Engines
The MD-11 is powered by three high-bypass-ratio turbofan engines in the 60,000-lb thrust class. As shown in .gure 1, two engines are mounted in underwing pods, 116 in. below and 331 in. outboard of the nominal CG, toed in 2° and up 1.5° . The third engine is at the base of the vertical tail 178 in. above the vertical CG and inclined 2.5° nose up. The test airplane was equipped with PW4460 engines with 60,000 lb of thrust each. These engines have FADEC systems. The crew normally controls the engines with electronic throttles that command a power setting based on engine pressure ratio (EPR). At sea level, EPR varies from just below 1.0 at idle to about 1.65 at maximum power; thus, each 0.1 EPR is about 10,000 lb of thrust. Figure 3 shows thrust as a function of EPR for low-speed operation. At Mach 0.2, approach idle is an EPR of about 1.04, with a thrust of approximately 5000 lb per engine. Flight idle with an EPR of about 0.99 has a thrust of approximately 2000 lb per engine.
The FADECs normally accept small (± 5 percent) EPR trim commands from the .ight management system to closely maintain engine limits or thrust settings and eliminate the need for throttle stagger to match engines to a given EPR. The alternate FADEC switches on the overhead panel select the FADEC control to backup software that does not require or accept any data bus inputs.
As is typical for high-bypass turbofan engines, thrust response near idle power is initially very slow. After reaching about 20 percent of full thrust, the thrust response improves dramatically, and in the mid-range, has a time constant of approximately 1 sec. Figure 4 shows a .ight time history of a small thrust step for one engine from a throttle angle of 58° to 65° . Note that the thrust has reached about 65 percent of the step change in a second, and that the maximum rate is about 20,000 lb thrust per second. When a step throttle reduction is performed, the thrust decay shows similar nonlinear effects: rapid at the higher thrust levels, but slow near idle thrust.
An approach idle setting raises the idle speed to improve engine response when the .aps are extended more than 27° . Figure 5 shows an offline engine simulation of an engine idle to full power snap acceleration beginning from .ight-idle and approach-idle power settings at sea-level-static conditions. At .ight (minimum) idle
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Figure 3. Thrust-EPR relationship for the PW4460 engine.
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Figure 4. Response of a PW4460 engine in the MD-11 to a step throttle increase, .aps up, gear down, PCA not engaged.
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