and allowance for future growth.

**Figure 1. F/A -** **18E/F Super Hornet** Airframe

– A larger wing planform area and longer fuselage result in a 33% increase in internal fuel capacity over the F/A- 18C/D, providing significant increases in mission radius and endurance. The addition of two new wing store stations, bringing the total number of store stations to eleven, enhance mission flexibility and lethality. Increases in wing, wing Leading Edge Extension (LEX) and empennage areas combine to expand launch payload capability by 28% and give the E/F three times the bring back ordnance capability of the C/D while reducing both launch and recovery Wi nd Over Deck (WOD) requirements. To aid directional controllability in the event of an engine failure, E/F vertical tail and rudder areas are 15% and 54% larger, respectively, than those of the C/D. Power Plant

– The E/F is powered by two General Electric F414-GE-400 engines derived from the highly reliable F404-GE-402 engines utilized by the C/D models. The F414s, like their F404 predecessors, employ augmented low bypass, twin -spool turbofans with afterburners and Variable Exhaust Nozzle (VEN) geometry. I mproved engine efficiency is achieved with the addition of a Full Authority Digital Engine Controller (FADEC).Copyright © 2000 by B.A. Marks, M.J. Heller, R.S. Traven, E.O. Etz 3 Flight Control System

– Improvements in the Super Hornet’s quad- redundant, digital flyby-wire flight control system provide greater reliability and maintainability. S ig nificant we ight reductions have been achieved by eliminating the mechanical back- up system and conventional speedbrake. **Ba ckground**  During shipboard operations, it is highly desirable to minimize the catapult end airspeed, and thus W ind Over Deck (WOD), required to launch an aircraft to safe fly-away speeds. Minimizing launch W OD provides greater operational flexibility by l owering ship steaming and catapult energy requirements. For the United States Navy, the minimum permissible catapult launch end airspeed is established based on the following six criteria defined in the Carrier Suitability Testing Manual, Reference 1: 1.) Hi gh Angle Of Attack Flying Qualities 2.) Proximity To Or Warning Of Impending Stall (Not To Exceed 0.9*CLmax) 3.) Minimum Level Flight Longitudinal Acceleration Capability (0.065 a/g) 4.) Flight Control Saturation 5.) Aircraft Center Of Gravity Sink Off Bow 6.) Single Engine, Minimum Control Airspeed (Vmc) The minimum safe catapult launch end airspeed is the lowest speed that satisfies all of these criteria. For normal fleet operations, a 15 kt margin is added to the minimum launch endspeed to account for variability in catap ult performance, WOD and other safety considerations. This paper focuses on the Vmc criterion and the significant improvements achieved for the Super Hornet with the implementation of flight control system revisions specifically designed to improve departu re characteristics below Vmc. It is important to note that operational endspeeds do not guarantee single engine fly-away (positive rate of climb). H owever, they are intended to ensure adequate controllability for all permissible store loadings (symmetric and asymmetric) to enable the pilot to safely execute emergency procedures and, if necessary, safely eject from the aircraft in the event of a catastrophic engine failure during catapult launch operations. **Maneuver Description** The Vmc characteristics of the F/A- 18E/F were established under the Engineering and Manufacturing Development (EMD) flight test program. The objectives of the Vmc flight testing were to establish full and half flap Vmc for symmetric and asymmetric store loadings. The results of this testing were used to establish catapult launch asymmetry limits, fleet operational launch airspeeds and recommended emergency launch and recovery procedures. Prior to flight demonstration, the Boeing St. Louis Manned Flight Hardware Simulator (MFHS) was used to