6 ms·

A plane going up and down 20-30 feet seems like it would be very unpleasant. Considering that there's longitude and latitude, isn't it more likely that the alti

• chatmasta 3y ago
  Maybe the plane is staying level but the ground is variable terrain.
  • rockostrich 3y ago
    Elevation is relative to sea level, not the ground.
    • funnyflywheel 3y ago
      This only holds true if you’re flying at or above the transition altitude. The transition altitude depends on where you’re flying: for example, in the USA and Canada it’s 18_000 feet MSL.
      • danbtl 3y ago
        It's still sea-level. The transition altitude just changes the altimeter setting from one that matches the current air pressure to a standard pressure setting.
        • funnyflywheel 3y ago
          I did some more reading, and it turns out I confused QNH with QFE.
    • chatmasta 3y ago
      The plane is measuring altitude, which is relative to a reference point, unlike elevation which is relative to sea level. And if the altitude is determined by pressure sensor, musn't it be relative to the ground directly below the plane, anyway? (Although personally, I agree with the sibling comment that the variability is likely an artifact of the sensor resolution.)
      • adastra22 3y ago
        Planes dont measure height relative to the ground. How would that even work? Their sensor is air pressure, which is treated as a function of elevation.
        • jjwiseman 3y ago
          They do sometimes! Via radar altimeters, when relatively close to the ground. And sometimes to keep autopilots from freaking out, we have to build radar reflectors to make the ground look level to radar even when it's really not. https://lustublog.com/2017/02/17/artificiel-mais-pas-superficiel/ https://lustublog.com/2017/02/17/artificiel-mais-pas-superfi...
      • jjwiseman 3y ago
        When you climb to the top of Mt. Everest, the air pressure is about 1/3 of what it is at sea level even though you're standing on the ground.
        • chatmasta 3y ago
          Yeah, you make a very good point. Fortunately the blast radius of my scientific hubris is limited to whatever code I manage to deploy to the internet, and I'm not involved in designing or building aircraft. btw: Aren't you the guy who tracks planes flying in circles? I follow you on Twitter. Such a cool project!
      • victortroz 3y ago
        It's relative to sea level. After transition altitude (18k feet in most places) the pressure setting to the altimeter is changed to standard (iirc 1013 hPa) so all aircrafts are in the same reference regardless of terrain.
        • epse 3y ago
          Most places in the US, but accurate
    • twothamendment 3y ago
      Yes, the elevation is based on sea level. I don't fly much and recently landed in Denver and was watching the altitude on the screen in front of me. As we were descending we landed well before I was thinking we would, about a mile in elevation above sea level.. it was "duh" obvious when it happened, but I was tired and clearly not thinking about it!
    • quickthrower2 3y ago
      And using the ground proximity to guide a landing instead of altitude has lead to some crashes I have read.
  • momirlan 3y ago
    it's the Earth vibrating ...
• dhritzkiv 3y ago
  20-30 feet change over what timeframe? The resolution of the chart data in the article is about 30 seconds. While I think the fluctuation is due to the accuracy of instrumentation, 20-30 feet change over the course of a minute seems like nothing.
  • xvedejas 3y ago
    That's significantly slower than a typical elevator, in fact. Slow elevators run at about 200 feet per minute.
    • pbj1968 3y ago
      My elevator takes almost a minute to go from the first floor to the second floor. I guess I have a slow one.
      • prmoustache 3y ago
        Nobody without disabilities would use such an elevator.
        • pbj1968 3y ago
          Connect the dots…
• danbtl 3y ago
  Planes report pressure altitude via their transponders. 20-30 feet up and down is very normal for an autopilot. GPS altitude is used for vertical guidance for certain types of GPS approaches (i.e. "LPV" approaches[1]) and requires the airplane's avionics to be equipped with a WAAS[2] receiver that provides accurate altitude information. [1] https://en.wikipedia.org/wiki/Localizer_performance_with_vertical_guidance https://en.wikipedia.org/wiki/Localizer_performance_with_ver... [2] https://en.wikipedia.org/wiki/Wide_Area_Augmentation_System https://en.wikipedia.org/wiki/Wide_Area_Augmentation_System
• altgoogler 3y ago
  When you take off, you're going up at a rate of 500 fpm to 2000 fpm. Even if you go from +1000 fpm to -1000 fpm over the course of several seconds, you aren't going to feel much. At cruise altitude, you're moving along at 500 mph, which is 777 feet per second. So going from +30 feet to -30 feet in a minute is just an adjustment of only about 5 degrees. You'd barely feel it, even walking down the isle. An acceleration of 33 ft/sec per sec is 1 g. You experience greater changes in vertical motion on any flight you go on. *edit: units
  • sokoloff 3y ago
    > So going from +30 feet to -30 feet in a minute is just an adjustment of only about 5 degrees. You'd barely feel it, even walking down the isle. You would pretty obviously feel a change in pitch of 5° walking down the aisle. You mixed feet per second and feet per minute. 60 feet of change across 777 feet of run is about 4.5° (inverse sin(60/777)), such as you'd experience if the change was in 1 second instead of in 1 minute. Calculating 60' change in 777*60 feet, inverse sin (60/(777*60)) is 0.07°, which is why you don't feel that change in inclination of the aisle.