For an Airbus A330 in emergency electrical configuration with both generators and APU FAIL (both engines working), why does the checklist ask you to select RAT MAN ON before slat extension and then LAND REC ON?
If we look at the Airbus A330 manual we will see that the RAT (Ram air turbine) can only power the
GREEN hydraulic system and when the EDP (Engine driven pump) of the green system is available, the emergency generator is run by the EDP and not the RAT.
The A330 has two levels of emergency electrical configurations. They are:
A fault in the main electrical buses of the aircraft (
AC BUS 1and
AC BUS 2). In this case, even if the engines and their generators are running the aircraft electrical system cannot be supplied. In this situation, the green hydraulic system's EDP powers the emergency electrical generator. This is the first level.
The aircraft falls into level two when there is a dual engine failure situation. So, there are no working generators to supply the aircraft electrics. Here, the RAT extends automatically. The RAT then runs the green hydraulic system which in turn runs the emergency generator.
The level one emergency configuration is always better because it provides the aircraft with more power.
When you are coming into land, while the aircraft is in level two emergency electrical configuration (RAT only), once the slats are extended, the emergency generator is automatically disconnected, and the aircraft is purely powered by the batteries. The reason for this is to allocate RAT only for the operation of the hydraulic systems so that the flight control power is prioritized. When you turn on the land recovery push button, the
LAND RECOVERY AC BUS and the
LAND RECOVERY DC BUS are automatically powered giving the aircraft the necessary electrical needs for the landing. This includes powering of the SFCC 1 (Slat flap control computer). But in the case of RAT powering the aircraft, only the slat channel is recovered. The landing has to be conducted flapless.
When the aircraft is in level one emergency electrical configuration (emergency generator powered by the green EDP), there is no disconnection of the emergency generator even when slats are extended. The reason why it is still required to extend the RAT in this situation is to allow the RAT to assist the green hydraulic system. The green system managing both the emergency generator and the load from the flight control demands can cause the emergency generator to shut off. With the RAT on, the green system thus gets that extra help to prevent an over demand situation. Here also, pushing the land recovery gives one of the SFCC computers. But with the EDP running the show, both the flaps and slats are available for the landing.