OT Safety Bulletin

Factors that can reduce the effectiveness of Ground Proximity Warning System (GPWS) equipment

Contributed by Len Cormier, Bermuda DCA

ICAO has for some years now mandated that States implement regulations to ensure that aircraft are equipped with a ground proximity warning system which has a forward looking terrain avoidance function (commonly referred to as Terrain Avoidance Warning System (TAWs) or Enhanced Ground Proximity Warning System (EGPWS)).   

There are factors that can reduce the effectiveness of GPWS equipment. Several low-cost but crucial measures can be taken to reduce the likelihood of false GPWS warnings or, more seriously, the system’s failure to provide a valid warning.

The GPWS safety issues that have been identified concern the upkeep of software on which the GPWS depends, as well as the obstacle, runway and terrain database and the provision of Global Navigation Satellite System (GNSS) positioning.

SOFTWARE UPDATE - Perhaps the most easily rectified shortcoming involves the software utilised by GPWS. Software updates are issued regularly, yet industry sources reveal these are not being implemented by all operators, or are not installed in a timely manner.

Most recently in August 2013, a Controlled Flight Into Terrain (CFIT) accident occurred in which an Airbus A300 crashed on a non-precision approach to a runway in Birmingham, Alabama.  As noted in the National Transportation Safety Board (NTSB) report on this accident, the software that offered enhanced alerting had been available since December 2003 but was not installed in this aircraft. If the aeroplane had been equipped with the latest version of EGPWS software available, the caution alert the flight crew received would have sounded about 6.5 seconds earlier, and 150 feet higher.

DATABASE UPDATE - Similarly, it is crucial to regularly update the obstacle, runway and terrain database provided by manufacturers for use with their equipment, since the proper functioning of the GPWS may otherwise be jeopardised. Again, updates are issued for these databases on a regular basis by equipment manufacturers. GPWS operation can also be undermined by the lack of suitable navigational input. The equipment was designed to function with a position update system, but not all installations are linked to GNSS receivers.  While the required position data can be acquired by using an effective ground-based navaid network, the most reliable of which is provided by DME/DME, such support for area navigation systems is not available everywhere. Use of GNSS, accessible worldwide, eliminates the possibility of position shift, which is another source of false warnings (or worse, the failure to provide a genuine warning).

Aircraft operators can obtain the greatest safety benefit from GPWS by following certain practices directly related to the equipment in use. They should:

• Update software to the latest available standard;
• Update databases to the latest available standard;
• Ensure that the GNSS position is provided to GPWS; and
• Implement any applicable service bulletins issued by manufacturers.

It is essential that other measures be undertaken to ensure CFIT prevention through effective use of GPWS. These measures include, but are not limited to: crew training; use of standard operating procedures; crew reporting and operator investigation of spurious warnings; and implementation of a safety management system by the operator.