Aircraft Traffic alert and Collision Avoidance System (TCAS)

>> Friday, August 28, 2009


The Traffic alert and Collision Avoidance System (or TCAS) is an aircraft collision avoidance system designed to reduce the incidence of mid-air collisions between aircraft. It monitors the airspace around an aircraft for other aircraft equipped with a corresponding active transponder, independent of air traffic control, and warns pilots of the presence of other transponder-equipped aircraft which may present a threat of mid-air collision (MAC). It is an implementation of the Airborne Collision Avoidance System mandated by International Civil Aviation Organization to be fitted to all aircraft with MTOM (maximum take-off mass) over 5700 kg (12,586 lbs) or authorised to carry more than 19 passengers.

Official definition from PANS-ATM (Nov 2007): ACAS / TCAS is an aircraft system based on secondary surveillance radar (SSR) transponder signals which operates independently of ground-based equipment to provide advice to the pilot on potential conflicting aircraft that are equipped with SSR transponders.

In modern glass cockpit aircraft, the TCAS display may be integrated in the Navigation Display (ND); in older glass cockpit aircraft and those with mechanical instrumentation, such an integrated TCAS display may replace the mechanical Instantaneous Vertical Speed Indicator (which indicates the rate with which the aircraft is descending or climbing).

TCAS basics

TCAS involves communication between all aircraft equipped with an appropriate transponder (provided the transponder is enabled and set up properly). Each TCAS-equipped aircraft "interrogates" all other aircraft in a determined range about their position (via the 1030 MHz radio frequency), and all other craft reply to other interrogations (via 1090 MHz). This interrogation-and-response cycle may occur several times per second.
Through this constant back-and-forth communication, the TCAS system builds a three dimensional map of aircraft in the airspace, incorporating their bearing, altitude and range. Then, by extrapolating current range and altitude difference to anticipated future values, it determines if a potential collision threat exists.
TCAS and its variants are only able to interact with aircraft that have a correctly operating Mode C or Mode S transponder. A unique 24-bit identifier is assigned to each aircraft that has a Mode S transponder. These identifiers can be decoded online using tools such as those at

The next step beyond identifying potential collisions is automatically negotiating a mutual avoidance maneuver (currently, maneuvers are restricted to changes in altitude and modification of climb/sink rates) between the two (or more) conflicting aircraft. These avoidance maneuvers are communicated to the flight crew by a cockpit display and by synthesized voice instructions.

Safety aspects of TCAS

Safety studies on TCAS estimate that the system improves safety in the airspace by a factor of between 3 and 5.
However, it is well understood that part of the remaining risk is that TCAS may induce midair collisions: "In particular, it is dependent on the accuracy of the threat aircraft’s reported altitude and on the expectation that the threat aircraft will not make an abrupt maneuver that defeats the TCAS RA. The safety study also shows that TCAS II will induce some critical near midair collisions..." (See page 7 of Introduction to TCAS II Version 7 (PDF) in external links below).

One potential problem with TCAS II is the possibility that a recommended avoidance maneuver might direct the flight crew to descend toward terrain below a safe altitude. Recent requirements for incorporation of ground proximity mitigate this risk. Ground proximity warning alerts have priority in the cockpit over TCAS alerts.

Some pilots have been unsure how to act when their aircraft was requested to climb whilst flying at their maximum altitude. The accepted procedure is to follow the climb RA as best as possible, temporarily trading speed for height. The climb RA should quickly finish. In the event of a stall warning, the stall warning would take priority.

Versions of TCAS

Collision Avoidance systems which rely on transponder replies triggered by ground and airborne systems are considered passive. Ground and airborne interrogators query nearby transponders for mode C altitude information, which can be monitored by third-party systems for traffic information. Passive systems display traffic similar to TCAS, however generally have a range of less than 7 nautical miles (13 km). Portable Collision Avoidance System.


