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Why do all trend vectors in the G1000 on airplanes indicate the status 6 seconds later? Why 6? Why not 5 or 10? The fact that the trend vector shows after 6 seconds allows us to turn 3 degrees per second in standard turns. Anyway, why 6 seconds for speed and altitude? I've also read that on heavy aircraft like the A320, this trend vector shows 10 seconds ahead. Is it related to the plane size?

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  • $\begingroup$ Speculation: A standard rate turn is 3 degrees/second. Maybe it's convenient to have the trend vector be a multiple of that value $\endgroup$
    – 60levelchange
    Commented Apr 2, 2023 at 6:03
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    $\begingroup$ More speculation: 6 seconds is also a nice fraction of 1 minute (one tenth) making it easy to extrapolate out to, say, feet per minute. $\endgroup$
    – TypeIA
    Commented Apr 2, 2023 at 9:02
  • $\begingroup$ The standard turn rate reference mark is located 18 degrees to the right and left of the luber line from the calculation of 6x3=18 degrees. If the trend vector was 5 seconds, it would be 5x3=15 degrees to the right and left. So it's not really about the standard turn speed of 3 degrees/second. @60levelchange $\endgroup$
    – pilot162
    Commented Apr 2, 2023 at 12:27
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    $\begingroup$ @pilot162 Most people can mentally multiply/divide by 10 much easier than by 12 or 6, for the obvious reason. $\endgroup$
    – TypeIA
    Commented Apr 2, 2023 at 12:50
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    $\begingroup$ Adding to the confusion: airspeed trend vector on this system is a ten second prediction. So why is six seconds preferred in one place and not another? $\endgroup$
    – RealAnswersNotAI
    Commented Aug 24, 2023 at 21:53

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It's a graphical space reason.

Vertical Speed

The altitude tape shows the current altitude approx. $\pm 300$ ft. Therefore, the maximum trend you can show is $\pm 300$ ft in $x$ seconds, thus $300/x$ is the maximum allowed vertical speed so that the trend bar fits the altitude tape. What is the maximum vertical speed which must considered acceptable? $3000$ ft/min? $\pm 3000$ feet/min = $\pm 50$ feet/sec. Then, $300/x=50 \to x=6$ seconds.

If they would have pick $x=5$, then the maximum would have been $3600$ ft/min which is too much for an aircraft equipped with a G1000 (and space for trend symbol would have been wasted), while $x=10$ would lead to $1800$ ft/min which is too small. $3000$ ft/min is a reasonable choice.

Airspeed

Same reasoning for airspeed. It shows $\pm 30$ kts, then $30/6$ means $5$ kts/s, which is a reasonable maximum acceleration for a GA.

Let's look at the 737, in this case $x=10$ seconds, but the tape displays $\pm 50$ kts, thus a maximum of $5$ kts/s, identical. In the A320, also in this case $x=10$ seconds and the tape displays $\approx\pm 42$ kts, thus a maximum of $4.2$ kts/s, which is sligtly lower but it still makes sense.

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