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In the case of ELAC 2 failure, ELAC 1 will take over. However, it is not allowed to dispatch an Airbus A320 with an inoperative ELAC 2. Why is that?

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    $\begingroup$ Is it permissible to dispatch with ELAC 1 inoperative? $\endgroup$ – Cpt Reynolds Jul 28 '18 at 11:48
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    $\begingroup$ @CptReynolds According to the MMEL, ELAC 1 may be dispatched inop with conditions, but there is no relief for ELAC 2. $\endgroup$ – user71659 Jul 28 '18 at 20:10
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    $\begingroup$ @user71659 If ELAC 2 is inoperative, could it be allowed to swap ELACs and dispatch under MEL with ELAC 1 inoperative? That would explain why ELAC 2 inoperative doesn’t have an MEL relief. $\endgroup$ – Cpt Reynolds Jul 28 '18 at 20:40
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As pointed out by this discussion on PPRune, ELAC 1 only controls elevator actuators hooked in to the Blue hydraulic system. ELAC 2 is split across Green and Yellow.

If ELAC 1 is the only operational computer and the aircraft enters emergency electrical configuration, blue hydraulics will be lost until the RAT deploys. Further, the fallback for ELAC 2 (G/Y hyd) is SEC 2, which is not powered in EEC. Thus, the aircraft will have no primary pitch control.

The solution is that if the fault is in the ELAC computers themselves, to swap the units before dispatch.

In general, if there's something weirdly asymmetric about the A320, the first guess should be something to do with EEC. This is because EEC, by design, tries to only power one unit, typically number 1, of each system.

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