Go No Go System in Aircraft
A very general query comes in mind of A320 engineers and pilots that while many systems in A320 has mainly no.1 computer as NO GO item as per MMEL, but in flight control system, ELAC 2 (elevator aileron computer) is NO GO instead of ELAC 1. Why so?
Here is the answer airbus has provided in it’s IN SERVICE INFORMATION (ISI).
In case of Normal electrical power supply loss (loss of GEN1 and GEN2) (Also known as Emergency electrical configuration)
- The electrical bus-bars 2PP (ELAC 2 / SEC 2 Normal supply) and 4PP (ELAC 1 / SEC 1 Normal supply) are lost, leading to the loss of ELAC 2, SEC 2, ELAC 1, SEC 1
And
- The blue electric pump is lost, leading to the loss of the blue hydraulic circuit.
In this configuration, the automatic RAT extension is launched. The RAT allows to pressurize the blue hydraulic circuit and to supply the electrical bus-bar 4PP.
However, it takes a few seconds for the RAT to be fully operative. During this short period:
- The blue hydraulic circuit is still not pressurized,
- ELAC 1 and SEC 1 are supplied through battery 1,
- ELAC 2 and THS motor 1 are supplied through battery 2
This configuration enables to operate the elevators with ELAC 2 on the left green servocontrol and the right yellow servocontrol, and the THS with the ELAC 2 on the motor 1.
If ELAC2 was under MEL, A/C Pitch authority would be temporarily lost (Elevator Servocontrols in centering mode with Surfaces set to neutral) until RAT is running. This is because ELAC 1 which is controlling elevators using blue hydraulic systems will actually have no blue hydraulic power to control elevators temporarily till RAT comes.
This is the reason why the ELAC 2 is NO GO. #
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