B-737 Hot weather operations engine start procedure.

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AZJim320
Posts: 23
Joined: Mon Jan 29, 2018 1:33 pm

B-737 Hot weather operations engine start procedure.

Post by AZJim320 »

This post is intended for B-737 simmers who like to follow real world procedures. Summer is fast approaching. We have already had days over 90 degrees in Phoenix where I live. When it gets to be 110+ degrees here the normal engine start procedure just doesn’t work By the time you get the packs back on after starting both engines the temperature in the cabin can get very uncomfortable. I learned this procedure back when I use to fly the B-737 based in Phoenix. This procedure can be used anytime but is essential when operating in very hot weather to help keep the cabin as cool as possible. Before I go into the procedure a quick pneumatic system review is necessary.

On the pneumatic panel on the overhead you have two pack switches and two engine bleed switches. We call these the four corner switches. Between the engine bleed switches you have the APU bleed. Notice on the diagram on the panel the APU bleed connects to the left side of the system.

Between the two pack switches you have the isolation valve switch. This switch has three positions, open, close, and auto. This switch is usually in the auto position during normal operations with the exception of the procedure I will give you, and when you you do a bleeds off takeoff. I can explain the proper way to do a bleeds off takeoff in a later post if anyone is interested. The other time you would move it from the auto position would be for pneumatic abnormal procedures like bleed trips, pack faults etc.

When you place the isolation switch to the closed position it holds the valve closed, isolating the left and right side from each other. When you place it in the open position the isolation valve is held open. When the isolation valve is in the auto position the valve will open and close as a function of the positions of the four corner switches, (left and right pack and left and right bleed switches). If ANY of the four corner switches are in the off position the isolation valve will open. If ALL four switches are in the ON position (bleeds on packs on or auto or high depending on which aircraft you are flying), the isolation valve will close.

The last thing I will discuss is the system pressure gage. This gage has two pressure indicators left and right. When the isolation valve is open the pressure will be equal and both needles will show the same pressure. When the isolation valve is closed each needle will read the pressure in the left or right side individually. Remember the minimum pressure for start is 30 psi at sea level. A good APU will give you about 40 psi at sea level.

So finally on to the procedure. We begin the same way as a normal engine start. APU bleed on, isolation valve auto and both packs off. You can identify the exact moment the pack is switched off when you are sitting back in the cabin. This is when most of the passengers reach up and try to adjust their gasper outlet. If you are sitting next to one of them next time you can impress them with your B-737 system knowledge and explain where the airflow went and the AC will be back on soon, hopefully the pilots are using this procedure so it will be back quicker as you will see.

We will start engine two first. This procedure is one reason B-737 pilots almost always start with the right engine. After the right engine is stabilized we are going to “ISOLATE AND VENTILATE”. You will place the isolation switch to close position and turn the right pack on or auto depending on which model 737 you are flying.
With the isolation valve closed the left and right pneumatic systems are isolated. The right engine bleed is supplying air to the right pack. You will notice the right pressure needle will drop a little because of the demand of the right pack. Most importantly, you now have air conditioning to the cabin again! Because the APU is on the left side of the pneumatic system it is available to provide air to start the left engine. After the left engine is stabilized you reconfigure the pneumatic switches. APU bleed off, left pack on or auto and isolation switch back to auto. The isolation valve will remain in the closed position because all four corner switches are now on.

So to summarize the procedure.

1. Normal engine start with engine two, (right engine)
2. After right is stabilized, isolation switch closed, right pack on or auto (ISOLATED AND VENTILATE).
3. Start engine one.
4. After engine one is stabilized reconfigure pneumatic panel. APU off, left pack on and isolation switch back to auto

Hopefully I explained this well enough but let me know if you have any questions.
Cheers,
Jim
overload
Posts: 41
Joined: Wed Nov 06, 2013 3:39 am

Re: B-737 Hot weather operations engine start procedure.

Post by overload »

Thanks Jim,
Very interesting and informative,
Eugene
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