On March 29, 2000, the FAA issued a final rule requiring the mandatory equipage of Terrain Awareness and Warning Systems (TAWS) equipment on turbine-powered airplanes that are configured to have six or more passenger seats. Aircraft operators had until March 29, 2005, to install the equipment and this rule is still in effect today.
- TAWS Requirements by Operator Type
- History of EGPWS and TAWS
- Evolution to EGPWS and TAWS
- System Protections
TAWS Requirements by Operator Type
TAWS applies to airplanes configured with six or more passenger seats, not to airplanes type certificated for six or more passenger seats. The rule requires the use of one of two types of systems, Class A or B, applicable as follows:
- Part 91
- Piston-powered airplanes and turbine-powered airplanes configured with fewer than six seats are unaffected by this rule.
- Turbine-powered airplanes configured for six or more passenger seats must have Class B TAWS.
- Part 135
- Turbine-powered airplanes configured for six to nine passenger seats must have Class B TAWS.
- Turbine-powered airplanes configured for 10 or more passenger seats must have Class A TAWS.
- Part 121
- Any turbine-powered airplane operating under this part must have Class A TAWS.
This chart describes the features provided by Class A and B TAWS.
| Class A | Class B |
|---|---|
| Imminent terrain impact | Imminent terrain impact |
| Premature descent | Premature descent |
| Excessive rates of descent | Excessive rates of descent |
| Negative climb rate or altitude loss after takeoff | Negative climb rate or altitude loss after takeoff |
| Flight into terrain when not in landing configuration | |
| Excessive downward deviation from ILS glide slope | |
| Descent of the airplane to 500 feet above the terrain or nearest runway elevation (voice callout “Five Hundred”) | Descent of the airplane to 500 feet above the terrain or nearest runway elevation (voice callout “Five Hundred”) |
| Class A TAWS also requires the installation of a situational display, which provides the operator with a visual display of the surrounding terrain and/or obstacles relative to the aircraft. | Class B TAWS does not require a situational display of surrounding terrain and/or obstacles relative to the aircraft. |
History of EGPWS and TAWS
GPWS was developed to combat controlled flight into terrain (CFIT) accidents, which were a leading cause of aviation fatalities in the 1960s and 1970s. A CFIT accident occurs when an airworthy aircraft, under the control of a qualified crew, is inadvertently flown into the ground, water or an obstacle with no prior awareness by the pilots.
Development and Implementation
Canadian engineer Donald Bateman, while working for Honeywell (then AlliedSignal/Sundstrand), is credited with inventing the first functional GPWS. His early systems, developed in the late 1960s and early 1970s, utilized the aircraft’s radar altimeter and other sensors to measure height above ground and descent rates. The system was designed to automatically issue aural and visual warnings, such as “SINK RATE” and the critical “PULL UP” command, if parameters indicating a potential collision were exceeded.
Following a series of fatal CFIT accidents and recommendations from the NTSB, the FAA mandated the installation of TSO-approved GPWS equipment on all large turbine and turbojet airplanes in 1974. The International Civil Aviation Organization (ICAO) followed with a similar recommendation in 1979.
Impact and Limitations
The introduction of GPWS had a dramatic effect on aviation safety. Prior to its mandatory implementation, large passenger aircraft experienced approximately 3.5 fatal CFIT accidents per year; this number fell to 2 per year in the mid-1970s and, by 2006, not a single passenger fatality in a large jet aircraft CFIT crash had occurred in U.S. airspace since the mandate.
Evolution to EGPWS and TAWS
The initial GPWS had a “blind spot.” It relied primarily on a downward-looking radar altimeter and could not provide sufficient advance warning for rapidly rising terrain directly ahead of the aircraft, such as a steep mountain slope. To address these limitations, an improved system, the enhanced ground proximity warning system (EGPWS), was introduced in 1996.
EGPWS incorporated a worldwide digital terrain and obstacle database and used GPS technology to determine the aircraft’s precise position and flight path. This allowed the system to look ahead and provide earlier, predictive warnings (forward-looking terrain avoidance function) and a visual terrain display in the cockpit.
The FAA later amended its rules in March 2000 to require the installation of an FAA-approved TAWS on most turbine-powered aircraft with six or more passenger seats, solidifying EGPWS as the new standard in ground proximity safety. The evolution of GPWS/EGPWS, credited largely to Don Bateman’s continuous innovation, is a cornerstone of modern aviation safety.
