B737 NG Evac recall items

[Global TM]

B737 NG
Evac recall items

During an evacuation, if there is no fire, what is the reasoning behind pulling the engine
fire warning switches?

May you never have to.
GTM

[Jhieminga]

My view on this is that it secures the engine nacelles. You'll have people running around out there pretty soon after and you don't want a fire risk, or a still running engine. Basically it's the quickest way to ensure a fully shut-down airplane.

Depending on the reason for the evac, you may have a busted airplane (gear not fully down) in which case you want the spar fuel valve shut to make sure it stays in the tanks.

[Blip]

Yes but the evacuation is not called for by the captain via the P.A. until the Engine Start Levers are positioned to Cutoff.

Doesn't this action achieve the same result?

It's a good question Global TM. As Jhieminga touched on, perhaps it's one of those things where Boeing are damned if they do, damned if they don't. Possible reasons for evacuating an aircraft are many. All are obviously very serious. The number of scenarios where pulling the fire warning switches and firing a extinguisher bottle are numerous enough to warrant it's inclusion on all occasions, just to be sure.

However it does seem like overkill at times I agree (when performed in the simulator).

[Jhieminga]

Yes but the evacuation is not called for by the captain via the P.A. until the Engine Start Levers are positioned to Cutoff.

Doesn't this action achieve the same result?

Pulling a fire switch will do the following:

1. Arm firing circuits
2. Allow fire switch to be rotated for discharge
3. Close engine fuel shut-off valve.
4. Trip the associated GCR (i.e. switches off the generator)
5. Close hydraulic supply to EDP & disarms its LP light (Not if APU)
6. Close engine bleed air valve (If APU will also close air inlet door)
7. Close thrust reverser isolation valve (Not if APU)
8. Spar fuel valve closes.
(Source: www.b737.org.uk)

So the fire switch accomplishes more than the fuel cut-off does.

[Centaurus]

I believe that the actuation of the fire bottles as part of pax evacuation drill is no longer required in the revised B737NG Boeing published checklist. Unless there is a engine fire of course?

In the B737 Classics and the -200, if you just pull the fire switches only in order to shut down the engines as part of the pax evacuation drill, the engines will continue to run for at least 20 seconds due to the position of the various fuel shut-off valves.

Actuation of the start levers to cut-off affords however immediate engine shut down. The Classic drill states (among other things) start levers-cutoff. Then order pax evacuation (engines running down by now) and finally fire switches - pull. If you reversed the order and pulled fire switches as first action and then ordered pax evacuation, it is possible that the escape slides would inflate and get blown away by the still idling engines especially if you failed to cut the start levers immediately after the pax evacuation order.

[ManaAdaSystem]

Evacuation check list is no longer a recall check list (new Boeing procedures).
You pull the handles, but do not rotate unless you have a fire light.

[alexban]

SNAM: the new QRH is changed.Now you only pull the fire switches.Only if you have an engine fire you should rotate the related fire switch and hold for 1 sec.
One question:for engine fire on take off,on a 737,you'll do the pax evac ?