GUY NORRIS / LOS ANGELES
Predictive alerting device designed for combat aircraft could help reduce the risk of controlled flight into terrain
Flight testing of an innovative combined terrain and traffic avoidance system (T2CAS) was completed in Arizona on 4 February, clearing the way for what the developer hopes will be the granting of US Federal Aviation Administration certification within weeks.
Aviation Communication & Surveillance Systems (ACSS), a joint L-3 Communications and Thales company, is developing T2CAS as an evolution of its TCAS 2000 with added terrain awareness warning system (TAWS) functionality. The combined product, says ACSS, provides an upgrade path for operators seeking compliance with new TAWS and TCAS requirements.
At the heart of T2CAS is the ground collision avoidance module (GCAM), a predictive alerting device derived from a terrain-following/terrain-avoidance system developed for the Dassault Mirage 2000N combat aircraft by Dassault Electronique (now part of Thales). GCAM uses a collision prediction and alerting (CPA) function that combines terrain and airport digital databases with a model of the aircraft's climb capability.
It is the latter feature which, according to ACSS, is the main differentiator between T2CAS and any previous commercial TAWS device. The CPA function predicts potential terrain conflicts from two main computational paths: terrain environment determination and aircraft flightpath prediction.
The former correlates precise aircraft position information from GPS and/or inertial sources with the digital database. The flightpath prediction uses current aircraft flightpath data to project the flightpath more than 2min ahead of the aircraft, and provides visual cautions up to 4min ahead.
Alerts are generated when the system predicts the flightpath might intersect the terrain elevation, providing terrain avoidance rather than just a warning.
"It's all about time," says flight safety adviser Greg Feith, a former National Transportation Safety Board investigator who led the inquiry into the Korean Air Boeing 747-300 controlled flight into terrain (CFIT) accident at Guam in 1997. "This [T2CAS] allows crews to make informed decisions, and to take alternative corrective actions. If I've got 4min to make a decision that's a heck of a lot better than 30s," he says.
Using a Beech King Air C90 as a testbed, the ACSS team has evaluated several scenarios to prove the overall capability of the system. On a demonstration for Flight International, three CFIT scenarios were shown including imminent terrain impact, a dynamic turn into terrain and a non-precision approach with premature stepdowns. Flying at the almost vertical 4,000ft (1,200m) rock wall of Arizona's Newman Peak the system first flags up a caution alert line that shows up in yellow on a conventional terrain hazard display. This tells the pilot where a caution will occur if the current vertical flightpath angle is maintained, and is one of the enhanced situational awareness improvements featured in the ACSS system.
Once triggered by the system, the length and direction of the displayed line mirrors a cautionary terrain segment within 30¼ either side of the flightpath, out to 2min ahead. If the aircraft reaches the yellow line without any action being taken the caution sensor issues a "TERRAIN AHEAD" aural warning and changes the relevant terrain on the display to a solid yellow. Failure to take further action results in a "TERRAIN AHEAD - PULL UP" aural warning and a solid red area on the display.
Using the climb model, specifically tailored to the performance of the host aircraft type, the system then issues an "AVOID TERRAIN" warning if it calculates the recovery manoeuvre is insufficient to clear the terrain. Red and black crosshatched areas indicate no-go terrain zones and the alert ceases once a safe escape trajectory is established with a left or right turn.
Once around the peak and in a dynamic turn, the system, which normally monitors a 3¼ field of view to reduce nuisance alerts, expands the terrain conflict search to 90¼ to left or right, depending on the turn direction. The look-ahead function in the system also issues an aural terrain warning on approach when, simulating inadvertent premature descent, the CPA envelope intersects the terrain data underlying the flightpath.
ACSS president Joe Hoffman sums up T2CAS as something that "won't tell you what to do, but based on what it knows will tell you what will happen if you don't do something!"
Source: Flight International
