lancaster sfty320 paper final

Running Head: TRAFFIC ALERT COLLISION AVOIDANCE SYSTEMS 1TRAFFIC ALERT AND COLLISION AVOIDANCE SYSTEMS AND HUMAN FACTORS

by:Aaron Lancaster

SFTY 320: Human Factors in Aviation SafetyEmbry-Riddle Aeronautical University

Worldwide CampusMay 2011

TRAFFIC ALERT COLLISION AVOIDANCE SYSTEMS 2

Traffic Alert and Collision Avoidance Systems (TCAS)

Flight rules, by their very nature, create certain problems that must be solved by the pilot

operating with in the confines of those rules. There are two primary sets of rules under which all

pilots operate in any class of airspace; Visual Flight Rules (VFR) and Instrument Flight Rules

(IFR). The factor commonly referred to as “aircraft separation” by Air Traffic Control (ATC)

services is in its most basic form, keeping aircraft far enough apart for safe operations to be

conducted.

Under IFR, the task of aircraft separation is the responsibility of ATC. However, ATC is

not perfect. Though this task is usually accompanied by other tasks which ATC will accomplish,

including aircraft sequencing and vectoring, it is still the responsibility of the pilot to “see and

avoid” (“Midair Collision Avoidance”, 2008).

The primary problem of VFR flight for the pilot has long been the regulatory

responsibility of “seeing and avoiding” other aircraft (“Pilot's Role in Collision Avoidance”,

1983). These aircraft vary greatly in size, shape, color and airspeed. Likewise, their pilots vary

greatly in age, experience, and certification level. These factors collectively contribute to the

recognition, perception, and reactions by the pilot to other aircraft in the vicinity. The problem of

“seeing and avoiding” other aircraft centers firmly on the task of collision avoidance. Systems to

handle this task have been the subject of ongoing research since at least the 1950's (“Intro to

TCAS II”, 2000).

Description & Applications

What is TCAS?

Traffic Alert Avoidance System or Traffic Alert and Collision Avoidance Systems

(TCAS) is a means to provide cockpit instrumentation of air traffic in the vicinity of an aircraft


while in flight. The FAA states, “The Traffic Alert and Collision Avoidance System (TCAS) is an

airborne system developed by the FAA that operates independently from the ground-based Air

Traffic Control (ATC) system. TCAS was designed to increase cockpit awareness of proximate

aircraft and to serve as a 'last line of defense' for the prevention of mid-air collisions.” (“TCAS

Home Page”, 2011).

The International Civil Aviation Organization further defines Airborne Collision

Avoidance System (ACAS) as, "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." (“Flimsy

3”, 2010). For the purposes of this paper, the terms “TCAS” and “ACAS” will be used

synonymously unless specific reference is being made to foreign collision avoidance systems.

TCAS manifests itself in the cockpit as a digital readout similar in appearance to a digital

Electronic Horizontal Situation Indicator (EHSI) or other Navigational Display (ND)

(“TCASII/ACASII User's Manual”, 2000). These displays are common to most cockpits of

either the glass or older steam-gauge type and are familiar to pilots.

Secondary Surveillance Radar System

Secondary Surveillance Radar (SSR) is a type of Air Traffic Control Surveillance System

that supplements primary radar. Primary ATC radar detects aircraft within range of the radar

ground station but only provides location information. SSR emits a radar signal which is detected

by airborne equipment aboard aircraft called the “transponder.” Transponders are also referred to

as Air Traffic Control-Radio Beacon Systems (ATC-RBS). The transponder responds to the the

interrogation signal of the SSR and replies with a code which the pilot has pre-programmed.

The Transponder code may have been provided by ATC to the pilot to uniquely identify


the aircraft to ATC. Certain codes are reserved for specific operating conditions, such as

emergencies, that serve to signal ATC (“FAR/AIM”, 2011). Transponders may also be

programmed to provide additional information to the interrogating SSR signal such as airspeed,

altitude, and heading. ATC uses all of these elements to make informed decisions concerning

traffic within the area (“TCASII/ACASII User's Manual”, 2000).

The TCAS relies on aircraft being equipped with Secondary Surveillance Radar (SSR)

Transponders. All commercial aircraft are equipped with this type of transponder since it is a

requirement for entering the certain classes of airspace that surround busy airports for the

purpose of traffic management (FAR/AIM, 2011). However, this requirement may not affect

General Aviation (GA) users who never enter these types of airspace and whose aircraft are

therefore not equipped with transponders.

