Europe’s safety regulator is concerned that crews are not being given full information on use of collision-avoidance systems to maintain safe separation, to the point where pilots might disregard their instructions.

The European Union Aviation Safety Agency says some aircraft flight manuals and operating manuals “do not provide adequate information” on separation procedures from nearby aircraft.

“In some cases the flight crew may interpret the available information…as authorisation to disregard a [conflict resolution advisory] based on their visual perception of the situation,” says EASA in a safety bulletin.

It points out that standard regulations require immediate crew response to such advisories, adding that a timely response is the “most important single factor” affecting the performance of collision-avoidance systems.

“Visually acquired traffic may not be the same traffic causing [the advisory],” warns the bulletin, adding that disregarding instructions will reduce safety levels. “The visual perception of an encounter may be misleading, particularly at night.”

EASA says it intends to monitor the situation and could consider further action. But for the time being it is recommending that interim measures be taken including reviewing and updating aircraft flight manuals to ensure that “no credit” is given to visual acquisition of traffic by the crew which “may be in contradiction” with alerts from a resolution advisory.