Definition
An additional aircraft class rating added to an existing pilot or flight instructor certificate that authorizes the holder to act as pilot in command of, or instruct in, airplanes with more than one engine. The add-on is earned by meeting the aeronautical experience requirements and passing the required practical test for the multiengine class; a separate written knowledge test is generally not required when adding a class rating at the same certificate level already held.
Plain English
It is an extra rating you add to a pilot or instructor certificate you already hold so you can fly or teach in airplanes with two or more engines.
Context Anchor
Seen in flight instructor endorsements when an instructor recommends an applicant for the multiengine add-on practical test.
Derivation
Add-on' is everyday English meaning something extra attached to what you already have. In FAA usage, the existing certificate stays in place and the multiengine privilege is simply added to it, rather than starting over with a new certificate.
Why Pilots Care
Allows a pilot to fly more capable aircraft, expanding training options and career opportunities.
Intuition Check
“Add-on” does not mean an informal extra lesson or a casual upgrade. In this context, it means an FAA rating added to an existing pilot certificate after the required steps are completed.
Example Sentence 1
After earning her commercial single-engine certificate, she began training for her multiengine add-on so she could fly the company's twin.
Example Sentence 2
With the multiengine add-on complete, the pilot began logging time in a twin-engine aircraft.