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Appendix 1 to ETSO-C147a - Changes to RTCA/DO-197A, "Minimum Operational Performance Standards for an Active Traffic Alert and Collision Avoidance System I (Active TCAS I)" applicable to Traffic Advisory System (TAS) airborne equipment

Available versions for ERULES-1963177438-9262

ED Decision 2016/029/R found in: CS-ETSO Amdt 17 - European Technical Standard Orders (Oct 2026)

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###### Appendix 1 to ETSO-C147a – Changes to RTCA/DO-197A, “Minimum Operational Performance Standards for an Active Traffic Alert and Collision Avoidance System I (Active TCAS I)” applicable to Traffic Advisory System (TAS) airborne equipment

*ED
Decision
2016/029/R*

*Note: This Appendix changes several sections of
DO-197A that have been modified by DO-197A Change 1. However, the
below changes adopt different requirements than those* *contained
in DO-197A Change 1.*

**1.0 Changes
Applicable to Both Class A and Class B Equipment.**

1.1 Receiver Characteristics.

1.1.1 In-band Acceptance. In lieu of paragraph 2.2.2.1 of RTCA
DO-197A, substitute the following requirement:

Given a valid transponder reply signal in the absence of interference
or overloads, the minimum trigger level (MTL) is defined as the input
power level that results in a 90% ratio of decoded to received
replies.

The MTL over the frequency range of 1,087 to 1,093 MHz shall be no
greater than -70 dBm.

1.1.2 In-band Acceptance. In paragraph 2.4.2.2.1 of RTCA DO-197A,
eliminate the following:

under Intruder Aircraft
eliminate the last line: “Scenario C and D ≥ -78 dBm.

under Test Description Success:,
eliminate the last sentence: For scenarios C and D, the ratio of
correctly decoded intruder replies to total input replies shall not
exceed 10%.

1.2 Transmission Frequency. In lieu of paragraph 2.2.3.1 of
RTCA/DO-197A, substitute the following requirement:

“The transmission frequency of Mode C interrogations shall be 1,030
±0.2 MHz.”

1.3 Transmitter RF Output Power.In lieu of paragraph 2.2.3.2 of
RTCA/DO-197A, substitute the following requirement:

When transmitting at full (unattenuated) output power, the peak RF
output power delivered to a quarter wave stub antenna shall be within
the following limits:

Maximum RF Power: 54 dBm (250W)

Minimum RF Power: 50 dBm (100W)

In the event that antenna gain differs from that of a quarter wave
stub antenna (3 dBi), the power limits shall be adjusted accordingly.
These limits are based upon range and interference limiting
requirements.

*Note: When transmitting at full (unattenuated) power, the RF power
radiated at the pattern peak shall be within the following limits:*

*Maximum EIRP: 57 dBm (500W)*

*Minimum EIRP: 53 dBm (200W)*

*It is assumed that the peak gain of a typical quarter wave stub
antenna is 3 dBi. EIRP = Effective Isotropic Radiated Power.*

*Note: As an alternative to the above, an active TAS may choose to
operate as a low power system at a fixed rate power product limit of
42 Watts per second, in which case the peak RF output power delivered
to a quarter wave stub antenna shall not exceed 46 dBm (40W).*

1.4 Transmitter Pulse Characteristics. In lieu of paragraph 2.2.3.5
of RTCA/DO-197A, substitute the following requirement:

ATCRBS interrogations from active TAS shall employ the Mode C format
illustrated in Figure 2-1.

The rise and decay times may be less than shown in the following
table, provided the sideband radiation does not exceed the spectral
limits tabulated in this standard. The amplitude of P3 shall be
within 0.5 dB of the amplitude of P1.