The TACS does not rely on SSR to provide an interrogation signal. Instead, TACS emit a

signal similar to SSR that causes SSR Transponders to reply. From these replies the TACS builds

a 3-D map of the airspace and charts where aircraft are located within it (“TCAS II/ACAS II

User's Manual”, 2000).

Precipitating Incidents

TCAS Research and Development (R&D) was spurred-on by several aircraft incidents.

Most of these incidents resulted in losses of many lives. These incidents took place between

1956 and 1986 (“Intro to TCAS II”, 2000). The three primary incidents which have most greatly

influenced TCAS R&D are listed here.

The first incident to draw major public and FAA attention to the need for collision

avoidance system R&D was the Grand Canyon midair collision of 1956. In this incident United

Airlines Flight 718 and TWA Flight 2 collided over Temple and Chuar Buttes of the Grand


Canyon, AZ on June 30. This incident resulted in 128 fatalities and provoked airline and aviation

authorities to begin R&D studies for systems in collision avoidance (“Intro to TCAS II”, 2000).

The second incident which played a major role in TCAS R&D was the San Diego, CA

midair collision of 1978. In this incident Pacific Southwest Airlines (PSA) Flight 182 (a Boeing

727-214) and a private Cessna 172 collided and crashed into the North Park neighborhood of San

Diego, CA. This crash resulted in 144 fatalities and 9 injuries, some of which were inhabitants of

North Park. It also led to the FAA's initiation of the formal R&D program for TCAS (“Intro to

TCAS II”, 2000).

The last of these incidents is the 1986 midair collision of Aeromexico Flight 498 (a DC-

9) and a private Piper PA-28-181 Archer over Cerritos, CA. This incident resulted in 82 fatalities

and 8 injuries including individuals on the ground. 36 of the commercial passengers were U.S.

Citizens and this led the U.S. Congress to mandate that certain categories of both foreign and

domestic aircraft be equipped with TCAS in order to conduct operations within U.S. airspace

(“Intro to TCAS II”, 2000).

Generations

In the late 1970's, a version of TCAS called Beacon Collision Avoidance System

(BCAS), was developed to listen to the transponder responses to two or more ATC SSR stations,

locate adjacent aircraft and provide alerts (Ricker, 2006).

Currently, there are two generations of TCAS in use. TCAS I, was the original design

specification, intended for GA users and regional airlines. TCAS I provides basic functionality

that helps pilots acquire other aircraft visually. TCAS I is mandatory in aircraft with 10 to 30

seats (“TCAS Home Page”, 2011). TCAS I was formally implemented in 1993 and became

required by Federal regulation in 1997 (“Flimsy 1 Attachment”, 2010).


TCAS II is a newer and more advanced generation that builds on the functionality of

TCAS I by adding the ability to create a projected flight path and analyze this information.

Newer software versions of TCAS II incorporate a transponder Mode-S datalink via which two

TCAS's can negotiate and coordinate deconfliction. TCAS II is an international requirement on

aircraft with more than 30 seats or weigh more than 15,000 kg. (“TCAS Home Page”, 2011).

TCAS II entered service in 199 and has been a U.S. requirement since 1993 and an international

requirement since 2003 (“Flimsy 1 Attachment”, 2010).

Other Generations

The most advanced calculations of TCAS II are only able to compute projected flight

paths accurately enough to provide deconfliction instructions to aircraft for maneuvering

vertically. Significant work was in progress to attempt to improve the power of the calculations

in order to enable more accurate RA to include horizontal maneuvering (Hahn, 1996). However,

after a great deal of testing and study it was determined that the errors in the system were too

great and the efforts into the development of TCAS III and TCAS IV were both abandoned in

favor of developing Automatic Dependent Surveillance – Broadcast (ADS-B) in which position

information is sent to ATC in real-time and aircraft with ADS-B receivers can see all other

aircraft in the airspace, not just those with the same equipment (Burgess, et. al., 1995). ADS-B is

part of the NextGen airspace management system under development by the FAA (“Fact Sheet –

NextGen”, 2007).

