TCAS

TCAS is the Traffic Alert & Collision Avoidance System. This text here describes the TCAS version available in the real world around 1999. That TCAS version is also modelled in Precision Simulator. So, the text here can be considered a tutorial about the real as well as the simulated TCAS. Both are identical.

In the meantime, newer TCAS versions may have been released in the real world which possibly differ from the Tau and DMOD values listed below. Basically, the purpose of this text is to show that the simulator’s TCAS is in no way a simplified “toy”, but a serious, detailed reproduction of a real-life TCAS as installed on a 747-400.


	TCAS I or TCAS II ?


	The 747-400 is equipped with TCAS II. That means the pilot is not only provided with traffic information, but also, if necessary, with pitch commands to resolve an upcoming conflict. TCAS I doesn’t provide pitch commands.


	How is traffic modelled in the simulator ?

Of course, in order to see TCAS in action one needs dynamic traffic. So, six dynamic target aircraft have been implemented in Precision Simulator. If desired, they fly in the vicinity of the own flight path coming from random directions and altitudes.

Two collision probability levels can be selected on the Instructor’s Page. Depending on this probability setting is whether or not the targets are fitted with TCAS II as well, so that their pitch commands are coordinated with the own TCAS II. Also, if a high probability is set, the targets will always enter the vicinity on direct collision courses. A target aircraft with a mode A transponder (mode A sends no altitude info) may be simulated as well. Traffic situations can be created and reloaded: Position, altitude, heading, vertical speed and airspeed of all targets can be saved on the hard disk.

The traffic visible in the outside window is rendered with realistic strobe, beacon and nav lights. Realistically, the red/green nav lights are only visible from the front. The traffic is visible by night and day if there are no clouds in between.

This screen shot here shows a climbing aircraft, 600 feet above and flying away from the own aircraft. The actual threat coming from the left is not visible in the outside window, yet.


	What is TCAS, in short words ?

TCAS scans the vicinity by interrogating the transponders of other aircraft. It then uses the received transponder signals to compute distance, bearing and altitude relative to the own aircraft.

The evaluated traffic information is displayed as symbols on the ND, the Navigation Display. Note that altitude and vertical motion information is only available if the received signal comes from a mode C or mode S transponder. Otherwise the associated symbol on the ND will have no altitude information and no vertical motion arrow.

As TCAS checks the other aircraft’s relative distance permanently in short time intervals, it can, therefore, also calculate the other aircraft’s closure rate relative to the own aircraft. The closure rate is the most important variable and indeed a very fail-safe key to a good collision prediction. Complex, trigonometric calculations of flight paths and ground speeds are unnecessary and would only result in unsharp and unreliable extrapolations. Heading or bearing information is not even required to compute a TCAS alarm.

When TCAS detects that an aircraft’s distance and closure rate becomes critical, TCAS generates aural and visual annunciations for the pilots.

If necessary, it also computes aural and visual pitch commands to resolve a conflict. If the other aircraft uses TCAS II as well, these pitch commands are coordinated with the other aircraft’s pitch commands so that both aircraft don’t escape to the same direction. Even three aircraft can be coordinated.

By the way, TCAS provides only vertical guidance, no lateral guidance. TCAS also ignores performance limitations. In other words, when flying at maximum altitudes TCAS may still generate a climb command.


	What systems are required to run TCAS ?


	To calculate traffic displays and advisories, TCAS requires input from the following systems:

	• IRS
		Attitude data, vertical motion data.
	• Gear position sensors
		As the extended gear disturbs the lower directional antenna, bearing detection of traffic flying below the own aircraft must be inhibited.
	• Radio altimeters
		TCAS must know the radio height as the alarm logic varies with the height above ground.
	• Air/ground relays
		Automatic standby control.
	• GPWS
		GPWS overrides TCAS advisories during a windshear or ground proximity warning.
	• AC Bus 3
		Electric power supply.


	What is a tau area ?

The tau area is a specific “protective area” around the own aircraft. When an intruder aircraft enters the tau area, TCAS triggers an alarm. The threshold of the tau area is defined by time. "Tau" means time. In other words, tau is the time-to-go to CPA, to the “Closest Point of Approach". The time-to-go is distance divided by closure rate -- both combined vertically and horizontally.

