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In another question, an answer mentions that the P-3 Orion uses a gearbox driven compressor for cabin pressurization, even though the aircraft uses T-56 turbine engines that supply bleed air in other designs. (How are turboprops and other propeller planes pressurized?)

Brief research suggests that the "EDC" exists, but I can not find any rationale for it. (http://blueaero.com/wp-content/uploads/2016/11/Heico_P3_EDC_Repair.pdf)

Does using gearbox driven compressors help in some way with the mission, possibly in dealing with the environment? Is it an anachronism/carryover from the L-188 Elektra? Is it not actually gearbox driven, but somehow an exchanger for bleed air?

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    $\begingroup$ Bleeding air from compressor reduces air available for combustion and also reduces compression ratio, so it impacts both the engine output and thermal efficiency. You can think of bleed air as the reverse of a supercharger. If engine performance is tight but weight budget is OK, then it make sense to take some power from the output shaft and drive a separate compressor. $\endgroup$ – user3528438 Apr 30 at 2:52
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    $\begingroup$ @user3528438 In other words, taking bleed air costs more power in lost combustion than the power which is extracted? $\endgroup$ – trognanders Apr 30 at 2:54
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    $\begingroup$ Even if it cost no power in lost combustion, you are still stealing power from the HP shaft after all, that alone costs as much power as an external compressor. But in reality lost combustion is many times that. $\endgroup$ – user3528438 Apr 30 at 3:04
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    $\begingroup$ This is only one possible rationale. P-3 may have other reasons that leads to this design. Also some more info here: aviation.stackexchange.com/questions/17030/… $\endgroup$ – user3528438 Apr 30 at 3:12
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    $\begingroup$ US Navy P-3s commonly operated with 2 or 3 engines shut down to conserve fuel - may have something to do with it - a single active engine would have to provide 4 times as much bleed air as it would with all active, and 2 active engines would need to each provide twice as much. $\endgroup$ – stretch Jun 4 at 14:48

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