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When do I have to turn on the engine bleed air and APU? In videos I have seen that they are always on, and sometimes they turn them off after take-off. I also have the same question about the packs.

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  • $\begingroup$ Related, related (although not specific to the 737) $\endgroup$ – Pondlife Jan 25 '18 at 17:06
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The general answer is, the engine bleeds stay on almost all the time.

The times they're turned off:

  • For a bleeds-off takeoff to get more power from the engines.
  • During deicing on the ground, so that deicing fluid doesn't get in the bleed air system.
  • When you are using the APU bleed air, the #1 engine bleed switch needs to be off IF that engine is running above idle power. If you're using APU bleed air & the isolation valve is open, the #2 engine bleed switch also needs to be off (if that engine is running above idle power).

The reason for the last requirement is because the engines, running above idle, put out greater bleed air pressure than the APU does, and you want to avoid the situation where the failure of a check-valve could put high pressure bleed air into the APU -- back-pressuring the APU, which could cause serious damage.

That scenario plays out if you're using the APU to run the cabin air conditioning (typically, on a hot day with lots of passengers), since it puts out a greater volume of bleed air than the engines do when they're operating at or near idle (i.e. typical taxi power settings). In that case, you close the engine bleeds, open the APU bleed and the isolation valve, and let the APU run both packs.

That scenario also plays out when starting an engine while taxiing; the switchology is reasonably intuitive but a little lengthy to describe.

There is no need when starting the engines initially to close the engine bleeds, since the normal procedure is to close the APU bleed & turn it off before taxiing the aircraft.

For a bleeds-off takeoff, the configuration is, engine bleeds off, APU bleed on, packs on, isolation valve closed. After takeoff, the steps are, #2 bleed on, pause, APU bleed off, pause, #1 bleed on, then isolation valve to Auto.

There is a procedure for a bleeds-off landing (to get more engine power for a go-around), and there are some QRH instances when you might turn a bleed switch off, but all of that is fairly rare.

If the OP is using a flight sim, the simple general answer, always leave the engine bleed switches on, will work fine. Turn off the APU bleed after start. If you get the Master Caution for "Dual Bleed", you forgot to turn off the APU bleed, do so.

Edit, pack switches: off for engine start, on otherwise (except as directed by the QRH).

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  • $\begingroup$ Is it possible to start engine 1 with both bleed air from the APU and engine 2 switched on? $\endgroup$ – user19440 Jan 25 '18 at 16:59
  • $\begingroup$ @user19440 With the switches on, yes. You wouldn't use bleed air from the #1 along with bleed air from the APU to do it, though. If #2 were running at idle, its bleed air output wouldn't be all that much compared to the APU. If the #2 were above idle, then you shouldn't have the APU bleed open. But it's entirely normal to start both motors with all 3 bleed switches in the on position. $\endgroup$ – Ralph J Jan 25 '18 at 17:02
  • $\begingroup$ "switchology" - I like that, I think I'll borrow it. :) $\endgroup$ – FreeMan Jan 26 '18 at 15:42
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APU Bleed air is usually turned on while starting the engine to supply them with enough pressure. After the engines have started, Engine Bleed is turned on an APU Bleed air is turned off as it's not needed anymore and the APU is being shut down. For takeoff you can decide whether you keep the Engine Bleeds on or not. Having them turned off during the takeoff makes the engines more powerful. After takeoff the engine bleed is turned on again to make sure the packs can supply fresh air to the cabin.

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