Gravity Wins: Electronic landing tools

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 TAWS

Class A TAWS has to provide three alerting functions including: