US-FAA14 CFR 121.159
Single-engine airplanes prohibited
Read the official ruleOfficial FAA regulation text (public domain)
No certificate holder may operate a single-engine airplane under this part.
Verbatim text of a US Federal Aviation Regulation (public domain), reproduced for reference — not legal advice and not for navigation. A plain-English explainer may be added later; always rely on the current official rule linked above.