Blocked Transmission

Simultaneous transmission by two stations result in one of the two (or both) transmissions being blocked and unheard by the other stations (or being heard as a buzzing sound or as a squeal). With the steady growth of air traffic worldwide there is a corresponding increase in the incidence of blocked or simultaneous transmissions. These frequently result in dangerous situations developing, especially when they go undetected.

Effects:

• All or part of a message is blocked;
• A pilot does not act on a clearance intended for him/her;
• A pilot acts on a clearance intended for another aircraft;
• Unacceptable delay in establishing RTF contact or in issuing a clearance or passing a message;
• The workload of controllers and pilots is increased due to the need to resolve the confusion.

Defences:

• Radio discipline;
• Anti-blocking devices (where fitted);

Typical Scenarios:

• A flight takes a clearance intended for another flight and takes action, e.g. alters heading or level, with resultant loss of separation;
• A flight misses all or part of a clearance intended for it and maintains its level and/or heading, bringing it into conflict with other flights (level bust);
• A controller assumes that a message received is from a different flight and issues inappropriate instructions;
• A controller fails to note error in read-back (including wrong call sign) and does not correct the error.

Solutions:

• Improved radio discipline by pilots and ATCOs;
• Pilot calls “blocked” whenever simultaneous transmissions are observed;
• Widespread use of radio anti-blocking devices.

Fonte: SkyBrary