develop a reliable test technique which was operationally relevant and enabled the dependency of Vmc on angle of attack (AOA) and lateral weight asymmetry to be quantified. The development of this technique and its relevance to the aircraft carrier environment is discussed in Reference 2. The resulting technique is described as follows:Copyright © 2000 by B.A. Marks, M.J. Heller, R.S. Traven, E.O. Etz 4 1.) Stabilize the airplane in wings level, steady heading flight at the specified initial conditions: a.) Landing gear extended, full or half flaps b.) Test altitude +500 ft, airspeed for target AOA, dual engine thrust required for slow rate of descent (~1,000 ft/min) 2.) Wh ile maintaining wings level, steady heading flight, reduce power on the critical engine to Flight Idle and advance the opposite throttle to the desired th rust level (Intermediate Rated Power or Maximum Af terburner) while passing through the test altitude. 3.) Ma intain initial AOA with neutral directional controls. Lateral inputs required to maintain a wings level condition are permitted. 4.) Apply recovery controls (rudder to oppose yaw, lateral stick to arrest roll) after 2 seconds has elapsed (simulated pilot reaction time) or a limiting factor has been reached: a.) 20° Angle Of Bank (AOB) change b.) 10° Angle Of SideSlip (AOSS) change c.) Pilot evaluation of yaw / roll rates (pilot comfort) d.) Full rudder deflection does not result in reversal in yaw rate slope 5.) Re-acquire steady heading flight within 5° AOB into the operating engine. Dynamic Vmc, as it pertains to the F/A- 18E/F catapult launch scenario, is defined as the ai rspeed (target throttle chop speed) below which recovery controls do not prevent AOB, AOSS or yaw / roll rates from exceeding the limiting factors defined in Step 4. Using this technique, maneuver entry speed for a given test AOA is controlled by varyin g aircraft weight. As fuel is burned and the aircraft becomes lighter, the airspeed required to stabilize in wings level flight at a given AOA is reduced. Consequently, the maneuver is repeated at successively lighter weights and lower airspeeds until Vm c is established. Developmental testing conducted in the MFHS revealed that dynamic Vmc was extremely sensitive to the magnitude of the lateral stick input used to arrest the sideslip- induced roll rate resulting from the throttle split. This characteristic was later verified in flight. Overreacting to this initial roll rate was found to aggravate the departure. It was found that judicious use of lateral control was required to arrest the roll rate without inducing additional sideslip due to the adverse yawing moment produced by the ailerons. Attempting to roll into the good engine fu rther aggravated the departure since, at the Vmc condition, full rudder deflection is required to oppose the thrust- induced yaw combined with the aileron - induced yaw generated by attempting to maintain an approximate wings- level condition. Consequently, the rudders are unable to generate the additional yawing moment required to coordinate a roll into the good engine. O ptimal lateral stick position for recovery was determined to be approximately 1/3 to 1/2 of the allowable travel. The use of full lateral stick at the Vmc condition often resulted in a yaw departure. To quantify the sensitivity of lateral stick deflection on dynamic Vmc, a simple pilot model was developed to replicate the pilot technique used to perform the flight test maneuver.Copyright © 2000 by B.A. Marks, M.J. Heller, R.S. Traven, E.O. Etz 5 This model permitted precise, repeatable off- line simulation of the maneuver for use in establishing Vmc sensitivities to variations in atmospheric conditions, angle of attack, latera l weight asymmetry, and most importantly for the purposes of this discussion, lateral stick input. Figure 2 illustrates the sensitivity of dynamic Vmc to the magnitude of the lateral stick input used to arrest the sideslip- induced roll rate.