***ACTIVE
TAS MODE PULSE SHAPES (All values in Microseconds)***

<table bgcolor="#d9d9d9" cellpadding="7" cellspacing="0">
<col/>
<col/>
<col/>
<col/>
<col/>
<col/>
<col/>
<tr>
<td bgcolor="#808080" valign="bottom">
Pulse
					Designator
</td>
<td bgcolor="#808080" valign="bottom">
Pulse
					Duration
</td>
<td bgcolor="#808080" valign="bottom">
Duration
					Tolerance
</td>
<td bgcolor="#808080" colspan="2">
Rise
					Time
</td>
<td bgcolor="#808080" colspan="2">
Decay
					Time
</td>
</tr>
<tr>
<td bgcolor="#d9d9d9" rowspan="2">
P1,
					P3
</td>
<td bgcolor="#d9d9d9" rowspan="2">
0.8
</td>
<td bgcolor="#d9d9d9" rowspan="2">
±
					0.075
</td>
<td bgcolor="#d9d9d9">
Min
</td>
<td bgcolor="#d9d9d9">
Max
</td>
<td bgcolor="#d9d9d9">
Min
</td>
<td bgcolor="#d9d9d9">
Max
</td>
</tr>
<tr>
<td bgcolor="#d9d9d9">
0.05
</td>
<td bgcolor="#d9d9d9">
0.1
</td>
<td bgcolor="#d9d9d9">
0.05
</td>
<td bgcolor="#d9d9d9">
0.2
</td>
</tr>
</table>

The pulse spacing tolerances shall be as follows: P1 to P3: 21 ±
0.10 microseconds

1.5 Mode S Broadcast Reception. In lieu of paragraph 2.2.4.2 of
RTCA/DO-197A, substitute the following requirement:

The Active TAS shall have the capability to receive 1,030 MHz Mode S
broadcast signals for the purpose of obtaining a count of TCAS
interrogators in its vicinity. Mode S reception may reside in an
associated Mode S transponder, or may by integral to the Active TAS
equipment, in which case those functions necessary to receive and
process Mode S broadcast signals for a TCAS count shall be
implemented and tested in accordance with RTCA/DO-181A.

*Note: As an alternative to the above, an active TAS may choose to
operate at a fixed rate power product limit of 42W/sec, in which case
the requirement to obtain a count of TCAS interrogators for the
purpose of interference limiting is eliminated.*

1.6 Interference Limiting.In lieu of paragraph 2.2.6 of RTCA/DO-197A,
substitute the following requirement:

To assure that all interference effects from Active TAS equipment are
kept to a low level, Active TAS equipment shall control its
interrogation rate or power or both to conform to the following
limits.

These limits are given in terms of:

RR = the Mode A/C reply rate of own transponder

NT = the number of airborne TCAS interrogators detected via Mode S
broadcast receptions with a receiver threshold of -74 dBm.

The Minimum Active TCAS shall have the capability to monitor RR and
NT and to use this information in interference limiting. Once each
scan period, NT shall be updated as the number of distinct TCAS
addresses received within the previous 20 second period.

The limits are as follows:

<table bgcolor="#d9d9d9" cellpadding="7" cellspacing="0">
<col/>
<col/>
<col/>
<thead>
<tr>
<td bgcolor="#808080">
NT
</td>
<td bgcolor="#808080" colspan="2" valign="top">
K 
Upper
						Limit for Σ P(k)
k=1
</td>
</tr>
<tr valign="top">
<td bgcolor="#808080">
 

