A321neo has on the top of the fin, backwards, a long tube, which looks like the pitot tube on the Boeing 707, but on the opposite way, and connected on it, a cable (I think because it is a test flight). What is the purpose of each one? Picture's credits: www.airbus.com/presscentre
By trailing a long tube behind the airplane, a nearly free-stream static pressure measurement can be taken. A perforated cone at the end of the tube acts as a drag device to keep the tube stable.
Image from NASA Airdata measurement and calibration
It is used in test aircraft. The image below shows the trailing cone attached to the 787 dreamliner (pictured at Paris airshow).
Image from flyingsquadron.com
This image shows the trailing cone assembly used in 787 dreamliner.
Image from airpigz.com
The device is specified by FAA and JAA as one of several methods that can be used to perform flight calibrations. According to FAA Advisory Circular 91-85:
Where precision flight calibrations are used to quantify or verify altimetry system performance they may be accomplished by any of the ollowing methods. ...
• Precision tracking radar in conjunction with pressure calibration of atmosphere at test altitude.
• Trailing cone.
• Pacer aircraft.
• Any other method acceptable to the FAA or approving authority.