The Aerodynamics Of Power Loss AUTHOR Major Mark C. Thoman, USMC CSC 1985 SUBJECT AREA Topical Issues EXECUTIVE SUMMARY Modern multi-engine aircraft are well designed and extremely reliable, and as a result the chances of engine failure haue been significantly reduced. But until that probability is reduced to zero, the training syllabus of a pilot learning to fly propeller driven, multi-engined aircraft must include a detailed analysis of the aerodynamics of partial powered flight. This analysis begins with a review of the basic aerodynamic forces that effect any aircraft during flight, and continues with an examination of the additional forces that result as a consequence of an engine failure. In turn, each phase of flights including take-off, cruise, descent, approach, wave-off and landing is discussed. Attention is focused on the particular aerodynamic forces most pronounced during each phase, and a study of practical flying techniques is completed. The entire analysis is intended to increase a pilots knowledge to a practical level so that he might respond accurately when confronted with a power loss emergency. THE AERODYNAMICS OF POWER LOSS OUTLINE THESIS STATEMENT: The training syllabus of a pilot learn in fly propeller driven, multi-engine aircraft must include a detailed analysis of the aerodynamics of partial power. I. AERODYNAMIC REVIEW A. Primary forces B. Drag 1. Induced drag 2. Parasite drag C. Power required D. Power available, excess power available E. Propeller aerodynamics 1. Center of thrust 2. Critical engine 3. Induced flow F. Control authority vs. airspeed II. AERODYNAMICS OF POWER LOSS A. Power available vs. power required B. Aircraft control 1. Yaw 2. Roll C. Vmc(air) 1. Definition 2. Variables D. Asymetric power technique 1. Aerodynamic forces 2. Angle of bank 3. Balance ball 4. Balanced flight vs. optimum performance III. CRUISE FLIGHT A. Emergency procedures completed B. Asymmetric technique applied C. Power available insufficient 1. Decelerate and or descend 2. Decrease parasite drag 3. Decrease induced drag 4. Drift-down performance charts IV. TAKE-OFF A. Aerodynamically the worst case B. Engine failure prior to refusal speed C. Engine failure after refusal speed 1. Continue take-off 2. Improve aerodynamic conditions a. Maintain maximum power available b. Reduce power required c. In extremis D. Keys to success 1. Study performance charts 2. Brief crew V. DESCENT AND APPROACH A. Aerodynamics of descent B. Aerodynamics of the approach 1. Increased power required 2. Maneuvering flight a. Airspeed b. Angle of bank 3. Power changes vs. rudder trim 4. Landing configuration C. Final approach 1. Airspeed 2. Glide slope VI. WAVE-OFF A. Aerodynamics, similar to take-off B. Success dependant upon planning 1. Landing site selection 2. Weight 3. Configuration 4. Airspeed C. Procedures 1. Stop rate of descent a. Nose attitude b. Power application 2. Asymmetrical technique applied 3. Retract landing gear 4. Accelerate, climb 5. Avoid turns VII. LANDING A. Glide slope B. Flare, touchdown 1. Runway length 2. Yaw effect C. Rollout aerodynamics 1. Reverse thrust 2. Asymetric lift 3. Cross wind D. Rollout procedures VIII. SUMMARY A. Asymmetric power technique B. Airspeed C. Throttles D. Destination E. Crew brief F. Knowledge TABLE OF CONTENTS I. AERODYNAMIC REVIEW 3 II. AERODYNAMICS OF POWER LOSS 6 III. CRUISE FLIGHT 12 IV. TAKE-OFF 13 V. DESCENT AND APPROACH 16 VI. WAVE-OFF 18 VII. LANDING 20 VIII. SUMMARY 24 IX. FOOTNOTES 26 X. BIBLIOGRAPHY 27 THE AERODYNAMICS OF POWER LOSS It had been a miserable flight. Nothing had gone right. Here we were on short final, in rainy weather, with both engines on the left wing feathered. I was exhausted. The last hour had been a continuous struggle between the emergency contitions and the worsening weather. A quick glance outside confirmed once again that number one and two were standing tall', but the sluggishness of the aircraft, and the difficulties I'd encountered in flying straight also told me that. We were approaching minimums, and the world outside the cockpit was growing darker. The co-pilot announced that he had the runway in sight. I allowed myself a sigh of relief, but my hopes of landing were shattered when the flight engineer shouted, 'There's an airplane on the runway, take it around!' Reacting in disbelief, I shoved number three and four throttles to maximum power, and raised the nose. As we commenced the climb, the aircraft began a yaw and roll to the left which I tried to counter with the flight controls. I yelled to the co-pilot to help, and together we applied full aileron and rudder. But nothing we did would stop the motion, and we continued to roll over on our back. I fought the inevitable with every ounce of my strength, but in the end all I could do was close my eyes, and wait for impact. I didn't want to watch. Thank goodness we were flying the simulator, and the only thing damaged was my pride. In