</td>
<td bgcolor="#808080">
If
						RR &lt; 240
</td>
<td bgcolor="#808080">
If
						RR &gt; 240
</td>
</tr>
</thead>
<tbody>
<tr valign="top">
<td bgcolor="#d9d9d9">
0
</td>
<td bgcolor="#d9d9d9">
250
</td>
<td bgcolor="#d9d9d9">
118
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
1
</td>
<td bgcolor="#d9d9d9">
250
</td>
<td bgcolor="#d9d9d9">
113
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
2
</td>
<td bgcolor="#d9d9d9">
250
</td>
<td bgcolor="#d9d9d9">
108
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
3
</td>
<td bgcolor="#d9d9d9">
250
</td>
<td bgcolor="#d9d9d9">
103
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
4
</td>
<td bgcolor="#d9d9d9">
250
</td>
<td bgcolor="#d9d9d9">
98
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
5
</td>
<td bgcolor="#d9d9d9">
250
</td>
<td bgcolor="#d9d9d9">
94
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
6
</td>
<td bgcolor="#d9d9d9">
250
</td>
<td bgcolor="#d9d9d9">
89
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
7
</td>
<td bgcolor="#d9d9d9">
250
</td>
<td bgcolor="#d9d9d9">
84
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
8
</td>
<td bgcolor="#d9d9d9">
250
</td>
<td bgcolor="#d9d9d9">
79
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
9
</td>
<td bgcolor="#d9d9d9">
250
</td>
<td bgcolor="#d9d9d9">
74
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
10
</td>
<td bgcolor="#d9d9d9">
245
</td>
<td bgcolor="#d9d9d9">
70
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
11
</td>
<td bgcolor="#d9d9d9">
228
</td>
<td bgcolor="#d9d9d9">
65
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
12
</td>
<td bgcolor="#d9d9d9">
210
</td>
<td bgcolor="#d9d9d9">
60
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
13
</td>
<td bgcolor="#d9d9d9">
193
</td>
<td bgcolor="#d9d9d9">
55
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
14
</td>
<td bgcolor="#d9d9d9">
175
</td>
<td bgcolor="#d9d9d9">
50
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
15
</td>
<td bgcolor="#d9d9d9">
158
</td>
<td bgcolor="#d9d9d9">
45
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
16
</td>
<td bgcolor="#d9d9d9">
144
</td>
<td bgcolor="#d9d9d9">
41
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
17
</td>
<td bgcolor="#d9d9d9">
126
</td>
<td bgcolor="#d9d9d9">
36
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
18
</td>
<td bgcolor="#d9d9d9">
109
</td>
<td bgcolor="#d9d9d9">
31
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
19
</td>
<td bgcolor="#d9d9d9">
91
</td>
<td bgcolor="#d9d9d9">
26
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
20
</td>
<td bgcolor="#d9d9d9">
74
</td>
<td bgcolor="#d9d9d9">
21
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
21
</td>
<td bgcolor="#d9d9d9">
60
</td>
<td bgcolor="#d9d9d9">
17
</td>
</tr>
<tr valign="top">
<td bgcolor="#d9d9d9">
						≥22
</td>
<td bgcolor="#d9d9d9">
42
</td>
<td bgcolor="#d9d9d9">
12
</td>
</tr>
</tbody>
</table>

P(k) = power (watts) of the kth interrogation each second. This is
the total radiated power (after all losses in cabling and antenna).
If the set of powers is not the same in each 1 second period, then Σ
P(k) represents the average value.

K = total number of interrogations in a 1 second period.

*Note 1: RR = the Mode A/C interrogation reception rate of own
transponder may be used instead of RR = the Mode A/C reply rate of
own transponder.*

*Note 2: As an alternative to the above, an active TAS may choose
to operate as a low power system at a fixed rate power product limit
of 42W/sec, in which case the requirement to further interference
limit based on RR or IR is eliminated*.

In lieu of paragraph 2.4.2.5 of RTCA/DO-197A, substitute the
following:

This test verifies that Active TAS is able to monitor its own
transponder reply rate and to derive a count of TCAS aircraft by
listening to TCAS broadcast interrogations and, based on these
values, adjust its transmit power-rate product to conform to the
Active TAS interference limits.

Inputs:

Active TAS

Aircraft Altitude = 8000 ft.

Altitude Rate = 0 FPM

Intruder Aircraft 1-22

Equipage = Active TCAS II

Range = Not Applicable

Relative Speed = Not Applicable

Altitude = Not Applicable

Altitude Rate = Not Applicable

TCAS Broadcast Interrogation Power = -50 dBm

ATCRBS Interrogation

Frequency = 1030 MHz

Type = ATCRBS Mode C

Power = -50 dBm

Rate

Scenario A = 230 per second

Scenario B = 250 per second

Conditions:

Active TAS initialized and operating at T = 0 seconds. Each of the 22
intruders is assigned a discrete address and transmits only TCAS
broadcast interrogations and only at the following times and rates:

Intruders 1-10 every 10 sec starting at T = 30 sec.

Intruders 11-15 every 20 sec starting at T = 70 sec.

Intruders 16-22 every 20 sec starting at T = 130 sec.

The timing of the TCAS broadcast interrogations and the ATCRBS
interrogations are controlled to prevent overlap of each other.

Scenario Description

The test involves use of an ATCRBS transponder which supplies reply
rate information to Active TAS. The transponder is interrogated in
Mode C at a 230 per second rate in Scenario A and at a 250 per second
rate in Scenario B. During each scenario, the value of Total Radiated
Power per second from Active TAS is measured by summing the
transmitter output powers of each Active TAS interrogation over a
scan period, determining the average per second value and accounting
for cable and antenna losses.

