Bleeds off takeoff procedure for the B-737

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

Bleeds off takeoff procedure for the B-737

Post by AZJim320 »

I have been asked by a pilot to explain the procedure for a bleeds off takeoff for the B-737, as I said I would from my hot weather procedure post. The first thing I want to explain is the difference between a bleeds off takeoff and a packs off takeoff. A bleeds off takeoff is when we takeoff with the engine bleeds off and use the APU bleed to pressurize the aircraft. A packs off takeoff is an unpressurized takeoff that usually happens when you need the performance of bleeds off, but are operating an aircraft with an inoperative APU. I will focus this post on the bleeds off procedure.

In most modern airliners the bleeds off procedure is very simple. The Airbus for example, the procedure is, APU bleed on. After climb power, APU bleed off. That’s it! The B-737 is a very unique airplane because it originally entered service back in 1967. It is still being produced new today with the NG series. There is only one type rating for all the variants. All you need to fly the different variants is difference training. Boeing could not change how the aircraft was operated too much, as the newer models went into service or the FAA would required a new type rating to fly it. This is why the overhead panel hasn’t changed that much and the procedures are not that different from the early 200’s to the newest models today.

Boeing could have produced a newer model 737 with flyby wire flight controls and modern push buttons on the overhead but the FAA would have required a new type rating. This would of been a hard sell to one of its best customers, Southwest Airlines. Back in the early days of Southwest you needed to already have a B-737 type rating just to interview there. Training expense was a big part of their business model. A new 737 requiring a new type rating would have been a large training expense for them.

So first why do we do bleeds off takeoffs? With the bleeds off the B-737 can lift an additional 2000 to 4000 lbs. depending on conditions. With the engine bleeds off there is no demand from the packs and the engines can produce more power and lift more weight. Because of the design of the pneumatic system it requires a procedure to not cause potential problems as you will see. For a quick review of the pneumatic panel see my hot weather procedure post.

There is one important thing to consider for a bleeds off takeoff and this relates to the duel bleed light. On the top of the pneumatic panel on the left side is the duel bleed light. This light is illuminated when the APU bleed is on and left engine bleed is on, and or, the right engine bleed is on with the isolation valve open. It is normal to see this light during engine start. The limitation with the duel bleed light illuminated is, do not operate the engines above idle thrust or it can back pressure the APU. This becomes especially important after a bleeds off takeoff when you reconfigure the pneumatic switches. If you do this incorrectly it could back pressure the APU bleed with climb thrust and possibly cause an APU fire!
What is important in this procedure is the order you move the switches. After both engines are started we leave the APU running. You can begin the bleeds off procedure either with a normal two engine start or with the hot weather procedure from my earlier post.
We are going to draw a, “Big C”, across the six pneumatic switches starting with the right pack switch on the top right of the panel.

1. Right pack on or auto,
2. isolation switch close,
3. left pack on or auto,
4. left engine bleed off,
5. APU bleed on,
6. Right engine bleed off.

It is important to move the switches in this order. Also delay step 6 till you are ready for takeoff, otherwise only the left pack is providing AC to the aircraft and the passengers will become very uncomfortable, if you were number 15 for takeoff! It is probably a hot day causing you to do a bleeds off takeoff for performance.

There is a gouge after you set up the switches to make sure you did it correctly. It is, “Squeeze, Spread, Squeeze”. This means the left two switches are pointing in, the middle two are pointing out and the right two switches are pointing in.

After takeoff we have to reconfigure the switches back to normal. This is very important to do correctly because there’s is the possibility of a duel bleed with the engines at climb thrust! If you were one of my students when I use to teach sim and caused a duel bleed I would give you an APU fire because this actually happened in the real world.

So to put the switches back to normal think “BACK” to normal do the, “BACKWARDS” C.
Start at the bottom right of the six pneumatic switches and draw a BACKWARDS C.

1. Right engine bleed on. The right engine bleed turns on the right pack. Wait till the pack has a chance to stabilize.
2. APU bleed off. The left pack will loose air but you already have the right pack going so the aircraft will continue to pressurize. Also with the APU bleed off there is no longer a possibility of a duel bleed.
3. Left engine bleed on. The left pack now has air again.
4. Left pack is already on, stays on.
5. Isolation switch back to auto. All four corner switches are on so isolation valve remains closed.
6. Right pack is already on and stays on.

The only thing left to do now is to shut down the APU. A tip for this is, “Cool and Blue”. Cool means let the APU run for at least one minute without a pneumatic load to cool down. Blue means make sure you see the, “APU GEN OFF” bus light before you shut it down. If you don’t it is connected to the electrical system! There is no auto transfer so you would put the electrical system into standby with emergency lights coming on which would not impress your passengers. Hopefully I explained this well enough. Happy to answer any questions otherwise.

Cheers,

Jim
CaptainMD
Posts: 45
Joined: Fri Jan 19, 2018 4:17 am

Re: Bleeds off takeoff procedure for the B-737

Post by CaptainMD »

Thank you so much for posting this Jim! I just performed a flight with the 737-800 from KPSP to KLAX. The outside temp at KPSP tonight was 95 degrees F :!:

I noticed that I was able to derate my thrust a bit more for takeoff using this procedure vs. utilizing the normal procedure under these temperatures.

Adding overheated irritable virtual customers to your flight plan adds another layer of thinking. It is all about them in real life when flying this aircraft type.
Massimo D'Alessandro
Simulator Pilot
Beech Baron 58, King Air 350, Premier 1A
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