An infringement of separation standards occurred 70 NM east of
Darwin, NT, between a descending Boeing 737-376 (737) and an
Embraer EMB-120 (Brasilia) that was maintaining level flight. The
event took place during the hours of darkness and in visual
meteorological conditions. The crew of the 737 intentionally flew
the aircraft through its assigned level in response to a traffic
alert and collision avoidance system (TCAS) warning. The Brisbane
sector controller also received a short-term conflict alert (STCA)
between the two aircraft from the Australian Advanced Air Traffic
System (TAAATS). The STCA alerted controllers when the radar
trajectories of two aircraft indicated that separation standards
might be infringed. The 737 and the Brasilia passed within 1.6 NM
horizontally and 600 ft vertically. The required separation
standard was either 3 NM or at least 1,000 ft.
TCAS is an airborne device that functions independently from the
ground based air traffic control system and provides collision
avoidance protection for a broad range of aircraft. The system
fitted to the 737, TCAS II version 6.04, provided recommended
escape manoeuvres in the vertical dimension, to either increase or
maintain the existing vertical separation between the aircraft. The
escape solution was communicated directly to the flight crew via a
cockpit display and a synthesised voice attention getter. The TCAS
alerts in the 737 used a liquid crystal display (LCD) instantaneous
vertical speed indicator (IVSI) with red and green markings to
indicate the vertical speeds to be avoided (red), and the desired
vertical speed to be flown (green). The display was 70 mm square
and had a fixed range of 6.5 NM. Aircraft were depicted using
geometric symbols, depending on their threat status. A partial
aircraft symbol was displayed at the extremity of the screen for
aircraft detected beyond the display range. TCAS information could,
in principle, have been incorporated in the electronic flight
instrument system display (EFIS) of that aircraft but wiring and
space available in the electronics racks of the aircraft would have
required a major modification programme. TCAS information that is
incorporated in the EFIS display allows the pilot to show traffic
at longer ranges.
TCAS equipment interrogates the transponders of other aircraft
to determine their range, bearing and altitude. Accordingly, the
TCAS does not provide protection against aircraft that do not have
an operating transponder. The Brasilia was fitted with an operating
mode C transponder (altitude encoding) but was not TCAS equipped
and the crew was not immediately aware of the infringement of
separation standards. The Brasilia, which had 30 passenger seats
and had a maximum take-off weight of 11,990 kg, was not required to
be TCAS equipped by the Australian legislation current at the time.
That legislation mandated the carriage of TCAS for all
turbine-powered aircraft with more than 30 passenger seats or a
maximum take-off weight greater than 15,000 kg. When both
potentially conflicting aircraft are fitted with TCAS, both TCAS
units co-ordinate their intentions to provide appropriate
co-coordinated avoidance manoeuvres.
TCAS II can issue two types of alerts:
- Traffic Advisory (TA) to assist the pilot in the visual search
for the intruder aircraft and to prepare the pilot for a potential
RA; and
- Resolution Advisory (RA) to recommend manoeuvres that will
either increase or maintain the existing vertical separation from
an intruder aircraft.
Once the risk is over, the TCAS issues a synthesised voice
`clear of conflict' message.
The 737 crew were on their fourth consecutive day of duty and
completing the last sector of a four-sector day when the incident
occurred. It took place at the transfer of control point between
Brisbane Centre and Darwin Approach. The 737 was en-route from
Cairns to Darwin and was on descent to FL220. The Brasilia was
enroute to Groote Island, under the control of Brisbane Centre,
with instructions to maintain flight level (FL) 210. Those routes
placed the two aircraft on almost reciprocal tracks. As the 737 was
passing FL235, the crew were instructed by the Brisbane sector
controller to contact Darwin approach control for further descent,
but were not advised of the opposite direction traffic.
The Manual of Air Traffic Services (MATS) gave guidance to
controllers on how best to manage the situation and advised:
`6.6.5.5 Frequency change management in relation to the transit
of an aircraft across airspace boundaries shall be arranged in a
manner that normally enables pilot communication with the unit
responsible for the airspace within which the aircraft is
operating. A lateral tolerance of 10 NM either side of the boundary
is permitted, except that when entering controlled airspace, the
frequency change shall be within the 10 NM prior to the
boundary.
6.6.5.10 Aircraft should normally remain on the frequency
appropriate to the airspace in which it is operating. However,
aircraft may be transferred to another ATS frequency provided
that:
- significant operational advantage will be gained;
- workload, communications and equipment capabilities will permit
the responsible controller to take such action as is necessary to
preserve the separation without delay;
- the actual separation is in excess of the minimum;
- there is no possibility of separation being reduced to the
minimum with the normal operation of the aircraft.'
