ACAS II operations

in the European RVSM environment

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 1
 Introduction

Training package on ACAS II performances in the RVSM operations:

A brochure detailing the interaction between ACAS II and RVSM
A quiz to verify that the basic knowledge is acquired
This set of slides as a support for academic training

Plan of the presentation:

Background
TCAS II description
Expected TCAS II alerts in RVSM airspace
Common encounters in RVSM operations in Europe
Operational implication

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 2
 Background

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 3
 ACAS II mandate in Europe (1/2)

Two steps:
1st January 2000: all civil fixed-wing turbine-engine aircraft having a maximum
take-off mass exceeding 15,000 kg, or a maximum approved passenger seating
configuration of more than 30 will be required to be equipped with ACAS II
1st January 2005: all civil fixed-wing turbine-engine aircraft having a maximum
take-off mass exceeding 5,700 kg, or a maximum approved passenger seating
configuration of more than 19 will be required to be equipped with ACAS II

ACAS II can trigger two types of advisories:

Traffic Advisories (TAs):
To prepare the crew for a possible RA and to help in the visual search

Resolution Advisories (RAs):

Avoidance manoeuvres in the vertical plane recommended to the pilot

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 4
 ACAS II mandate in Europe (2/2)

TCAS II version 6.04a:

Is not fully compliant with the ICAO ACAS II SARPs
Is not operationally compatible with RVSM

TCAS II version 7.0:

Was developed to improve the general TCAS II performances
Addresses the issue of TCAS II version 6.04a operational incompatibility with
RVSM

Practical implementation issues (supply, installation, certification)

temporary exemptions until 30 September 2001

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 5
 RVSM implementation (1/2)

Principle: reduction of the separation to 1000 ft from FL290 to FL410

Goal: increased airspace capacity and flight efficiency
Key dates:
March 1997: first implementation in the North Atlantic Region (NAT)
February 2000: implementation in the Pacific Region
January 2002: implementation in Europe

Early introduction in Germany/Austria (tactical), Ireland/UK (strategic)

ACAS II mandatory carriage and RVSM implementation are not linked
ACAS II is not, itself, a prerequisite for RVSM

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 6
 RVSM implementation (2/2)

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 7
 Experience of TCAS II in NAT RVSM

RVSM implemented in NAT since 27 March 1997

Aircraft equipped with TCAS II version 6.04a have flown in this airspace
since this date

A preliminary study had highlighted that TCAS II version 6.04a was not
adapted to RVSM and anticipated some operational issues

Operational feedback:
Many undesirable long duration TAs (for aircraft flying on adjacent flight levels
with similar speeds)
A few undesirable RAs (mainly due to turbulence or imperfect altitude keeping)

No approved operational procedure to avoid long duration advisories

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 8
 TCAS II description

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 9
 Principles (1/2)

System designed to prevent mid-air collisions and near mid-air collisions

Autonomous and independent of any aircraft navigation equipment and
ground systems

TCAS II is based on two main concepts:

time-to-go to the Closest Point of Approach (CPA); modified according to
altitude bands (sensitivity level)

In a one-second cycle, TCAS II:

Monitors other aircraft by interrogating their transponder
Selects an advisory when a range test and an altitude test are satisfied
Presents the information to the pilot

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 10
 Principles (2/2)

RAs are co-ordinated between TCAS II equipped aircraft

Pilots should follow their RAs as accurately as possible

Closest Point of
Approach (CPA)
Climb, Climb
Descend, Descend
Standard
manoeuvre

Prohibited Standard
manuvre manoeuvre

A manoeuvre in the opposite sense to the RA is hazardous

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 11
 Advisory triggering: range test (1/2)

TCAS II computes the time-to-go to the CPA

Based on the closure rate and the distance between aircraft

TCAS
Intruder

< 48 s (TA)
< 35 s (RA)

If this time is lower than the set threshold, the range test is satisfied

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 12
 Advisory triggering: range test (2/2)

Specific case of very low closure rates between aircraft:

Intruder

(very low closure rate)

TCAS
1.3 NM (TA)
1.1 NM (RA)

If the distance between aircraft is lower than the set protection distance,
the range test is satisfied

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 13
 Advisory triggering: altitude test (1/2)

TCAS II computes the time to reach co-altitude

Based on the relative altitude between aircraft and the relative vertical speed

TCAS

Intruder

If this time is lower than the set threshold, the altitude test is satisfied

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 14
 Advisory triggering: altitude test (2/2)

Specific case of level aircraft:

No advisory

850 ft (TA)
700 ft (RA)
TCAS
850 ft (TA)
700 ft (RA)
Advisory triggering

If the relative altitude between aircraft is lower than the set threshold, the
altitude test is satisfied

