Opening the FO sliding window is essentialy the only move left when everything else has been done. This a desperate situation.
In order to understand why we would do such a thing, we must fully understand each and every step of the QRH.
The case upon which I would like to give my understanding, is the B737 (NG).
First of all we are directed to the "Smoke, Fire or Fumes" Non Normal Checklist, from know on to be refered as "NNC".
The first steps involve the ability of the crew to fight the hazard, so, if Smoke goggles and/or O2 masks are required, we will make use of these. (And establish comms via interphone),
Next, we are required to prepare the electrical system, in order to avoid any unwanted electrical power transfer form one transfer bus to another (737 has 2 transfer busses), due to the fact that from know on we are going to attempt to isolate a posible electrical problem that could be the culprate.
Next we disconnect the most frequent and therefore the most possible source of the fire. This includes all equipment located at the galleys (ovens, beverage makers, water heaters..... etc).
Following this, the Recirculation fans arwe disconnected. Why? 2 reasons: No1. We dont want to re circulate smoke.
No2. The recirculation fan/s (plastic and very toxic) may have set alight, and by disconnecting this, the fire may extinguish, and once again we a void recirculating hazardous smoke or fumes.
Next the APU BLEED air switch is set to OFF. The apu bleed duct travels form the back of the aircraft, right to the centre of the aircraft, where it joins the bleed duct on the left hand side of the X-bleed valve. This distance is long, and there may be a leak at any point. Extremely hot air from such a leak, will easily create a fire if in contact with glass fibre, cables, etc...
Up until now, all preliminary preparations have been done, and all highly possible sources have been dealt with. From now on, anytime smoke is the greatest threat, we will be directed to the "Smoke or Fumes Removal" checklist, if this is not the case, we continue with this NNC.
Next steps involves observation. Are the fumes/smoke decreasing? Can we confirm the source and disconnect this? If not, we continue with other possibilities, though these are less probable to be the cause.
Thes steps include switching OFF equipment cooling fans, and switching ON ALTN equipment cooling fans, the motive is the same as with the recirculation fans.
Passenger cabin lights are disconnected and reading lights switched on, this is basically to enhance visibility throughout the passenger cabin.
Final steps are time consuming, so we must be very carefull as to not delay the diversion to an airport whilst taking care of these steps. Take into account that a fire on board is extremely dangerous, so landing is prefered asap, even if items have not yet been completed. Many (me included) believe fire to be the greatest threat one will ever be challenged with during flight.
Following steps include disconnecting 1 pack (isolation valve closed) and waiting at least 2 minutes to see it smoke/fumes decrease. If this does not work, try with the other pack.
NNC offers no other possibility. If smoke/fumes continue (or if they stop, but it is considered neccesary)we can only try to put these outside of the aircraft. T
This takes us to the Smoke/fumes revomal NNC.
Steps during this NNC depend on the configuration of the Air/cond and Press system, more to the point, these depend on wether we may use the packs or not (obviuosly, if we disconnected a pack which was causing smoke/fumes, we will never switch it on).
If pack/s is/are available, we set them to HIGH, and LAND ALT is set to 10,000'. In normal conditions, the 737 cabin will be at 8000' when flying above Fl370. If we set 10.000' on the LAND ALT window, the cabin will raise from 8000 to 10.000', thus further opening the outlow valve and causing a strong stream of air towards the rear (outflow valve is located at the rear) which pulls smoke with it.
If smoke is still uncontrollable, we descend to 10000', and when below 14000', we are instructed to set the outflow valve to MAN, and fully open this. Just like before, this causes a strong stream of air towards the rear end, and smoke is dragged with it.
However if smoke is confirmed to be IN the flight deck, AND packs are OFF, there is only one alst way possible to evacuate smoke from the flight deck. This involves slowing to holding speed (normally between 210 and 220kts) and opening the FO sliding window (Flight deck door must be closed to avoid smoke coming in from the passenger cabin). This is very loud, but comms are still posible. The idea is as follows. A stream of relatively calm and stream line air flows outside of the window, this creates a region of lower pressure and by laws of physics air, and therefore smoke too, are drawn towards this region, putting the FO in quite an awkward position, but hopefully letting the CAPT read his instruments.
This is an awfully long explanation, but I beleive that in order to know why the sliding window would ever be opened, we must first fully undertand each and every step and scenario simulated by the QRH, from begging to end. Starting by isolating the most probable causes, and working towards an uncontrollable situation where drastic and desperate measures have to be taken.
Opening the window is not normally desired, but if we have no bleed air available, and the smoke is in the Flight deck, this is the most effective way to tackle teh situation.
Finally, please excuse my English! This is not my mother language.