140 160 200 250 AIRSPEED KNOTS IAS TAS 40 60 80 100 120 140 160 180 200 260 Figure 12-1.  Airspeed indicator markings for a multiengine  airplane. many twins are not required to do this in any configuration,  even at sea level. The current 14 CFR part 23 single-engine climb performance  requirements for reciprocating engine-powered multiengine  airplanes are as follows.  • More than 6,000 pounds maximum weight and/or  V$_{SO}$ more than 61 knots: the single-engine rate of  climb in feet per minute (fpm) at 5,000 feet mean  sea level (MSL) must be equal to at least .027 V$_{SO}$ 2.  For airplanes type certificated February 4, 1991, or  thereafter, the climb requirement is expressed in terms  of a climb gradient, 1.5 percent. The climb gradient  is not a direct equivalent of the .027 V$_{SO}$ 2 formula.  Do not confuse the date of type certification with the  airplane’s model year. The type certification basis of  many multiengine airplanes dates back to the Civil  Aviation Regulations (CAR) 3. • 6,000 pounds or less maximum weight and V$_{SO }$ 61 knots or less: the single-engine rate of climb at  5,000 feet MSL must simply be determined. The rate  of climb could be a negative number. There is no  requirement for a single-engine positive rate of climb  at 5,000 feet or any other altitude. For light-twins  type certificated February 4, 1991, or thereafter, the  single-engine climb gradient (positive or negative) is  simply determined. Rate of climb is the altitude gain per unit of time, while climb  gradient is the actual measure of altitude gained per 100 feet  of horizontal travel, expressed as a percentage. An altitude  gain of 1.5 feet per 100 feet of travel (or 15 feet per 1,000,  or 150 feet per 10,000) is a climb gradient of 1.5 percent. At the smaller blade angles near the flat pitch position, the  drag added by the propeller is very large. At these small blade  angles, the propeller windmilling at high rates per minute  (rpm) can create such a tremendous amount of drag that the  airplane may be uncontrollable. The propeller windmilling  at high speed in the low range of blade angles can produce  an increase in parasite drag, which may be as great as the  parasite drag of the basic airplane. Feathering is necessary because of the change in parasite  drag with propeller blade angle.  [Figure 12-3]  When the  propeller blade angle is in the feathered position, the change  in parasite drag is at a minimum and, in the case of a typical  multiengine airplane, the added parasite drag from a single  feathered propeller is a relatively small contribution to the  airplane total drag. The propellers of the multiengine airplane may outwardly  appear to be identical in operation to the constant-speed  propellers of many single-engine airplanes, but this is not the  case. The propellers of multiengine airplanes are featherable,  to minimize drag in the event of an engine failure. Depending  upon single-engine performance, this feature often permits  continued flight to a suitable airport following an engine  failure. To feather a propeller is to stop engine rotation with  the propeller blades streamlined with the airplane’s relative  wind, thus to minimize drag.  [Figure 12-2] This section deals with systems that are generally found on  multiengine airplanes. Multiengine airplanes share many  features with complex single-engine airplanes. There are  certain systems and features covered that are generally unique  to airplanes with two or more engines. There is a dramatic performance loss associated with the loss  of an engine, particularly just after takeoff. Any airplane’s  climb performance is a function of thrust horsepower, which  is in excess of that required for level flight. In a hypothetical  twin with each engine producing 200 thrust horsepower,  assume that the total level flight thrust horsepower required  is 175. In this situation, the airplane would ordinarily have  a reserve of 225 thrust horsepower available for climb. Loss  of one engine would leave only 25 (200 minus 175) thrust  horsepower available for climb, a drastic reduction. Sea level  rate of climb performance losses of at least 80 to 90 percent,  even under ideal circumstances, are typical for multiengine  airplanes in OEI flight. Operation of Systems Propellers 12-3