Advisories

TCAS's of both generations are pre-programmed with standardized safety ranges based

on RTCA, Inc. document DO-185A (December, 1997). These range dimensions can be as much

as 14NM (“Overview of ACAS II/TCAS II”, 2009). These range dimensions vary with airspeed to


provide the pilot with visible and audible advisories classified as, “Intruder”, “Traffic Advisory

(TA)”, “Resolution Advisory (RA)”, and “Clear of Conflict” based on the proximity and action

required (“TCAS II/ACAS II Collision Avoidance System User’s Manual”, 2000).

Pilot Actions

Pilots are trained to react to RA by maneuvering the aircraft vertically by means of

climbs or descents. The RA indicates to the pilot the rate at which the pilot should climb or

descend.

The information available to the TCAS is more current and accurate relative to the

aircraft than the information ATC has at its disposal due to the cycle time of the ATC SSR. TCAS

does not have this limitation because it transmits and receives signals on multiple antennas. For

this reason, pilots are trained to prioritize TCAS RA over ATC instructions, should a conflict

arise. This means that if ATC gave a clearance to one or more aircraft in order to prevent a

collision and these instructions were immediately followed by TCAS RA that were in conflict a

collision could be caused rather than prevented (“Investigation Report”, 2004). This issue will

be revisited later in this paper.

Vertical Clearance

A problem develops when the aircraft does not have sufficient clearance below to respond

to RA that directs a descent. The situationally aware pilot would realize this and not respond to

the RA. However, this is not always the case and this issue has been the subject of continuing

research and study related to TCAS.

TCAS HF Implications

In this part we will look at TCAS from a Human Factors (HF) perspective. Specific

attention will be given throughout to the human factors interfaces as defined by the SHEL-L


model and their relationship to the design, implementation, and use of TCAS. Next, we will take

a look at Cockpit Resource Management (CRM) implications. Finally, we will examine several

aircraft incidents and how they relate to HF in TCAS today.

Design & Implementation

An inherent limitation of the design process is the lack of knowledge of future

technologies and relationship to the current technologies that are being developed (concurrent).

This largely is a Liveware-Liveware interface problem in the design specialty of the industry.

This could be reduced or even solved by greater amounts of information sharing between

industry agencies and corporations. Perhaps this might best manifest itself through standards

groups. One barrier to such information sharing is the proprietary nature of ideas and inventions.

TCAS designers are also restricted by the current cockpit environment. TCAS must be

incorporated into the cockpit where other systems already in use have set precedents and even

established closed standards which define the cockpit environment and way pilots interface with

those systems. This is a flaw in both the Liveware-Hardware and Liveware-Software interfaces.

There is little that can be done concerning the manifestation of instrumentation in the cockpit

aside from a completely new approach. This is likely to be very costly and therefore impractical.

Perhaps one approach to overcoming this is to determine the ultimate end-state of these systems

and focus on “bridge-technologies that will enable future implementation of that end-state. As

stated earlier in this section, standards organizations and collaborative efforts are the best way to

ensure universal integration and adaptability of new systems.

The need for implementation into cockpit environments which do not support TCAS such

as small aircraft, which may not have a transponder, has led to the development of Portable

Collision Avoidance Systems (PCAS), such as the “MRX” produced by Zaon Flight Systems.


These systems which resemble a police radar detector such as those used in many automobiles

and operate on internal battery power enable pilots who own or use such aircraft or even multiple

aircraft to increase their situational awareness and see and avoid other aircraft (“Owner's

Manual: PCAS MRX”, 2008).

TCAS Use & Cockpit Resource Management

TCAS use has drastically changed since its advent. Initially, TCAS was received poorly

because early software versions were flawed and provided the crew with a high number of false

TA's and RA's. Some of these were much more severe than others. For example, two of the

original, and fatal, aircraft incidents that spurred the R&D of TCAS dealt with aircraft

simultaneously changing altitudes. In the early days of TCAS, it was not uncommon for pilots

who were on final approach, which involves a drastic change in altitude, to receive false RA's.

For this reason, many pilots largely regarded TCAS as an annoyance or merely another system

that crowded an already over-crowded cockpit, increasing workload. TCAS was also viewed as

a distractor, creating Cockpit Resource Management (CRM) difficulties. These issues were with

the original TCAS II software version 6.00. They were later mostly resolved with software

version 6.04. With the improvement of this Liveware-Software interface, pilots began to view

TCAS as a way to gain greater situational awareness, streamline CRM, and even use it in new

and creative ways such as avoiding wake turbulence from aircraft in their flight path and

increasing fuel efficiency (Ricker, 2006).

