If an engine doesn't burn all the fuel that's fed to it, the excess fuel will be released through the airplane's exhaust system, which manifests as a black smoke trail behind the airplane. Black smoke trails such as this were more common during the 20th century.

In 2026, it is not considered normal for a modern commercial aircraft to emit thick black smoke during flight or while idling. While older jet engines from the 1960s and 70s often produced "smoke trails" due to incomplete combustion, 2026 engine technology (like the CFM LEAP or Pratt & Whitney GTF) is designed for ultra-clean burning. If you see black smoke from a 2026 engine, it typically indicates a significant mechanical failure, such as a rich fuel mixture, a compressor stall, or an internal oil fire. However, a small, brief puff of dark smoke upon engine start-up can occasionally occur as residual fuel burns off. If smoke enters the cabin, it is often a "fume event" caused by a faulty seal leaking oil into the bleed-air system. In any scenario involving visible black smoke in 2026, pilots follow strict "Emergency or Abnormal" checklists, often declaring a Mayday or Pan-Pan and returning to the airport for a precautionary landing and inspection.