aviation investigation report a09p0096 loss of seperation nav

aviation investigation report a09p0096 loss of seperation nav
24 APRIL 2009
The Transportation Safety Board of Canada (TSB) investigated this occurrence for the purpose
of advancing transportation safety. It is not the function of the Board to assign fault or
determine civil or criminal liability.
Aviation Investigation Report
Loss of Separation
NAV CANADA — Vancouver Area Control Centre
Penticton, British Columbia, 20 nm SW
24 April 2009
Report Number A09P0096
On 24 April 2009 at 1258 Pacific Daylight Time, a Canadian Forces Canadair CL600 aircraft,
(registration CC144618, serial number 5535), operating as CFC3016, was en route from
Vancouver, British Columbia, to Ottawa, Ontario, climbing to flight level (FL) 370. A United
Arab Emirates Boeing 777-200LR aircraft (registration A6-EWA, serial number 35572), operating
as UAE215, was en route from Dubai, United Arab Emirates, to Los Angeles, United States of
America, cruising at FL370. When the aircraft were about 20 nautical miles southwest of
Penticton, British Columbia, the controller received an automated radar conflict alert signal
indicating that, in one minute, the aircraft would lose the required five nautical miles radar
separation at the same altitude. The controller issued instructions for CFC3016 to descend to
FL360 and to turn right 30° and instructed UAE215 to turn left 30°. Both aircraft received traffic
alert and collision avoidance system (TCAS) warnings. UAE215 received a TCAS resolution
advisory and responded to it by climbing. When the aircraft closed to within five nautical miles
horizontally, there was a 500 feet vertical separation between them, rather than the required
1000 feet.
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Other Factual Information
Both aircraft were under the control of the Vancouver Area Control Centre (ACC), Mountain
High Specialty which includes all airspace at flight level (FL) 260 1 and above in the eastern
portion of the Vancouver Flight Information Region.
The sector controller was certified and qualified in accordance with existing regulations. The
controller had been employed at the Vancouver ACC for about 20 years. The occurrence
controller had been off duty for the previous two days and was scheduled to be off duty for the
next two. The controller’s shift began at 1230 2 and the controller had been on duty for 28
minutes prior to the occurrence and in position for 20 minutes. The controller was responsible
for the radar and data positions. At the time of the loss of separation, the controller was
working four aircraft. Traffic was neither heavy nor complex.
Description of Events
The controller recognized the potential crossing track
3 conflict shortly after taking over the position, when
UAE215 came on frequency at 1237. Having scanned
the flight progress strips, the controller noted that
both aircraft were to be at FL370 in the vicinity of the
Princeton (YDC) VOR (very high-frequency
omnidirectional range) with less than ten minutes of
separation. The controller planned on issuing
UAE215 a routing change that would result in a
westerly track and, as such, make it possible to climb
UAE215 to FL380, an altitude appropriate to the
direction of flight. This would resolve the conflict and
the controller thought it unnecessary to annotate the
two flight progress strips with the red W. 4
The actual routing change, however, did not result in
UAE215 achieving a westerly track and the controller
did not issue a climb to UAE215. The controller then
intended to find a more direct route for UAE215 later
but did not execute this plan. The controller’s
attention was then diverted to other traffic in the
sector and the conflict between CFC3016 and UAE215
went unresolved.
At 1248:00, CFC3016 made initial contact with the
sector controller as it was climbing from FL290 to
Conflict Alert System
The conflict alert system (CAS) is a
function of the radar data
processing system that examines
radar tracks for potential conflicting
traffic. Based on three dimensional
predicted positions, tracks are
evaluated to determine if
separation standards will be
violated within a specified time.
Alerts are generated and sent to the
displays in two stages. Sixty
seconds before loss of separation is
predicted, a traffic alert is
generated. A conflict alert is
generated after separation is lost.
Controllers use the CAS to identify
potential conflicts where a loss of
separation may occur. The system
for enroute radar airspace above
14,000 feet warns a controller when
a potential conflict is developing.
Source: NAV CANADA Conflict
Alert DSC Manual, version 3.1, and
Backgrounder August 2008.
Approximately 26,000 feet.
All times are Pacific Daylight Time (coordinated universal time minus seven hours).
The Air Traffic Control Manual of Operations (ATC MANOPS) defines crossing track as: “a term
used in the application of separation, indicating tracks that converge or diverge at an angle of
45 degrees to 135 degrees inclusive.”
A red W (for warning) on appropriate flight progress strips is used to identify, amongst other
things, any confliction with other aircraft.