### Predicted Dynamic Minimum Control Speeds

3 Tank Interdiction Loading, Full Flaps Standard Day, 14° AOA, 200 ft Hp

120 125 130 135 140 145

0 1 2 3 4 5 **Max Lateral Stick Deflection @ Vmc** **Condition (in) M** **i n** **i m** **u m** **C o** **n t** **r o** **l A** **i r** **s p** **e e** **d (** **k c** **a s** **) Max Lateral** 

**Stick Travel** Increasing Bank Into Bad Engine

**Max Lateral Stick Deflection @ Vmc**  **Condition (in) M** **i n** **i m** **u m** **C o** **n t** **r o** **l A** **i r** **s p** **e e** **d (** **k c** **a s** **)** 5 kt

**Figure 2 . Dy** **namic Vmc Sensitivity To Lateral Stick Input**  Simulated engine-out maneuvers were initiated from the symmetric throttle position required for 1g, level flight at a given AOA. The throttle on the critical engine was reduced to idle one second af ter a trim condition was established. For symmetrically loaded configurations, Vmc is independent of the direction of

the throttle split. H owever, for the asymmetrically loaded configurations evaluated to date, the critical engine corresponds to the engine on the wing - heavy side of the aircraft. Vmc is greatest for this scenario due to the additional aileron bias required to balance the lateral weight asymmetry. This was demonstrated with the F/A- 18E/F for various store loadings during manned simulator and fli ght demonstration testing. Full rudder to oppose the thrust asymmetry resulting from the throttle split was initiated two seconds after completion of the throttle split. As described in Step 4 of the Maneuver Description, this two second delay is intended to account for potential pilot delays in recognizing and reacting to a dynamic engine failure and is defined in the F/A- 18E/F Weapon System Detail Specification, Reference 3. Longitudinal stick was input as required to maintain the target AOA throughout the maneuver. Lateral inputs required to maintain a wings level condition we re permitted in executing the flight test demonstration maneuver. H owever, to simulate a worst case scenario, lateral stick inputs were initiated two seconds af ter completion of the throttle split during off- lineCopyright © 2000 by B.A. Marks, M.J. Heller, R.S. Traven, E.O. Etz 6 simulations. Lateral stick was input as required to attempt to capture and maintain a 5° angle of bank into the good engine, the maximum permitted by the Reference 3 Detail Specification. The pilot model lateral stick proportional gain was also permitted to vary with roll rate to aid in the initial recovery and more accurately reflect the lateral stick characteristics exhibited by the test pilots during manned simulation and flight demonstration testing. The Vmc impact of pilot aggressiveness in the lateral axis was established by varying the pilot model lateral stick proportional gains. As Figure 2 illustrates, applying full lateral stick during the recovery increases Vmc by 10 kt relative to the Vmc obtained when l ateral stick inputs are limited to 1/2 the available travel. This can have serious ramifications in the catapult launch environment. Recall that for normal fleet operations, a 15 kt margin is added to the minimum launch endspeed to account for variability in catapult performance, WO D and other safety considerations. I n the event of a dynamic engine failure at holdback release, the catapult end airspeed can be as much as 5 kt lower than the targeted value. T his reduction in endspeed results from the redu ction in total acceleration associated with the loss of thrust on the failed engine. I n establishing Super Hornet Vmc speeds, lateral stick inputs were limited to 1/3 to 1/2 the available travel. Applying full lateral stick to oppose the sideslip- induced roll resulting from a catastrophic engine failure at holdback release could exhaust the remaining 10 kt of the original 15 kt operational launch margin, potentially resulting in an out-of-control departure at an altitude approximately 60 ft above the wate r. Consequently, warnings were added to the NATOPS Emergency Catapult Flyaway procedure, Reference 4, instructing the fleet pilot to limit lateral stick inputs to 1/2 the available travel. T he pilot was also warned of the potentially severe consequences of

a pplying full lateral stick in conjunction with a catastrophic engine failure. This approach had been successfully implemented for other aircraft, specifically the F- 14, which exhibited similar lateral stick- induced yaw departure tendencies. I n all l ikelihood, no further action would have been taken for the Super Hornet had it not been for an out-of-control departure experienced during follow-on Vmc demonstration testing at an altitude of 2,000 ft. **Vmc Testing Leads to an Out -** **Of -** **Control Event** The techniques described above resulted in a very successful program for most of the configurations tested. H owever, one configuration resulted in an unexpected departure and is the focus of this paper. The configuration labeled “5-WET” is the F/A- 18E/F proposed tanker configuration. The 5-WET label represents the 5 five external fuel stations loaded on the aircraft, as illustrated in Figure 3.

**Figure 3. 5 WET Configu ration** Copyright © 2000 by B.A. Marks, M.J. Heller, R.S. Traven, E.O. Etz 7 Using the dynamic Vmc technique described previously, the pilot repeatedly executed the maneuver at progressively s lower speeds as gross weight was reduced in an attempt to establish a full flap, 14° AOA, 5-WET Vmc. As the pilot approached this speed, execution of the Vmc maneuver and flying qualities were deemed to be excellent. There was very little roll -off noted and the pilot was easily able to maintain wings level with very little lateral stick (less than 1/3 of that available). S ideslip was contained well within the knock- it-off criteria and there was no perception in the cockpit of being near a Vmc condition. However, on the event that led to the departure the pilot did experience a call from monitoring station that sideslip had exceeded the knock- it-off limit of 10°. The pilot reduced AOA and used lateral stick to oppose the sideslip- induced roll -off. As si deslip and roll angle continued to build, the pilot instinctively increased lateral stick input to hold the wings level. W ithin approximately 8 seconds from the completion of the throttle split, sideslip had built to nearly 30° and the aircraft no longer responded to lateral stick input, which by this time had reached the maximum available. The pilot experienced a rapid roll reversal opposite to his stick input and an ensuing departure that resulted in an inverted position 60° nose low at 2,000 feet. The pilot regained control of the aircraft after this snap roll to inverted and was able to roll the aircraft upright and avoid impacting the water by approximately 200 ft. Time histories comparing the maneuver flown just prior to the event with the actual event are provided in Figure 4. The mechanics of

