Qantas Introduces TCAS

In simple terms, TCAS is an independent onboard alerting system which is designed to calculate and display the range, bearing, and relative altitude of other transponder (mode C) equipped aircraft. Using this information it calculates, from the current flight paths, the closest point of approach.

Qantas TCAS systems are set to display up to six aircraft at the one time; this has been determined as the maximum number to prevent overload inthe provided. Should TCAS detect a collision risk, it will give a voice warning and at the same time, indicate pitch commands on the primary flight display (PFD).

The 747-400 with its electronic flight instrument system (EFIS) is ideally configured to accept the new equipment, which displays traffic information on the multi function navigation display (ND), located alongside the PFD.

TCAS information is controlled by the crew and can be selected to off, traffic advisory (TA) or resolution advisory (RA). With TA selected, traffic information only is displayed on the ND and resolution advisories are inhibited. In the RA position, in addition to the traffic information, voice commands or annunciations are given together with a pitch command on the PFD. Should the equipment be switched off by the crew, this is indicated by an “off” on the ND.

Symbols for each type of advisory or traffic display appear on the ND when an intruder aircraft is detected, and if the latter is transponder equipped, a data tag in the same colour will also appear.

TAS are indicated in amber and RAs in red.

The data tag indicates the height of the intruder aircraft, relative to the detecting aircraft; this is shown in hundreds of feet with a plus or minus sign above or below the symbol. Should the intruder have a vertical movement of greater than 500ft a minute, an arrow in the direction of movement is displayed to the right of the symbol.

Traffic Advisories

TAs are in the nature of a caution and are displayed when a transponder equipped aircraft is detected which TCAS assesses will pass close, but with a safe separation. A voice annunciation “traffic, traffic” is given approximately 40 seconds before the intruder aircraft reaches the closest point of approach. The voice warning is not repeated.

Should the intruder aircraft be outside the range presently set on the ND, increasingly the range setting will display the aircraft and its relative track can then be monitored.

TCAS will also display “proximate” traffic and “other traffic”; the former is traffic that is near but does not present a risk of collision, and the latter is traffic detected but well clear.

Resolution Advisories (RAS)

An RA is a voice warning given when TCAS detects an intruder equipped with a transponder, which it calculates will pass without adequate separation. The warning is given 25 seconds before the intruder reaches the closest point of approach; at the same time a pitch command is displayed on the PFD. The pitch command is given in the form of a large truncated “V” symbol pointing up or down which indicates the direction and magnitude of the change of pitch necessary to clear the intruder. The voice warning is given as either “climb, climb, climb” or “descend, descend, descend”.

On receipt of the command, the pilot flying the aircraft disconnects the auto pilot and smoothly applies a pitch input to place the pitch datum on the horizontal truncated section of the “V”.

There are a further three voice announcements to cope with changing circumstances; “increase climb, “climb, climb now, and “monitor vertical speed; each given twice.

The “increase climb” command is self explanatory and indicates a command for an increase in pitch to avoid the intruder. “Climb, climb now” reverses a previous command to descend. “Monitor vertical speed” is given when TCAS senses that the pilot flying the aircraft has over pitched in meeting the command.

When TCAS determines that a conflict no longer exists the voice annunciation “clear of traffic, clear of traffic” is made and the pitch command on the PFD is removed. All responses to RA commands are made in the pitching plane only; turns must not be made.

Finally, should TCAS detect an intruder aircraft above and another below, both being in possible conflict, it will analyse and display the best path to avoid both.

Inhibits

TCAS commands and annunciations are inhibited in the following situations:

⚫ increase descent RAS are inhibited below 1450ft radio altitude

⚫ descend RAS are inhibited below 1000ft radio altitude

⚫all RAS are inhibited below 500ft radio altitude and TCAS reverts to the TA only mode all TA voice annunciations are inhibited below 400ft radio altitude and finally, ground proximity or wind shear warnings will cut out TCAS voice commands and PFD pitch guidance will be terminated.

747 TCAS Simulator Demonstration

Following the audio visual briefing given to me by Captain Gary Cox, who is a senior 747-400 Check Captain and Captain Faulkner’s deputy, it was off to the simulator to see how well the equipment performed in practice.

Fortunately, having had an introduction to EFIS in the Ansett A320 simulator and a familiarisation ride in the Beechjet 400A, the 747-400 instrument panel was not entirely a new learning experience. However, since space does not permit, I will leave my impressions of the 747 to another day and concentrate on TCAS Once airborne, we ran through a series of traffic and collision situations and it was soon apparent that here was a piece of equipment that was indeed worth its salt.

As soon as the warning “traffic, traffic” was given by synthetic voice annunciation, a glance at the ND immediately indicated the relative bearing and range of the intruder. The information given was easy to read and understand, without any ambiguity. I found that I could get an immediate mental picture of the location of the other aircraft, vertically and in azimuth, together with its trend.

Under the flight conditions being simulated, which were quite good visibility but half light, it was some time after the warning before the intruder was detected visually. In RA situations, the pitch command on the PFD was positive and left no doubt as to the corrective action necessary.

Without corrective action, the situations being simulated would have lead to at least a very near miss, if not a collision, however the warning information given, and the time provided to take action, ensured a safe response.

All in all, it was a very convincing demonstration of the efficacy of the system.

Summary

There is no substitute for an active and systematic visual lookout and TCAS is not meant to replace this. However, as a piece of equipment that can see well beyond the human eye, it is a welcome addition to the cockpit, providing valuable information on possible conflicting traffic situations.

The only downside to TCAS is that it needs other aircraft to be mode Ĉ transponder equipped otherwise it can’t determine relative heights and hence give traffic resolution commands. Fortunately however, TCAS is being seen more and more worldwide as a mandatory item of equipment and in fact will be a requirement for RPT aircraft to enter US airspace from December 31 1993.

Qantas sees TCAS as a cost effective item of equipment which enhances safety, particularly in the vast areas of international airspace which have marginal traffic information systems. From my brief exposure to the equipment, I would give it an unqualified thumbs up as a positive step forward in flight safety.

My thanks to Qantas for the opportunity to see TCAS in operation.