| From the transitioning pilot's point of view, the
basic difference between a light twin and single engine airplane is the
potential problem involving engine failure. The information that follows
is confined to that one basic difference.
The term "light twin" as used here pertains to the propeller driven airplane having a maximum certificated gross weight of less than 12,500 pounds, and which has two reciprocating engines mounted on the wings.
Before the subject of operating technique in light twin engine airplanes can be thoroughly discussed, there are several terms that need to be reviewed. "V" speeds such as Vx, Vxse, Vy, Vyse, and Vme are the main performance speeds the light twin pilot needs to know in addition to the other performance speeds common to both twin engine and single engine airplanes. The airspeed indicator in twin engine airplanes is marked (in addition to other normally marked speeds) with a red radial line at the minimum controllable airspeed with the critical engine inoperative, and a blue radial line at the best rate of climb airspeed with one engine inoperative (Fig. 16-5).
Vx - The speed for best angle of climb. At this speed the airplane will gain the greatest height for a given distance of forward travel. This speed is used for obstacle clearance with all engines operating. However, this speed is different when one engine is inoperative, and in this handbook is referred to as Vxse (single engine).
Vy - The speed for the best rate of climb. This speed will provide the maximum altitude gain for a given period of time with all engines operating. However, this speed too will be different when one engine is inoperative and in this handbook is referred to as Vyse (single engine).
Vme - The minimum control speed with the critical engine inoperative. The term Vme can be defined as the minimum airspeed at which the airplane is controllable when the critical engine is suddenly made inoperative, and the remaining engine is producing takeoff power. The Federal Aviation Regulations under which the airplane was certificated, stipulate that at Vme the certificating test pilot must be able to: (1) stop the turn which results when the critical engine is suddenly made inoperative within 20 degrees of the original heading, using maximum rudder deflection and a maximum of 5 degree bank into the operative engine, and (2) after recovery, maintain the airplane in straight flight with not more than a 5 degree bank (wing lowered toward the operating engine). This does not mean that the airplane must be able to climb or even hold altitude. It only means that the heading can be maintained. The principle of Vme is not at all mysterious. It is simply that at any airspeed less than Vme, air flowing along the rudder is such that application of rudder forces cannot overcome they asymmetrical yawing forces caused by takeoff power on one engine and a powerless windmilling propeller on the other. The demonstration of Vme is discussed in a later section of this handbook.
Many pilots erroneously believe that because a light twin has two engines, it will continue to perform at least half as well with only one of those engines operating. There is nothing in FAR, Part 23, governing the certification of light twins, which requires an airplane to maintain altitude while in the takeoff configuration and with one engine inoperative. In fact, many of the current light twins are not required to do this with one engine inoperative in any configuration, even at sea level. This is of major significance in the operations of light twins certificated under Part 23. With regard to performance (but not controllability) in the takeoff or landing configuration, the light twin engine airplane is, in concept, merely a single engine airplane with its power divided into two individual units. The following discussion should help the pilot to eliminate any misconceptions of single engine operation of light twin airplanes.
When one engine fails on a light twin, performance is not really halved, but is actually reduced by 80 percent or more. The performance loss is greater than 50 percent because an airplane's climb performance is a function of the thrust horsepower which is in excess of that required for level flight. When power is increased in both engines in level flight and the airspeed is held constant, the airplane will start climbing - the rate of climb depending on the power added (which is power in excess of that required for straight and level flight). When one engine fails, however, it not only loses power but the drag increases considerably because of asymmetric thrust and the operating engine must then carry the full burden alone. To do this, it must produce 75 percent or more of its rated power. This leaves very little excess power for climb performance.
As an example, an airplane which has an all engine rate of climb of 1.860 FPM and a single engine rate of climb of 190 FPM would lose almost 90 percent of its climb performance when one engine fails.
Nonetheless, the light twin does offer obvious safety advantages over the single engine airplane (especially in the enroute phase) but only if the pilot fully understands the real options offered by that second engine in the takeoff and approach phase of flight.
It is essential then that the light twin pilot take proficiency training periodically from a competent flight instructor.