6 ms·

“According to this data, the plane’s altitude was only fluctuating by about 20-30 feet. This is more stable than I expected!” Autopilots are very good and they

• jfim 3y ago
  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 altitude is coming from GPS, which is notoriously inaccurate with regards to elevation?
  • 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.
      • 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.
• yread 3y ago
  I guess they got a lot more precise with implementation of Reduced Vertical Separation Minimum (RVSM) - planes had to be separated by 2000 ft and this was reduced in early 2000s to 1000ft
  • ceejayoz 3y ago
    It was probably fairly precise already. To get their license, a private pilot must demonstrate via a checkride the ability to stay within 100 feet of an assigned altitude, even in a steep turn.
    • wkipling 3y ago
      Not quite how it works. These are the instruments we are referring to not the ability of pilots. In fact in RVSM airspace the autopilot must be used. Instruments must be very accurate given the reduced separation in RVSM airspace. Often on modern aircraft multiple altimeters are compared and voted to provide a single output provided to the displays and autopilot.
      • ceejayoz 3y ago
        That's missing the point. If a human can manage to keep it within 100 feet of a desired altitude, an autopilot most certainly can; it didn't require new technology in the 2000s. Autopilots in the 1960s/1970s weren't seesawing all over the skies.
        • sokoloff 3y ago
          RVSM is overwhelmingly about instrumentation accuracy and precision, not pilot capability. [0] The pressure difference between 5K MSL and 10K MSL at standard conditions is 14.6 kPa. The pressure difference between 30K MSL and 35K MSL at ISA is 6.3 kPa. For a given amount of aircraft-to-aircraft variability in their precision altitude sensing equipment, the resulting difference in actual altitude is more than double in RVSM airspace than in the lower altitude range above. That's the reason for RVSM: there is less change in pressure with change in altitude, coupled with a very busy altitude range (such that controllers would have an operational need to pass traffic overhead with only vertical separation rather than being able to use vectoring to achieve lateral separation between aircraft). It's not a linear relationship, but if I take an airplane with a 0.75 kPa absolute error in one direction and pass traffic with a 0.75 kPa absolute error in the other direction 1000' indicated above them, at low altitude, that 1.5 kPa total error is a little over 500 feet while IFR-IFR separation is 1000 feet minimum outside of RVSM. (These aircraft would likely be right on the border of passing a non-RVSM static system check.) If I take those same two aircraft into the mid flight levels and pass one over the other at 30K and 31K feet, the total error is around 1200 feet, which is why non-RVSM aircraft cannot be separated by 1000 feet in RVSM airspace, because you don't know that they'll miss each other. Improve the accuracy and precision of the static system and improve the examination criteria, making the airplane RVSM-capable, and now you can pass that traffic over each other at 1000' of indicated separation and be sure they'll miss. [0] - There is a pilot training requirement, which is focused on knowing the rules for RVSM and does not involve a checkride.
• jjwiseman 3y ago
  I don't know what sensors are feeding the API from the post, but most passenger jets do broadcast information about the accuracy of their sensed position, including vertical position/altitude. If you click on an aircraft on the map at https://globe.adsbexchange.com/ https://globe.adsbexchange.com/, and scroll the left sidebar all the way to the bottom you'll see a section labeled "Accuracy". ADS-B Exchange doesn't show Rc/v, the vertical position accuracy, but it does show other values. See https://mode-s.org/decode/content/ads-b/7-uncertainty.html https://mode-s.org/decode/content/ads-b/7-uncertainty.html for more information.
  • ssaannmmaann 3y ago
    I went down a rabbit hole by clicking on globe.adsbexchange.com :)
  • dramm 3y ago
    Pressure encoders, as I said. That's what feeds all aviation altitude data... i.e. anytime you see the word 'altitude' and its not qualified with 'GPS altitude' which is effectively not normally used. ADS-B Out concurrently transmits GPS height about the ellipsoid data as well as pressure altitude data. No use is normally made of the GPS height data. We are discussing pressure altitude data here, that's what aviation works off of. The accuracy and reliability metrics in the ADS-B broadcast you are referencing refers to the GPS data not the pressure transducer/encoder data. In cases of encoder failure being detected a flag is broadcast and the pressure aka baro altitude data field is set to all 0. ADS-B cannot give information about the pressure altitude accuracy or reliability like it does for GPS metrics. It relies on the encoders being better than their +/- 125' accuracy requirement and that's tested for periodically. ADS-B can in principle broadcast 100' or 25' resolution encoders, that info is in the messages. The ones here will be 25'. (I've got a long background with ADS-B related technology, currently helping the FAA out on some niche stuff). edit: trying to improve clarity/correctness but there is too much to cover here.
    • jjwiseman 3y ago
      Cool, thanks for the info. (BTW I wasn't disagreeing with anything you wrote, I just wanted to mention, since the topic of resolution/accuracy came up, that lots of aircraft broadcast information about the accuracy of some of the sensor data they're reporting, which you can do some pretty cool stuff with. E.g. that's how https://gpsjam.org/ https://gpsjam.org/ works.)
• cragfar 3y ago
  No idea how true it is, but I overheard someone on a flight say that whenever you feel a real sudden jolt on a plan it's really only moving like 2-3ft.