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 15
 Improvements of TCAS II version 7.0

Modification of altitude band boundaries: FL200-FL300 FL200-FL420

TA RA

Time Altitude Time Altitude Target vertical

threshold threshold threshold threshold miss distance

TCAS II v6.04a 48 s 1200 ft 35 s 800 ft 700 ft

TCAS II v7.0 48 s 850 ft 35 s 700 ft 600 ft

Additional improvements:
Decrease of the reduced time threshold for level aircraft (30 s 25 s)
Introduction of a Miss-Distance Filtering to filter RAs in the horizontal plane
Fewer RAs that go against the aircraft vertical trajectory
Triggering of weakening RAs to reduce deviations in very slow divergence
Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 16
 Expected TCAS II advisories
in RVSM airspace

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 17
 The ACASA project

Airborne Collision Avoidance System Analysis

EUROCONTROL co-ordinated project
Investigates several areas related to TCAS II operations in Europe
Includes a study of ACAS/RVSM interaction in Europe:
Objective: investigate technical and potential operational issues
Method: use of multiple sources of data
Modified radar data
Real-time and fast-time simulations
Automatic and non-automatic artificial encounters
Comparison between current situation in the FL250-FL290 altitude band and
the forecast situation above FL290

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 18
 Traffic Advisories

Increased number of TAs in comparison with CVSM but lower

number than in the FL250-FL290 altitude band

Expected TA rate with TCAS II version 7.0:

1 TA every 20 flight hours in RVSM
1 TA every 60 flight hours in CVSM
1 TA every 5 flight hours in the FL250-FL290 altitude band

Expected TA rate with TCAS II version 6.04a:

1 TA every flight hour in RVSM (operational issue for the pilots)

Repetitive TAs are a potential operational issue for pilots

Long duration TAs are not expected to be an issue
Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 19
 Resolution Advisories

Increased number of RAs in comparison with CVSM but lower

number than in the FL250-FL290 altitude band

Expected RA rate with TCAS II version 7.0:

1 RA every 330 flight hours in RVSM
1 RA every 1000 flight hours in CVSM
1 RA every 70 flight hours in the FL250-FL290 altitude band

Expected RA rate with TCAS II version 6.04a:

1 RA every 165 flight hours in RVSM (high number of TAs without RAs)

Undesirable RAs mainly triggered during 1000 ft level-off encounters but

in similar proportion than in the FL250-FL290 altitude band

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 20
 Concluding Remarks

TCAS II version 7.0 is compatible with RVSM:

Increased number of advisories in comparison in CVSM
But lower number than between FL250 and FL290

TCAS II version 6.04a is not operationally compatible with RVSM

Mixed equipage (low percentage of TCAS II version 6.04a):
No expected significant implication for controllers
Operational issue for the pilots of TCAS II version 6.04a equipped aircraft

No TCAS II safety issues related to the implementation of RVSM

TCAS II efficiency is maintained in RVSM airspace

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 21
 Common encounters for RVSM operations in
Europe

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 22
 Standard RVSM vertical separation (1/2)

Both aircraft are level and separated by 1000 ft

Intruder

FL340

1,000 ft no TA
850 ft

FL330

TCAS

TCAS II version 7.0:

Altitude thresholds: 850 ft for TAs and 700 ft for RAs
The altitude test is not satisfied No advisory

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 23
 Standard RVSM vertical separation (2/2)

TCAS II version 6.04a:

Altitude thresholds: 1200 ft for TAs and 800 ft for RAs
The altitude test for TAs is satisfied Triggering of TAs

Concluding remarks:
TCAS II version 7.0 is compatible with 1000 ft vertical separation
Pilots of TCAS II version 6.04a equipped aircraft will experience a large
number of TAs
If a TA is triggered,
Pilots shall comply with the airline operational instructions
Standard pilots behaviour is expected
No controllers action since TAs should not be notified by pilots

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 24
 Vertical offset (1/3)

Two level aircraft at adjacent flight levels have a vertical offset

FL340

> 850 ft Nothing

< 850 ft TA
< 700 ft RA

FL330

Considerations about altitude:

RVSM approved aircraft can have a 65 ft maximum vertical offset
Aircraft report their altitude by either 25 ft or 100 ft quanta
TCAS II can overestimate vertical offsets (e.g. 75 ft or 100 ft instead of 65 ft)

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 25
 Vertical offset (2/3)

With a 100 ft quantization:

The intruder is seen with a 100 ft offset if the real offset 51 ft
The relative altitude seen by TCAS II can be as low as 835 ft
Combined offset: 100 ft (intruder) + 65 ft (own) = 165 ft

