If gpg-key is an issue, then download RPi .deb package ver 1.0.23.8 directly from fr24 repository. This does not need gpg-key. The key is required for apt-get method.
Code:
cd /home/pi/ wget -O fr24feed_1.0.23-8_armhf.deb http://repo-feed.flightradar24.com/rpi_binaries/fr24feed_1.0.23-8_armhf.deb sudo dpkg -i fr24feed_1.0.23-8_armhf.deb
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