the departure can best be explained when viewed from a total controllability standpoint. As Figure 4 illustrates, the rudders were deflected to their position limits within a very short time of the engine l oss, mainly due to the sideslip feedback to the rudder in the control laws. The pedal that was applied shortly after the engine loss also contributed, however, most of the rudder had already been applied automatically by the flight control system. As a g eneral rule, control augmentation is in question when a control surface can no longer react to the motion changes it is intended to augment, which this incident clearly possessed. The presence of the position - limited rudders with increasing lateral stick deflection is a bad combination in two ways. F irst, the directional stability augmentation desired with the sideslip feedback can no longer be achieved. Second, the roll coordination function of the rudder can not be performed when additional aileron d eflection is commanded from the lateral stick. The result is an uncoordinated roll input, which in conjunction with the inability to contain yaw rate, has the effect of rapidly increasing adverse sideslip due to the lateral stick input. Given the stable dihedral effect exhibited by the F/A- 18E/F, the rolling moment produced by the sideslip growth became large and dominated the other moments acting on the aircraft, rolling the aircraft opposite to the pilot input. Alternately stated, this was a case of two axes simultaneously desiring, and then overwhelming, the same control surface to effectively defeat the basic intentions of that surface. The result was a large sideslip excursion that spawned the departure from controlled flight. **Solution** In establi shing the Emergency Catapult Flyaway procedure for the Super Hornet, it was reasoned that by instructing the fleet pilot to limit lateral stick inputs to 1/2 the available travel, sufficient safety margin existed to prevent an out-of-control departure in the event of acatastrophic engine failure during launch. Recall that a 15 kt operational margin is added to Vmc when establishing target end airspeeds. F urthermore, it was reasoned

.Copyright © 2000 by B.A. Marks, M.J. Heller, R.S. Traven, E.O. Etz 8 **120 130**  **140 150** 

**0 4**  **8 12**  **16**

**0 20**  **40 60** 

**-20 0**  **20 40** 

**-2 0**  **2 4** 

**-2 0**  **2 4**  **-80 -60**  **-40 -20**  **0 20** 

**-160 -120**  **-80 -40**  **0 40**  **Ca lib**  **rate d A** **ir spe**  **ed ~**  **kt**

**T h rottle**  **H andle**  **A ngle**  **~ d**  **eg FCS**  **An gl**  **e Of** **A ttack ~**  **d eg** 

**IN S**  **Si de**  **slip A ngle**  **~ d**  **eg**

**Longitudinal S tick D**  **ef le**  **ction ~ in** 

**Late ral S**  **tick D ef** **le ction ~ in**  **Pi tch A**  **ttitude ~**  **d eg** 

**Rol l A**  **ttitude ~**  **d eg**  **Right THA** 

**Le ft**  **THA E1**  **, F light 427, R**  **ec 31 (E**  **asily C**  **ontrolle d)**  **E1 , F**  **light 427, R ec** **32 (D ep**  **arture )** 

**10 kt**

**-100 0**  **100 200** 

**-200 -160**  **-120 -80**  **-40 0** 

**-80 -60**  **-40 -20**  **0 20**  **40**

**-40 -30**  **-20 -10**  **0 10**  **20**

**-20 0**  **20 40**  **60 80** 

**0 2**  **4 6**  **8 10**  **12 14**  **16 -40**  **-30 -20**  **-10 0**  **10 20** 

**0 2**  **4 6**  **8 10**  **12 14**  **16 Pe**  **dal Force ~**  **lb Hea**  **ding ~ de g** 

**Rol l R**  **ate ~**  **d eg**  **/se c**  **Yaw R ate**  **~ de**  **g/ se**  **c**

**Di ffe**  **re nt**  **ial A ile**  **ron D ef** **le ction ~**  **d eg**  **Col le**  **ctiv e**  **Rudde r D**  **ef le**  **ction ~ de**  **g**