the next hour I would get to try this same approach again, and I knew what my mistake had been. I had failed to plan ahead, and had allowed the aircraft to get too slow. When I applied full power on the remaining engines, there was not enough control authority to counter the unbalanced thrust. It had been a valuable lesson. Statistics have shown that power loss in modern aircraft is becoming increasingly rare.1 Additionally, the superb design of these aircraft have given them good flying characteristics, even under partial power conditions. But, when an engine does fail, the successful outcome of the flight will be based largely on the knowledge and skill of the pilot and his crew. Thus, the training syllabus of a pilot learning to fly propeller driven, multi-engined aircraft, must include a detailed analysis of the aerodynamics of flight under partial power conditions. It is the purpose of this paper to make that detailed analysis; To review the basic areodynamic factors that impact on partial powered flight. To explore how asymetric thrust effects the different phases of a flight. To study how the aerodynamics of the propeller complicates the problem. And of course to discuss general flying techniques that can be applied. If a pilot understands the aerodynamic forces effecting his aircraft then he will be well prepared for the day he must face a partial power emergency. I. AERODYNAMIC REVIEW Before studying the details of partial power, let's briefly review the aerodynamic conditions that are found in normal flight. For an aircraft to maintain itself aloft, the primary forces of weight and drag must be balanced by equal forces of lift and thrust. As the wing moves through the atmosphere, it creates the lifting force sufficient to carry the total weight of the aircraft. In the process of creating lift, it also creates a significant amount of drag. This component of drag is called 'induced drag', and it is proportional to the weight of the aircraft.2 The larger the force of lift required, the larger the component of induced drag will be. The second component of drag is called 'parasite drag', and it is created by the movement of the entire airframe through the atmosphere.3 Parasite drag is directly proportional to airspeed. The faster the aircraft moves through the atmosphere, the greater the resistance, or parasite drag. Additionally, parasite drag is greatly effected by the configuration of the aircraft. At an equal airspeed, an aircraft with the landing gear extended creates much more parasite drag than one with the landing gear retracted. To maintain a constant airspeed, the total forces of both induced and parasite drag must be overcome by the thrust of the propulsion system. This specific amount of thrust is called the 'power required', and it is different for every combination of aircraft weight, airspeed, or configuration.4 The total thrust that the propulsion system of the aircraft is able to produce is called the 'power available'.5 The difference between the power required and the power available translates directly into the performance capabilities of the aircraft. Visualise a situation where the power available is equal to the power required. At this point the aircraft is at the limit of its performance capabilities, and cannot accelerate, climb, or carry any additional weight. Modern mulit-engine aircraft have a considerable excess of power available when all engines are operating. This excess power available provides for good acceleration and climb characteristics with a designed payload. But, when engines fail the power available drops rapidly, and in some cases may fall below that of the power required. As we shall see, this is when a pilot's training becomes very important. The aerodynamics of multi-engine aircraft are further complicated by the peculiar aerodynamic characteristics of the propeller. As any pilot of a propeller driven aircraft will tell you, the center of thrust created by a propeller is not at its center of rotation. Due to a number of design and flight factors, the plane of rotation of the propeller is not exactly perpendicular to the flow of air. In fact, the plane of rotation is tilted upward, and as a result causes a higher angle of attack, and a stronger thrust, on the descending propeller blade. The standard propeller, spinning clockwise, will create a center of thrust well to the right of the center of rotation. On a multi-engine aircraft, the thrust of the propellers on the right wing are farther from the center of the aircraft than those on the left wing. The thrust of the number four propeller, outboard on the right wing, is the farthest of all from the center of the aircraft. During normal flight the thrust of the number four propeller is balanced by the thrust of the number one propeller. But in the event number one should fail, the thrust of the number four propeller has the greatest imbalancing effect. As a result, aerodynamic