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For free flight in visual meteorological conditions (VMC) terrain and obstacle avoidance capabilities must be provided by different means.

Numerous helicopter accidents occur each year in VMC, involving either a collision with the terrain or with known charted obstacles such as power lines. The reasons often involve the pilot having too high a workload to continuously monitor the relative positions of all obstacles.

To reduce the number of accidents of this type, onboard DTM and obstacle databases can be continuously scanned by computer, producing a warning if the aircraft's flight path gets too close to any obstacle.

A DTM database on its own is not sufficient for safe navigation. It must be supplemented by a comprehensive obstacle database.

The obstacle database contains the position and peak height above mean sea level of all significant obstacles in the area.

Currently defined obstacle database categories are:

• buildings
• masts and towers
• pylons and power lines
• survey trig stations
• spot heights

The obstacle category determines whether an extra error allowance is made during calculations. For example a standard vegetation allowance is added to spot heights to allow for any tree growth in the vicinity.

Plot of Obstacles in Database using TGS

Close up View of Obstacles in Database

Digital Design Ltd. has developed software to prove the feasibility of computer generated routes involving terrain and obstacle avoidance. The software processes the DTM and obstacle databases and calculates route parameters such as headings and distances, the minimum safe altitude (MSA) and minimum set heading altitudes. This is all computed automatically, and takes only seconds to complete.

The criteria used for these routes are existing ICAO PANS-OPS (Doc 8168) standards for conventional navaids, but can easily be adapted to TERPS criteria, or criteria for GPS based route navigation. The use of existing standards was necessary to enable the computer generated results to be compared to the parameters of existing promulgated routes.

A high degree of correlation was found, with the only anomalies identified being due to the varying standards used over time for the existing manually designed routes.

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