Since the shock of the disastrous collision in July 2002 of the Bashkirian Tupolev Tu-154M with a DHL Airlines Boeing 757 over southern Germany, safety agencies have been developing advice for pilots faced with conflicting instructions from air traffic control (ATC) and airborne collision avoidance systems (ACAS, also known as TCAS). It had become clear as a result of the collision that not all states give their pilots identical advice on the sometimes conflicting priorities that determine what the pilot does.
Just after the collision, Eurocontrol issued a "safety flash" version of its ACAS II bulletin, summarising advice not only on how to act in the event of an ACAS resolution advisory (RA) that conflicts with ATC instructions, but the technical reasons and the logic. This year Eurocontrol has updated the bulletin with advice on situations - such as simultaneous horizontal and vertical convergence - that can generate an apparently illogical or unnecessary RA.
The "safety flash" basic advice is that "it is critical that pilots respond accurately and promptly to resolution advisories". To eliminate doubt, it made it clear that pilots should comply with RAs even if:
there is an opposite instruction by the controller; the aircraft is at the top of its altitude envelope and a climb RA is generated, "climb at least a little, but do not descend opposite to the RA"; the controller gives traffic information that seems to conflict with an RA. The reason for trusting the ACAS first is that the update rate of the controller's radar is not as fast and accurate as that generated by the ACAS, particularly if the aircraft is climbing or descending fast; an aircraft has been visually acquired, "follow the RA because the wrong aircraft might have been identified and the situation wrongly assessed".Pilots are reminded to tell ATC of their action in following an RA as soon as possible, for example: "[callsign] TCAS climb".
The most recent edition of the bulletin does not change any of the basic advice, but introduces techniques for reducing the likelihood of avoidable RAs. The main point is to reduce rate of climb or descent to 1,000ft/min (5.1m/s) or less when the aircraft is within 1,000ft (300m) of its cleared altitude. Eurocontrol says it wants this advice to become required procedure, pointing out that it is incorporated into aeronautical information publications in "two core area European states".
Finally, the bulletin states that following an RA caused by fast vertical convergence will still guarantee separation, even if the pilots judge it to be operationally unnecessary because they know they are about to level off. Following RAs caused by "level busts" - aircraft passing through their cleared altitude by mistake - is also the best way of ensuring separation in such events, says Eurocontrol.
Source: Flight International