engineers have termed the number one engine the 'critical engine', because its loss will create the most difficult control situation.6 A propeller creates thrust by accelerating a large mass of air. Those portions of the wing located directly behind the propeller, are exposed to this accelerated air, and as a result create more lift than those portions exposed only to the free flow of the atmosphere. This accelerated air is called 'induced flow', and the additional lift it creates is most apparent at slower airspeeds. The induced flow remains balanced across the wing span of the aircraft when all engines are operating. But as we shall see, it can become a factor when a propeller is feathered and the induced flow is not balanced. Lastly in our aerodynamic review we must examine the relationship between flight control authority, and airspeed. To be effective, flight control surfaces require a flow of air to create the forces sufficient to maneuver the aircraft. The forces created are directly proportional to the airspeed. As an aircraft accelerates, the flight controls creat an increasingly greater force, and as a result are more effective. Conversly, as an aircraft decelerates, the flight controls become less effective. As the deceleration continues a point will be reached where the flight controls no longer create sufficient force to maintain control of the aircraft. As we shall see this will be compounded by the factors involved during partial powered flight. II. AERODYNAMICS OF POWER LOSS Having completed the review of basic aerodynamics, let's begin the detailed examination of the aerodynamic conditions associated with partial power flight. Visualize a four engine aircraft, during cruise flight, where the forces of weight and drag are balanced by equal forces of lift and thrust. The power being applied by the engines is equal to the power required, and as a result constant altitude and airspeed are being maintained. Additionally, the total forces of thrust and drag are aligned with the aerodynamic center of the aircraft. When a single engine fails, the total power available is immediately decreased by twenty-five percent. The power required is increased, due in part to the additional parasite drag created by the unpowered propeller. If the effected propeller can be feathered then the increased drag will be minimal, however, if the effected propeller cannot be feathered, and remains windmilling, the increase in drag will be significant. As stated earlier, the aircraft's performance capabilities are a direct function of the excess power available. Depending upon the flight conditions, the loss of a single engine could eliminate all excess power. The pilot must then take appropriate steps to reduce the power required. These procedures will be discussed in detail for each regieme of flight. As the thrust from the propeller is lost, an abrupt yawing motion will become apparent. This yawing motion results from the shifting of the centers of both thrust and drag away from the aerodynamic center of the aircraft. The center of thrust shifts toward the operating engines as the center of drag shifts in the opposite direction, toward the failed engine. The amount of yaw will be dependant upon how much thrust is being produced by the operating propellers, and how much drag is created by the failed propeller. If the propeller cannot be feathered the yawing motion could be severe. The amount of yaw will also be dependant upon which engine failed. An outboard engine will have a much greater effect than an inboard engine, and the failure of number one will have the greatest yawing effect of all. The entire yawing motion will be significantly compounded by the failure of two engines on the same side of the aircraft. For the purposes of our discussion, assume there is sufficient airspeed to create the aerodynamic forces required to counter this yawing motion. Directional control of the aircraft can then be maintained. As the thrust from the propeller is lost, there will also be a slight rolling motion. When the propeller ceases functioning, the induced flow over the effected portion of the wing will also cease. This results in a loss of lift on that portion of the wing, and an imbalance of lift across the entire wing span, creating a rolling motion toward the unpowered propeller. At cruise speeds this rolling motion will hardly be noticeable, and is easily controlled by the ailerons. The rolling motion will be most prominant at lower speeds, and higher power settings, where the phenomenon of induced flow is most apparent. To briefly recap, our example aircraft has lost the thrust from a single propeller. The pilot has been able to balance the asymmetric thrust and lift through the simple use of the flight controls and is able to maintain a constant airspeed and altitude. The control forces required to balance the asymmetric thrust and lift are dependant upon airspeed. If