TCAS I is the first generation of collision avoidance technology. It is cheaper but less capable than the modern TCAS II system, and is mainly intended for general aviation use. TCAS I systems are able to monitor the traffic situation around a plane (to a range of about 40 miles) and offer information on the approximate bearing and altitude of other aircraft. It can also generate collision warnings in the form of a "Traffic Advisory" (TA). The TA warns the pilot that another aircraft is in near vicinity, announcing "traffic, traffic", but does not offer any suggested remedy; it is up to the pilot to decide what to do, usually with the assistance of Air Traffic Control. When a threat has passed, the system announces "clear of conflict".


TCAS II is the second and current generation of instrument warning TCAS, used in the majority of commercial aviation aircraft (see table below). It offers all the benefits of TCAS I, but will also offer the pilot direct, vocalized instructions to avoid danger, known as a "Resolution Advisory" (RA). The suggestive action may be "corrective", suggesting the pilot change vertical speed by announcing, "descend, descend", "climb, climb" or "Adjust Vertical Speed Adjust" (meaning reduce vertical speed). By contrast a "preventive" RA may be issued which simply warns the pilots not to deviate from their present vertical speed, announcing, "monitor vertical speed" or "maintain vertical speed". TCAS II systems coordinate their resolution advisories before issuing commands to the pilots, so that if one aircraft is instructed to descend, the other will typically be told to climb — maximising the separation between the two aircraft.

As of 2006, the only implementation that meets the ACAS II standards set by ICAO is Version 7.0 of TCAS II, produced by three avionics manufacturers: Rockwell Collins, Honeywell, and ACSS (Aviation Communication & Surveillance Systems; an L-3 Communications and Thales Avionics company).


TCAS III was the "next generation" of collision avoidance technology which underwent development by aviation companies such as Honeywell. TCAS III incorporated technical upgrades to the TCAS II system, and had the capability to offer traffic advisories and resolve traffic conflicts using horizontal as well as vertical manouevring directives to pilots. For instance, in a head-on situation, one aircraft might be directed, "turn left, climb" while the other would be directed "turn right, descend." This would act to further increase the total separation between aircraft, in both horizontal and vertical aspects. Horizontal directives would be useful in a conflict between two aircraft close to the ground where there may be little if any vertical maneuvering space. All work on TCAS III is currently suspended and there are no plans for its implementation.

Current implementation
Although the system occasionally suffers from false alarms, pilots are now under strict instructions to regard all TCAS messages as genuine alerts demanding an immediate, high-priority response. (Only stall warnings and Ground Proximity Warning System warnings have higher priority than the TCAS.) The FAA and most other countries' authorities' rules state that in the case of a conflict between TCAS RA and air traffic control (ATC) instructions, the TCAS RA always takes precedence (this is mainly because of the TCAS-RA inherently possessing a more current and comprehensive picture of the situation than air traffic controllers, whose radar/transponder updates usually happen at a much slower rate than the TCAS interrogations). If one aircraft follows a TCAS RA and the other follows conflicting ATC instructions, a collision can occur, such as the July 1, 2002 Ɯberlingen disaster. In this mid-air collision, both airplanes were fitted with TCAS II systems which functioned properly, but one obeyed the TCAS advisory while the other ignored the TCAS and obeyed the controller; both aircraft descended into a fatal collision.

Current TCAS Limitations
While the benefits of TCAS are undisputable, it can be assumed that TCAS' true technical and operational potential (and thus its possible benefits) is not yet being fully exploited because of the following limitations in current implementations (most of which will need to be addressed in order to further facilitate the design and implementation of Free flight):

• TCAS is limited to supporting only vertical separation advisories, more complex traffic conflict scenarios may however be more easily and efficiently remedied by also making use of lateral resolution maneuvers; this applies in particular to traffic conflicts with marginal terrain clearance, or conflict scenarios that are similarly restricted by vertical constraints (e.g. in busy RVSM airspace)

• ATC can be automatically informed about resolution advisories issued by TCAS only when the aircraft is within an area covered by a Mode S, or an ADS-B monitoring network. In other cases controllers may be unaware of TCAS-based resolution advisories or even issue conflicting instructions (unless ATC is explicitly informed by cockpit crew members about an issued RA during a high-workload situation), which may be a source of confusion for the affected crews while additionally also increasing pilot work load. In May 2009, Luxembourg, Hungary and the Czech Republic show downlinked RAs to controllers.

