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Airbus AP/FD
TCAS mode:
a new step towards safety Improvement

By: Paule BOTARGUES 2 Current TCAS

Engineer, Automatic Flight Systems,
Engineering interface and
procedures
Traffic Advisory (TA)
1 Introduction When the TCAS considers an intruder to be a
potential threat, it generates a TA.
This advisory aims at alerting crews to the intruder’s
The Traffic Collision Avoidance System (TCAS) has position. TAs are indicated to the crew by:
been introduced to reduce the risks associated • An aural message, “Traffic, Traffic”
with mid-air collision threats. Today this safety goal • Specific amber cues on the Navigation Display,
has globally been reached. which highlight the intruder’s position.

However, surprise and stress created by TCAS No specific action is expected from the crew
Resolution Advisories may lead to non-optimum following a TA.
crew response, resulting in a lack of proper
communication with ATC, undue aircraft altitude
deviations, injuries in the cabin and the jeopardizing GS 260 TAS 250
of the aircraft’s safety. 260 / 12 12
09
This article will review the current TCAS interface 15
and procedures. It will then present the Auto Pilot/
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Flight Director (AP/FD) TCAS mode function

developed by Airbus, and its numerous operational
18

+06
benefits, which further enhance the pilot interfaces.
03

+70
22

5
00

10 24
33
VOR1 27 ADF2
30

Figure 1: Navigation Display in case of TCAS TA

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Resolution Advisory (RA) • Green / red zones on the Vertical Speed Indicator
If the TCAS considers an intruder to be a real (VSI) specifying the type of maneuver the pilot
collision threat, it generates an RA. has to perform.
In most cases, the TCAS will trigger a Traffic In order to fly the required maneuver, the pilot
Advisory before a Resolution Advisory. selects both the Auto Pilot (AP) and Flight Directors
RAs are indicated to the pilots by: (FD) to OFF, and adjusts the pitch attitude of the
• An aural message specifying the type of vertical aircraft as required, so as to reach the proper
order (Climb, Descent, Monitor, Adjust…) Vertical Speed (V/S). This unfamiliar flying technique
• Specific red cues on the Navigation Display increases the stress level already induced by the
materializing the intruder triggering of the Resolution Advisory.

Red area indicating

the forbidden vertical
speed domain

"Adjust V/S, Adjust"

Figure 2: TCAS RA HMI without AP/FD TCAS mode

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3 AP/FD TCAS Note: At any time, the crew keeps the possibility
mode concept to disconnect the AP and the FDs,
and is capable to respond manually
Airbus has carried out an in-depth analysis of: to a TCAS RA by flying according to the
• Needs expressed by airline pilots “conventional” TCAS procedure (i.e. flying
• Human factor studies linked to the TCAS system the vertical speed out of the red band).
• Recommendations given by airworthiness
authorities. The AP/FD TCAS mode will behave differently
depending on the kind of alert triggered by the
This resulted in the development of a new TCAS:
concept called AP/FD TCAS guidance, via the • In case of Traffic Advisory (TA), the AP/FD TCAS
Auto Flight System (AFS), to support pilots mode is automatically armed, in order to bring
flying TCAS RAs. crew awareness on the TCAS mode engagement
if the TA turned into an RA.
The AP/FD TCAS mode is a vertical guidance
mode built into the Auto Flight computer. It controls • In case of Corrective RA (“CLIMB”, “DESCEND”,
the vertical speed (V/S) of the aircraft on a vertical “ADJUST”, etc aural alerts), the aircraft vertical
speed target adapted to each RA, which is speed is initially within the red VSI zone. The
acquired from TCAS. requirement is to fly out of this red zone to reach
With the Auto Pilot engaged, it allows the pilot to the boundary of the red / green V/S zone.
fly the TCAS RA maneuvre automatically. Consequently:
With the Auto Pilot disengaged, the pilot can fly – The TCAS longitudinal mode engages. It
the TCAS RA maneuvre manually, by following the ensures a vertical guidance to a vertical speed
TCAS Flight Director pitch bar guidance. target equal to the red / green boundary value
It has to be considered as an add-on to the (to minimize altitude deviation) ± 200 ft/min
existing TCAS features (traffic on Navigation within the green vertical speed zone, with a
Display, aural alerts, vertical speed green / red pitch authority increased to 0.3g
zones materializing the RA on the Vertical Speed – All previously armed longitudinal modes are
Indicator). automatically disarmed, except the altitude
capture mode (ALT*) in case of an “ADJUST
In case of a TCAS RA, the AP/FD TCAS mode V/S” alert. This prevents an undue altitude
automatically triggers the following: excursion: indeed, in this type of RA, reaching
• If both AP and FDs are engaged, the AP/FD 0 ft/min is always safe, as this value is never
vertical mode reverts to TCAS mode, which within the red vertical speed zone. Therefore,
provides the necessary guidance for the Auto if the altitude capture conditions are met, the
Pilot to automatically fly the TCAS maneuver TCAS mode will safely allow to capture the
• If the AP is disengaged and FDs are engaged, targeted flight level
the TCAS mode automatically engages as the – The Auto Thrust engages in speed control
new FD guidance. The FD pitch bar provides an mode (SPEED/MACH) to ensure a safe speed
unambiguous order to the pilot, who simply has during the maneuver
to centre the pitch bar, to bring the V/S of the – The current engaged lateral mode remains
aircraft on the V/S target (green zone) unchanged.
• If both AP and FDs are OFF, the FD bars will
automatically reappear with TCAS mode guiding
as above.
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SPEED TCAS NAV AP1

