Are amphibious airplanes considered land or sea planes for pilot certification? Can an ASEL-only pilot fly an amphibious single-engine plane if (s)he only takes off and lands on land? Can an ASEL-only pilot take off from the left seat, then let the ASES-only pilot in the right seat take over and alight on water?
For pilot certification purposes there is no "amphibious" class (see 14 CFR 61.5 (b)(2) ). Consequently, again for pilot certification purposes, (the rating held on your pilot certificate), practical tests (flight checks) taken in amphibious airplanes can result in either a "sea" class rating, "land" class rating, or possibly both. This would depend upon the class rating the pilot trained for, applied for, and was tested for, under 14 CFR Part 61. (See 14 CFR 61.45 (a) and (b))
So, for example, in my opinion, if a pilot holds a Private pilot certificate with only an Airplane Single-engine Land rating then he/she could fly an amphibious Single-engine airplane, but could only takeoff and land on "land." In the alternative, if the pilot held a Private pilot certificate with only an Airplane Single-engine Sea rating, he/she could fly an amphibious Single-engine airplane, but could only takeoff and land on "water."
This is an informative FAA legal interpretation (Fitch) which states, in pertinent part:
Title 14 C.F.R. § 61.45(a)(l)(i) states, in relevant part, that" ... an applicant for a certificate or rating issued under this part must furnish (1) An aircraft of U.S. registry for each required test that-(i) Is of the category, class, and type, if applicable, for which the applicant is applying for a certificate or rating ...." Additionally, the aircraft required for a practical test must have "[t)he equipment for each area of operation required for the practical test" and no operating limitations that would prohibit its use in any of the areas of operation required for the practical test. 14 C.F.R. § 61.45( b)(l )(i) -(ii). Under 14 C.F.R. § 6 l.45(b)(2), if an applicant for a certificate or rating uses an aircraft with operating characteristics that preclude the applicant from performing all of the tasks required for the practical test, the applicant's certificate or rating will be issued with an appropriate limitation.
Also note the wording of the last sentence, in the next to last paragraph, of the Fitch legal interpretation linked above. This wording seems to state that an "amphibious" airplane can be both an ASEL and ASES.
However, both practical tests may be administered for ATP: ASEL and ASES using an amphibious airplane, as long as that airplane is certificated and maintained in accordance to regulation as an ASEL and ASES amphibious airplane.
(emphasis is mine)
Amphibious aircraft are in the same class as seaplanes. There are no single-engine or multi-engine amphibious ratings, it's either land or sea, and if the aircraft can land in water you need the seaplane rating, regardless of whether it also has landing gear and you only ever operate from land. https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/seaplane_handbook/media/faa-h-8083-23-2.pdf