the aircraft were allowed to decelerate, an airspeed would soon be reached where the aerodynamic forces created by the flight controls would be inadequate to balance the forces created by the asymetric thrust and lift. The pilot cannot maintain directional control of the aircraft at or below this airspeed, which is known as 'air minimum control speed' or Vmc(air).7 Air minimum control speed is a value effected by many variables. Any condition that effects the critical balance between the flight control forces, and the asymetric lift and thrust will ultimately effect the air minimum control speed. The aerodynamic factors that effect Vmc(air) include, the basic design of the aircraft, the weight of the aircraft, the angle of bank, the condition of the unpowered propeller, the number of engines that have failed, and whether the aircraft is flying in or out of ground effect. The atmospheric conditions that effect Vmc(air) include temperature and altitude. The mechanical factors include individual engine performance, the current power setting, and the hydraulic pressure available to the flight control booster system. Individual aircraft performance charts attempt to give an air minimum control speed for as many different conditions as possible. They are sufficiently accurate to inform the pilot at approximately what airspeed he can anticipate directional control difficulties. However, with this many variables involved, these performance figures may not be precise. Once again let us assume that sufficient airspeed is being maintained to establish control of the asymmetrical thrust and lift, the wings are level and the balance ball is centered. At this point most pilots believe that the aircraft is in balanced flight, with the oncoming air flow aligned with the center of the aircraft. In reality, the aircraft is flying in a rather large sideslip. Let us examine why this is true. For the purposes of our discussion, let's assume that the number one engine has been feathered. Due to the asymmetric forces, the nose of the aircraft yawed to the left. The pilot applied rudder to create a balancing force on the tail in an attempt to counter the yawing motion. Like the yawing motion, this balancing force on the tail is also to the left. Even though the balance ball is centered, these two forces are pushing the aircraft to the left, resulting in a rather large sideslip. The sideslip produces an additional increase in parasite drag, and air minimum control speed. To eliminate this sideslip the pilot simply creates an equal force to the right by holding a slight angle of right bank toward the operating engines. Thus, the pilot compensates for the sideslip and allows the aircraft to regain balanced flight. The amount of bank required depends largely upon the asymmetric thrust which must be countered. In accordance with Federal Aviation Administration regulation, aircraft manufacturers are limited to five degrees of bank angle when conducting Vmc(air) performance evaluations.8 Additionally, as the angle of bank is applied, the original force on the rudder, required to control the yawing motion, can be reduced. It is important to note at this point, that while flying at airspeeds near Vmc(air), even a shallow turn into the failed engine can cause loss of directional control. The force of the sideslip combines with the lateral force of the turn and rapidly developes to a point beyond the flight control authority. flight testing has shown that Vmc(air) can increase as much as thirty-five knots with only a five degree angle of bank turn into the failed engine.9 Turns into the failed engine can be executed only when there is a safe margine between actual airspeed and air minimum control speed. While maintaining this angle of bank into the operating engines, the pilot will notice that the balance ball is no longer centered. Though uncomfortable, the ball is in fact displaying balanced flight. As designed, it is seeking the lowest point in the gauge, but because of the constant angle of bank, the lowest point is no longer in line with the normal markings. Having regained balanced flight, the optimum performance capabilities of the aircraft are also regained, and the power required for flight is maintained at a minimum. For the purposes of this paper, I will refer to this maneuver as 'asymmetric power technique'. III. CRUISE FLIGHT When an engine falls during cruise flight, the aircrew will initially be preoccupied with the completion of all appropriate emergency procedures. Simultaneously, the pilot flying the aircraft should apply the asymmetric power technique to maintain balanced flight. As cruise airspeeds are normally well above air minimum control speed, loss of directional control is not an immediate concern. But, in many instances the power required to maintain optimum cruise altitude and airspeed is quite high, and the remaining power available may not be adequate. The pilot has