• In the above context, TCAS lacks automated facilities to enable pilots to easily report and acknowledge reception of a (mandatory) RA to ATC (and intention to comply with it), so that voice radio is currently the only option to do so, which however additionally increases pilot and ATC workload, as well as frequency congestion during critical situations.

• In the same context, situational awareness of ATC depends on exact information about aircraft maneuvering, especially during conflict scenarios that may possibly cause or contribute to new conflicts by deviating from planned routing, so automatically visualizing issued resolution advisories and recalculating the traffic situation within the affected sector would obviously help ATC in updating and maintaining situational awareness even during unplanned, ad hoc routing changes induced by separation conflicts.

• Today's TCAS displays do not provide information about resolution advisories issued to other (conflicting) aircraft, while resolution advisories issued to other aircraft may seem irrelevant to another aircraft, this information would enable and help crews to assess whether other aircraft (conflicting traffic) actually comply with RAs by comparing the actual rate of (altitude) change with the requested rate of change (which could be done automatically and visualized accordingly by modern avionics), thereby providing crucial realtime information for situational awareness during highly critical situations.

• TCAS equipment today is often primarily range-based, as such it only displays the traffic situation within a configurable range of miles/feet, however under certain circumstances a "time-based" representation (i.e. within the next xx minutes) might be more intuitive.

• Lack of terrain/ground and obstacle awareness (e.g. connection to TAWS, including MSA sector awareness), which might be critical for creating feasible (non-dangerous, in the context of terrain clearance) and useful resolution advisories (i.e. prevent extreme descent instructions if close to terrain), to ensure that TCAS RAs never facilitate CFIT (Controlled Flight into Terrain) scenarios.

• Aircraft performance in general and current performance capabilities in particular (due to active aircraft configuration) are not taken into account during the negotiation and creation of resolution advisories (as it is the case for differences between different types of aircraft, e.g. turboprop/jet vs. helicopters), so that it is theoretically possible that resolution advisories are issued that demand climb or sink rates outside the normal/safe flight envelope of an aircraft during a certain phase of flight (i.e. due to the aircraft's current configuration), furthermore all traffic is being dealt with equally, there's basically no distinction taking place between different types of aircraft, neglecting the option of possibly exploiting aircraft-specific (performance) information to issue customized and optimized instructions for any given traffic conflict (i.e. by issuing climb instructions to those aircraft that can provide the best climb rates, while issuing descend instructions to aircraft providing comparatively better sink rates, thereby hopefully maximizing altitude change per time unit, that is separation)

• TCAS is primarily extrapolation-oriented, as such it is using algorithms trying to approximate 4D trajectory prediction using the "flight path history", in order to assess and evaluate the current traffic situation within an aircraft's proximity, however the degree of data- reliability and usefulness could be significantly improved by enhancing said information with limited access to relevant flight plan information, as well as to relevant ATC instructions to get a more comprehensive picture of other traffic's (route) plans and intentions, so that flight path predictions would no longer be merely based on estimations but rather actual aircraft routing (FMS flight plan) and ATC instructions. If TCAS is modified to use data that is used by other systems, care will be required to ensure that the risks of common failure modes are sufficiently small.

• TCAS is not fitted to many smaller aircraft mainly due to the high costs involved (between $25,000 and $150,000). Many smaller personal business jets for example, are currently not legally required to have TCAS installed, even though they fly in the same airspace as larger aircraft that are required to have proper TCAS equipment on board. TCAS can however only develop its true operational potential once all aircraft in any given airspace can be safely assumed to have a properly working TCAS unit on board.


Aircraft ADC HG480B13

Aircraft B727 and B737 Honeywell Air Data Computer (ADC) p/n HG480B13

A) Reason for the study
Air Data Computer p/n HG480B13/B20 is among the top 5 of unscheduled removal in June 2004 component reliability review. This study is to analyse the common reasons of such failures over 57 month period (Jan 02 to Sep 04) and to identify possible measures to improve reliability.