A combined offset of 150 ft is needed to trigger a TA (threshold = 850 ft)

A TA will be triggered if both aircraft have an offset 51 ft
A combined offset of 300 ft is needed to trigger a RA (threshold = 700 ft)
No RA can be triggered

With a 25 ft quantization:
The intruder is seen with a 75 ft offset if the real offset 63 ft
The relative altitude seen by TCAS II cannot be lower than 860 ft
No advisory can be triggered
Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 26
 Vertical offset (3/3)

TCAS II version 6.04a:

Altitude thresholds: 1200 ft for TAs and 800 ft for RAs
Triggering of TAs between level aircraft separated by 1000 ft with or without
vertical offsets

Concluding remarks:
TAs triggered by TCAS II version 7.0 because of vertical offsets are rare
events:
Two offsets 51 ft and
100 ft altitude report quantization (a small proportion of the fleet)

If a TA is triggered,
Pilots shall comply with the airline operational instructions
Standard pilots behaviour is expected

No controllers action since TAs should not be notified by pilots

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 27
 Oscillation (1/2)

Two level aircraft at adjacent flight levels oscillate in the altitude keeping

FL340

> 850 ft Nothing

< 850 ft TA
< 700 ft RA

FL330

RVSM approved aircraft can oscillate with a 65 ft maximum amplitude

This configuration is very similar to the vertical offset:
All calculations for vertical offsets are valid for oscillations

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 28
 Oscillation (2/2)

Concluding remarks:
TAs triggered by TCAS II version 7.0 because of oscillations are rare
events:
Two simultaneous opposite oscillations 51 ft and
100 ft altitude report quantization (a small proportion of the fleet)

TCAS II version 6.04a triggers TAs between level aircraft separated by

1000 ft with or without oscillations
When a TA is triggered,
Pilots shall comply with the airline operational instructions
Standard pilots behaviour is expected

No controllers action since TAs should not be notified by pilots

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 29
 Turbulence (1/3)

Due to turbulence, an aircraft deviates towards another aircraft level at an

adjacent flight level

FL340

Turbulence
High instantaneous vertical speed
TA or pop-up RA

FL330

TCAS II detects a high vertical closure rate configuration

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 30
 Turbulence (2/3)

Similar behaviour of both aircrafts TCAS II

If the predicted time to co-altitude is
Lower than the time thresholds for TAs and RAs
A pop-up RA is triggered
Lower than the time threshold for TAs only
A TA is triggered

TCAS II version 6.04a:

Triggering of TAs between level aircraft separated by 1000 ft with or without
turbulence
Slightly greater number of pop-up RAs for level aircraft

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 31
 Turbulence (3/3)

Concluding remarks:
These events should be rare in the European airspace
More likely in NAT
When a TA or an RA is triggered,
Pilots shall comply with the airline operational instructions
Normal pilots response is expected

When a controller is informed of a manoeuvre in response to an RA, the

standard procedure shall apply

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 32
 1000 ft level-off encounter (1/4)

One aircraft is levelling off at 1000 ft from another aircraft

FL340
High vertical rate
Short time to co-altitude
TA (and RA) before level-off

FL330

TCAS II has no knowledge of the intent of own and intruder aircraft

Assumes that the current flight profiles will be maintained

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 33
 1000 ft level-off encounter (2/4)

Altitude test based on the time to reach co-altitude

Example: TA: 48 s 1920 ft
2400 fpm
RA: 35 s 1400 ft

FL340

FL330
TA: 48 s 1920 ft
RA: 25 s 1000 ft No RA if correct level off

TCAS II of the descending aircraft triggers a TA and an RA

TCAS II of the level aircraft triggers only a TA

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 34
 1000 ft level-off encounter (3/4)

RAs are more likely if both aircraft are manoeuvring

TCAS II version 7.0 tries to avoid Climb RAs for descending aircraft
(and vice-versa)

A 1000 fpm vertical rate by 1000 ft from the cleared altitude should:
Prevent RAs
Minimise the number of TAs

TCAS II version 6.04a is expected to trigger more RAs than version 7.0:
Larger time threshold for level aircraft (30 s)
More RAs requesting a vertical trajectory change
Many improvements provided by TCAS II version 7.0

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 35
 1000 ft level-off encounter (4/4)

Concluding remarks:
TAs and RAs can be triggered during 1000 ft level-off encounters
These RAs can be necessary if an aircraft does not level-off as required
With a 1000 fpm vertical rate by 1000 ft from the cleared altitude :
no advisory is expected to be triggered with one manoeuvring aircraft
only TAs are expected to be triggered with two manoeuvring aircraft
When a TA or an RA is triggered,
Pilots shall comply with the airline operational instructions
Normal pilots response is expected