several options at this point, but ultimately he must reduce the power required. By ensuring the aircraft's configuration is as 'clean' as possible, parasite drag can be held to a minimum. Parasite drag can also be decreased by a simple reduction in airspeed. If maintaining altitude is a primary concern, due to terrain clearance for example, induced drag can be decreased by reducing the weight of the aircraft. This can be accomplished by jettisoning either fuel, cargo, or both. If range is a major concern, as in a trans-oceanic flight, then a descent to a lower altitude would allow the wings and the propellers to operate more efficiently. Drift-down charts are provided in operating manuals to ensure optimum performance during this type of power loss situation.10 Obviously, the option selected by the pilot will be entirely dependant upon the circumstances surrounding the particular engine failure, and the flight profile. Generally during normal cruise flight, maintaining control of the aircraft, and remaining airborne is not an immediate problem. IV. TAKE-OFF The take-off phase of any flight is aerodynamically the worst possible time to suffer a power loss. The aircraft is heavier at take-off than it is at any other time during the flight. The aircraft's configuration is 'dirty', and the flight profile requires both an acceleration and a climb. As a result, the power required to perform the take-off often approaches the total power available. Additionally, the aircraft must accelerate through a period where it is airborne, but may be flying at an airspeed less than the air minimum control speed. During this critical phase of flight, a power loss will demand immediate response by the pilot and his crew. With that clear in our minds, lets examine the aerodynamic problems that will be encountered, and recommend some practical solutions. If an engine should fall during the take-off roll, and the airspeed is still less than the computed refusal speed, the take-off must be aborted. The primary concern of the pilot is stopping the aircraft within the length of the runway while maintaining directional control. This situation is aerodynamically similar to the landing maneuver, and the details will be examined in that portion of the paper. If an engine should fail after the aircraft has accelerated beyond the computed refusal speed, then the aircraft is commited to flight. Given this situation the pilot will have to react guickly to control the asymmetric thrust, to maintain the maximum power available, and to reduce the power required to a minimum. The greatest difficulty will be experienced if power is lost during the acceleration from refusal speed to air minimum control speed. During this critical period, if the effected engine is still producing thrust, consideration should be given to letting it continue to run. If the propeller is producing a great deal of drag, then it must be feathered immediately. Whenever the engine is feathered, an immediate application of the asymetric power technique will assist in the control of the asymetric thrust, and also reduce the parasite drag associated with a sideslip. At this point the pilot must also ensure that a maximum power setting is placed on all operating engines. Terrain permitting, the rate of climb should be held to a minimum to allow for the maximum acceleration. Turns should be made only to avoid obstacles, kept as shallow as possible, and never made in the direction of the failed engine until the airspeed is well above Vmc(air). Raising the landing gear on most aircraft will significantly reduce the parasite drag, and this should be performed as soon as possible. Raising the flaps will also reduce the parasite drag, but in most aircraft this will decrease the wing area to a point where it will not sustain flight. Flap operation during take-off is discussed in detail in individual aircraft flight manuals. An immediate reduction in the weight of the aircraft will decrease the induced drag. One of the most effective ways to accomplish this is to jettision fuel. If the asymmetric thrust can be controlled, level flight maintained, and an acceleration continued, then the take-off will be successful. If the power loss occurs during the critical acceleration to air minimum control speed, the asymmetric thrust may not be controllable. In this extreme situation, it would be preferable to reduce asymmetric power and land straight ahead, with the wings level, rather than impact while the aircraft is out of control. When, a power loss is suffered on take-off, many procedures will have to be accomplished in a very short period. The key to success is preparation, and this takes two simple forms. First, a complete study of all the applicable performance charts will allow the pilot a thorough knowledge of the capabilities of his aircraft and the current take-off conditions. If the performance perameters are critical then he can either take