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I've realized that some pilots turn the air conditioning units (also called 'packs') OFF during takeoff, like in this video:

LH456 Takeoff Video

Is there any operational reason for that?

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  • $\begingroup$ it leaches power, they only turn it off for the inside engines $\endgroup$
    – ratchet freak
    Aug 31, 2014 at 22:06
  • $\begingroup$ I also noted that both pilots didn't have their hands behind the throttle when pushing it to max power. Isn't that supposed to be done? $\endgroup$
    – shortstheory
    Sep 1, 2014 at 3:07
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    $\begingroup$ @shortstheory That's normal, because when they engage the Takeoff / Go Around mode, the throttle automatically moves to the correct position, so, they need to release, and right after the trust is correctly set, they hold the throttles again. $\endgroup$
    – Ygor Montenegro
    Sep 1, 2014 at 23:39

3 Answers 3

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The "packs" (or A/C on the aircraft) are powered by engine bleed air. When bleed air is extracted from the engines, less thrust can be produced. This may be a problem if the airplane is taking off at a high elevation airport or the departure path requires a certain climb gradient to clear obstacles. This action does create some discomfort in the cabin so it's usually not preferred unless required for safety.

Some newer airplanes are manufactured with an "APU to pack" configuration - i.e. the pilots leave the APU running during takeoff, which provides bleed air to power the packs. The workload on the engines are now reduced.

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  • $\begingroup$ I was wondering why they don't close the bleeds instead of the air conditioning packs? If they close the bleeds, there would be no loss, am I right? $\endgroup$
    – Ygor Montenegro
    Mar 16, 2015 at 2:34
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Pilots will turn off the packs for takeoff if maximum engine performance is needed or if there is a need to prioritize a high demand bleed draw. The particulars of the situations calling for a packs-off takeoff is airframe specific.

In the EMB-145, normal takeoff was with the APU bleed powering the packs. The specific configuration calls for engine bleeds off, APU bleed on, cross-bleed valve open and packs on. This lets the APU be the sole source of bleed air and the packs the sole consumer of bleed air. This is a typical configuration, but assumes that the APU is operational and that anti-ice will not be needed.

If the APU is inoperative or anti-ice is called for, then the takeoff configuration becomes engine bleeds on, APU bleed off, cross-bleed off, packs off. This isolates the APU from the bleed system (but keeps it available as a 5th generator) and makes the engines the bleed sources with no draw from the packs. This configuration with the APU inop allows the engines to avoid a performance hit from the bleed air taps (drawing compressed air at the 9th and 14 compressor stages of this particular engine). For anti-ice takeoffs, this configuration provides the ability to provide anti-ice as the APU bleed source is not powerful enough for this on the EMB-145.

The particular pneumatic configuration details will vary by airframe, and as mentioned in comments to the other answers, some airplanes have limitations on bleed valves being closed (e.g. to keep a pneumatic powered hydraulic pump operational) and some do not.

In summary, the reason for a packs off takeoff is likely for takeoff performance or for high demand needs such as anti-ice.

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Best answer that I can think of, without the exact engineering or design info, is that since the A/C packs are run with engine bleed air, if you need the extra thrust from the engines, you'd turn off the packs for that extra power for take off...

When I did my dispatcher certificate, we used the 727... and from my notes, you'd get a 1500 lbs increase in runway limit performance, with the Packs off.

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  • $\begingroup$ but why don't just turn off the bleeds, instead of the packs? $\endgroup$
    – Ygor Montenegro
    Sep 1, 2014 at 1:27
  • $\begingroup$ My guess would be how the system is designed, but I honestly don't know. $\endgroup$
    – slookabill
    Sep 1, 2014 at 1:32
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    $\begingroup$ No, you can never turn off the bleeds of an engine. The flight control surfaces are powered by hydraulics, which are powered by bleed air. Disabling bleed air would result in very degraded maneuverability (minimal control is provided by backup systems). $\endgroup$
    – kevin
    Sep 1, 2014 at 5:37
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    $\begingroup$ @YgorMontenegro On the 747-100 and -200 you can turn off the bleeds individually by engine. However, you would be removing some of the hydraulic power redundancy. Each engine has two hydraulic pumps, one powered by the engine itself and one powered by the pneumatic duct, which is pressurized by the engine bleeds. That latter pump is not brought into play as I remember unless hydraulic pressure from the engine pump drops below a set value. $\endgroup$
    – Terry
    Sep 1, 2014 at 6:20
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    $\begingroup$ @kayson starts, packs/pressurization and a/ice. If we took off into icing we used a engine bleeds on, crossbleed off, apu bleed off, packs off config. $\endgroup$
    – casey
    Sep 1, 2014 at 12:59

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