Evaluating a Direct-To Routing Terrain Awareness and Warning Systems TAWS A and TAWS B Risk: Silencing TAWS Alerts TAWS Alerts you to upcoming terrain. At a closure time of approximately  1 minute, a “Caution! Terrain!” alert is issued. This alert  changes to the more serious “Terrain! Terrain!” alert when the  closure time reaches 30 seconds. In some areas of the world,  this terrain warning may very well be too late, depending on  the performance of the aircraft. You need to determine the  equipment’s criteria and note if the unit makes allowances for  lower power output of the powerplant(s) at higher elevations,  resulting in lower climb rates than may be programmed into  the unit for that aircraft. One of the risks involved in proceeding directly to a waypoint  is that you may be yet unaware of any significant terrain  between the present position and the waypoint. A terrain  display offers a convenient way of seeing clearly what lies  between here and there as reported and documented in the  database. Always consult the MEF values along the path of  flight to ensure terrain and obstruction clearance. A second type of aural alert warns about excessive descent  rates sensed by the system (“Sink Rate!”) or inadvertent loss  of altitude after takeoff (“Don’t Sink!”).  A terrain awareness and warning system (TAWS) offers  you all of the features of a terrain display along with a  sophisticated warning system that alerts you to potential  threats posed by surrounding terrain. A terrain awareness  and warning system uses the aircraft’s GPS navigation signal  and altimetry systems to compare the position and trajectory  of the aircraft against a more detailed terrain and obstacle  database. This database attempts to detail every obstruction  that could pose a threat to an aircraft in flight. The introduction of terrain awareness and warning systems  has sharply reduced the number of CFIT accidents. Despite  this significant leap forward in safety, incidents and accidents  involving terrain still happen. In the modern TAWS‑equipped  cockpit, some of these incidents have been related to pilot  reaction to TAWS alerts. TAWS sometimes gives nuisance  alerts that desensitize the pilot to TAWS alerts, which can  result in the pilot’s decision to ignore a valid alert deemed  unnecessary by the pilot. Most TAWS systems contain  software logic that attempts to recognize and remain silent in  situations in which proximity to terrain is normal. This logic  is partly based on the aircraft’s distance from the runway  of intended landing. For example, flying at an altitude of  200 feet AGL when 3,500 feet away from the runway is  reasonable, but flying at an altitude of 200 feet AGL when  5 miles from the runway is not reasonable. TAWS’ logic  attempts to silence itself in normal situations, and to sound  in abnormal situations. There are presently two classes of certified terrain awareness  and warning systems that differ in the capabilities they  provide to the pilot: TAWS A and TAWS B. A TAWS A system provides indications for the following  potentially hazardous situations: Despite efforts to minimize nuisance alerts, they still occur  occassionally. For this reason, most TAWS systems offer a  terrain inhibit switch that allows you to silence TAWS alerts.  There have been cases in which pilots have used the inhibit  switch or ignored TAWS alerts, thinking they were nuisance  alerts, when in fact the alerts were valid indications of a  dangerous situation. For this reason, you should train yourself  to respond to TAWS alerts just as you would to any other sort  of emergency. Always, if in any doubt, set “Full Power and  Climb” at V$_{X}$  or V$_{Y}$, depending on the equipment manual  and AFM/POH. The practice of simply ignoring or disabling  TAWS alerts based on pilot intuition has not proved to be a  safe one. Your manufacturer’s reference manual and aircraft  flight manual supplement will prescribe specific procedures  for responding to TAWS alerts. A TAWS B system provides indications of imminent contact  with the ground in three potentially hazardous situations: Aural alerts issued by a terrain awareness and warning system  warn you about specific situations that present a terrain  collision hazard. Using a predictive “look ahead” function  based on the aircraft’s ground speed, the terrain system alerts  The only current, fully certified systems, known as TAWS,  are certified under Technical Standards Order (TSO)-C151.  5-8 1. Excessive rate of descent 2. Excessive closure rate to terrain 3. Altitude loss after takeoff 4. Negative climb rate 5. Flight into terrain when not in landing configuration 6. Excessive downward deviation from glideslope 7. Premature descent 8. Terrain along future portions of the intended flight  route 1. Excessive rate of descent 2. Excessive closure rate to terrain (per Advisory Circular  (AC) 23‑18, to 500 feet above terrain) 3. Negative climb rate or altitude loss after takeoff