The Air Navigation Order 1995

2.  Position and Size of Marks

(1) The position and size of marks on heavier than air aircraft (excluding kites) shall be as follows:

(a) on the horizontal surfaces of the wings:

(i) on aircraft having a fixed wing surface, the marks shall appear on the lower surface of the wing structure and shall be on the port wing unless they extend across the whole surface of both wings. So far as is possible the marks shall be located equidistant from the leading and trailing edges of the wings. The tops of the letters shall be towards the leading edge of the wing;

(ii) the height of the letters shall be:

(aa) subject to sub-paragraph (bb) at least 50 centimetres;

(bb) if the wings are not large enough for the marks to be 50 centimetres in height, marks of the greatest height practicable in the circumstances;

(b) on the fuselage (or equivalent structure) and vertical tail surfaces:

(i) the marks shall also appear either:

(aa) on each side of the fuselage (or equivalent structure), and shall, in the case of fixed wing aircraft be located between the wings and the horizontal tail surface; or

(bb) on the vertical tail surfaces;

(ii) when located on a single vertical tail surface, the marks shall appear on both sides. When located on multi-vertical tail surface, the marks shall appear on the outboard sides of the outer-surfaces. Subject to sub-paragraphs (iv) and (v) below, the height of the letters constituting each group of marks shall be at least 30 centimetres;

(iii) if one of the surfaces authorised for displaying the required marks is large enough for those marks to be 30 centimetres in height (whilst complying with sub-paragraph (v) below) and the other is not, marks of 30 centimetres in height shall be placed on the largest authorised surface;

(iv) if neither authorised surface is large enough for marks of 30 centimetres in height (whilst complying with sub-paragraph (v) below), marks of the greatest height practicable in the circumstances shall be displayed on the larger of the two authorised surfaces;

(v) the marks on the vertical tail surfaces shall be such as to leave a margin of at least 5 centimetres along each side of the vertical tail surface;

(vi) on rotary wing aircraft where owing to the structure of the aircraft the greatest height practicable for the marks on the side of the fuselage (or equivalent structure) is less than 30 centimetres, the marks shall also appear on the lower surface of the fuselage as close to the line of symmetry as is practicable and shall be placed with the tops of the letters towards the nose. The height of the letters constituting each group of marks shall be:

(aa) subject to sub-paragraph (bb) at least 50 centimetres; or

(bb) if the lower surface of the fuselage in not large enough for the marks to be of 50 centimetres in height, marks of the greatest height practicable in the circumstances;

(c) wherever in this paragraph marks of the greatest height practicable in the circumstances are required, that height shall be such as is consistent with compliance with paragraph 3 of this Part.

(2) The position and size of marks on airships and free balloons shall be as follows:

(a) in the case of airships the marks shall be placed on each side of the airship. They shall be placed horizontally either on the hull near the maximum cross-section of the airship or on the lower vertical stabiliser;

(b) in the case of free balloons, the marks shall be in two places on diametrically opposite sides of the balloon;

(c) in the case of both airships and free balloons the side marks shall be so placed as to be visible from the sides and from the ground. The height of the letters shall be at least 50 centimetres.