Consequently, the combination of technology, equipage of aircraft and effective use, according to a study issued by Airbus in 2020, the rate of CFIT accidents in airlines reduced by 89% from 0.18 per million flight hours in 1999 to 0.02 per million flight hours in 2019.
System Protections
Basic Modes
Basic modes are independent of position on the earth, rather they use proximity to the terrain below the aircraft or deviation from glideslope to alert the crew of a potential erosion of safety margin. These modes have no forward-looking prediction of flight path.
| Mode | Description | Example |
|---|---|---|
| Mode 1 | Excessive Descent Rate | “SINK RATE”, followed by “PULL UP” if the condition persists |
| Mode 2 | Excessive Terrain Closure Rate | “TERRAIN, TERRAIN”, followed by “PULL UP” |
| Mode 3 | Altitude Loss After Takeoff | “DON’T SINK” if significant altitude loss occurs after liftoff or a go-around |
| Mode 4 | Unsafe Terrain Clearance | “TOO LOW, GEAR” (if gear is not down), “TOO LOW, FLAPS” (if flaps are not in landing position), or “TOO LOW, TERRAIN” |
| Mode 5 | Excessive Deviation Below Glideslope | “GLIDESLOPE” (soft or hard alert depending on deviation and altitude) |
| Mode 6 | Advisory Callouts | Provides automated altitude callouts (e.g., “FIVE HUNDRED”), minimums alerts, and “BANK ANGLE” warnings |
| Mode 7 | Windshear Detection and Alerting | “WINDSHEAR”, “GO-AROUND”, or “GO-AROUND, WINDSHEAR” |
Watch a video showing the logic of the basic TAWS system modes.
Enhanced Functions
In addition to the standard seven modes, EGPWS incorporates advanced, database-driven functions that provide predictive warnings and enhanced situational awareness. These modes derive alerts using proximity to terrain and anticipation of the flight path trajectory to predict terrain conflicts and alert the crew accordingly.
| Terrain Look-ahead Alerting | Uses position data and an internal terrain database to predict potential conflicts with terrain along the projected flight path, providing cautions and warnings 30 to 120 seconds in advance (e.g., “CAUTION TERRAIN”, “TERRAIN, TERRAIN, PULL UP”) |
| Terrain Clearance Floor (TCF) | Creates a protective floor based on the aircraft’s position relative to the nearest or destination runway in the database, regardless of the landing gear or flap configuration, providing “TOO LOW TERRAIN” alerts if the aircraft descends below this floor |
| Terrain Awareness and Display (TAD) | Presents a graphical display of surrounding terrain on the navigation display or a dedicated screen, using colors to indicate terrain elevation relative to the aircraft (e.g., red for a high threat area, yellow for caution, green for safe clearance) |
| Predictive Windshear | Provides advance warning of potential windshear conditions using the terrain database and other inputs |
Each mode provides different warning levels based on the aircraft rate of closure to terrain or obstacles. All systems are dependent on the aircraft entering the caution or warning envelope.
| System Type | Alert Type | Time to Impact |
|---|---|---|
| GPWS (older systems) | Warning (general) | 20 seconds or less |
| Fixed Wing (EGPWS) | “Caution Terrain” | ~60 seconds |
| “Warning – Terrain, Pull Up” | ~30 seconds | |
| Rotorcraft (TAWS) | “Caution Terrain” | ~30 seconds |
| “Warning Terrain” | ~20 seconds |
Software Version
There are two aspects to EGPWS operational software which are important, the underlying software containing the operating logic and the terrain database.
Operating Software
The EGPWS operating software consists of sophisticated algorithms that integrate various aircraft parameters with an internal terrain database to predict and alert flight crews to potential conflicts with the ground or obstacles.
Having the most current version of this operating software is critical to ensure the most current algorithms are used to accurately predict potential conflicts while simultaneously reducing nuisance alarms. Updated software also ensures compatibility with other avionics systems in the cockpit, which may have also received updates. Maintenance providers can determine the current version of software and make updates as necessary.
Terrain Database
The terrain database is an internal database of worldwide terrain, obstacles and runways. Software updates often include enhancements to these databases or improvements in how that data is processed, ensuring the system has the most current and high-resolution information. Using outdated databases can lead to incorrect or missing warnings.
EGPWS terrain and obstacle databases are typically updated every six months, or on an “as-needed” basis when significant changes in terrain or obstacles are identified. Updating the terrain and obstacle database should occur as soon as practical after a new version is issued.
System manufacturers provide access to the updated software and current terrain and obstacle database through their website.
International Business Aviation Council Ltd.