Success: The Total Radiated Power per second shall not exceed the
following values:

Scenario A

250 watts/sec measured at T = 20 sec

245 watts/sec measured at T = 60 sec

158 watts/sec measured at T = 120 sec

42 watts/sec measured at T = 180 sec

Scenario B

118 watts/sec measured at T = 20 sec

70 watts/sec measured at T = 60 sec

45 watts/sec measured at T = 120 sec

12 watts/sec measured at T = 180 sec

*Note: For fixed rate power systems, total radiated power is
constant and shall not exceed 42 watts/sec.*

1.7 Active TAS Antenna System. In lieu of paragraph 2.2.10 of
RTCA/DO-197A, substitute the following requirement:

The equipment shall transmit interrogations and receive replies from
at least one directional antenna mounted on the top or bottom of the
aircraft.

1.8 Pilot Advisory Functions. In lieu of paragraph 2.1.5 of
RTCA/DO-197A, substitute the following requirement:

TAS is an airborne traffic alert system that interrogates ATC
transponders in nearby aircraft and uses computer processing to
identify potential and predicted collision threats. The system is
designed to protect a volume of airspace around the TAS equipped
aircraft. The system will provide appropriate aural and visual
advisories to assist the flightcrew in visually acquiring the threat
aircraft when TAS predicts a penetration of the protected airspace.
Traffic advisories indicate the relative positions of intruding
aircraft that meet certain range and altitude criteria and are
approximately 30 seconds from closest point of approach. They assist
the flightcrew in visually acquiring the intruding aircraft. The
system provides a traffic display (Class A systems only) and aural
and visual alerts. These indicate the relative position and altitude
of ATC transponder-equipped aircraft. Traffic advisories can be
generated for aircraft with operative Mode S, Mode C or Mode A
(non-altitude reporting) transponders. The TAS equipment is viewed as
a supplement to the pilot who, with the aid of the ATC system, has
the primary responsibility for avoiding collisions. The TAS system
provides no indication of aircraft without operative transponders.
For Class A systems, it shall be acceptable for the TAS system to use
shape as the only discriminate for traffic threat levels.

This will allow the use of a monochrome display representation of the
TCAS symbology. For Class A systems, it shall also be acceptable to
provide a blinking TA symbol to allow further discrimination of the
traffic alert symbol.

**2.0 Changes
Applicable Only to Class A Equipment.**

2.1 Pilot Advisory Functions, Active TCAS I Pilot Interface and Aural
Alert. In lieu of paragraphs 2.1.5, 2.2.12 and 2.2.15 of
RTCA/DO-197A, substitute the following requirements:

1. A traffic display shall be provided to indicate the presence and
location of intruder aircraft. The traffic display may be combined
with other aircraft displays. The traffic display shall provide the
crew with the intruder’s range, bearing, and, for altitude
reporting intruders, relative altitude and vertical trend.

2. Two levels of intruder aircraft shall be displayed; those causing
a TA, and other traffic. Other traffic is defined as any traffic
within the selected display range and not a TA.

*Note: The use of TCAS threat levels as defined in DO-197A is an
acceptable alternative to the requirements defined in this section.*

3. As a minimum, the traffic display shall depict the following
information to aid in the visual acquisition of traffic and assist in
determining the relative importance of each aircraft shown:

*Note: TCAS I symbology as defined in the FAA Memorandum titled
‘Interim Guidance for Airworthiness Approval and Operational Use of
Traffic Alert and Collision Avoidance System (TCAS I)’ dated June
16, 1995 is an acceptable alternative to the symbology requirements
defined in this section.*

*In addition, the use of TCAS symbology with a monochrome display
is also an acceptable means of depicting traffic information.*

a. Symbolic differentiation among traffic of different relative
importance. TA, other traffic (see i, j, k, l, & m below).

b. Bearing

c. Relative altitude (for altitude reporting aircraft only)

(1) Above or below own aircraft (+ and - signs)

(2) Numerical value

d. Vertical trend of intruder aircraft (for altitude reporting
aircraft only).

e. Range. The selected range shall be depicted.

f. The display must be easily readable under all normal cockpit
conditions and all expected ambient light conditions from total
darkness to bright reflected sunlight.

g. The display shall contain a symbol to represent own aircraft. The
symbol shall be different from those used to indicate TA and other
traffic. The display shall be oriented such that own aircraft heading
is always up (12 o’clock).