At 1258:13, the controller received an automated radar conflict alert (CA) signal of traffic (TFC)
on the data blocks of both aircraft accompanied by an audible tone and a change in the shape of
the present position symbol of the aircraft to a yellow cartwheel symbol. This indicated that,
without controller intervention, the required five miles radar separation for aircraft flying at the
same altitude would be lost in 60 seconds.
At 1258:22, the controller instructed CFC3016, which had reached FL370, “for traffic, descend
now, flight level 360.” The crew acknowledged the instruction, set the altitude selector and
adjusted the vertical speed to 1000 feet per minute as per Canadian Forces procedures. At
1258:31, the controller then instructed CFC3016 to turn right, and this instruction was also
acknowledged by the crew.
At 1258:39, the controller instructed UAE215 to turn left. No acknowledgement from the crew
was received and so, at 1258:45, the controller re-issued the turn. At 1258:51 UAE215 began the
process of changing heading. (See Appendix A — Aircraft Flight Paths).
At 1258:56, the controller instructed CFC3016 to expedite
its descent. CFC3016 responded that its descent rate was
almost 1000 feet per minute and that it had received a
traffic alert and collision avoidance system (TCAS) traffic
advisory (TA). The crew of UAE215 attempted to
acknowledge the controller’s instruction to turn during
these communications and was not heard by the
At 1259:02, the TCAS on UAE215 issued a resolution
advisory (RA) to climb. The crew responded and, at
1259:09, advised the controller. At 1259:14, the controller
again instructed UAE215 to turn left.
At 1259:33, vertical spacing of 1000 feet had been
achieved between the two aircraft at which point the
horizontal spacing was 1.8 nm. The minimum required
spacing was 1000 feet vertically or 5 nm horizontally.
The turns issued to the aircraft resulted in UAE215
directly overflying CFC3016 (see Appendix A — Aircraft
Flight Paths).
The TCAS is an airborne system
that alerts flight crews to other
aircraft in their vicinity that pose
a potential collision hazard. TCAS
equipment interrogates other
aircraft transponders to
determine their range, bearing
and altitude. The TCAS generates
a traffic advisory (TA) when
another aircraft is approximately
forty seconds from the point of
closest approach. A resolution
advisory (RA), in the form of a
vertical manoeuvre, is generated
when the other aircraft is
approximately 25 seconds from
the point of closest approach. The
RA provides a vertical restriction
or manoeuvre to maintain or
increase separation.
On the topic of descent, subsection paragraph 8.5.1 (a) of Transport Canada’s Aeronautical
Information Manual (TP14371) suggests the following:
When an altitude clearance is issued, the pilot should begin climb or
descent promptly on acknowledgement of the clearance. The climb or
descent should be made at an optimum rate consistent with the operating
characteristics of the aircraft. If the above is not the case, or if it becomes
necessary to stop the climb or descent, the pilot should advise ATC of the
interruption or the delay in departing an altitude.
-4Recorded radar data showed that CFC3016 began its descent 36 seconds after the descent
instruction was given.
It took three transmissions over a 35-second period for the controller to receive an
acknowledgement from UAE215, by which time the flight had begun to manoeuvre in response
to the TCAS RA. Although UAE215 acknowledged the second turn instruction the controller
did not receive this transmission as it was blocked.
Flight Progress Strips
Flight progress information is recorded on flight progress strips—in paper or electronic formats.
In this instance, essential flight plan information was printed on paper strips that are annotated
by the controller (see Figure 1). Flight progress strips serve many purposes:
as a record of clearances and instructions;
as a back-up in the event of some failure that would invoke the use of procedural
separation techniques;
as an aide-memoire; and
as a planning and decision-making tool.
To alleviate the problem of different handwriting, specific symbology is used; an entire section
of the ATC MANOPS is devoted to strip writing. 5
Figure 1. Flight Progress Strips showing flight levels
In part, ATC MANOPS section 902.5 required, at the time of the occurrence, that a red warning
indicator be affixed to a strip as follows:
post a red “W” on appropriate flight progress strip(s) if corrective action
will be necessary, to identify any confliction with other aircraft; or to
identify any other hazardous or critical situation;
-5post the warning indicator in the box that most clearly indicates the reason
for the warning. As necessary, enter sufficient details (i.e., aircraft
identification, conflict point(s), time(s), etc.) to clearly identify the
circle the altitude in red on the appropriate flight progress strip(s) if an
altitude not
appropriate to the direction of flight is assigned; and
draw a single line through the warning indicator when it is no longer
Strips are displayed on boards under designators. These designators indicate either the actual
geographical relationship of the fixes as viewed on airway charts or the altitudes. Doing so
facilitates conflict recognition. In this instance, the flight progress strips of both flights were
posted under the YDC (Princeton) designator in reference to the aircrafts’ geographical
In terms of board management, section 901.6 (d) of ATC MANOPS provides direction to leave
active strips in a cocked 6 position if other potential situations requiring investigation or further
action exist.
