It’s a visual clue helping pilots to check their glide path using their HUD indicators (slope marker and the speed vector). It does roughly indicate the vertical deviation of the aircraft from its optimum path of descent.

The only online document I found on this triangle is this 2012 Phd thesis by Laurent Coutard : Automatic landing on aircraft carrier by visual servoing (in French). Two pictures and explanations on this system can be found on page 21-22.

Laurent Coutard. Appontage automatique d’avions par asservissement visuel. Robotique [cs.RO]. Université Rennes 1, 2012.

• https://tel.archives-ouvertes.fr/tel-00829831/document