Terrain Awareness and Warning System (TAWS) uses databases and onboard sensors to calculate the position of the aircraft relative to surrounding terrain. Its core functionality delivers terrain awareness and predictive warnings required for commercial and general aviation safety. When operating at low altitudes in close proximity to terrain, the awareness system may naturally produce many unwanted warnings, yet safety doctrine states that a stall warning or windshear warning takes precedence over an RA.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the difference between TAWS and TCAS?
The difference between TAWS and TCAS is that TAWS is land-based and protects against terrain, while TCAS is airborne and protects against other aircraft. TAWS provides a hard warning that the aircraft is in a dangerous situation and immediate action is required. TCAS provides traffic advisories (TAs) and, in the case of TCAS II, resolution advisories (RAs) with recommended vertical maneuvers.
Because terrain is immovable and must be maneuvered around, TAWS alerts take precedence over any TCAS RA. When TAWS issues an alert a signal is sent to the TCAS to force it into TA-Only Mode, so the TCAS can only issue TAs which are advisory and require no action.
TAWS uses radio-altimeter antennas that measure height above terrain, whereas TCAS uses a directional antenna to determine bearing and the interrogation-and-response round-trip time to determine range, building a three-dimensional map of transponder-equipped aircraft in the airspace.
Terrain proximity is evaluated only by TAWS, which uses radio-altimeter reading and terrain-closure rates derived therefrom to detect hazardous proximity. TCAS does not sense terrain and instead monitors the relative location and altitude of other transponder-equipped aircraft, analyzing projected flight path and time to the Closest Point of Approach to decide when to generate a TA or an RA.
When is a TAWS system test deemed OK?
A functional test of any Terrain Awareness and Warning System is deemed OK only when every loudspeaker delivers the exact, standard phraseology in the prescribed order. During an initial self-test the equipment must voice the caution words SINKRATE, TERRAIN, DON'T SINK, TOO LOW, GEAR, TOO LOW, FLAPS, TOO LOW, TERRAIN, and FIVE HUNDRED without distortion. The same test must also prove that GLIDESLOPE and the hard warning PULL UP, PULL UP will be issued when the aircraft is deviating far below the glideslope or approaching terrain.
If these calls are faint, clipped or absent, the test is failed. The FAA warns in every Information for Operators notice that muting or ignoring the alerts invites CFIT accidents. In flight, the system must continue operation in the landing configuration and must keep the terrain awareness display alive. If either function drops out, the test is again deemed unsatisfactory. Pilots treat the cockpit speaker check as a red-tag item: the moment any caution or warning word is missing, the TAWS system is not OK and the aircraft stays on the ground.