**Ti me**  **~ se**  **c T i**  **me ~**  **se c**  **Figure 4. 5 WET Vmc Testing Leads to Out -** **Of -** **Control Event**Copyright © 2000 by B.A. Marks, M.J. Heller, R.S. Traven, E.O. Etz 9 that by establishing Vmc at 14° AOA, the maximum AOA allowab le during catapult launch operations, additional margin was built- in to the procedure (Vmc increases significantly with increasing AOA). D uring normal, "hands-off" catapult launch procedures at or above the Vmc- based minimum launch endspeed, the flight co ntrol system is designed to capture and maintain a 12° AOA with only a transitory excursion to no greater than 14° AOA during the initial rotation following main wheel roll -off. The additional margin results from the unlikelihood that, if the pilot entered the loop, he/she would actively maintain a 14° AOA any longer than necessary to arrest the abnormal aircraft settle off the bow and establish a positive rate of climb. H owever, the out-of-control event which occurred during Vmc demonstration testing in the 5-WET configuration graphically illustrated that attempting to mitigate the risk of ad eparture through pilot training alone was not sufficient. As described previously in this paper, the mechanics of the Vmc departure are well understood. By limiting lateral stick input, and thus limiting the adverse effects associated with large differential aileron deflection at a condition where no additional rudder power is available to counter them, the severity of the departure characteristics at Vmc can be g reatly reduced. H owever, the use of l ateral stick to hold the wings level is very instinctive to the pilot. I n the end, it was decided that with the digital technology available to the F/A- 18E/F, it was unreasonable to ask the fleet pilot to act against that instinct, particularly wh en distracted by adverse circumstances, possibly at night in inclement weather. The result is a modification to the flight control sof tware, which reduces aileron authority when additional rudder deflection is unavailable. Modification of

the control laws to desensitize the effects of the lateral stick in a single-engine situation was viewed as the most robust option for preventing this type of incident in the future. There were, however, many issues that required consideration

– most associated with the concern that modifying the control laws at an extremely late stage in the development program would change the basic flying qualities and invalidate the results that had taken years of development and testing to identify. G iven these concerns, the goals of the control law modification were: 

##  Fix the problem, focusing on the higher angle of attack problem area above 12° AOA. 

##  Don’t affect flying qualities near the most- used flight conditions

– on-speed at 8° AOA. 

##  Don’t affect normal maneuvering during two-engine operation at the higher AOAs. The solution to the problem focused on the rudder needs of the directional and lateral axes during single-engine operation: Rudder Usage for the Directional Axis - The directional axis is most straightforward, in that the rudder is used to offset the asymmetric yawing moment from the single engine. Rudder is commanded in two ways within the control laws. F irst, the pedal commands rudder to counter yaw rate – which is the normal pilot control input for the engine-out case. And second, sideslip feedback also commands rudder to augment the directional stability. I n single-engine operation, sideslip generation is normal and rudder is automatically applied by the sideslip feedback to oppose it. At low speeds near Vmc, practically all of the available rudder deflection is commanded via the sideslip feedback, with the pedal application typically making only a minor additional contribution. Rudder Usage for the Lateral Axis - The lateral axis requires rudder to coordinate roll maneuvers, i.e. keep sideslip small as the aircraft rolls. There are two factorsCopyright © 2000 by B.A. Marks, M.J. Heller, R.S. Traven, E.O. Etz 10 addressing sideslip control. F irst, the control surfaces used to roll the aircraft (ailerons and differentially deflected stabilator) typ ically possess the side effect of producing adverse yawing moments that tend to generate sideslip in the direction to oppose the roll. Rudder must be applied with the roll to offset those moments and maintain low sideslip. Second, the kinematic effect of the roll itself conducted at higher angles of attack generates sideslip, and yaw rate must be generated in the same direction to offset this effect when coordinating a velocity vector roll. This additional yaw rate requirement makes another significant d emand of the rudder at higher angles of attack. The rudder deflection required to offset these two factors is automatically applied during lateral stick inputs within the F/A- 18E/F flight control system via the Rolling Surface to Rudder Interconnect (RSRI ). The basis of

