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This question applies to aircraft being tested, presumably within a certification program.

I've seen what looks like either a ball or "shuttlecock" at the end of a tether attached to the top/rear extremity of an aircraft's vertical fin. An example can be seen toward the end of this video.

Screenshot from linked video: enter image description here

What is its purpose? Is it part of or does it contain any instrumentation, or is it there to provide a direct visual indication of something - such as trailing airflow patterns?

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That is a static pressure line. The "shuttlecock" device on the end pulls the line out behind the aircraft, where it can get a more accurate pressure reading, away from the air flowing around the aircraft that affects the regular pitot-static system. It will also be more accurate during flight conditions that normal aircraft rarely experience, but need to be accurately analyzed during testing.

Static pressure thingy
Cropped from this image

Static pressure has an effect on the performance of the aircraft, so during testing that is one value they want to measure accurately during flights. Once testing is complete there is no longer a need for that information, and the regular pitot-static system takes the measurements needed.

The line can be seen in this photo, and in this video, you can see the reel inside the rear of the aircraft that is used to pull in the line when it is not in use.

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    $\begingroup$ I've seen high-performance aircraft (e.g. jet fighters) fitted with long forward-pointing spikes during testing. I presume they are for the same purpose? If so, what would call for a spike vs a trailing line? $\endgroup$
    – Anthony X
    Commented Apr 19, 2015 at 21:56
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    $\begingroup$ @AnthonyX Yeah, similar purpose, but not identical. The spike is for measuring pitot or impact pressure; the shuttlecock is for static. $\endgroup$
    – egid
    Commented Apr 19, 2015 at 22:32
  • $\begingroup$ Oh OK I thought a spike would have had both pitot and static ports. $\endgroup$
    – Anthony X
    Commented Apr 21, 2015 at 2:10
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    $\begingroup$ @AnthonyX The pitot boom or YAPS boom (the spike) does have both pitot and static ports. As seen here: aerosonic.com/products/flight-test-equipment $\endgroup$
    – Mark Jones Jr.
    Commented Jan 13, 2018 at 13:47
  • $\begingroup$ Does this cause drag? $\endgroup$
    – Boeing787
    Commented Oct 20, 2021 at 0:13

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