h. A ring shall be placed at a range of 2 NM from own aircraft symbol
when a display range of 10 NM or less is selected. The ring shall
have discrete markings at each of the twelve clock positions. The
markings shall be of a size and shape that does not clutter the
display.

i. Symbol fill shall be used to discriminate traffic by threat levels

j. The symbol for a TA is a filled rectangle, and, when appropriate,
a data field and vertical trend arrow as described in m. & n.
below.

k. The symbol for other traffic shall be an open rectangle, and, when
appropriate, a data field and vertical trend arrow as described in m.
below.

l. Overlapping traffic symbols should be displayed with the
appropriate information overlapped. The highest priority traffic
symbol should appear on top of other traffic symbols. Priority order
is; 1) TA traffic in order of increasing tau, i.e., the time to
closest approach and the time to coaltitude, 2) other traffic in
order of increasing range.

m. A data field shall indicate the relative altitude, if available,
of the intruder aircraft and shall consist of two digits indicating
the altitude difference in hundreds of feet. For an intruder above
own aircraft, the data field shall be preceded by a “+”
character. For an intruder below own aircraft, the data field shall
be preceded by a “-” character. For coaltitude intruders, the
data field shall contain the digits “00”, with no preceding “+”
or “-” character. The data field shall be wholly contained within
the boundaries of the rectangular traffic symbol. For TA traffic,
(filled symbol), the data characters shall be depicted in a color
that contrasts with the filled symbol color. For other traffic, the
data field shall be the same color as the symbol. The height of the
relative altitude data characters shall be no less than 0.15 inches.

n. A vertical arrow should be placed to the immediate right of the
traffic symbol if the vertical speed of the intruder is equal to or
greater than 500 fpm, with the arrow pointing up for climbing traffic
and down for descending traffic. The color of the arrow shall be the
same as the symbol.

o. Neither a data field nor a vertical arrow shall be associated with
a symbol for traffic which is not reporting altitude.

p. The display shall be capable of depicting a minimum of three
intruder aircraft simultaneously. As a minimum, the display shall be
capable of displaying aircraft that are within 5 NM of own aircraft.

q. The display may provide for multiple crew-selectable display
ranges.

r. When the range of the intruder causing a traffic advisory to be
displayed is greater than the maximum range of the display, this
shall be indicated by placing no less than one quarter of the traffic
advisory symbol at the edge of the display at the proper bearing. The
data field and vertical trend arrow shall be shown in their normal
positions relative to the traffic symbol.

s. The size of the traffic symbol shall be no less than 0.2” High.

4. “No bearing” advisories shall be presented for an intruder
generating a TA when the intruder’s relative bearing cannot be
derived. The “no bearing” advisory shall be an alphanumeric
display shown in tabular form. The display shall be in the form of
“TA 3.6 -05”, which translates to a TA at 3.6 nautical miles, 500
feet below. “No bearing” TA’s against non-altitude reporting
intruders shall include the range only, e.g. “TA 2.2”, which
translates to a non-altitude reporting, no bearing TA at 2.2 nautical
miles. The advisory shall be centered on the display below the own
aircraft symbol. The display shall include provisions to display at
least two “no bearing” TA’s.

5. Aural Alerts. Each TAS aural alert shall be announced in a
high-fidelity, distinguishable voice.

a. The aural alert message “Traffic-Traffic”, spoken once, shall
be used to inform the crew of a TA.

b. All TAS aural alerts should be inhibited using the following order
of precedence;

(1) Below 400 ±100 feet AGL when TAS is installed on an aircraft
equipped with a radio altimeter.

(2) For aircraft without a radio altimeter, the aural annunciations
shall be inhibited when the landing gear is extended.

*Note: When the TAS is installed on a fixed gear aircraft without a
radio altimeter, the aural annunciations will never be inhibited.*

2.2 Traffic Advisory Criteria. Replace the second section in
paragraph 2.2.14 of RTCA/DO-197A, with the following text:

The TAS equipment shall provide two levels of advisories: Other
Traffic (OT), and Traffic Advisories (TA). TAs are issued based on
either tau, i.e., the time to closest approach and the time to
coaltitude, or proximity to an intruder aircraft. The range tau is
defined as the range divided by range rate and the vertical tau is
defined as the relative altitude divided by the altitude rate.