the control law change is that the rudder cannot perform its intended function in either axis if it is unavailable. Therefore, the concept is to give one axis priority of the rudder over the other. The control law solution implemented gives the directional axis that priority. The reasoning is that the job of constraining the sideslip induced by the asymmetric thrust is of primary importance over rolling the aircraft, and should be done first. The priority assignment looks at the total rudder command from the pedal and sideslip feedbacks and determines how much lateral authority can be granted given the commanded load. For instance, if the pedal and sideslip contributions are small, then full lateral authority can be allowed since only the lateral axis is using the rudder. However, in the case where directional activity is high, the lateral command must be limited to avoid over-saturating the rudder. This is achieved by implementing a variable limiter on the lateral command path as a fu nction of the pedal+sideslip feedbacks. The computation for the limit is such that the limited lateral command plus the existing directional feedbacks will not over-saturate the rudder (some lateral command is always allowed). Logic is also included to l imit the lateral authority only in the direction that adds to the rudder, allowing unlimited command in the opposite direction. I n order to affect only the higher angles of attack, the added logic was incorporated on an AOA schedule so that the limiting would be fully active at angles of attack greater than 12°, and inactive at angles of attack less than 10°. This schedule ensures that the on-speed flying qualities remain unchanged. A b lock diagram of

the modification is shown in Figure 5. Before the logic was released for flight, several manned simulations were conducted to ensure that normal maneuvering was not affected. I n addition, the new logic was fed into the time histories of p revious roll maneuvers to show that the logic would not interfere with normal maneuvering. All of these efforts confirmed that the modified control laws would not impact normal maneuvering, yet would be invoked as needed in the engine-out case. **Vmc Regression Flight Testing With New Control La ws**  The new control laws were first evaluated in the simulator. These tests indicated that Vmc dependency on lateral stick was eliminated. I n fact, full lateral stick inputs at and below previously determined Vmc speeds would allow the aircraft to roll in the commanded direction without the adverse sideslip build - up that resulted in a yaw departure prior to modifying the control laws. Additionally, the simulator was used to

.Copyright © 2000 by B.A. Marks, M.J. Heller, R.S. Traven, E.O. Etz 11 **Rudde r Com**  **man d**  **fro m Ped**  **al**

**Rudde r Com**  **man d**  **fro m Side**  **sl ip**  

**+K 2**  **-K 2** 

**+50 -50** 

**RS MAX**  **RS MI**  **N RS**  **M A X** 

**RS M I**  **N Rol**  **l C ommand**  **K1 AOA Sche**  **dule Lim**  **it er** 

**Lim it** **er (K** **2 < 50)**  **(K 1 >**  **50) +**  **+ +** 

**+ -** 

**+**

**Pr io**  **ritiz ed**  **Rol l**  **Co mmand**  **Figure 5. Block Diagram of Control Law Modification** ensure that there would be no transient rol l "ratcheting" as a result of the gain changes at and around 12° AOA regardless of the engine-out situation. The pilots were unable to detect any lateral stick gain changes as a result of the new control law architecture even when transitioning through th e affected AOA range of the modification. Of particular interest was the control margin available for roll even with large lateral weight asymmetries. This was also found to be acceptable even with an engine out-condition on the critical side of the asym metry. Flight testing was then accomplished to verify s imulator results and certify the new control laws. F light testing in the clean aircraft showed no degradation in roll control or signs of s ideslip build - up at high AOA. In the 5-WET configuration, the aircraft was very we ll behaved. Lateral control power was more than adequate for the fly-away task. Additionally, full lateral stick inputs were intentionally inserted near Vmc. P rior to the control law modification, inputs of this nature near Vmc would result in significant adverse sideslip build - up and provoke loss of a ircraft control, as the Figure 4 5-WET departure illustrates. H owever, with the control law modification implemented, the aircraft remained well behaved with no signs of s ide force build - up or yaw- induced departure noted by the test pilot or indicated in the test data. **Impact to the Fleet Operator** Arguably, a settle off the catapult during carrier operations at sea is the most critical emergency a naval aviator will encounter. Any one of an umber of

