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In all ships capable of operating with helicopters, you see these white lines painted over the flight decks and even flat surfaces, all of them serving an specific purpose, which are the following:
A square outline, which delimits the place of the flight deck in which no object can protrude from the surface.
A circle, many times with a spot at the center, serving as a target where the helicopter can land safety and with no risk of hitting anything, this is also the place where it can be refueled and rearmed.
The “line-up line”, which as its name implies is for the helicopter to align itself while on approach for landing, telling the pilot where its safe to do so.
Line-up lines may vary depending on the design of the ship, here we can see both parallel and perpendicular approaches in the first instance, or sideways approaches, single and dual, in the second and third, since these are dictated by the amount of obstacles that surround the flight deck.
VERTREP (VERTical REPlenisment) line, a series of T’s that shows the limit in which the center of the rotor hub of the helicopter’s rotor can go, in order to avoid any obstacle while hovering over the flight deck, it’s usually aligned with the very center of the landing spot, but sometimes is further back due to the presence of a mast or weapon.
A variant of the VERTREP is the red TOTO line, for use of heavier helicopters that have bigger rotors, when it’s present it also means said helicopter cannot land in the flight deck.
Many ships have decks in which helicopters cannot land, but can be used for vertically replenishment, here the square marks where the cargo can be unloaded, and the dotted line the approach pattern in the first case, the rotor hub center limit in the second (allowing for multiple approach patterns), and the third a corridor that lets the helicopter know how much space it has to maneuver.
When ships can’t allow for VERTREP deliveries, there’s also this small X inside a circle, indicating a place in which a helicopter’s hoist can land or pick up things or personnel, usually there for emergency use only.
And the final symbol, the HIFR (Helicopter In Flight Refueling), a small H denoting a place equipped with a fuel hose that can be brought up by the helicopter’s hoist for hovering mid-air refueling, used in decks that can’t be used for landing.
There are more or less universal symbols, although some navies give them slight variations, but the idea behind them is to show pilots what they can and cannot do while operating on any ship.