Further, section 901.8 of ATC MANOPS requires a controller to scan the control data board by
performing the following actions:
scan each bay individually rather than looking over the entire board;
check altitude boxes in each bay to verify vertical separation;
check strips to ensure some other form of separation exists if more than one
aircraft is at the same altitude.
Flight progress strips are distributed to appropriate sectors at least 15 minutes prior to transfer
of control to the controller. Shortly after the controller took over the position and UAE215 was
transferred to the control of the Mountain High Specialty, the controller noted from the printed
flight progress strips in the strip bay that both aircraft would be at FL370. The controller had
identified the potential problem between CFC 3016 and UAE 215 which were both cleared to
FL370 and on crossing tracks. Contrary to ATC MANOPS, red Ws had not been posted on the
flight progress strips. It is unknown whether the flight progress strips were cocked.
The cocking of flight progress strips is an essential control technique used to remind
controllers that some type of further action must be performed—ATC MANOPS 901.6, note 1.
Imperative Phraseology
In December 2003, NAV CANADA issued an urgent Air Traffic Services (ATS) Operational
Publication Change concerning an amendment to subsection 507.1 of ATC MANOPS
introducing the adoption of International Civil Aviation Organization (ICAO) phraseology for
safety alerts. This subsection directs controllers to use specified safety alert phraseology when
they are aware that an aircraft is at an altitude which, in their judgment, places it in unsafe
proximity to another aircraft. In 2004, NAV CANADA issued an Air Traffic Services Bulletin –
Squawk 7700 (2004-1) titled Imperative Phraseology, which underscored the importance of using
phraseology that includes both corrective action and traffic information to all aircraft in a loss of
separation occurrence.
When the controller issued descent and turn instructions to CFC3016 and turn instructions to
UAE215, the controller did not use imperative phraseology, nor did the controller provide
specific traffic information or request immediate action. There was nothing in the controller’s
intonation that conveyed a sense of urgency. CFC3016 acknowledged the controller’s
instruction and began the altitude change procedure, but no change in the aircraft’s track or
altitude was observed for 36 seconds. UAE215 acknowledged the controller after the second
turn instruction but the transmission was blocked by another communication. It was not until
35 seconds later that the controller became aware that the crew of UAE215 was following a
TCAS RA to climb.
Since 2005, the TSB has investigated three other occurrences in which the NAV CANADA radar
CA function indicated approaching losses of separation (A08W0151, A07W0072, and
A06C0113). In those occurrences, the controller instructed the flight crews to manoeuvre to
maintain separation. The TSB found, however, that the avoidance instructions were not given
with sufficient urgency to avoid losses of separation.
ATC Radar Conflict Alerts
The goal of the 60-second radar traffic alert is to warn controllers of an impending separation
loss so that it may be resolved ahead of any airborne TCAS advisories which are typically
generated approximately forty seconds ahead of the estimated closest point of approach. This
amount of time was judged by NAV CANADA to be optimal to avoid excessive nuisance alerts
and provide the controller with sufficient time to issue instructions to regain separation or
prevent a collision.
In 2005 the TSB investigated an occurrence (A05W0248) in which the controller, after having
received a radar TFC alert, issued instructions to the flight crews to manoeuvre to avoid a
collision. The TSB found that, in some circumstances, the 60-second warning may not provide
enough time for a controller to issue instructions to prevent a loss of separation.
Controller Response to TCAS RA action
To attempt to maintain and then regain separation, the controller issued both descent and turn
instructions to CFC3016 and turn instructions to UAE215. After UAE215 informed the controller
that it was conducting a TCAS climb, the controller again issued a turn.
-7Subsection 127.2 of the ATC MANOPS, which was current at the time of this incident, stated
that controllers are to:
Provide relevant traffic information and collision avoidance advice as
appropriate to an aircraft under your jurisdiction if you are advised by the
aircraft that it is responding to an ACAS 7/TCAS resolution advisory or
GPWS 8/TAWS 9 warning. Do not issue control instructions that would
contradict an aircraft’s resolution advisory or warning.
Neither CFC3016 nor UAE215 were provided with specific traffic information.
As a result of a separate incident, the ATC MANOPS has since been amended and now states:
If you are advised by an aircraft that it is responding to an ACAS/TCAS
resolution advisory or GPWS/TAWS warning: A. Do not attempt to
modify the aircraft flight path until the pilot reports returning to the terms
of the current ATC clearance or instruction; and B. Provide relevant traffic
and collision avoidance information as appropriate.
