Blackbird Sims / Milviz Forum Archive

I knew it wasn't me going crazy when I couldn't get the engines to start! Is there a patch to fix this available?

you can still start the engines. just close the iso valve and then put it on auto.

Jettrader wrote:

signmanbob wrote:

mbmorgan wrote: Actually there is a difference - I discussed this a few months ago in the CaptainSim Forum (as you could only start their 732 with the PACKS ON) and then asked Westjet Pilot, who now flies the NG but has experience on the old 732. As far as I now have heard from many sources, the pressurization system of the older 737-200 is different from the NG variants and startup procedure differs in some ways

Hi Johann,

nice to find you here

A few years ago it happened that I got a Lufthansa 727 maintenance manual, and also some sections of the 732. So here is an actual diagram of the 732 bleedair system:



In the lower middle you can see the APU, our usual source for pneumatic power. The isolation valve right above to the right seperates the right side from the left side, including the APU.
What happens in the MilViz 732 is, if you close the isolation valve you will get around 30PSI, with the valve open 16PSI. This is wrong.. On a closed system, no matter if the room is a bit bigger (valve open) or smaller (valve closed) the pressure is always the same. So if you have more pressure when you supply the left side only, there must be a consumer on the right side who is responsible for the pressure drop.. let's do some research by looking at the things connected to the right side where pressure could be lost.
1. ground connection- negative, it's a check valve. Unless it's defect, the pressure can't leave here.
2. pack valve- this is closed, so same applies here.
3. purge valve- also closed.
4. wing anti ice- closed too.
5. the last option is the engine bleed system now.. the bleed valve, which should be open (THIS is a good question now! does the starter switch open the bleed valve or does it have to opened by the pilot? If so, engine start should only be possible with engine bleed on!). From there we have the relief valve, which is closed, the 8th stage bleed air, which has a check valve, and the 13th stage bleed valve, which is closed. And the starter of course.

As you can see, the pressure is not able to leave the system anywhere unless you switch on the packs, the wing anti ice or the starter. With this fact in mind, the pressure has to be the same on both the left and the right duct regardless of the isolation valve. Therefore you can start the right engine first or the left engine first, without ever touching the isolation valve switch, given it is open or auto.

By the way, the 30PSI achieved by the MilViz APU at idle are the minimum start pressure. Usually the APU would provide ~40PSI idling, ~20PSI with one pack (2 packs not approved with the Garrett 85-129).

I actually have that same diagram in my source materials. I've also reworked the pneumatic section as much as I intend to.

MatzeH84 wrote: A few years ago it happened that I got a Lufthansa 727 maintenance manual, and also some sections of the 732. So here is an actual diagram of the 732 bleedair system

It does look almost the same as the NG's system, only more basic.

Well, no need to change things that work.. you can still find some parts of the 707 on the 737NG.

MatzeH84 wrote:Well, no need to change things that work.. you can still find some parts of the 707 on the 737NG.

That's true. Boeing had a winning design with the 707 and just built on it.

For those who are interested: I just got answer from a professional; according to him, and as the diagram I posted already suggested, the engine bleed air valves HAVE to be open to perform an engine start. The switches are usually left in the on position (may vary on each operator) and are only moved in non-normal situations.

Would be nice to have the normal startup with correct valve settings as the 732 is such a great plane and it's a niggle that distracts from the overall quality.