- Forward Looking Terrain Avoidance (FLTA) enabling the system to determine reduced required clearance when the aircraft does an approach to landing. It also has to predict imminent terrain impact with ample margin to allow the crew to react and implement evasive maneuvers.
- Premature Descent Alert (PDA). This advises the crew when the aircraft has deviated downward from its predicted vertical path.
Basic Ground Proximity Warning Functions include:
1. Excessive rate of descent
2. Excessive closure rate to terrain
3. Negative climb rate or altitude loss after take-off
4. Flight into terrain when not in landing configuration
5. Excessive downward deviation from the Instrument Landing System Glide Slope.
6. Descent of the aircraft to 500 feet above the terrain or nearest runway elevation and provide a voice callout "Five Hundred"
Class A, TAWS also requires the installation of a flight deck mounted display, which will place an aircraft symbol on a digital terrain map. This presentation may be included on a Weather Radar screen or other type of Electronic Flight Instrument System (EFIS) and must include aircraft position information provided by the Flight Management System (FMS) or GPS along with other navigation systems spelled out in Advisory Circular AC 25-23.
Class A TAWS equipment must be able to present the flight crew with a display that illustrates the aircraft position so that the pilot can estimate the relative bearing and distance to any terrain that may represent a hazard. In addition, the display has to give the crew the ability to identify variations in ground elevations within 2,000 feet of the aircraft. A separate distinguishable alert must be provided in the event of an encounter with hazardous terrain. In the event of a system failure or a condition where the TAWS has been inhibited, some type of alert conforming to the flight deck design philosophy has to be presented to the flight crew.
Class B TAWS
Class B Terrain Awareness and Warning Systems have the same three alert functions but the basic GPWS functions only have to include:
1. Excessive rates of decent.
2. Negative climb rate or altitude loss after take-off.
3. Descent of the aircraft to 500 feet above ground level or nearest runway elevation with a voice callout "Five Hundred".
The Class B version of this system will not require a flight deck display, however, provisions should be made so that a visual indication could be added later.
Horizontal position information is still required with an approved GPS being called out in the Advisory Circular. In addition, there is no requirement for the interface of a Radio Altimeter.
Phase of flight
One of the most important features of TAWS is the ability to recognize the "Phase of Flight."
There are four phases:
- En-route Phase: The aircraft is more than 15 nautical miles from the nearest airport or whenever the conditions for the remaining three phases are not met.
- Terminal Phase: The aircraft is within 15 nautical miles of the nearest runway and the distance to the runway is decreasing and the aircraft is at 3,500 feet or below. This phase also applies when the aircraft is within five nautical miles and is flying at or below 1,900 feet above the nearest runway altitude.
- Approach Phase: Distance to the nearest runway threshold is equal to or less than five nautical miles and height above the nearest runway threshold is equal to or less than 1,900 feet and distance to the nearest runway threshold is decreasing.
- Departure Phase: This condition should be realized by some reliable device that determines the aircraft is in a weight on wheels state at initial power up. Once the aircraft reaches 1,500 feet above the departure end of the runway the departure phase is terminated.
GPWS operating modes are automatically actuated when the aircraft is in one of the following flight configurations:
1. Excessive descent rate relative to cruise and approach phase altitude
2. Excessive rate of closure to terrain relative to altitude, flight phase, and airspeed
3. Altitude loss during take-off phase
4. Insufficient terrain clearance relative to flight phase and airspeed
5. Inadvertent deviation below glide slope beam during ILS approach phase
6. Call-outs warning of descent below pre-established altitude and selected decision height
7. Flying in dangerous wind shear conditions during take-off or final approach phase
Many aircraft will utilize flap position along with landing gear position information, capitalizing on the capabilities of the radio altimeter. In fact most airframe manufacturers will provide both operational as well as functional testing data for EGPWC. Frequently these assessments will require the use of specialized test equipment including air data simulation equipment. In most cases all airframe sensors that supply the TAWS will be verified as operational. Devices like TAWS provide the worldwide aviation maintenance community with significant challenges, such as the impact of an improperly rigged flap. In the case of TAWS equipped aircraft, this may signify that the aircraft is in a different mode of operation than in reality. In the event the system does not include the artificial intelligence needed to inform the flight crew of a disagreement in aircraft configuration, the crew may not be receiving the true picture as what would be considered hazardous terrain.
Airframe or TAWS manufacturers documentation should always be consulted to determine what airframe devices are tied to the TAWS and subsequent testing methods should always be employed to ensure continued airworthiness of the system.
Yes, it is still a given that gravity is going to triumph, but now it is almost a sure bet that we will only let it win on our terms.