TCAS Incidents & Associated Human Factors

Three main incidents resulted from the improper use of TCAS in which HF was a

primary finding of the incident investigation.

Incident #1: The first of these was the 2001 Japan Airline mid-air incident. In this incident


Japan Airlines Flight 907, a Boeing 747-446, and Japan Airlines Flight 958, a DC-10-40D,

almost collided with each other. The pilots of the B747 ignored the RA issued by their TCAS to

stop their climb, choosing instead to follow ATC instructions, contrary to their training. At the

same time, the crew of the DC-10 followed their RA. Both planes had received conflicting

instructions from ATC. As a result, the pilots and controllers failed to properly deconflict the

aircraft resulting in a near mid-air collision. During the evasive maneuvering of the B747 many

passengers were injured and a galley cart was even thrown through the ceiling of the aircraft. The

pilots stated they estimate the aircraft cleared each other by only 35 feet. Had this been an actual

mid-air collision, it would have been the largest loss of life in aviation history. Findings from the

incident investigation showed that the ATC controllers mixed up flight numbers, forgot about the

DC-10, and the ATC trainee lost his composure and issued an errant instruction. Additionally, the

pilots mis-prioritized the instructions of ATC and TCAS choosing to follow ATC instructions

over TCAS RA. As indicated in an earlier section entitled “Pilot Actions,” this is incorrect. The

investigation findings also indicate that the crew did not properly communicate to the Captain

the poor judgement of ignoring the RA (Tomita, 2005).

Incident #2: The Second incident was the Uberlingen mid-air collision of 2002. In this incident

Bashkirian Airlines Flight 2937, a Tupolev Tu-154M passenger jet, and DHL Flight 611, a

Boeing 757-23APF cargo jet, collided over Uberlingen, Germany resulting in 71 fatalities. The

ATC controller was working two stations simultaneously. He lost situational awareness as he

struggled to use a malfunctioning telephone and issued an errant clearance to de-conflict the

aircraft and also missed a transmission from the DHL flight that TCAS RA was in effect.

Additionally, the crews of both aircraft ignored their TCAS RA that were issued immediately


after the errant ATC instructions were issued. These factors combined with various ATC systems

being under maintenance at the time lead to the incident and subsequent loss of life. This incident

was followed by the murder of the ATC controller some time later by a family member who had

degraded psychologically form losing his wife and daughter in the crash (“Investigation

Report”, 2004).

Incident #3: The third incident that resulted from human factors in the use of TCAS was the

mid-air collision of Gol Airlines Flight 1907 a Boeing 737-8EH with an Embraer Legacy 600

business jet (N600XL) over Mato Grosso, Brazil in 2006. This incident resulted in the B737

crashing into the dense rainforest below and 154 fatalities. However, the Embraer jet landed with

no injuries. The CENIPA investigation findings indicate that an improper ATC handoff was

conducted. The the receiving controller did not realize that the inbound aircraft had not been

issued the proper altitude clearance and resulted in the two aircraft utilizing the same airway, at

the same altitude, and in opposite directions (“Midair Collision Final Report 1907 English

Version”, 2006).

This was not recognized by ATC or the flight crews because the Embrear's transponder

was non-operational unknown to the flight crew. Neither ATC or the flight crew recognized that

the ACAS and transponder were non-operational and the flight crew did not check to ensure it

was operational. Additionally, two way communication was lost between the Embraer and the

ATC, which went unrecognized and uncorrected by both parties (“US Summary Comments”,

2006).

Conclusion

In this paper we have seen how TCAS plays a very important role in collision avoidance.


In the first part of this paper we learned about what TCAS encompasses, its technological

generations and complementary technologies, how it alerts pilots to a potential problem, what the

pilot does in response, and some limitations of the system. In the second part we focused on the

HF implications of TCAS and the affected interfaces, how TCAS can degrade or enable CRM

and examined HF-related TCAS incidents. Throughout this paper we have seen how a very

powerful technology can be used as a great enabler or to the mortal detriment of aviation

operations. TCAS is central to avoiding aircraft incidents involving mid-air collisions and will

continue to play an important role in the future of aviation safety until replaced by a more

advanced, more reliable technology such as ADS-B.


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