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IBAC
Bulletin B 02-3
Subject: ACAS (TCAS) Alert Bulletin
Over the past several months,
there have been several TCAS events in which the improper response to a TCAS
RA resulted in the vertical miss distance between two aircraft being less
than that which TCAS is designed to provide.
In addition, the final report of a near mid-air collision between two
TCAS-equipped aircraft in Japan in January 2001 indicates that one of the
contributing factors to this event was the improper response of one pilot
to the RA received during the encounter.
With
this in mind, it is essential for all operators to review the previously published
guidance related to responding to RAs.
In
this regard, please refer to the attachment hereto.
The
International Civil Aviation Organization (ICAO) currently has this subject
under consideration with a view to issuing a related State Letter. A copy
of the State Letter will be the subject of an ensuing Bulletin.
Prepared
by:
Peter
R. Ingleton
IBAC Director, ICAO Liaison.
28
August 2002
Attachment
TCAS
Transition Program (TTP) Industry Alert Bulletin
August 12, 2002
Background
Over the past several months, there
have been several TCAS events in which the improper response to a TCAS RA
resulted in the vertical miss distance between two aircraft being less than
that which TCAS is designed to provide. In
addition, the final report of a near mid-air collision between two TCAS-equipped
aircraft in Japan in January 2001 indicates that one of the contributing
factors to this event was the improper response of one pilot to the RA
received during the encounter.
With
this in mind, the TTP believes it is essential for all operators to review the
previously published guidance related to responding to RAs.
The following material is taken from the Federal Aviation
Administration Advisory Circular (AC) 120-55B, entitled AIR CARRIER OPERATIONAL APPROVAL AND USE OF TCAS II.
Although the title refers to air carriers, the guidance contained in
the AC is applicable to all TCAS operators.
1.
Pilot Responsibilities. TCAS is intended to serve as a backup to visual
collision avoidance, application of right-of-way rules, and air traffic
separation service. For
TCAS to work as designed, immediate and correct crew response to TCAS
advisories is essential. Delayed
crew response or reluctance of a flightcrew to adjust the aircraft’s flight
path as advised by TCAS due to Air Traffic Control (ATC) clearance provisions,
fear of later FAA scrutiny, or other factors could significantly decrease or
negate the protection afforded by TCAS. Flightcrews
are expected to respond to TCAS RAs in accordance with the following
guidelines:
(a) When
an RA occurs, the pilot flying (PF) should respond immediately by direct
attention to RA displays and maneuver as indicated, unless doing so would
jeopardize the safe operation of the flight or the flightcrew can assure
separation with the help of definitive visual acquisition of the aircraft
causing the RA. By not responding to an RA, the flightcrew effectively takes
responsibility for achieving safe separation.
In so choosing, the following cautions should be considered:
(i) The
traffic may also be equipped with TCAS and it may maneuver in response to an
RA that has been coordinated with your own TCAS.
Note: If the traffic is
TCAS-equipped, its RA will be in the opposite direction to that which you
receive.
(ii) The
traffic acquired visually may not be the same traffic causing the RA.
(iii) Visual
perception of the encounter may be misleading.
Unless it is unequivocally clear that the target acquired visually is
the one generating the RA and there are no complicating circumstances, the
pilot’s instinctive reaction should always be to respond to RAs in the
direction and to the degree displayed.
(b) Satisfy
RAs by disconnecting the autopilot, if necessary, using prompt, positive
control inputs in the direction and with the magnitude TCAS advises.
To achieve the required vertical rate (normally 1,500 feet per minute
(fpm) climb or descent), first adjust the aircraft’s pitch using the
suggested guidelines shown in the table below.
Then refer to the vertical speed indicator (VSI) and make all necessary
pitch adjustments to place the VSI in the green arc. On aircraft with pitch guidance for TCAS RA displays, follow
the RA pitch command for initial, increase, and weakening RAs.
|
SPEED |
PITCH
ADJUSTMENT |
|
.80
MACH |
2° |
|
250
KIAS below 10,000 feet |
4° |
|
APPROACH
below 200 KIAS |
5°
to 7° |
(c) For
TCAS to provide safe vertical separation, the initial response is expected
within 5 seconds of when the RA is first displayed.
Vertical speed responses should be made to avoid red arcs or outlined
pitch avoidance areas and, if applicable, to accurately fly to the green arc
or outlined pitch guidance area. Excessive
responses to TCAS RAs are inappropriate and may increase interference with
other traffic and result in additional RAs.
(d) Respond
immediately to any “increase” or “reversal” RA maneuver advisories. Initial vertical speed response to an increase or reversal RA
is expected by TCAS within 2 1/2 seconds after issuance of the advisory.