B) Data

1. Repair and Findings Data since 1 Jan 2002 till 30 Sep 2004 from Spares Department is reviewed to classify the types of common defect and shop findings; s/n and/or aircraft with repeated removals (attachment 5A). Total 31 units were removed unscheduled.

Defect confirmed - 24(78%)
Defect Not Confirmed - 6(19%)
Under repair - 1(3%)

Time since fitted (TSF) ranged from 21 to 416 hrs for those with known TSF.
Mod status was known for some s/n based on out going mod status from shop reports.

2. 1 cancellation and 2 delays were due to ADC.

3. PIREPs were reviewed for the same period. B737 1 defect and B727 8 out of total 17 defects were rectified by ADC replacement.

C) OEM Repair shop data

Unscheduled removal data has been provided to Honeywell and Boeing for their feedback

OEM (Honeywell) feedback:
1. Apparently, every LRU removed is old (approx >20 years), the youngest one being 16 years. Most of them were removed for calibration related issues and some for solder joints. These LRUs being this old, one would expect these problems

2. It appears an inordinate amount of problems with DADC accuracy and calibration of the transducer card (a dual pressure sensor that takes in the Ps and Pt pressures from the aircraft pitot and static ports and converts them into frequency outputs that are used by the DADC to make altitude and airspeed calculations).

3. This transducer is essentially a dual pressure to frequency converter. It is calibrated by measuring and recording the frequency output at controlled pressures and at controlled temperatures. A polynomial lookup table is created and loaded into the "calibration memory" on the CCA based on these measurements. The calibration memory for each sensor relates the applied pressure at a specific temperature to the actual output frequency. The DADC receives two frequency outputs from the transducer card and uses the calibration memory to determine the applied pressures.

4. There are several other calibrated circuits in the HG480 i.e. Synthesized Q-Pots, Hold circuits and Altitude fine synchro #1. However, the transducer assembly is the most frequently recalibrated (fine adjusted to nominalize scale error testing during the pressure station tests).

5. Following are the cause of transducers to go "Out of Calibration":

a) DADC pressure ports should be disconnected from the aircraft plumbing if the there is any related maintenance work especially when using compressed air to "blow out" the pitot or static lines. Overstressing the sensors beyond about 60psi with compressed air will damage the sensors and will cause noticeable output errors.

b) Any foreign material blown into the sensors itself will cause output errors. However, this is rather rare.

c) Pulling a hard vacuum on the ports or sensors for any length of time will deform the diaphragm and cause noticeable output errors.

6. Design MTBUR for HG480B13/100 is 8,000 hrs. However ADCs on B737 had MTBUR of 2126 hrs and those on B727 had MTBUR of 1734 hrs.

7. Alpha numeric s/n’s are pre 1988. Numeric s/n’s were built since 1988 with first two digits indicating the year e.g. 97062209 was manufactured in 97.

D) Current Maintenance Programme review
Current maintenance programme for B727 and B737 Equipment Cooling System were reviewed as per OEM’s feedback since inadequate cooling can cause equipment premature failure.

Perform during 3A operational check of E&E equipment cooling system. In particular, verify that alternate blower operates

Perform visual inspection of pressure controller, temperature regulator and equipment cooling blower motor during 5C.

All components are Condition Monitored (CM) ie fly to failure.

E) Discussion
1. Summary of defects

Reason of removal
Altitude Indication - Total 5 - 3 (DC) ; 2 (DNC) ; 0 (Under Repair)
Mach/ASI - Total 12 - 8 (DC) ; 3 (DNC) ; 1 (under Repair)
Auto Pilot Data Mode - Total 13 - 10 (DC) ; 3 (DNC) ; 0 (Under Repair)
Others - Total 5 - 5 (DC) ; 0 (DNC) ; 0 (Under Repair)

2. For 80% of the ADC removals, transducer calibration/replacement was the most common shop rectification for defects like ASI flag in view, ASI and Altimeter indication inaccurate. It’s normal for transducer being number one fault for these ADC’s per OEM.