2.3 Display Overload. In lieu of paragraph 2.2.17 of RTCA/DO-197A,
substitute the following requirements:

If the number of targets exceeds the display capability, excess
targets shall be deleted in the following order:

a. Other traffic beginning with the intruder at the greatest range.

b. TAs beginning with the intruder having the largest tau. Once a TA
has been generated against an intruder, it cannot be removed as a TA
until the TA criteria are no longer satisfied even though it may be
dropped from the display.

*Note: This exception does not apply when TCAS I symbology and
threat levels are used.*

**3.0 Changes
Applicable Only to Class B Equipment.**

3.1 Pilot Advisory Functions, Active TCAS I Pilot Interface, and
Aural Alert. In lieu of paragraph 2.1.5, 2.2.12, and 2.2.15 of
RTCA/DO-197A, substitute the following requirements:

1. A visual “Traffic” annunciation, shall be provided for the
duration of the TA.

2. Aural Alerts. For aircraft without a radio altimeter, the aural
annunciations shall be inhibited when the landing gear is extended.

*Note: When the TAS is installed on a fixed gear aircraft without a
radio altimeter, the aural annunciation will never be inhibited.*

a. Aural alert messages shall be annunciated in threat priority
sequence, greatest threat first.

(1) Initial aural traffic advisories shall be spontaneous and
unsolicited. The unsolicited annunciations shall be as follows:
“Traffic-<X>O’Clock”, spoken once, (where <X> is
the clock position of the intruder, such as 1 o’clock, etc.). If
surveillance bearing information is not available on the intruder,
“Traffic, No Bearing”, shall be annunciated.

(2) The current relative bearing to intruder aircraft shall be
annunciated as a traffic advisory update upon crew command.
Additional information such as relative altitude, range of intruder,
and vertical trend (i.e. climbing, descending) may also be
annunciated.

(3) The acceptability of these aural annunciations must be reviewed
during flight test. The following factors, at a minimum, must be
evaluated for acceptability: quantity of unsolicited annunciations,
duration of annunciations, annunciation clarity, and volume. This
evaluation shall occur under normal cockpit workload conditions
during departure, cruise, and approach and landing phases of flight
and should include evaluation of suitability in a normal air traffic
control voice communication environment.

(4) Control means shall be provided to request a traffic advisory
update, mute a current aural advisory, and cancel/restore aural
advisories (turning the equipment off is an acceptable means of
providing the cancel aural advisories function). The default
condition of the equipment at power on shall be aural advisories
active.

b. All TAS aural alerts should be inhibited using the following order
of precedence;

(1) Below 400 ±100 feet AGL when TAS is installed on an aircraft
equipped with a radio altimeter.

(2) For aircraft without a radio altimeter, the aural annunciations
will never be inhibited in flight but may be inhibited on the ground
when the aircraft is equipped with a weight-on-wheels system.

3.2 Traffic Advisory Criteria. Replace the first and second sections
in paragraph 2.2.14 of RTCA/DO-197A, with the following text:

The Active TAS equipment shall provide two levels of advisories:
Other Traffic (OT), and Traffic Advisories (TA). Other traffic is
defined as any traffic within the selected display range and not a
TA. TAs are issued based on either tau, i.e., the time to closest
approach and the time to coaltitude, or proximity to an intruder
aircraft. The range tau is defined as the range divided by range rate
and the vertical tau is defined as the relative altitude divided by
the altitude rate.

3.3 Display of intruders on the ground. In lieu of paragraph 2.2.16
of RTCA/DO-197A, substitute the following requirements:

The Active TAS equipment shall provide logic to inhibit TAs of
altitude reporting intruders which are on the ground. This logic
shall be used when the TAS-equipped aircraft is below 1,700 feet AGL.
The 1,700 foot threshold shall include hysteresis of + 50 feet.

*Note: This represents a requirement for a capability within the
Active TAS avionics. When Active TAS is installed on an aircraft
which does not have a radio altimeter, there is not a requirement for
this logic to function.*

3.4 Display overload. In lieu of paragraph 2.2.17 of RTCA/DO-197A,
substitute the following requirements:

If the number of intruders exceeds aural memory storage capacity,
excess intruders shall be deleted in the following order:

a. Other traffic beginning with the intruder at the greatest range.

b. TAs beginning with the intruder having the largest tau. Once a TA
has been generated against an intruder, it cannot be removed as a TA
until the TA criteria is no longer satisfied even though it has been
dropped from the list of aural warnings.

[Amdt ETSO/12]