1FD2
ATHR
1FD2
A /THR

TCAS

"Adjust V/S, Adjust"

Figure 3: PFD upon a Corrective TCAS RA with AP/FD TCAS mode

• In case of Preventive RA (e.g.” MONITOR V/S” – The current engaged lateral mode remains
aural alert), the aircraft vertical speed is initially unchanged.
out of the red VSI zone. The requirement is to
maintain the current vertical speed • Once Clear of Conflict, vertical navigation is
Consequently: resumed as follows :
– The TCAS longitudinal mode engages to maintain – The AP/FD longitudinal mode reverts to
the current safe aircraft vertical speed target the “vertical speed” (V/S) mode, with a
– All previously armed longitudinal modes are smooth vertical speed target towards the
automatically disarmed, except the altitude FCU target altitude. The ALT mode is armed
capture mode (ALT*). Indeed, as for an to reach the FCU target altitude (ATC
“ADJUST V/S” RA, levelling-off during a cleared altitude)
Preventive RA will always maintain the vertical – If an altitude capture occurred in the course
speed outside of the red area. So if the altitude of a TCAS RA event, once Clear of Conflict,
capture conditions are met, the TCAS mode the AP/FD longitudinal mode reverts to the
will allow to safely capture the targeted level, altitude capture (ALT*) or to the altitude hold
thus preventing an undue altitude excursion (ALT) mode
– The Auto Thrust engages in speed control – The lateral mode remains unchanged.
mode (SPEED/MACH) to ensure a safe speed
during the maneuver
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CLEAR OF
ADJUST V/S CONFLICT

TRAFFIC CLIMB !
TRAFFIC CLIMB !

ALT CRZ ALT CRZ TCAS TCAS V/S -1000

ALT
TCAS

0 0 0 0 0 0 0

Figure 4: FMA and VSI during a TCAS sequence with AP/FD TCAS mode

FL 270

Safety margin increased CLEAR OF

CONFLICT

FL 260 (selected level)

ADJUST V/S ALT*

TCAS
ALT

Sub-mode ALT* 0
TCAS
TRAFFIC ALT
TRAFFIC

CLB
ALT
TCAS

CLB
ALT TCAS mode initially reduces the V/S and
then captures the ALT level selected

Figure 5: Safe altitude capture in TCAS mode

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4 Operational The AP/FD TCAS mode was demonstrated to a

large panel of pilots from various airlines, and was
benefits perceived by them as a very simple and intuitive
solution. It was deemed to be consistent with the
The operational benefits of the AP/FD TCAS mode Airbus cockpit philosophy and Auto Flight system.
solution are numerous; the system addresses
most of the concerns raised by in-line experience All agree that the AP/FD TCAS mode represents
feedbacks: a safety improvement.
• It provides an unambiguous flying order to the
pilot
• The flying order is adjusted to the severity of the
RA; it thus reduces the risks of overreaction 5 Certification schedule
by the crew, minimizes the deviations from
trajectories initially cleared by ATC, and adapts The certification of the AP/FD TCAS mode function
the load factor of the maneuvre is expected:
• The availability of the AP/FD TCAS mode makes • On the A380: by May 2009
it possible to define simple procedures for the • On the A320 family:
aircrews, eliminating any disruption in their flying – with CFM engines, by end 2009
technique: the procedure is simply to monitor – with IAE engines, by July 2010
the AP, or to manually fly the FD bars, when the • On the A330/A340, depending on the aircraft
TCAS mode engages, while monitoring the VSI. type, from the beginning of 2010 (A330 PW/RR)
to the end of 2011 (A340-500/600).
By reducing the crews’ workload and stress
level, the AP/FD TCAS mode should therefore The certifcation dates for all required retrofit
significantly reduce: standards are not yet frozen.
• Inappropriate reactions in case of Resolution
Advisory (late, over or opposite reactions)
• Misbehaviours when Clear of Conflict
• Lack of adequate communications with ATC.

Note: For ATC controllers, the AP/FD TCAS

mode is totally transparent in terms of
expected aircraft reactions.