Controlled Flight Into Terrain (CFIT), and other troubles are common for all aircraft. Obstacles are out there, the trick is avoiding them. The FAA has come up with various equipment that can help minimize the surprises obstacles may offer.
It wouldn't be an FAA requirement if there wasn't a technical standard order (TSO) to go with it. TSO-C151B covers terrain awarness and warning systems (TAWS). Ground Proximity Warning Systems (GPWS) are covered in this as well. Helicopter Terrain Awareness and Warning Systems (HTAWS) are covered in their own TSO-C194. For the most part, all the systems work similarly, the main difference is the performance of the aircraft.
Most TAWS systems work by knowing where the aircraft is and checking a terrain database to know if there are obstacles in the path of the aircraft. Knowing the trends (is the aircraft climbing or turning, etc) will allow a computer to know how much of the database needs checking. A current altitude source is needed. Sometimes this could be the barometric altitude, other time it may be a radar altitude.
There are various databases of terrain available. Either from the FAA or other sources. The FAA makes available the current database, and a daily update that will include construction cranes, and other changes to obstacles. The daily update file (DDOF) is available as the whole current database, and also just the changes.
The TAWS must be looking along the path of flight to function. The distance ahead is critical, especially at cruise altitude and speed. While most 121 aircraft are capable of clearing all but the highest mountains in the world, the assigned flight level or performance with fuel and passengers may not allow the current flight to climb over all the enroute terrain.
During approach to landing, and departure, there can be many more obstacles. Buildings, trees, construction cranes, and radio towers can be near airports. The TAWS must also consider these obstacles locations and provide safe clearance to avoid them.
Generally the TAWS should provide warnings if the aircraft is or will be between 100ft and 400ft of an obstacle during departure, and less than 1000ft during enroute portion of flight.
Within the TAWS there are various classes of capabilities provided by the systems. The classes are broken down as:
- Class A: Alerting based on; excessive rates of descent, excessive closure rate to terrain, negative climb rate after take-off, flight into terrain when not in landing configuration and excessive downward deviation on ILS approach. It will have voice alerting and sweep tones. It must also have a terrain display.
- Class B: Similar alerts, but with wider tolerances as the class A alerts. Class B assumes no Radar Altimeter, and the base threshold of the runway altitude. There must be an altitude call out for 500ft and other alerts, but class B TAWS does not require a display.
TAWS systems are required on some aircraft. All part 121 turbine aircraft must have a class A terrain awareness and warning system (121.354). All part 135 turbine aircraft that seat 6-9 passengers must have a class B TAWS (135.154). Even part 91 turbine aircraft that seat more than 6 seats must have a class B TAWS. More details are in AC25-23.
While TAWS have not eliminated all CFIT accidents, there are many situations where the systems have proven to save lives.