action to change them or at least be forewarned. Secondly, a complete take-off brief, given by the pilot, will allow the crew to work as a team. They will then know his intentions and can perform many of the emergency procedures, while he flies the aircraft. This preparation will allow the pilot and his crew the maximum margine for success in the event of power loss during take-off. V. DESCENT AND APPROACH Whether the engine failed on take-off, or during cruise flight, at some time the aircraft will have to be landed. Lets continue our examination of power loss through the descent and approach phases of flight. The power required during a descent is minimal, and as a result the adverse effects of asymmetric thrust and lift will almost disappear. The aircraft responds normally, and turns in either direction can be executed safely. But this apparent return to normal flight can be a trap for an unsuspecting pilot. If airspeed is allowed to decay during the descent, when power is reapplied directional control difficulties may be experienced. Recovery is simple, but the solution is to maintain an airspeed well above air minimum control speed, regardless of the power setting. During the approach phase, the aircraft must be maneuvered to the final approach course and configured for landing. As a result, the power requirements for the approach phase can be quite high. The asymetric power technique should be applied continuously. Airspeed should be maintained well above air minimum cotrol speed, and maneuvering turns should be kept to a minimum angle of bank. Above fifteen degrees angle of bank the induced drag, increases dramatically thus adding to the power requirements. Turns into the failed engine can be executed, but only if the airspeed is well above Vmc(air). Performing the approach phase will require some large power changes, which due to the asymmetric thrust, will induce some large yawing moments. To minimize this effect, power changes should be executed smoothly, and rudder inputs anticipated. Rudder trim should be used throughout to maintain constant heading control and reduce pilot workload. Lowering the flaps and landing gear will significantly increase the parasite drag. If the resulting power required then exceeds the total power available, airspeed will decay and the aircraft will be committed to a descent. To avoid this, the pilot must carefully plan when to extend the flaps and landing gear. Again, each individual aircraft's flight manual will give specific instructions for this situation. Once established on the final approach course, there are several factors that should be kept in mind. As in all phases of power loss flight, the airspeed must be kept well above air minimum control speed. This will not have a direct effect on the approach, but will certainly be a factor if a wave-off must be executed. The asymmetric power technique should be applied continuously, but because of the reduced power setting during the final approach it is not as critical. The most comfortable glide slope is flown slightly more steeply than normal. A steeper glide slope allows the power to be reduced and the aircraft to be flown under more symmetric conditions. With the steeper glide slope, it is also easier to maintain an airspeed above Vmc(air). A well flown approach will make a successful landing, or wave-off, much easier. VI. WAVE-OFF Aerodynamically a wave-off is similar to a take-off. The flight profile of a wave-off requires both an acceleration and a climb, and the aircraft is often in a 'dirty' configuration. During a wave-off a very high power setting is required, and a large amount of asymetrical thrust and lift must be controlled. But, there are several factors that will allow success if they are planned for well in advance. The planning must first begin with the selection of a landing site. The pilot should not only study the weather, and the length of the available runway, but he must also give careful consideration to the terrain surrounding the airfield, and the profile of the published wave-off. If the wave-off requires a steep climb, or a turn into the failed engine, then the landing site may be unsuitable. Secondly, prior to executing the approach, the pilot should reduce the total weight of the aircraft to a point which will ensure a successful wave-off. Thirdly, during the approach the pilot should avoid putting the aircraft in a configuration that will prohibit a wave-off. Finally, the approach speed must be maintained well above air minimum control speed. If this is done, then there will be little difficulty in controlling the asymmetric thrust and I when maximum power is applied. The procedures for executing a wave-off are similar to that of a take-off. The rate of descent must first be arrested, and this is accomplished by setting the take-off attitude and adding maximum power on the operating engines. The asymmetric power technique should be applied