s ituations may result in a settle off the bow of the ship (i.e., “sof t” catapult shot, degraded engine performance, or total single engine loss of thrust), but all are sensitive to pilot reaction time and pilot input in a manner unmatched in most shore- based operations, since the aircraft is immediately catapulted at only 60 feet above the water. The carrier environment is rarely forgiving of p ilot error, and an aviator’s initial response to a settle off the catapult has historically been the determining factor as to whether the aircraft flies away safely, or impacts the water, with or without the pilot still at the controls. Consequently, procedures for a safe “Emergency Catapult Flyaway” are criticallyCopyright © 2000 by B.A. Marks, M.J. Heller, R.S. Traven, E.O. Etz 12 important to carrier pilots to en sure aircraft recovery. P riorities during emergency catapult flyaway are to establish control of the aircraft, arrest aircraft settle, and accelerate for climbout away from the water. In the event the single engine rate of climb remains negative, it is h ighly desirable to at least slow the descent rate and maintain aircraft control to accomplish external stores jettison and continue flyaway attempt, or perform a safe, upright ejection. Although catapult launch bulletins should ensure an endspeed at least 15 knots above Vmc, wind over deck variations and loss of thrust may erode that planned margin, and the effects of

p ilot overreaction, both in pitch and roll, cannot be ignored. Years of fleet experience and simulator testing have demonstrated that the transition from a “hands-off” catapult to immediate “pilot- in -the- loop” control for emergency catapult flyaway can lead to an initial over- rotation to AOA’s greater than 14°, primarily resulting from instinctive pilot reactions in the presence of extreme "g round" rush. Prior to implementation of

the Vmc control law modification, this action, coupled with lateral stick input of g reater than approximately 1/2 stick could have resulted in an immediate adverse yaw departure at less than the carrier deck height of 60 feet. A d eparture at that altitude would not have provided any opportunity for controlled stores jettison, or ability for safe ejection. **Conclusions** The Vmc control law modification developed for the Super Hornet provides a forgiving environment for variances in pilot reaction that may be encountered during single engine catapult events. Specifically, at higher AOA’s, the decreased sensitivity to lateral stick input will prevent adverse yaw departures. Additionally, it is apparent from simulation that at speeds below Vmc, where immediate departure would have been the likely result of a ny lateral stick activity, the modification allows for lateral control of the aircraft to accomplish stores jettison or controlled ejection within a safe envelope. App lications of up to full lateral stick are now acceptable to attempt to arrest roll rates induced by the asymmetric thrust in a single engine failure scenario. The result is a flight control system that inspires confidence in the naval aviator in his/her ab ility to handle a single engine failure off the catapult in even the worst environmental conditions, specifically at night in bad weather. This Vmc control law modification provides the carrier aviator time to procedurally step through a decision matrix th at will allow him/her to control the Super Hornet with an engine loss at the most critical moment. At the very least, the modification will provide a controllable aircraft for a wings- level ejection that will save the pilot's life. **Lessons Learned** Where flight safety is concerned, full advantage should be taken of the digital technology available to today's flight control designers. W hen the critical connection between dynamic Vmc and lateral control inputs was first identified during the F/A- 18E/F EMD flight test program, it was believed that the risk of an out-of-control departure following a catastrophic engine failure during catapult launch could be sufficiently mitigated through pilot training. H owever, as the 5-WET departure discussed in this pape r graphically illustrated, training alone does not adequately protect even the most highly skilled pilot when that training requires him/her to act against what is instinctive. For the Super Hornet, fly- by-wire technology was successfully utilized to virtually eliminate lateral stick- induced yaw departures below Vmc.Copyright © 2000 by B.A. Marks, M.J. Heller, R.S. Traven, E.O. Etz 13 **References** 1. SA- FTM-01, “Carrier Suitability Testing Manual,” Revision 2, 30 September 1994. 2. SFTE European Chapter 10th Annual Symposium Proceedings, "Single Engine Minimum Control Airspeed Evaluation Of The F/A- 18E/F," LCDR Ken Ham, Henry Melton, Bret Marks, 14- 17 June 1998. 3. SD-565-3- 1, "Detail Specification For Model F/A- 18E/F Aircraft Weapon System," Vol. I, 27 Aug 1993. 4. (DM)A1 - F18EA- NFM-000, "Preliminary NATOPS Flight Manual Performance Charts

– Navy Model F/A- 18E/F 165533 And Up Aircraft," 1 March 1999