In this video, the B738 starts takes off without the plane's pressurization system engaged. The video specifically notes that the craft is being pressurized fairly soon after the takeoff has happened.

Is this because of the icing conditions they had on the ground? Or is there another reason why it would take off unpressurized?

Screen capture for the extremely busy:

enter image description here

  • $\begingroup$ Was WAI (wing anti ice) selected? Then I would expect that the pilot wanted to use all available bleed air for heating the leading edge slats right after take-off, avoiding the additional strain of pressurizing the hull. $\endgroup$ Jul 28, 2015 at 12:47
  • $\begingroup$ @PeterKämpf On the 737, the wing anti-ice switch snaps to the off position when the thrust levers are advanced for takeoff (it's solenoid-held on, spring-loaded to off). So wing anti-ice is always off for the takeoff roll itself by the design of the system. Once airborne you can turn it on. At which point, though, you're clean (Type 4 deicing fluid), and I can't think of a scenario where the extra bleed air during the early climbout (until the packs are turned on) matters that much or would show up in performance charts. $\endgroup$
    – Ralph J
    Jul 28, 2015 at 13:31
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    $\begingroup$ On reflection, the performance charts are Bleeds On / Bleeds Off (and off = neither packs nor wing anti-ice getting engine bleed air), and Wing A/I On or Off. There's no option in any charts I've seen for Engine Bleed On, Wing A/I On, Packs Off. Because the assumption would be that you always pressurize the cabin, rather than shedding that for performance (APU inop + engine bleed off for T/O being the sole exception). Inop APU with a Bleeds Off takeoff driven by performance considerations (runway braking action) seems like the only plausible scenario, IMHO. $\endgroup$
    – Ralph J
    Jul 28, 2015 at 13:36

2 Answers 2


The 737 like most modern airliners (notable exception, the 787) is pressurized by bleed air, from either the engines or the APU. Drawing bleed air from the engines reduces the thrust available for takeoff.

From most common to least common, takeoff configurations are:

  1. Engine bleeds on (supplying bleed air to the pneumatic system), APU off. This is a pressurized takeoff & generally the most common scenario.

  2. Engine bleeds off, APU bleed supplying air to the pneumatic system. This also results in a pressurized takeoff, with more engine thrust available. Used when max thrust is desired, typically due to heavy weights, short and/or contaminated runways, close-in obstacles requiring steep climb gradients, and similar circumstances.

In the case that the APU is inoperative but taking off with the engine bleeds off is still required, you get:

  1. Engine bleeds off, APU bleed off. This results in the unpressurized takeoff referenced in the question. At a suitable altitude (typically 2000 to 3000 feet into the climb, a minute or two after takeoff), the engine bleeds are turned back on & the aircraft pressurizes normally.

This is uncommon, but still considered a normal procedure (not an abnormal one out of the QRH).


Link to Pneumatic System description for the 737

Link to PDF discussing pneumatics & pressurization on the 737

  • $\begingroup$ This seems to contradict KeithS' answer. Do you have some citations for what you are saying? Keep in mind the aircraft in question is a brand new 737-800 (which may or may not make a difference I guess.) $\endgroup$
    – Jae Carr
    Jul 27, 2015 at 21:54
  • $\begingroup$ @RalphJ: Climbing is not so bad for ears. Even divers, who experience much larger pressure differentials, have to swallow or blow their ears when diving, but surfacing is rarely a problem. I do agree that the cabin starts pressurizing immediately though. In fact, it usually starts pressurizing even before takeoff. $\endgroup$
    – Jan Hudec
    Jul 28, 2015 at 6:19
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    $\begingroup$ Maybe the pilot wanted to have more bleed reserves to support the wing anti ice system, which runs on bleed air from the engines and is activated after take-off by a weight-on-wheel switch if it has been selected on by the pilot. $\endgroup$ Jul 28, 2015 at 12:41
  • $\begingroup$ @PeterKämpf That would make a lot of sense. I didn't hear anything along those lines from the video, but I wouldn't know what I'm listening for. I don't doubt it though, considering there were ground icing conditions (they do say that.) $\endgroup$
    – Jae Carr
    Jul 28, 2015 at 12:53
  • $\begingroup$ @JanHudec Agree that unpressurized rapid descents are WAY worse for the ears than a climb, but it'd still be clearly noticeable. Normal cabin rate is maybe 250'/minute; it might not damage any ear drums, but 3000'/min rate would be a lot less pleasant. $\endgroup$
    – Ralph J
    Jul 28, 2015 at 13:41

The air pressure outside decreases as the aircraft climbs. However up until a certain altitude, which I happen to forget, you can fly without the aircraft being pressurized. This is why we don't need to throw on oxygen masks while flying light aircraft (up until a certain altitude anyways)

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    $\begingroup$ I'm pretty sure that in most cases commercial flights take off with pressurization, that's why this flight had pressurization after takeoff as a special procedure. I'm wondering why they decided to use the special procedure instead of the normal one. All the same, you are correct (in the general sense), and welcome to the site :). $\endgroup$
    – Jae Carr
    Jul 27, 2015 at 20:08
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    $\begingroup$ I'm afraid you omit an important thing: The fact that the aircraft is at the correct pressure is different from the fact whether the pressurization switch is on or off. $\endgroup$
    – yo'
    Jul 27, 2015 at 20:11
  • $\begingroup$ @yo' Excellent point, modified the question so it's a bit clearer. $\endgroup$
    – Jae Carr
    Jul 27, 2015 at 20:14
  • $\begingroup$ I'm sorry @yo' -- what exactly is this "Pressurization Switch" you're referring to? I'm pretty familiar with the 737, and "the pressurization switch" is a new one on me. $\endgroup$
    – Ralph J
    Jul 28, 2015 at 2:46
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    $\begingroup$ This is all correct but it doesn't answer the question, which is why a commercial flight might take off unpressurized, not why doing so is safe. $\endgroup$ Jul 28, 2015 at 9:32

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