to control the asymmetric thrust, and reduce parasite drag. If the airspeed is near Vmc(air), directional control may present a problem, and the pilot may be forced to reduce the thrust on the asymmetric engine until directional control can be maintained. It is at this point when a good margine of airspeed over air minimum control speed is most critical. The aircraft should be 'cleaned up' by retracting the landing gear and flaps in accordance with the flight manual. Terrain permitting, the flight profile should include an acceleration and then a climb. Turns, especially into the failed engine, should be avoided until the airspeed is well above Vmc(air). If a turn is required, then it should be kept to a minimum angle of bank. A wave-off is not a difficult maneuver, but its success is entirely dependent upon sound prior planning. VII. LANDING Landing an aircraft that has suffered a power loss can prove to be a somewhat tricky maneuver. The largest problem continues to be one of directional control, so let us examine the aerodynamic conditions that are involved and discuss some of the practical solutions. If the glide slope has been flown as recommended, the aircraft will arrive over the threshold of the runway with ample airspeed and a reduced power setting. With minimal asymmetric thrust to control, the flare and touchdown will be very near normal. While executing the flare, the pilot should avoid an over rotation which can cause the aircraft to float. It is important to avoid this float for two reasons. First, it will use up valuable runway that could be much more effectively used in stopping the aircraft. Secondly, as the aircraft decelerates during the float, the rotating propellers on the operating engines continue to create an increasing amount of asymetric thrust. This results in a yawing tendancy which can be difficult to correct as the rudder effectiveness rapidly decreases.11 The best way to avoid both of these problems is to land at the computed touchdown speed near the approach end of the runway. Following touchdown, the next problem the pilot quickly faces is stopping the aircraft within the confines of the runway. There are several aerodynamic forces that will cause directional problems during the landing rollout. Let's examine each individually and then apply some practical solutions. The largest single force that can cause directional control difficlty is the application of asymmetric reverse thrust. If the pilot should mistakenly apply full reverse on the asymmetric engine, the aircraft will immediately turn into the operating engines. No combination of flight controls or nose wheel steering will keep the aircraft on the runway. Obviously, careful application of the reverse thrust should be foremost in the pilots mind. During the rollout, reverse thrust on the symmetric engines should be applied to begin the deceleration. Reverse thrust on the asymetric engine can then be applied as necessary, and only in amounts that can be controlled. The asymmetric reverse will cause an imbalance in the lift across the wing span. As the operating propellers are put in reverse pitch, they blank out the flow of air over that portion of wing directly behind them. But behind the feathered propeller the wing is still exposed to the free air stream and is still creating lift. This results in an inbalance in lift across the wing span and a rolling motion away from the failed engine. If left uncorrected, the induced rolling motion can cause a swerve into the failed engine. This is especially the case in aircraft configured with tricycle type landing gear systems.12 This rolling motion can be controlled effectively by timely application of the ailerons. A cross wind can also create a directional control problem during the landing rollout. As the relative wind moves to the side of the aircraft, it causes the upwind wing to rise and the nose to weathercock into the wind which will cause the aircraft to swerve into the wind. This swerve can easily be corrected by the flight controls and the nose wheel steering. The direction of the swerve created by the asymetric reverse thrust will always be toward the operating engines. The direction of the swerve created by the asymetric lift will always be into the failed engine. The direction of the swerve created by the cross wind will of course be dependant upon the direction the cross wind is coming from. Given a choice, it is advantageous to put the cross wind on the same side as the failed engine. Then the turning force of the cross wind can be used to balance the turning force created by the asymetric reverse thrust. Let's follow our example aircraft, with the number one propeller feathered, through the landing rollout. Right rudder has been used throught the flight to control the asymmetric thrust. Touchdown was completed in an on-speed condition near the approach end of the runway and the nose wheel was lowered to the ground. As