However, there are occasions when this routine is disrupted - and this is when the weaknesses in VHF radio system show through. Sometimes it is the routine itself, which is the cause e.g. premature anticipation of an expected message. This is the commonest event and one that leads to pilots anticipating messages from ATC that were actually destined for other aircraft. A key fact to be born in mind is that two messages are lost or damaged and this adds greatly to both the level of risk encountered in blocked transmissions. Essentially, Read-back procedures attempt to recover from incorrect communications, but are not always successful.
Prevention is always better than cure.
In addition, brief summaries of a list of Occurrence Reports involving blocked transmissions in the UK between January 1994 and June 1998 are also included. It is hoped that these summaries will give a clear indication of the widespread nature of the problem and the need to do something about it. Both the UK and the US operate comprehensive accident and incident reporting schemes. Some of these incidents have tragic yet preventable consequences. It does this by monitoring VHF channel activity and automatically inhibiting inadvertent transmission into an already busy frequency.