Official rule text as published in the U.S. Coast Guard Navigation Rules
Where it is impracticable for a seaplane or a WIG (wing-in-ground) craft to exhibit lights or shapes of the characteristics or in the positions prescribed in Rules 20-31, she shall exhibit lights and shapes as closely similar in characteristics and position as is possible.
MyCOLREGS explanation
Summary
Seaplanes and WIG (wing-in-ground) craft cannot always carry lights and shapes in the exact positions this set of rules describes for surface vessels. When that is impracticable, Rule 31 simply requires them to come as close as possible to the required characteristics and placement — the same idea, adapted to the aircraft shape.
Real World Example
A floatplane taxiing among anchored boats will show lights and shapes that approximate a normal vessel's as closely as her airframe allows - don't expect an exact match to what Rule 23 or Rule 30 would otherwise require. Rule 31 only asks for the closest practical equivalent; it says nothing about who has to keep clear of whom.
Practice Questions
Q1. A seaplane on the water has her navigation lights mounted on her wingtips and fuselage, but her airframe still can't match the heights, vertical separation, or exact positions Rules 20-31 prescribe for an ordinary vessel's masthead, side, and stern lights. What does Rule 31 require of her instead?
A. She must display a special aircraft-only strobe light instead.
B. She must remain at anchor until the light can be repositioned.
C. She must exhibit lights and shapes as closely similar in characteristics and position as possible.
D. She is exempt from carrying any lights.
Q2. Besides conventional seaplanes, what other type of craft does Rule 31 name?
A. Tugs with rigid tows
B. Hovercraft only
C. Submarines
D. WIG (wing-in-ground) craft
Practice Question Answers
Q1.
C. She must exhibit lights and shapes as closely similar in characteristics and position as possible. Rule 31 recognizes that a seaplane's design may make exact compliance impracticable and requires the closest possible equivalent instead. A invents a special light type the rule doesn't describe. B has her waiting indefinitely, which defeats the practical "as closely as possible" accommodation the rule actually makes. D wrongly assumes seaplanes are exempt from lighting altogether.
Q2.
D. WIG (wing-in-ground) craft Rule 31 by its terms applies to "a seaplane or a WIG craft," extending the same practicability allowance to wing-in-ground-effect craft. A, B, and C each name a real category of vessel or craft, but none of them appear in Rule 31 - the rule specifically pairs seaplanes with WIG (wing-in-ground) craft.