the pilot brought the throttles into the ground idle position, the aircraft swerved and rolled to the right. This was caused by the asymetric reverse thrust and the resulting imbalance in lift. An application of left rudder and left aileron maintained the directional control of the aircraft. The symmetrical engines, the inboard ones in this case, were brought into full reverse pitch and the brakes were applied. The reverse thrust of the asymmetric engine, in this case number four, was carefully applied so as not the create a loss of directional control. The effectiveness of the rudder can be increased during the landing roll out by ensuring that the rudder trim is zeroed out before touchdown. This will minimize the problems that can be induced during the rapid rudder reversal that occurs when transitioning from positive to reverse asymmetric thrust. The effectiveness of the nose wheel steering can be increased by ensuring the aircraft's weight is applied to the nose wheel. If the nose wheel is held off the ground, only the rudder is available for directional control. The rudder loses its effectiveness rapidly during the deceleration, and the nose wheel steering must be available to maintain directional control during the remainder of the landing roll out. A successful landing is not difficult, but it requires the pilot to plan ahead, to brief his crew carefully, and to execute the procedures smoothly. Upon touchdown he cannot relax. In fact, a lapse in attention has caused more than one pilot to lose control of his aircraft after a successful touchdown. If a loss of control occurs, the most likely cause is a misapplication of the asymmetric reverse thrust. The throttles should be immediately placed back into the ground idle position, and the directional control of the aircraft regained by the use of the flight controls and the nose wheel steering. A careful reapplictaion of reverse thrust will then bring the aircraft to a stop while maintaining directional control. VIII. SUMMARY There are several key factors that impact upon all phases of a flight in which a power loss has been experienced. * A continuous application of the asymmetric power technique will minimize the difficulties of maintaining directional control, and will ensure the optimum performance characteristics of the aircraft. * Maintaining an airspeed well above the air minimum control speed will ensure continued directional control, and allow for maneuvering turns in either direction. * Remember, the asymmetric thrust and lift can cause the immediate loss of control of the aircraft. This means that during a power loss situatuation the aircraft must be flown as much with the throttles as with the flight controls. * When researching a destination, keep in mind that in a reduced power condition the ability to wave-off is significantly reduced. * Always brief your crew. If they do not understand your intentions they cannot support you when the situation is most critical. * Finally, the single most important key to success in any emergency, and most particularly a power loss emergency, is knowledge. The more knowledge that can be applied to any airborne emergency the greater the opportunity for success. GOOD LUCK! FOOTNOTES 1Steve R. Last, Richard T. Turner, 'Shut-down Rates,' Aviation Week & Space Technology, vol.122 no.6 (February 11, 1985), p.94. 2H.H. Hurt, Jr., Aerodynamics for Naval Aviators (U.S. Navy, 1960), p.66. 3Hurt, p.87. 4Hurt, p.96. 5Hurt, p.150. 6Mr. R.A. Eldridge, 'Single-engine Procedure in Twin-engine Aircraft,' Approach, vol.26 no.10 (April 1981), p.6. 7Eldridge, p.3. 8Eldridge, p.3. 9Air Minimum Control Speeds (Marietta, Ga: Lockheed-Georgia Co.) p.2. 10Naval Air Systems Command, USN, KC-130F NATOPS Flight Manual, NAVAIR 01-75GAA-1, (Washington, D.C., 1981), p.12-115. 11Ibid, p.5-50. 12H.H. Hurt, Jr., Aerodynamics for Naval Aviators (U.S. Navy, 1960), p.305. BIBLIOGRAPHY Air Minimum Control Speeds. Marietta, Ga: Lockheed-Georgia Co. Eldridge, R.A. 'Single-engine Procedure in Twin-engine Aircraft.' Approach, vol.26 no.10 (April 1981), 2-6. Hopewell,R.S. 'Minimum Control Speed-Air Vmc(Air).' Approach, vol.20 no.7 (January 1975),27. Hurt,H.H. Aerodynamics for Naval Aviators. Washington D.C.: U.S. Navy, 1960. Last,S.R. and Turner,R.T. 'Shut-Down Rates.' Aviation Week & Space Technology, vol.122 no.6 (11 February 1985), 94. Miller,J.J. 'Engine Failure and Vmc(Air) in Multiengine Aircraft.' Approach, (February 1982), 12-13. Miller,J.J. 'P-3 and C-130 Engine-Out Landing Techniques.' Approach, vol.28 no.10 (April 1983), 18-20. 'Mishap with a Moral.' The MAC Flyer, vol.XXVI no.8 (August 1979), 10-11. Structural Aspects C-130 Operating Limitations. Marietta, Ga: Lockheed-Georgia Co. 'Understanding Fin Stall.' The MAC Flyer, vol.XXV no.9 (September 1978), 13-15. U.S. Navy. Chief of Naval Operations. Naval Air Systems Command. KC-130F NATOPS Flight Manual, NAVAIR 01-75GAA-1. Washington D.C., 1981.