Example: Say, a tau value of 30 seconds is used for an alarm threshold. As soon as the intruder's time-to-go to the CPA is dropped to 30 seconds, TCAS triggers an alarm.

A fast approaching intruder may be, for example, 6 miles away from the CPA when crossing its tau threshold, while another intruder half as fast may be only 3 miles away respectively.

In additon to this logic, TCAS uses different tau values at different altitudes. The higher the own altitude, the larger the tau area -- the greater the sensitivity level. The exact numbers are listed on the table below.


	What means DMOD ?

There are cases when a closure rate is so slow that the tau area will never be entered, while the physical separation, however, may be just a fraction of a mile. In such a critical scenario, the calculated closure rate is no longer useful as a sudden increase in the closure rate would leave no room for an advance warning.

This problem has been eliminated by an additional "Distance Modification" -- short "DMOD". Time or closure rate are not a factor here. DMOD affects only the physical separation between the own aircraft and the target.

DMOD values, too, vary with altitude. The exact alarm thresholds are listed on the table further below.


	What is a TA ?

TA is a so-called Traffic Advisory. TAs are given to the pilot in form of the word TRAFFIC displayed in yellow on the ND, and the aural voice annunciation "traffic, traffic". This is not the highest alert level. Its purpose is first to call attention to a possible conflict.

TCAS triggers a TA as soon as an intruder enters the TA specific DMOD or tau area (see table below). If no altitude data is available from the intruder aircraft, TCAS assumes the intruder’s relative altitude is within 1200 feet.

If bearing information is available, the intruder can be identified on the ND by a yellow, solid circle. Otherwise, the circle is removed and lateral distance and relative altitude with vertical motion arrow (if motion is detected) is displayed in yellow numbers under the word TRAFFIC.

This screen shot on the left shows a target approaching 900 feet below the own aircraft (“-09” means minus 900). It has just crossed the tau threshold for a TA.


	What is an RA ?

RA means Resolution Advisory, the highest alert level. Its purpose is to resolve a conflict by providing the pilot with aural and visual pitch commands. The pilot has to disengage the autopilot immediately as the escape maneouver has to be flown manually. Flight director commands as well as ATC advisories have to be ignored. The pitch command of an RA has always the highest priority.

TCAS triggers an RA when a target enters the RA specific DMOD or tau area (see table below). If no altitude data is available from the target, an RA will not occur.

If bearing information is available, the intruder can be identified on the ND by a red, solid square. Otherwise, the square is removed and lateral distance and relative altitude with vertical motion arrow (if motion is detected) is displayed in red numbers under the word TRAFFIC.


	What alarm thresholds are set ?

By the way, the PS1.3 Operations Manual mentions on page 236 some TA/RA tau thresholds of 25 and 40 seconds. These are rather “benchmark figures” to simplify the text in the manual. DMOD is not mentioned at all. But Precision Simulator is, in fact, more complicated -- just like the real TCAS. The simulator, too, uses different tau thresholds at different altitudes and it also incorporates DMOD and relative altitude thresholds:

Own Altitude	Threshold Tau ________ TA	Threshold Tau ________ RA	Threshold DMOD ________ TA	Threshold DMOD ________ RA	Threshold Relative Altitude ________ TA	Threshold Relative Altitude ________ RA
Feet	Seconds	Seconds	NM lateral	NM lateral	Feet	Feet
0 - 500 Radio	20	Inhibited	0.100	Inhibited	1200	Inhibited
501 - 2500 Radio	35	20	0.100	0.066	1200	300
2501 - 10000 Baro	40	25	0.300	0.066	1200	300
10001 - 20000 Baro	45	30	1.000	0.082	1200	300
20001 - 30000 Baro	45	30	1.000	0.105	1200	300
above 30000 Baro	45	30	1.000	0.122	1200	300

As mentioned in the introduction, Precision Simulator models the TCAS version available in the real world around 1999. Today, newer TCAS versions may be available which possibly use different values. The problem TCAS developpers have is finding the optimum thresholds for every imagineable scenario: Desired are, of course, useful alarms rather than “nuisance” alarms. So they’ve been running millions of computer simulations. With growing experience, further TCAS modifications may be released in the future.


