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  • Groundspeed without GPS

    If an airplane does not have a GPS, is there any other way it can get a groundspeed reading, or is all it gets an indicated airspeed reading?
    "The Director also sets the record straight on what would happen if oxygen masks were to drop from the ceiling: The passengers freak out with abandon, instead of continuing to chat amiably, as though lunch were being served, like they do on those in-flight safety videos."

    -- The LA Times, in a review of 'Flightplan'

  • #2
    yup, its pretty easy too. When you plan a flight, you plan it using set checkpoints on the ground. Beforehand, you figure out how far apart those checkpoints are. Once you pass over the first one, you start a stopwatch. Once you reach the next one, you stop the watch. Now that you know how far between the points and how long it took you to fly that distance, you can figure out your groundspeed. We do this with this flight "computer" (more like a sliderule)...now back to my studying.

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    • #3
      Originally posted by screaming_emu
      yup, its pretty easy too. When you plan a flight, you plan it using set checkpoints on the ground. Beforehand, you figure out how far apart those checkpoints are. Once you pass over the first one, you start a stopwatch. Once you reach the next one, you stop the watch. Now that you know how far between the points and how long it took you to fly that distance, you can figure out your groundspeed. We do this with this flight "computer" (more like a sliderule)...now back to my studying.
      Makes sense. Nowadays is groundspeed usually calculated from the airspeed or vice-versa, if you know what your headwind or tailwind is? Or does it not matter. Is it more accurate done one way or the other?
      "The Director also sets the record straight on what would happen if oxygen masks were to drop from the ceiling: The passengers freak out with abandon, instead of continuing to chat amiably, as though lunch were being served, like they do on those in-flight safety videos."

      -- The LA Times, in a review of 'Flightplan'

      Comment


      • #4
        Originally posted by indian airlines
        Makes sense. Nowadays is groundspeed usually calculated from the airspeed or vice-versa, if you know what your headwind or tailwind is? Or does it not matter. Is it more accurate done one way or the other?
        Its a lot more accurate to calculate it using the distance and time. The reasons for this are that a) The ony weather product dealing with winds aloft is simply a forecast. Sometimes they're right, sometimes they aren't. b) Wind changes with altitude. While you can take the forecasted wind for two altitudes and interpolate between them, there's no gaurentee that is going to be right. But with fairly exact calculations that you have made (within about half a mile for the distances, and a couple seconds for the time) you should be able to come up with a pretty good number.

        Some of the more advanced avionics setups (mostly the glass cockpits) will actually compare your true airspeed, heading, ground track, and ground speed to figure out what the wind is. These usually put a little arrow as well as telling you the direction the wind is coming from and its strength. Pretty nifty.

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        • #5
          Suppose you're about to land, and you know there's a 20kt tailwind. Your airspeed reads 120 (assume that's the landing speed for the plane we're talking about - I have a question here too, when they say landing speed, do they mean groundspeed or airspeed?), but since you have a 20kt tailwind, your groundspeed is 140kts. Do you need to be concerned that you're flying 20kts too fast?
          "The Director also sets the record straight on what would happen if oxygen masks were to drop from the ceiling: The passengers freak out with abandon, instead of continuing to chat amiably, as though lunch were being served, like they do on those in-flight safety videos."

          -- The LA Times, in a review of 'Flightplan'

          Comment


          • #6
            Originally posted by indian airlines
            Suppose you're about to land, and you know there's a 20kt tailwind. Your airspeed reads 120 (assume that's the landing speed for the plane we're talking about - I have a question here too, when they say landing speed, do they mean groundspeed or airspeed?), but since you have a 20kt tailwind, your groundspeed is 140kts. Do you need to be concerned that you're flying 20kts too fast?
            When they say landing speed is 120, its going to be 120 kts of airspeed. Think of it this way. If it was 120 kts of groundspeed, but you had a 120 kt tailwind (extremely exagerated, but it will help prove the point), what would the airplane's airspeed be?

            You are correct that with an airspeed of 120 kts, and a tailwind of 20 kts, your groundspeed is 140 kts. While it isn't really going to change the way the airplane is flying, it will affect your landing considerably. Because of the increased groundspeed, your landing roll is going to be quite a bit longer. Also in bigger airplanes, landing at such a high groundspeed may get you near your max tire speeds. That is why whenever possible, its best to land with a headwind. Not only will it decrease the speed which your wheels have to accelerate to when you touch down, but it can decrease your landing roll by quite a bit. It also gives you more time to think about the landing because stuff is coming at you slower.

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            • #7
              Awesome, thanks. I get it now.
              "The Director also sets the record straight on what would happen if oxygen masks were to drop from the ceiling: The passengers freak out with abandon, instead of continuing to chat amiably, as though lunch were being served, like they do on those in-flight safety videos."