MATS does not require controllers to provide traffic information
to either crew in these circumstances.
In order to comply with MATS 6.6.5.10, the Brisbane sector
controller could have either:
- Initially assigned the crew of the 737, FL230 on descent,
instead of FL220, then assigned the responsibility for separation
to the Darwin approach controller, coordinating the terms of the
transfer of the 737 with him, before transferring the aircraft to
the Darwin approach frequency; or
- Assumed the separation responsibility, coordinated the terms of
the transfer of the 737 with the Darwin approach controller, and
transferred the aircraft once the 737 had passed the Brasilia and a
horizontal separation standard existed.
Airservices Australia believed that MATS section 6.6.5.10 did
not apply to a change of frequency in these circumstances.
According to Airservices Australia the action of the Brisbane
sector controller, in assigning the crew of the 737 FL220, was
consistent with MATS in that the controller applied a 1,000 ft
separation standard between the two aircraft and the 737 crew was
transferred to the Darwin approach controller within 10 NM of the
lateral boundary between the Brisbane sector and Darwin
airspace.
Shortly after acknowledging the instruction to change frequency,
the 737 crew received an aural `traffic, traffic' warning and a
display indication of an aircraft 5 NM ahead. The pilot in command
stated that the TA quickly changed to a RA with a `descend,
descend, descend' aural alert. As the aircraft was approaching its
assigned level he disconnected the autopilot and pitched the
aircraft nose down with the intention of following the RA commands.
He stated that the required rate of descent shown on the IVSI was
1,200-1,500 ft/min. On passing FL220 the TCAS command abruptly
reversed to a climb RA (aural `climb, climb now') which was
followed positively. The climb annunciation continued until the
aircraft was at FL225. No more commands were issued and there was
no TCAS `clear of conflict', which is normally generated once a RA
is removed.
Analysis of recorded data indicated that as the 737 descended
through FL230, its rate of descent was approximately 2,900 ft/min.
At FL227, the automatic flight system commenced a transition
manoeuvre to achieve level flight at FL220. At approximately FL225
the autopilot was disengaged and the descent was continued manually
at a rate of descent in excess of 3,200 ft/min to FL215. That was
followed by a climb to FL225 at 2,900 ft/min as the pilot responded
to the RA reversal (`climb now' advisory).
Maintenance files from the TCAS computer were examined and no
indication of TCAS failure was found. Technical expertise was
requested from the manufacturer of the TCAS equipment. Their
evaluation of the event presented two possible scenarios.
`Explanation 1. The reported `descend' advisory was actually a
`reduce descent' advisory that was misunderstood by the crew. A
`reduce descent' would be consistent with the expected TCAS
response per the reported geometry of the aircraft. Because the
advisory was misinterpreted, the rate of descent was increased
rather than decreased until the aircraft was below the TCAS
required 700 feet vertical separation. Thus the TCAS was required
to issue a `climb now' advisory. The lack of a `clear of conflict'
annunciation is explained in the following paragraph.
`Explanation 2. There is a possibility that the intruder
aircraft's (equipped with mode C transponder) altitude report was
not correctly received by the TCAS. There have been instances when
a Mode C reply will not contain all the appropriate pulses in the
message or it transmits pulses that are too narrow for the TCAS to
detect. This could cause differing altitude reports and could
result in multiple unstable tracks at different altitudes for the
same intruder aircraft. This being the case, the TCAS could have
issued a `descend' advisory for the intruder because it appeared
(due to erroneous altitude report) that it was actually above its
own aircraft. If subsequent replies had the correct altitude, the
erroneous track would be dropped by the TCAS and the TCAS would
issue a `climb now' advisory on the track with the correct
altitude.
`The reason that `clear of conflict' was not annunciated can be
attributed to low track firmness of the intruder aircraft. Mode C
equipped aircraft are typically only equipped with a single antenna
mounted on the lower hull. Since the intruder aircraft was below
the 737 aircraft, it is likely that the TCAS was not able to
receive regular replies at close proximity. The TCAS computer unit
will `coast' the track of a previously established intruder file if
it does not receive a valid or reasonable interrogation response.
If the track of the intruder that generated the RA is coasted
during the time of the associated RA, then the `clear of conflict'
is not announced.'
Since the incident, the operator's TCAS software has been
updated to Version 7. The objectives of the Version 7 update were
to further increase the safety benefits of TCAS, make TCAS more
compatible with the procedures used by ATC and to address
operational concerns identified by pilots operating the older
versions of TCAS. Improvements to the aural annunciations included
a change from `reduce descent, reduce descent' to `adjust vertical
speed, adjust'.
