Gravity overcomes lift in every successful landing, and those of us involved in aviation do our best to make sure it is a photo finish more often than not. However, obstacles like mountains and man-made structures can present themselves in an aircraft's flight path. When visibility is greatly diminished, flight around dangerous terrain is still a possibility. Prior to 1975 the worldwide commercial airline fleet was averaging eight reported accidents per year where flight crews flew aircraft into the ground. This alarming situation did not go unnoticed and in response the FAA issued amendments to the FARs governing scheduled airlines and certain air charter operators. The amendments required the installation of an airborne system with the capability to forecast an impending Controlled Flight Into Terrain (CFIT). Technical Standard Order TSO -C92c identified requirements of the first Ground Proximity Warning Systems (GPWS). This computer-based system provides the flight crew with an advanced warning in the event of inadvertent infringement of the aircraft with terrain or obstacle safe clearances, taking into account crew recognition and reaction times. CFIT may occur during landing or possibly after take off if the aircraft should get into a negative rate of climb situation. Encounters can also be associated with flights into mountains or even man-made obstacles. Terrain Awareness and Warning Systems (TAWS) have to be able to calculate and anticipate most situations that could result in CFIT. Therefore, a wide array of aircraft sensors are required to provide the TAWS computer with pertinent data. In response to perceived hazards the computer can respond with both visual and aural cues.Ground mapping tools Corrected Barometric Altitude has been the primary source of altitude indication when the use of ground based landmarks is not possible, with corrections being made by an Air Data Computer (ADC). This corrected pressure may be erroneous under non-standard atmospheric conditions such as extreme temperature or strong pressure gradients. For years the use of weather radar as a ground-mapping tool has helped pilots determine geographic position and avoid dangerous obstacles in their path. Instrument Landing Systems (ILS) have given the aircraft a predictable and obstacle-free decent path to the runway. The availability of the global positioning system (GPS) has given precision navigation a whole new meaning. In fact, the use of Differential GPS will enable the system to not only calculate horizontal position over the earth but determine with some accuracy the height of the aircraft above ground level. This calculation, unlike Barometric Altitude, is not adversely affected by atmospheric conditions. The Radio or Radar Altimeter (RA) can be a tremendous asset in assessing potential terrain encounters, although this device does not usually become active until the aircraft is within 2,500 feet above ground level. TAWS are considered to be Enhanced Ground Proximity Warning Systems (EGPWS). This involves the use of a computer with a database constructed with information resulting from the electronic mapping of the earth's geographical regions. Blending this information with data from Radio Navigation systems (RNAV) or GPS gives the system the ability to perform geometric calculations to determine any possibility of an unwanted terrain encounter well in advance. The maintenance angle The FAA maintains close scrutiny on these new developments and has come up with some mandates on who will be required to install these new systems. Two classifications of equipment are currently being prescribed as a remedy for CFIT. Class A TAWS is required for FAA Part 121 and certain Part 135 operators. Class B will fulfill the mission for the remainder of Part 135 along with FAR Part 91 operators. Technical Standard Order TSO-C151a provides requirements and conditions that must be satisfied and Advisory Circular AC 25-23 provides the Airworthiness criteria for installation and approval.Class A TAWSClass A TAWS has to provide three alerting functions including:
- 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.
