Alternate static air

Pfew, I tried to come up with a short description of the pitot-static system, to explain to everybody who’s confused about today’s strip what “alternate static air” really means. But during writing I found that I would either have to write a few paragraphs, or spend an hour to make it as concise as possible. Since I’m really under time pressure today, can somebody else maybe explain it in a few words?

Anyway, here’s the link to the relevant Wikipedia Article.

Tagged with: , , , ,
10 comments on “Alternate static air
  1. CaptAPJT says:

    An aircrafts speed and altitude are calculated using a pitot static system. The static system gives us barometric altitude (like a barometer, lower pressure=higher altitude) a comparison between the pitot (ram pressure measure in the direction of flight) and static gives us airspeed. The alternate static air takes barometric pressure from a secondary source in case of a blockage of the primary.

    Or something like that…

  2. Andre Goulao says:

    I would say in layman’s terms that it is an alternative way to get the value of the air pressure surrounding the airplane, if the main pressure port becomes blocked by ice or other contaminant

  3. Hristo says:

    I really like this strip – it’s funny as it’s educational. The pitot-static system is demanding in terms of understanding as it is essential to flight and is exposed to a number of external factors which directly affect its performance. On light aircraft, when in doubt of the readings on altitude instruments (altimeter and VSI), the pilot can change the source of static pressure measurement. Static pressure is just as essential for airspeed measurement as it feeds a capsule in the ASI/Machmeter. The primary static port is located on the skin of the plane in such a place as to cause minimal instrument error due to air flowing around it and the alternate port can often be found in the cockpit of non-pressurized aircraft.The alternate port on a Cessna 172 for example is placed in the cockpit, behind the instrument panel. Pressure measurements from primary and alternate static ports will normally vary by a small amount as the air flowing around the cockpit causes aerodynamic suction which in turn leads to a slight pressure drop inside a non-pressurized aircraft. This is the shortest I could come up with on alternate static port/air.
    For Chuck to best vent the plane he’d have to slow the Cessna to a speed at which he’d be able to open a window for a minute.

  4. markm says:

    I suspected what this meant, but to be sure I had to go read the Wikipedia article on “Pitot-static system”.

  5. Karl Winters says:

    Another option is to break the glass on one of the pitot-static instruments — altimeter, airspeed, or vertical speed indicator. The latter is preferred as it is least important. I can imagine Chuck smashing the altimeter.

  6. Jean Loup says:

    Now I’m really confused!!

  7. Quill says:

    Others here have fairly eloquently explained the pitot-static system: small holes (typically two, one on each side of the plane) used to measure outside air pressure, which is important for several cockpit instruments, and there’s a knob to use an alternate source if the main one is plugged. The key thing to note in this comic though is that it’s for running the instruments, switching to alternate static source would have no effect whatsoever on a foul odor in the cabin, hence Julio’s “gross incompetence” remark. To fix that would need to open cabin vents, possibly a window, and clean all of Chuck’s trash out of the plane (this has been mentioned in previous comics).

  8. winterbeastie says:

    so if it stinks outside, it will stink inside is what I am reading. aren’t there air fresheners for aircraft like there are for cars?

  9. Jim Hammond says:

    Now that is a really clever strip, well done!

  10. YawningMan says:

    If you suspect your instruments aren’t accurately telling you how fast you’re going, how high you’re climbing, or how fast you’re changing altitude, pulling alternate static might be the best way to get a better read.

Leave a Reply

Your email address will not be published. Required fields are marked *