In the other Boeings (757,767,747), I'm very used to turning off the packs during the runway entry prior to takeoff, and setting them back on after reducing engines to climb thrust on the climbout. Now with the 737NG, I have learned that the packs are pretty much left on during takeoff, and that the alternate procedure is to turn off the engine bleed switches instead. This can be done either by having the APU provide air conditioning/pressurization via it's bleed switch being left on, or if the apu is inoperative, the aircraft is flown without pressurization with the engine bleeds off during this brief interval of time before climb thrust is enabled.I tend to takeoff with the APU bleed on/Isolation valve closed during all my flights, regardless of weather. I wanted to know what the real world NG crews do, especially during winter-spring when the outside temps are not as hot as during the summer months. Does it make sense to fly with the engine bleeds on during takeoff if temps are low or the runway is long enough? I use TOPCAT for takeoff performance analysis during preflight, and I see there is the option of having air conditioning on or off during takeoff. I interpret this selection as air conditioning on as engine bleeds on, and air conditioning off as engine bleeds off with apu bleeds on. Does this make sense?

Depends if you're heavy or not or if you need the extra power from the engines. I interpret air conditioning as packs.From an airline I know:Engine bleeds on:

• Air Conditioning Packs - ON
• Isolation Valve - AUTO
• Engine Bleeds - ON
• APU Bleed - OFF

Engine bleeds off:

• Air Conditioning Packs - ON
• Isolation Valve - CLOSED
• Engine Bleeds - OFF
• APU Bleed - ON (using APU for air conditioning)

So for the bleeds off takeoff procedure, the airco is then provided by the Left pack only, since the ISO valve is closed... correct ?

I think that hot outside temps demand more power from the engines than colder temps, since the air is thinner when hot. So it is more likely to use packs off in the summer, not during winter-spring. I can be wrong though.

So for the bleeds off takeoff procedure, the airco is then provided by the Left pack only, since the ISO valve is closed... correct ?

Correct.

Correct.

Is there a specific reason why the Isolation valve needs to be closed, or is this more a specific company SOP thing?

What operators do a bleeds off takeoff on a beast like an 757 or 747?

What operators do a bleeds off takeoff on a beast like an 757 or 747?

 

In the case of an op like BAW with the 744 - BLEEDs ON - PACKS OFF for each & every take off >300t.

Isol valve must be closed if a single pack operation is wanted (less stress on APU) but in AUTO position, and with engine bleeds OFF the isolation valve is open and must be forced closed or both packs will run.

Okay, so it's to put less stress on the APU; it makes sense, thanks.

Most operators do a packs off takeoff.... Because this allows a higher flex.. Thus reducing engine wear.

 

At the other end of the scale in limiting situations packs off will allow you more performance.

single pack operation is wanted (less stress on APU)

 

Not an NG expert but this page says single pack ops is actually harder on the NG's APU -> http://www.b737.org.uk/apu.htm#Pack_Operation.

I'm a old-school APU man, not used to all this talk of a seperate compressor with IGV's and such for APU pneumatic pressure...I perfer the old Garretts ;P

I have seen many flights from the flight deck of 777s and only once have I seen the crew opt for a packs-off takeoff. it was a pretty hot day in Mexico city (High density altitude) and we were as heavy as you could imagine...

 

As for the 737NGs, they call it BLEEDS-OFF takeoffs and not PACKS OFF, because you literally leave the bleed valves closed during the first climb segment as opposed to the 777 where the valves have an AUTO position and you merely turn the packs off.

 

That being said, I´ve talked to a couple friends who fly the SFP 800s for Gol airlines which regularly operates out of SBRJ (4000' rwy at sea level in Rio de Janeiro where in the summer we can get a chilly 40ºc at noon). They do sometimes opt for bleeds-off takeoffs but they say you don´t gain that much in terms of performance (You might get to carry an extra 800 kgs of cargo - one of them said), so it is rarely used.

 

I know of no operators who routinely perform bleeds-off TOs but since I´m not in the business I´ll take your word for it.

Not an NG expert but this page says single pack ops is actually harder on the NG's APU -> http://www.b737.org....#Pack_Operation.

I'm a old-school APU man, not used to all this talk of a seperate compressor with IGV's and such for APU pneumatic pressure...I perfer the old Garretts ;P

I'm also more familiar with classics.

If the above sop is correct and is used by an airline with NGs probably is made for classic similarities, you're right about the NG APU stress with single pack operation, but probably the sop is written for a fleet with both ngs and classics. If not, it is strange that they move the isolation valve when not required.

I always told that isolation valve must be left in AUTO position all the time, and eventually moved from that position only when the automatic logic is not correct for what we want.

I'm also more familiar with classics.

If the above sop is correct and is used by an airline with NGs probably is made for classic similarities, you're right about the NG APU stress with single pack operation, but probably the sop is written for a fleet with both ngs and classics. If not, it is strange that they move the isolation valve when not required.

I always told that isolation valve must be left in AUTO position all the time, and eventually moved from that position only when the automatic logic is not correct for what we want.

 

It's SOP to close the Isolation valve on the NG for a bleeds off takeoff. It's even Boeing SOP,..not a company specific thing. Jumpseated on Shanghai Airlines, MALEV, easyJet ,SKY, GOL, Ryanair as well and they all did it for a bleeds off takeoff, and i have yet to come across a FCOM which doesn't say you have to set it closed.

 

In AUTO the isolation valve is open if any corner switch (pack or bleed) is in OFF position. The isolation valve is closed if all corner switches are ON, which is normal (prevents a single pneumatic duct failure from loss of pressure to all of the manifold). The isolation valve is switch sensitive, it looks at pack and bleed switches not the valves, but it's not affected by position of APU bleed switch.

 

With the isolation valve in auto when operating one pack from the APU, such as during a bleeds-off takeoff, pack will auto switch to high air flow rate if both engine bleed switches are OFF. Automatic switching to high airflow occurs if both engine bleed air switches are OFF and the APU bleed air switch is ON, regardless of flap position, air-ground status, or number of packs operating.

 

Regards