Find the switch to stop the fan blade and crawl through | Chapter 11: Revolution Metro: LL Guide
Last update: 11 May 2016
This objective will not appear in your log straight away, but it is good to know about it beforehand. The plan is to reach the northern part of A2, because that is where the huge fan between the armory areas (A1 and A2), is located as well as its control panel. On your way there, collect the filter, and approach the room occupied by the enemy forces.
I suggest that you start by approaching the box with the switch located to your right. Use it to turn some of the nearby lamps off. Unfortunately, the halogen lights hanging down from the ceiling can't be shut down nor destroyed, just like in the previous part.
The other end of the large location can be reached by in several ways. The most risky one is to follow the man inspecting the installations. If you chose to follow this way, remember that you will often have to balance between staying in hiding, and eliminating the enemies that walk away from their groups. You, of course, need to do it only in dimly-lit areas.
A way better idea, in my opinion, is to locate the ladder shown in the above screenshot. It leads to balconies that aren't guarded by anyone. Examine the supply crates and walk over the narrow pipes to get near the control room with the main switch.
Both of the above variants will allow you to decrease the distance between Artyom and the main objective. The main path leading to the control room is well-lit, but this should be no problem because you can examine the part of the A2 area to the left (the western one). You will find there a manhole to the sewers, shown in the above screenshot.
Explore the sewers to find several supply crates. Do not assume non-linearity, because only one of the paths doesn't lead to a dead end and it leads to the right (the eastern one) part of area A2. Approach the sewer exit carefully and destroy the lamp above. Wait for the situation to quiet down and make your way upstairs. Eliminate the guard there who should be around.
Note that thanks to the sewers you can reach the north-most part of A2, i.e. the one located right behind the main control room. This location is patrolled by at least, two enemies, but do not worry. Their fields of vision do not overlap, so you will be able to eliminate them easily.
You can now take one of the ladders to the balconies above the control room. Carefully, approach the room and deal with the guard here. Devote now several moments to locate the filter shown in the above screenshot and the Artyom's diary page.
If there are no more enemies around, you should explore the entire A2 area, and find numerous supply crates (do not forget to search the corpses of the killed soldiers!). If you do not feel like doing it, remain in the control room and interact with the switch.
Leave the control room and approach the fan that you have just switched off. It is located in the north-western part of the A2 area. Jump into the hole between its blades to reach A1.