	This photograph was shot during a descent command. An aircraft is approaching very slowly from behind. The closure rate is too small to satisfy any tau criteria. In this case, the RA is triggered because the intruder enters the DMOD area.


	What pitch commands may occur ?

When a new RA is computed, one of the following aural annunciations may be triggered:

• If a target is approaching at the same altitude:
   “Climb, climb, climb!” -- or --
   “Descend, descend, descend!”

• If a target is approaching from above or below and
   will cross our altitude:
   “Climb, crossing climb! Climb, crossing climb!” -- or --
   “Descend, crossing descend! Descend, crossing descend!”

• If targets are approaching above and below the own
   flight path and the present vertical speed should be
   maintained:
   “Monitor vertical speed! Monitor vertical speed!”

When a pitch change to an existing RA is required, one of the following aural annunciations may be triggered:

• If the existing vertical speed is not sufficient anymore:
   “Increase climb! Increase climb!” -- or --
   “Increase descent! Increase descent.”

• If the previous pitch command is cancelled and a
   direction change is now required:
   “Climb, climb now! Climb, climb now!” -- or --
   “Descend, descend now! Descend, descend now!”

The respective visual pitch commands are displayed on the Attitude Indicator. Some airlines may also have an additional red band on the Vertical Speed Indicator. No matter which instrument, the pilot’s job simply is to get out of the red zone. Normally, TCAS commands a vertical target speed of 1500 to 2000 fpm.

When the traffic separation is increasing, the pitch command is removed from the PFD accompanied by the aural annunciation “clear of conflict”.


	This photograph shows one intruder aircraft approaching 900 feet above and another aircraft 800 feet below the own aircraft, triggering an RA. The aural voice annunciation sounds: “Monitor vertical speed...”.


	When are RAs inhibited ?

For obvious reasons, TCAS will not issue a descent command when the aircraft is at low altitudes.

Below 1450 feet radio altitude:
All "increase descent" RAs are inhibited.

Below 1000 feet radio altitude:
All "descend" RAs are inhibited.

Below 500 feet radio altitude:
All RAs are inhibited -- “TA only” mode.

Below 400 feet radio altitude or during a windshear warning:
All aural TCAS annunciations are inhibited.


	What is “proximate traffic” ?


	It’s a TCAS term for targets detected within a range of 6 NM from the own aircraft and -- if the target sends altitude information -- within 1200 feet relative altitude. These targets are represented on the ND as solid white diamonds. They do not represent an alarm.


	What is “other traffic” ?


	It’s a TCAS term for targets detected within a range of 40 to 7 NM from the own aircraft and -- if the target sends altitude information -- within 2700 and 1201 feet relative altitude. These targets are represented on the ND as outlined white diamonds. They do not represent an alarm.


	How can TCAS be controlled by the pilot ?


	TCAS modes and options can be controlled through the EFIS Control Panel and the ATC Transponder Panel.

The TFC push button on the EFIS Control Panel enables and disables the display of the aircraft symbols on the ND.

These symbols are only generated if the ND is in MAP, MAP CTR, VOR or APP mode (same conditions as for the weather radar display).

Note that the TFC switch controls only the aircraft symbols. It can not deactivate the red or yellow TRAFFIC alert messages. All TCAS alerts remain enabled.

On the ATC Transponder Panel, the pilot can switch the entire TCAS unit off, or enable “TA Only” mode, or enable both RA and TA modes.

Some airlines have the optional ABV and BLW switch. This switch expands the vertical display range of other traffic from 2700 to 7000 feet -- above (ABV) or below (BLW) accordingly.


	What do other aviation professionals say about this simulator?


	[Simulator][Reviews][Order][Support][Forum (vertical frame)][Forum (horizontal fr.][Home]

	© 1995-2002 Hardy Heinlin. All rights reserved.