Again, avoid red arcs or outlined pitch avoidance areas and fly to the
green arc or outlined pitch guidance area.
(e) If
a TCAS RA response requires deviation from an ATC clearance, expeditiously
return to the current ATC clearance when the traffic conflict is resolved, the
TCAS “clear of conflict” message is heard, or follow any subsequent change
to clearance as advised by ATC. In
responding to a TCAS RA that directs a deviation from assigned altitude,
communicate with ATC as soon as practicable after responding to the RA.
AC 20-155B, Appendix 5 recommends that the following phraseology be
used for notifying that an RA is in progress:
“New York Center, Universal 602, TCAS
Climb (Descent)”
(f) If
a TCAS RA maneuver is contrary to other critical cockpit warnings, then those
other critical warnings are respected as defined by TCAS certification and
training (that is, responses to stall warning, windshear, and Ground Proximity
Warning System (GPWS) take precedence over a TCAS RA, particularly when the
aircraft is less than 2,500 feet Above Ground Level (AGL)).
(g) The
potential consequences of improperly maneuvering the aircraft in response to
an RA include:
(i)
An
aircraft seen visually may not necessarily be the aircraft causing the RA or
may not be the only aircraft that TCAS considers a threat.
(ii)
It
is difficult to visually determine the vertical displacement of other aircraft
especially when ground reference information is unreliable or at cruise
altitudes where the earth’s horizon is obscured.
Therefore, disregarding RA information and maneuvering vertically based
solely on visual acquisition may result in a loss of safe separation.
(iii)
ATC
may not know when TCAS issues RAs. It
is possible for ATC to unknowingly issue instructions that are contrary to the
TCAS RA indications. Safe
vertical separation may be lost during TCAS coordination when one aircraft
maneuvers opposite the vertical direction indicated by TCAS and the other
aircraft maneuvers as indicated by TCAS.
Accordingly, during an RA, do not
maneuver contrary to the RA based solely upon ATC instructions.
(iv)
Maneuvering in a direction opposite to a
TCAS RA will negate safe separation as a TCAS-equipped intruder follows its
RA, which has been coordinated to be in the opposite direction.
2.
ATC Responsibilities. ATC responsibilities relating to TCAS are contained
in FAA Order 7110.65, Air Traffic Control.
Excerpts of information contained in this Order are highlighted below.
(a)
Controllers
will not knowingly issue instructions that are contrary to RA guidance when
they are aware that a TCAS maneuver is in progress.
When an aircraft deviates from its clearance in response to an RA, ATC
is still responsible for providing assistance to the deviating aircraft as
requested until:
(i)
The pilot informs ATC that the RA conflict has
cleared; and
(ii)
The aircraft has returned to the previously assigned
altitude; or
(iii)
Alternate ATC instructions have been issued and
acknowledged.
3.
ICAO Guidance. The FAA guidance contained in AC 120-55B is similar
to the guidance on ACAS provided by various ICAO documentation.
The following ICAO documents contain references to ACAS and procedures
for responding to ACAS advisories.
1.
Annex 2 — Rules of the Air, Chapter 3, paragraph 3.2.2,
Right-of-way.
2.
Procedures for Air Navigation Services — Air Traffic Management
(Doc 4444), Chapter 15, Procedures Related to Emergencies, Communication
Failure, and Contingencies, Section 15.6.3, Procedures in regard to aircraft
equipped with airborne collision avoidance systems (ACAS).
3.
Procedures for Air Navigation Services — Aircraft Operations, Volume
I — Flight Procedures (Doc 8168, PANS-OPS), Part VIII, Secondary
Surveillance Radar (SSR) Transponder Operating Procedures, Chapter 3,
Operation of ACAS equipment.
4.
Annex 10 — Aeronautical Telecommunications, Volume IV —
Surveillance Radar and Collision Avoidance Systems, Attachment A, paragraph
3.5.8.10.3, Contrary pilot response.
5.
State letter AN 7/1.3.72-97/77, dated 8 August 1997, Attachment E,
Proposed ACAS performance -based training objectives, page E-10, paragraph 12,
which provides guidance in relation to pilot action when ATC instructions to
maneuver conflict with an ACAS resolution advisory.
4.
Summary. The
safety benefits provided by TCAS are directly dependent on a pilot’s correct
response to an RA. The pilot’s
instinctive reaction to an RA should always be to respond to the RA in the
direction and to the degree displayed.
5.
Additional Information. Questions
related to the information contained in this alert bulletin should be
addressed to:
Dan Tillotson
ARINC
1 215 493 8016