Definition
An additional class rating added to an existing pilot certificate that authorizes the holder to act as pilot in command of an aircraft equipped to operate from water, such as a single-engine sea (ASES) or multiengine sea (AMES) airplane. It is earned by completing the required training and a practical test (or, in some cases, a proficiency check) with an authorized instructor and examiner, and is endorsed onto the pilot's existing certificate rather than issued as a new certificate.
Plain English
It is the extra rating a pilot earns to legally fly airplanes that take off and land on water. The pilot already holds a pilot certificate, and this rating is added to it after the required training and check.
Context Anchor
Seen in flight instructor endorsement discussions when an instructor signs off a pilot for training or testing to add seaplane privileges.
Derivation
"Add-on" is plain English for something added to what already exists. In certification, it signals that the rating attaches to a certificate the pilot already holds rather than replacing it.
Why Pilots Care
It expands the types of aircraft a pilot can legally fly, opening opportunities for operations on water.
Intuition Check
Add-on does not mean an informal extra lesson or an aircraft accessory. Here it means an official FAA privilege added to an existing pilot certificate.
Example Sentence 1
After finishing his single-engine sea training in Florida, he received the seaplane add-on endorsement from his instructor and passed the practical test the following week.
Example Sentence 2
The seaplane add-on allows the pilot to operate both landplanes and seaplanes under the same certificate.