              -- The LA Times, in a review of 'Flightplan'

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              • #8
                Excellent EMU. Many moons ago when I learned to fly we did it your way. GPS wasn't around in the 70's.

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                • #9
                  Originally posted by ptbodale
                  Excellent EMU. Many moons ago when I learned to fly we did it your way. GPS wasn't around in the 70's.
                  yeah, everybody is still taught that way, even though the GPS is waaaay easier. I can attest that its so easy to rely on the GPS for everything. I get the feeling that some people once they graduate from here, once they go to fly something without a moving map they're completely lost.

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                  • #10
                    Slightly related question...suppose you did infact have a 120 kt tailwind on a plane whose landing speed was 120 kts. This efffectively makes its groundspeed 0kts right? So what happens when the plane touches down? You would expect it to stop instantly right, since it has a groundspeed of 0kts, but wouldn't the tailwind affect it when it's on the ground too?
                    "The Director also sets the record straight on what would happen if oxygen masks were to drop from the ceiling: The passengers freak out with abandon, instead of continuing to chat amiably, as though lunch were being served, like they do on those in-flight safety videos."

                    -- The LA Times, in a review of 'Flightplan'

                    Comment


                    • #11
                      Hey Guys, This is one of the most interesting thread I have ever read !Thanks to Tanuj of his curiousity and Joe knowledge on that issue !

                      Thanks guys ! Liked reading your posts !
                      Inactive from May 1 2009.

                      Comment


                      • #12
                        Originally posted by indian airlines
                        Slightly related question...suppose you did infact have a 120 kt tailwind on a plane whose landing speed was 120 kts. This efffectively makes its groundspeed 0kts right? So what happens when the plane touches down? You would expect it to stop instantly right, since it has a groundspeed of 0kts, but wouldn't the tailwind affect it when it's on the ground too?
                        Well, with a 120 kt tailwind, you would actually have a groundspeed of 240.

                        Look at it this way, When you have a headwind, the way you would figure out a groundspeed is Airspeed-headwind component. If it is a tailwind, its Airspeed+tailwind.

                        Now if you had a 120 kt headwind, and a current airspeed of 120 kts, then you'll actually just be staying in place. Give it a shot on flight sim, use a plane like the 172 and set the winds so that they're about 55-60 kts pointing straight down the runway. Or if you want to use something bigger, such as the 747, just set the windspeed to something around 180-200. Pretty interesting trying to land like that

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                        • #13
                          Originally posted by screaming_emu
                          Well, with a 120 kt tailwind, you would actually have a groundspeed of 240.

                          Look at it this way, When you have a headwind, the way you would figure out a groundspeed is Airspeed-headwind component. If it is a tailwind, its Airspeed+tailwind.

                          Now if you had a 120 kt headwind, and a current airspeed of 120 kts, then you'll actually just be staying in place. Give it a shot on flight sim, use a plane like the 172 and set the winds so that they're about 55-60 kts pointing straight down the runway. Or if you want to use something bigger, such as the 747, just set the windspeed to something around 180-200. Pretty interesting trying to land like that
                          Yeah, I'll try that.

                          I should have asked my previous question a little better...what I mean is, if you have a 120kt tailwind on a plane with a specified 120kt landing speed, the plane would have to set its own speed (i.e. velocity input from the plane) to 0, so that the airspeed would be 120kts. Right? Or am I just confusing myself more.

                          So if that is right then, when the plane touches down, it has no "speed" of its own - what happens then?
                          "The Director also sets the record straight on what would happen if oxygen masks were to drop from the ceiling: The passengers freak out with abandon, instead of continuing to chat amiably, as though lunch were being served, like they do on those in-flight safety videos."

                          -- The LA Times, in a review of 'Flightplan'

                          Comment


                          • #14
                            Originally posted by indian airlines
                            Yeah, I'll try that.

                            I should have asked my previous question a little better...what I mean is, if you have a 120kt tailwind on a plane with a specified 120kt landing speed, the plane would have to set its own speed (i.e. velocity input from the plane) to 0, so that the airspeed would be 120kts. Right? Or am I just confusing myself more.

                            So if that is right then, when the plane touches down, it has no "speed" of its own - what happens then?
                            I think you have it backwards. Wind is always described in the direction its coming from. Like if you have a westerly wind, its coming from the west, if you have a headwind, its coming from in front of the pane, a tailwind is coming from the rear of the airplane.

                            But to answer your question, its like a boat in a stream. Say you were in a helicopter. You're hovering so that your airspeed stays zero. The wind is going to pick you up and push you at the same speed it is goin. Just like if you're in a kayak in a stream, you dont have to row, the stream will pull you along. But if you're goin upstream, if you want to keep the same speed relative to the shore (instead of the water) you're going to have to work twice as hard.

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                            • #15
                              So in theory you can make a plane “hover” in place if you have a wind speed greater than the stall speed (or whatever the term is) of the aircraft.

                              Would be kinda neat to see that

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