CAP 437 ??? ???For certain operational or technical reasons an installation may have to prohibit helicopter operations. In such circumstances, where the helideck cannot be used, the ???closed??? state of the helideck should be indicated by use of the international standard ???landing prohibited??? signal, yellow cross on red background, given in the Rules of the Air and Air Traffic Control Regulations, except that it has been altered in size to cover the letter ???H??? inside the aiming circle.???