• GuB-42 3y ago
  I have read somewhere that so much precision could actually be dangerous in some circumstances. This is because this way, if a pilot goes 3000 ft for instance, it will be exactly 3000 ft, if another pilot also wants to go 3000 ft on a collision trajectory, it will be a guaranteed collision. When altitudes are not that accurate, there is a higher chance it being just a near miss. The solution, I think, was to simply avoid round numbers. So now, it is 2950 ft, 3050 ft,... I may have the details wrong, but I am quite sure about that problem being seriously considered.
  • _moof 3y ago
    Yes, it's called the navigation paradox, and it mostly came about with the advent of GPS. It's the reason we now have what's called "strategic lateral offset procedure," or SLOP, whereby aircraft on heavily trafficked oceanic routes fly zero, one, or two miles off the centerline, randomly chosen.
    • darkerside 3y ago
      This is really interesting. But it seems like it could make collisions more likely, and the better solution would be separate corridors for east vs west traffic. Are there really 5 bidirectional lanes?
      • _moof 3y ago
        It's a bit more complicated than that. The routes I'm talking about are the North Atlantic Tracks, which are used for most traffic between North America and Europe. There are multiple tracks and SLOP is used within each track. All of the tracks run in the same direction at the same time, switching directions twice a day. They go eastbound at night, westbound during the day. SLOP is a mitigation to prevent aircraft in sequence on the same track from colliding. There are, of course, many other systems and procedures in place to prevent such collisions, but it's belt-and-suspenders up there.
  • spixy 3y ago
    how is it any better when 2 opposite planes choose same 2950ft?
• BWStearns 3y ago
  For small planes a 20-30 foot range isn’t abnormal for hand flying if you’re paying attention. I’m sure in cruise an airliner is using an autopilot though. I once had ATC ask if everything was cool on flight following after a hundred foot drop and I was surprised they were paying that much attention. I had forgotten to put my life jacket on before a water transit and while I was putting it on handed it off to my wife who hadn’t taken lessons yet (she later got her license!). It was interesting to see that their tracking was precise enough for them to chime in.
• svag 3y ago
  When I am on a flight and the flight does not provide the flight information, I am using the OsmAnd, https://osmand.net/ https://osmand.net/, to monitor the flight altitude, speed and direction.
• phkahler 3y ago
  >> Autopilots are very good and they are servoing to the pressure altitude. It would have been cool to use a phone to record a GPS track with altitude and compare them. Pressure != GPS. Also wonder if there would be distinct jumps in the difference if they reset the pressure based altimeter to a different AWOS. Not sure how it works in big planes, but in little ones you need to set your altimeter based on the local weather. The weather stations measure barometric pressure at their elevation and "correct it to sea level" you get this corrected reading over the radio and set it in your altimeter so your pressure-based altitude reading is corrected for local weather variations. Just going out flying for an hour the altimeter setting when returning to the same place might be off by a few millibar.
  • gfo 3y ago
    You use standard pressure (29.92 inHg) above transition altitude, which, in the United States, is 18,000 feet. Pilots wouldn't be changing the altimeter after climbing past this point, and would start using local values once descending through it again. Of course, your initial point is still correct: there could be slight variations if using those local settings and getting different values, but you'd only see that below transition altitude.
  • tim333 3y ago
    At high altitude you do this stuff "When you set your altimeter to 29.92, you're flying at standard pressure altitude." The idea is all the planes use the same setting so the one at FL35 doesn't hit the one at FL36. But those are not exactly 35000 and 36000 feet above sea level.
    • inoffensivename 3y ago
      nitpicking: that would be FL350 and FL360
  • dramm 3y ago
    The QHN/Kollsman window setting only affects what is displayed to the wetware. When you strip away all that the autopilot is just servoing to a pressure altitude. But sure if you are flying below the transition altitude and are flying between areas with different QNH settings when you adjust the setting the autopilot will climb or descend as needed because you told it to servo to a different pressure altitude. There are many EFB (e.g. Foreflight), or log book, or other flight recorders you can use on an iPhone. And some can record the pressure transducer in the iPhone to record an approximate "pressure altitude". e.g. Naviter SeeYou Navigator intended for gliders can do that (but it's not unusual for modern gliders to have an array of sophisticated air data sensors and specialized variometers and flight computers that would feed the app this data over Bluetooth). Popular EFB software Foreflight will not use the iPhone pressure transducer, if you want pressure data there you need to drive that through an external interface like a Sentry ADS-B receiver that has a pressure sensor built into it -- or much better if the aircraft is equipped with ADS-B Out can receive the "own-ship" ADS-B Out broadcast pressure altitude from it's high accuracy encoder). Any in-cabin pressure traducer will be sensitive to the difference between calibrated static pressure and cockpit pressure, things like opening or closing vents, or varying the airspeed significant (and ram air pressure or suction on the cockpit exit vents) can cause observable changes. And when using an iPhone or similar, especially without a great GPS satellite overhead view (e.g. in high wing aircraft) you are likely not to get high-quality GPS altitude data. think best case ~ +/- hundred feet, worse case with little overhead GPS sat view, much worse... but those consumer GPS app is likely to happily display multiple decimal points of precision :-)
• drumttocs8 3y ago
  What kind of telemetry (protocol, topology, etc) is used here? I'm in utilities, so am used to seeing modbus, DNP, etc, but also some OPC-UA nowdays.