ENGINE FAILURE ON THE MULTIENGINE AIRPLANE
In the case of the single-engine airplane, powerplant failure leaves only the alternatives of effecting a successful power-off landing or abandoning the airplane. In the case of the multiengine airplane, the failure of a power- plant does not necessarily constitute a disaster since flight may be continued with the remaining powerplants functioning. However, the performance of the multiengine airplane with a powerplant inoperative may be critical for certain conditions of flight and specific techniques and procedures must be observed to obtain adequate performance.
The effect of a powerplant failure on the multiengine turbojet airplane is illustrated by the first chart of figure 6.8 with the variation of required and available thrust with velocity. If half of the airplane powerplants are inoperative, e. g., single-engine operation of a twin – engine airplane, the maximum thrust available at each velocity is reduced to half that available prior to the engine failure. The variation of thrust required with velocity may be affected by the failure of a powerplant in that there may be significant increases in drag if specific procedures are not followed. The inoperative powerplant may contribute additional drag and the pilot must insure that the additional drag is held to a minimum. In the case of the propeller powered airplane, the propeller must be feathered, cowl flaps closed, etc., as the increased drag will detract considerably from the performance.
The principal effects of the reduced available thrust are pointed out by the illustration of figure 6.8. Of course, the lower available thrust will reduce the maximum level flight speed but of greater importance is the reduction in excess thrust. Since the acceleration and climb performance is a function of the excess thrust and power, the failure of a power – plant will be most immediately appreciated in this area of performance. As illustrated in figure 6.8, loss of one-half the maximum available thrust will reduce the excess thrust to less than half the original value. Since some thrust is required to sustain flight, the excess which remains to accelerate and climb the airplane may be greatly reduced. The most critical conditions will exist when various factors combine to produce a minimum of excess thrust or power when engine failure occurs. Thus, critical conditions will be common to high gross weight and high density altitude (and high temperatures in the case of the turbine powered airplane) as each of these factors will reduce the excess thrust at any specific flight condition.
The asymmetrical power condition which results when a powerplant fails can provide critical control requirements. First consideration is due the yawing moment produced by the asymmetrical power condition. Adequate directional control will be available only when the airplane speed is greater than the minimum directional control speed. Thus, the pilot must insure that the flight speed never falls below the minimum directional control speed because the application of maximum power on the functioning powerplants will produce an uncontrollable yaw if adequate directional control is unavailable. A second consideration which is due the propeller powered airplane involves the rolling moments caused by the slipstream velocity. Asymmetrical power on the propeller airplane will create a dissymmetry of the slipstream velocities on the wing and create rolling moments which must be controlled. These slipstream induced rolling moments will be greatest at high power and low velocity and the pilot must be sure of adequate lateral control, especially for the crosswind landing.
The effect of an engine failure on the remaining range and endurance is specific to the airplane type and configuration. If an engine fails during optimum cruise of the turbojet airplane, the airplane must descend and experience a loss of range. Since the turbojet airplane is generally overpowered at QLjD)max, a loss of a powerplant will not cause a significant change in maximum endurance. If an engine fails during cruise of a reciprocating powered airplane, there will be a significant loss of range only if the maximum range condition cannot be sustained with the remaining powerplants operating within the cruise power rating. If a power greater than the maximum cruise rating is necessary to sustain cruise, the specific fuel consumption increases and causes a reduction of range. Essentially the same relationship exists regarding maximum endurance of the reciprocating powered airplane.
When critical conditions exist due to failure of a powerplant, the pilot must appreciate the reduced excess thrust and operate the airplane within specific limitations. If the engine-out performance of the airplane is marginal, the pilot must be aware of the very detrimental effect of steep turns. Due to the increased load factor in a coordinated turn, there will be an increase in stall speed and—of greater importance to engine-out performance—an increase in induced drag. The following table illustrates the effect of bank angle on stall speed and induced drag.
The previous table of values illustrates the fact that coordinated turns with less than 15° of bank cause no appreciable effect on stall speed or induced drag. However, note that 30° of bank will increase the induced drag by 33.3 percent. Under critical conditions, such an increase in induced drag (and, hence, total drag) would be prohibitive causing the airplane to descend rather than climb. The second graph of figure 6.7 illustrates the case where the steep turn causes such a large increase in required thrust that a deficiency of thrust exists. Whenever engine failure produces critical performance conditions it is wise to limit all turns to 15° of bank wherever possible.
Another factor to consider in turning flight is the effect of sideslip. If the turn is not coordinated to hold sideslip to a minimum, additional drag will be incurred due to the sideslip.
The use of the flaps and landing gear can greatly affect the performance of the multiengine airplane when a powerplant is inoperative. Since the extension of the landing gear and flaps increases the parasite drag, maximum performance of the airplane will be obtained with airplane in the clean configuration. In certain critical conditions, the extension of the landing gear and full flaps may create a deficiency of thrust at any speed and commit the airplane to descend. This condition is illustrated by the second graph of figure 6.8. Thus, judicious use of the flaps and landing gear is necessary in the case of an engine failure.
In the case of engine failure immediately after takeoff, it is important to maintain airspeed in excess of the minimum directional control speed and accelerate to the best climb speed. After the engine failure, it will be favorable to climb only as necessary to clear obstacles until the airplane reaches the best climb speed. Of course, the landing gear should be retracted as soon as the airplane is airborne to reduce parasite drag and, in the case of the propeller powered airplane, it is imperative that the wind milling propeller be feathered. The flaps should be retracted only as rapidly as the increase in
THRUST REQ’D LANDING CONFIGURATION WING LEVEL FLIGHT
THRUST REQ’D CLEAN
BUT TURNING FLIGHT
REDUCED THRUST AVAILABLE DUE TO ENGINE FAILURE
airspeed will allow. If full flap deflection is utilized for takeoff it is important to recall that the last 50 percent of flap deflection creates more than half the total drag increase but less than half the total change in CLmax – Thus, for some configurations of airplanes, a greater reduction in drag may be accomplished by partial retraction of the flaps rather than retraction of the landing gear. Also, it is important that no steep turns be attempted because of the undesirable increase in induced drag.
During the landing with an engine inoperative, the same fundamental precautions must be observed as during takeoff, i. e., minimum directional control speed must be maintained (or exceeded), no steep turns should be attempted, and the extension of the flaps and landing gear must be well planned. In the case of a critical power condition it may be necessary to delay the extension of the landing gear and full flaps until a successful landing is assured. If a waveoff is necessary, maximum performance will be obtained cleaning up the airplane and accelerating to the best climb speed before attempting any gain in altitude.
At all times during flight with an engine inoperative, the pilot must utilize the proper techniques for control of airspeed and altitude, e. g., for the conditions of steady flight, angle of attack is the primary control of airspeed and excess power is the primary control of rate of climb. For example, if during approach to landing the extension of full flaps and landing gear creates a deficiency of power at all speeds, the airplane will be committed to descend. If the approach is not properly planned and the airplane sinks below the desired glide path, an increase in angle of attack will only allow the airplane to fly more slowly and descend more rapidly. An attempt to hold altitude by increased angle of attack when a power deficiency exists only causes a continued loss of airspeed. Proper procedures and technique are an absolute necessity for safe flight when an engine failure occurs.