"Going feet wet" is something you hear in movies about bomb runs to and from a carrier or in pilot circles among those of us who like to add a bit of drama to what we do for a living. There is no real definition for what a wet footprint is other than folklore. There is a point of confusion that we can clear up at the start.
I've seen in various pilot forums the discussion of wet footprints and ETOPS. The two having nothing to do with each other. ETOPS has to do with a multiple engine aircraft losing an engine and still being able to make it to a qualified airport. More about this: Extended Operations (ETOPS).
A wet footprint occurs when an airplane is far enough away from land that if it were to lose ALL engines it will end up in the water. As far as I can tell, there is no prohibition against this. Under U.S. rules, the only restriction is that an aircraft operated for hire must include some gear (as shown below). Under ICAO rules, there are similar restrictions imposed on general aviation aircraft. And that is it. (If you know otherwise, please "Contact Eddie" below.)
Everything here is from the references shown below, with a few comments in an alternate color.
In U.S. FAA and ICAO regulations, the discussion of gliding distance only pertains to how the airplane needs to be equipped.
[14 CFR 91, §91.205(b)(12)] If the aircraft is operated for hire over water and beyond power-off gliding distance from shore, approved flotation gear readily available to each occupant and, unless the aircraft is operating under part 121 of this subchapter, at least one pyrotechnic signaling device. As used in this section, “shore” means that area of the land adjacent to the water which is above the high water mark and excludes land areas which are intermittently under water.
[ICAO Annex 6, Part I, ¶220.127.116.11] Land planes shall carry the equipment prescribed in 18.104.22.168: a) when flying over water and at a distance of more than 93 km (50 nm) away from the shore, in the case of land planes operated in accordance with 5.2.9 or 5.2.10; b) when flying en route over water beyond gliding distance from the shore, in the case of all other land planes; and c) when taking off or landing at an aerodrome where, in the opinion of the State of the Operator, the takeoff or approach path is so disposed over water that in the event of a mishap there would be a likelihood of a ditching. 22.214.171.124 The equipment referred to in 126.96.36.199 shall comprise one life jacket or equivalent individual flotation device for each person on board, stowed in a position easily accessible from the seat or berth of the person for whose use it is provided.
[ICAO Annex 6, Part II, ¶188.8.131.52] Single-engined landplanes. Recommendation.— All single-engined landplanes:
a) when flying en route over water beyond gliding distance from the shore; or
b) when taking off or landing at an aerodrome where, in the opinion of the pilot-in-command, the take-off or approach path is so disposed over water that in the event of a mishap there would be a likelihood of a ditching;
should carry one life jacket or equivalent individual floatation device for each person on board, stowed in a position easily accessible from the seat or berth of the person for whose use it is provided.
14 CFR 91, Title 14: Aeronautics and Space, General Operating and Flight Rules, Federal Aviation Administration, Department of Transportation
ICAO Annex 6 - Operation of Aircraft - Part 1 Commercial Aircraft, International Standards and Recommended Practices, Annex 6 to the Convention on International Civil Aviation, Part I, July 2010
ICAO Annex 6 - Operation of Aircraft - Part 2 General Aviation, International Standards and Recommended Practices, Annex 6 to the Convention on International Civil Aviation, Part II, July 2008
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