3. Approx. 50 milli-inches of field correction is available for calibration before the sensor assembly needs to be replaced. S/n L1324 and G1653 were sent to Aero Inst and recommended scrapped due to expensive transducer need to replaced

4. Currently, Barfield Pitot Static Tester is used to c/out leak test and vacuum. The test set enables protection limit settings which, recommends the operator always set their desired protection limits before use. Therefore the test system is considered adequate.

5. MRO do disconnect pitot static plumbing and connections to necessary equipments before flushing/blowing action. Nitrogen/compressed air with very limited pressure is used. MM limitation for blow out pressure is 15 psi: less than OEM’s recommended <60 psi. Test lead and hoses are inspected for any obstruction before carrying out the flushing or leak test to prevent foreign object damage. Any obstruction will show up as test error comparing the Capt and F/O’s instruments when system will be isolated and flushed accordingly.

6. Auto pilot air data mode being the second highest reason of removal. It is quite common to aging equipment as per H/well.

7. ASI indicator fault being the third highest reason of removal. Inadequate equipment cooling system could be possible reason for these failures.

8. For B737 OP/C of Equipment cooling system. In particularly, verify that alternate blower operates during 3A. But for B727 only general visual inspection is called out. Boeing does agree heat build up may be a contributor to premature part removals and tests on the B727 is an acceptable preventive maintenance action.

9. Since most removals are DC it can be assumed proper trouble-shooting was done prior to removals of ADC. MM is not reviewed for trouble-shooting guide.

10. 7 units (s/n P129-1, M1134, N1005/2, P401, J1485, M119 and G1653) had repeated removals. If these were excluded, the removals would be reduced from 31 in 54 months (~10 per year) to 12 in 54 months (~4 per year). Thus these units appeared to have skewed the no of removals in the fleet.

11. 3 out of 9 s/n’s with repeated removals had been scrapped: P549, P548, and G1653.

12. Another 2 s/n’s without repeated removals had been scrapped due transducer requires replacements and BER. (i.e J1396 and L1324)

13. Two aircraft accounted for 12 of the 31 removals: TGA and TGH: 6 each and and most of them were DC. Per H/well these units are aging be an possible cause of high removals.

14. Recommended SB’s for model HG480B13/B20 will be reviewed once Honeywell has provided their study. The recommended SB’s will be incorporated at next shop visit.

15. Data does not indicate if any of the shop is giving better results. Units repaired by H/well the OEM did not perform that well for one of the s/n with repeated removals. However one of the unit sent back for warranty claim to Miraj was DNC and did not perform well. The strip report by Miraj (UK) was also delayed.

16. The shop report of Miraj (UK) does not have incoming and outgoing mod status.

17. Some of the s/n or acft regn stated on the US label/RO did not tally with data in CAMS. This made analysis rather difficult for such discrepancies.

18. There are provisions on the CCA (4062841-105) to do minor “tweaks” in the frequency output without changing the calibration memory so that the transducer cards can be adjusted in the field, albeit a very narrow range. The range should be enough to handle the normal small output errors that occur as the transducer ages.

19. To reduce “Mach flag in view (8 removals)” defects, we can install CCA 4071160-901.

20. OEM recommends ADC be checked for accuracy about every two years per FAA guidelines.

21. 95% of unscheduled ADC removals are alpha-numeric s/n’s. These mean they were made before 1988, at least 26 years old. We are facing ageing problem with these units.

F) Conclusions
Air Data Computer with multiple failures had skewed the removal rate. These 6 units should be critically examined in the shop and scraped if nothing positive can be done. Reliability on these units is expected to reduce over time but recommendations above can improve it to industry levels.
These units are aging and not many in the market at the moment. With implementation of RVSM, this units will be exchanged./modified to HG480B100 for B727. But for B737 p/n HG480B13/B20 will still be used. With the recommendation above and upgrading the unit for RVSM requirements, the number of removals is expected to be reduced.

Boeing message no. 1-PL70I dated 25 Sep 2004 and 1-OVSAQ dated 16 Sep 2004
H/well message dated 1 July 2004 and 7 Oct 2004
H/well ADC SB’s
B727 MM p/n D6-27705137
B737 MM p/n D6-371295


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