- Star Trek: The Motion Picture
- Star Trek II: The Wrath of Khan
- Star Trek III: The Search for Spock
- Star Trek IV: The Voyage Home
- Star Trek V: The Final Frontier
- Star Trek VI: The Undiscovered Country
- Star Trek Generations
In 2267, Appel served as an engineer at the underground pergium mining colony on Janus VI. Appel was one of the first to encounter the Horta, a "monster" that could burrow through solid rock, and was destroying machinery and killing the miners. Upon his encounter, Appel used a hand phaser and injured the creature.
When it was learned that the Horta was an intelligent lifeform, and a mother protecting her eggs, Appel and the other miners reached a peaceful alliance with the creature.