When a controller is informed of a manoeuvre in response to an RA, the

standard procedure shall apply

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 36
 Low closure rate encounter (1/3)

Two level aircraft at adjacent flight levels follow the same route. The
trailing aircraft has a slow overtake

Slow overtake:
By itself does not trigger any advisory, but
Induces longer duration advisories when triggered by the previous
configurations
T0: Start of TA T0 + 5 min: End of TA
FL340

480 kts 1.3 NM < 1.3 NM

< 850 ft

FL330
450 kts

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 37
 Low closure rate encounter (2/3)

For low closure rates, the range test is based on the protection distance:
1.3 NM for TAs
1.1 NM for RAs

Advisories last about the time that the aircraft remains within the
protection distance

TCAS II includes mechanisms for an early termination of advisories:

TAs and RAs end:
When the aircraft are diverging
Even if they are still slightly within the protection distance

Possible secondary short duration advisories

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 38
 Low closure rate encounter (3/3)

TCAS II version 6.04a:

Higher frequency because of the greater number of TAs
No weakening RAs larger deviations

Concluding remarks:
These events should be rare in the European airspace
More likely in NAT

When a TA or an RA is triggered,
Pilots shall comply with the airline operational instructions
Normal pilots response is expected

When a controller is informed of a manoeuvre in response to an RA, the

standard procedure shall apply

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 39
 Operational implication

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 40
 Controllers (1/2)

Increased number of TCAS II advisories in comparison with CVSM

But lower number than between FL250 and FL290
No effect on RVSM operations

TAs should not be notified by pilots to controllers

No standardised phraseology to report TAs

When a pilot reports a manoeuvre induced by an RA, the controller:

Shall not attempt to modify the aircraft flight path but
Shall provide traffic information as appropriate (PANS-RAC Doc. 4444)

The controller is not responsible for the provision of separation for

an aircraft, which is deviating from a clearance in response to an RA

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 41
 Controllers (2/2)

Increased number of TAs in comparison with CVSM :

Possible increase of the number of traffic information to provide because of
pilots requests

Increased number of RAs in comparison with CVSM :

Increased number of pilots response to RAs, but:
Only a subset inducing a deviation from the clearance with TCAS II version 7.0
Lower number than in the FL250-290 altitude band

Deviations should be limited when pilots follow properly the RAs

Induced encounters with a third aircraft expected to be unlikely
TCAS II includes a Multiple Threat Encounter logic

Provision of traffic information if practical during encounters that can result in

RAs (e.g. 1000 ft level-off)

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 42
 Pilots (1/2)

Increased number of TCAS II advisories:

Expected advisories with TCAS II version 7.0:
1 TA every 20 flight hours
Lower than in the FL250-FL290 altitude band
1 RA every 330 flight hours
Expected advisories with TCAS II version 6.04a:
1 TA per flight hour
1 RA every 165 flight hours

Recommendation for 1000 ft level-off encounters:

1000 fpm vertical rate by 1000 ft from the cleared altitude:
No advisory when one aircraft is manoeuvring
Only TAs if both aircraft are manoeuvring

Eurocontrol ACAS Programme ACAS II operations in the European RVSM environment 03/08/01 - v1.0
ACTOR/wp3/SLID/D Slide 43
 Pilots (2/2)

Standard behaviour (cf. PANS OPS Doc. 8168 and JAA TGL 11)
Pilots response is the same whatever the TCAS II version and the airspace
Pilots shall comply with their airline operational instructions
When a TA is triggered, pilots:
Should not request for a traffic information
Shall not manoeuvre in response to TAs only
When an RA is triggered, pilots:
Should follow the RA as accurately as possible
Shall not manoeuvre in the opposite sense of the RA
Shall limit the deviation to the minimum extent necessary to comply with the RAs
Shall promptly return to the initial clearance after the Clear of Conflict

To take a lateral offset to stop a long duration advisory is not an

approved procedure

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ACTOR/wp3/SLID/D Slide 44
 Conclusion

TCAS II is a last resort safety net designed to prevent mid-air collisions

Valid in RVSM as in any other airspace

TCAS II version 7.0 is compatible with RVSM

TCAS II version 6.04a is not operationally compatible with RVSM
The implementation of RVSM shall not modify pilots and controllers
behaviours

Pilots shall not manoeuvre in the opposite sense of the RAs

Maximum safety is obtained through compliance with RAs

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ACTOR/wp3/SLID/D Slide 45