and barometric altitude to determine the aircraft’s position  relative to the ground. The system uses this information in  determining aircraft clearance above the Earth and provides  limited predictability about aircraft position relative to rising  terrain. It does this based upon algorithms within the system  and developed by the manufacturer for different airplanes or  helicopters. However, in mountainous areas the system is  unable to provide predictive information due to the unusual  slope encountered.  This inability to provide predictive information was evidenced  in 1999 when a DH-7 crashed in South America. The crew  had a GPWS onboard, but the sudden rise of the terrain  rendered it ineffective; the crew continued unintentionally  into a mountain with steep terrain. Another incident involved  Secretary of Commerce Brown who, along with all on board,  was lost when the crew flew over rapidly rising terrain where  the GPWS capability is offset by terrain gradient. However,  the GPWS is tied into and considers landing gear status, flap  position, and ILS glideslope deviation to detect unsafe aircraft  operation with respect to terrain, excessive descent rate,  excessive closure rate to terrain, unsafe terrain clearance while  not in a landing configuration, excessive deviation below an  ILS glideslope. It also provides advisory callouts. Generally, the GPWS is tied into the hot bus bar of the electrical  system to prevent inadvertent switch off. This was demonstrated  in an accident involving a large four-engine turboprop airplane.  While on final for landing with the landing gear inadvertently  up, the crew failed to heed the GPWS warning as the aircraft  crossed a large berm close to the threshold. In fact, the crew  attempted without success to shut the system down and attributed  the signal to a malfunction. Only after the mishap did the crew  realize the importance of the GPWS warning. A TAWS uses GPS positioning and a database of terrain and  obstructions to provide true predictability of the upcoming  terrain and obstacles. The warnings it provides pilots are  both aural and visual, instructing the pilot to take specific  action. Because TAWS relies on GPS and a database of  terrain/obstacle information, predictability is based upon  aircraft location and projected location. The system is time  based and therefore compensates for the performance of the  aircraft and its speed.  [Figure 5-59]  The HUD is a display system that provides a projection of  navigation and air data (airspeed in relation to approach  reference speed, altitude, left/right and up/down glideslope)  on a transparent screen between the pilot and the windshield.  The concept of a HUD is to diminish the shift between  looking at the instrument panel and outside. Virtually any  An early application of technology to reduce controlled  flight into terrain (CFIT) was the GPWS. In airline use  since the early 1970s, GPWS uses the radio altimeter, speed,  TCAS II is a more sophisticated system which provides the  same information of TCAS I. It also analyzes the projected  flightpath of approaching aircraft and issues resolution  advisories to the pilot to resolve potential mid-air collisions.  Additionally, if communicating with another TCAS II  equipped aircraft, the two systems coordinate the resolution  alerts provided to their respective flight crews.  [Figure 5-58] RNG 5  1  .5  0  1  .5  4  2  6  4  2  12  -05  03  Terrain Alerting Systems  Ground Proximity Warning System (GPWS) Head-Up Display (HUD) Terrain Awareness and Warning System (TAWS) Figure 5-57. Alert System by Avidyne (Ryan). Figure 3-52.  TCAS II  Figure 5-58. An example of a resolution advisory being provided  to the pilot. In this case, the pilot is requested to climb, with 1,750  feet being the appropriate rate of ascent to avoid traffic conflict.  This visual indication plus the audio warning provide the pilot with  excellent traffic awareness that augments see-and-avoid practices. 5-34