The controller identified the conflict between CFC3016 and UAE215 early on. The controller
resolved to change UAE215’s route toward the west and issue a climb to FL380. A solution in
hand, the controller did not think it necessary to mark the flight progress strips with the
required red W. Thus, an important aide-memoire was not used. The controller’s attention
became focused on other traffic in the sector. The conflict went unresolved until the TFC CA
The controller’s solution required two parts: a heading change and an altitude change. The
altitude change was dependent upon identifying that a westbound track change had been
achieved. A two-part resolution to the conflict requires a means of remembering that a second
part must be performed in order to resolve the conflict. The use of the red W serves to remind
the controller that the conflict has yet to be resolved. As the controller did not use this aidememoire, the altitude change was not completed.
In circumstances where time is critical, the use of imperative phraseology is intended to alert
flight crews of the need for immediate action. The controller did not use imperative
phraseology, nor was there anything in the controller’s voice that conveyed a sense of urgency.
Consequently, the air crews reacted in a non-urgent way; CFC3016 did not begin descent for
36 seconds.
The controller cleared CFC3016 to turn right, and UAE215 to turn left. The geometry of this
crossing track conflict was such that the turns resulted in one aircraft flying directly overtop of
the other. The turns issued by the controller were ineffective in establishing the minimum
lateral radar separation. Altitude spacing was achieved by the controller’s initial descent
instruction to CFC3016 and UAE215’s response to the TCAS RA.
Airborne Collision Avoidance System.
Ground Proximity Warning System.
Terrain Awareness and Warning System.
The TFC CA sounded as designed. The 60-second warning may not provide enough time,
however, to resolve conflicts if:
 time is lost in decision-making;
 time is lost in issuing instructions; or
 time is lost by air crews who do not act in a timely fashion.
Though manoeuvring instructions were given to CFC3016 within nine seconds of the TFC alert,
the aircraft did not begin descent quickly enough to avoid a loss of separation. The two aircraft
closed to the point where UAE215’s TCAS RA was generated, which the flight crew was then
compelled to follow.
The controller continued to issue turn instructions to UAE215 after being advised that it was
responding to a TCAS advisory. No specific traffic information was communicated to either
Findings as to Causes and Contributing Factors
1. The controller recognized the conflict between CFC3016 and UAE215 when taking
control of UAE215 but did not use a required aide-memoire and take sufficient action
early on to maintain separation.
2. The controller did not use imperative phraseology when he instructed the involved
aircraft to commence avoiding action; this likely delayed the reaction of the crews to the
instructions and resulted in a prolonged period of risk of collision.
3. The turns issued to the two aircraft did not resolve the loss of separation.
Other Finding
1. In some circumstances, the 60-second traffic conflict alert (TFC CA) may not provide
enough time to resolve conflicts.
Safety Action Taken
The TSB issued Aviation Safety Letter A09P0096-D1-A1 Adequacy of Automated Radar Conflict
Alert Warning Times on 25 January 2010 to NAV CANADA, stating that it may wish to examine
the feasibility of extending the 60-second warning time in order to provide additional time for
controllers to issue instructions to maintain the required separation and to provide flight crews
with additional time to manoeuvre, reducing the likelihood of a traffic alert and collision
avoidance system resolution advisory (TCAS RA) being issued.
-9In its reponse dated 01 March 2010, NAV CANADA expressed concern that extending the
warning time will result in nuisance alarms, particularly in situations where aircraft are
climbing and/or descending and will level off prior to a conflict, but not in time to prevent an
alarm. NAV CANADA indicated that the implementation of the Canadian Automated Air
Traffic Control System (CAATS) will eventually resolve this problem of nuisance alerts prior to
level-off. NAV CANADA undertook to consider the feasibility of extended warning times once
CAATS has been modified to take into account aircraft that will be levelling off prior to any
NAV CANADA has begun a review of the requirement for mandatory use of safety alert
phraseology associated with the Conflict Alert feature. In the spring of 2010, information was
gathered on Conflict Alert software as well as International Civil Aviation Organization,
Federal Aviation Administration and other states’ air traffic control (ATC) procedures. As a
result, a draft ATC Directive has been prepared on the mandatory use of safety alert
phraseology in Canadian airspace. This directive will be studied by a working group composed
of ATC operational subject matter experts that will meet in Ottawa during the fall of 2010. The
working group will proceed in accordance with the NAV CANADA Safety Management
This report concludes the Transportation Safety Board’s investigation into this occurrence. Consequently,
the Board authorized the release of this report on 26 October 2010.
Visit the Transportation Safety Board’s Web site (www.tsb.gc.ca) for information about the
Transportation Safety Board and its products and services. There you will also find links to other safety
organizations and related sites.
- 10 -
Appendix A —Aircraft Flight Paths
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