A terrain awareness and warning system for any airplane
The Sandel ST3400 terrain awareness and warning system (TAWS), a self-contained unit that is small enough to mount on the pilot's primary panel, shows that Sandel Avionics truly understands the requirements of adding cutting edge airline quality instrumentation to GA airplanes.
The FAA has decreed that TAWS must be installed in turbine-powered airplanes that carry six or more passengers, including commuter, air taxi, and private (Part 91) aircraft. The deadline for fleetwide TAWS installations is March 5, 2005.
Sandel startled the avionics world five years ago when it introduced SN3308 — the first electronic horizontal situation indicator (EHSI) priced low enough to compete with vacuum-powered mechanical HSI units. Sandel has done it again with the ST3400 — it has produced a feature-filled TAWS that sells for under $20,000. Some similarly featured systems sell for twice that price. TAWS combines a ground proximity warning system (GPWS) part and a forward-looking terrain avoidance (FLTA) part. Explained very simply, the GPWS looks down and the FTLA looks ahead. By crunching the data from a very detailed internal terrain database and a GPS, the unit shows on a color screen where the airplane is in relation to the terrain, and where it will be in relation to out-front terrain in Sandel's exclusive Predictive Altitude mode.
The Sandel TAWS unit
Other manufacturers have chosen to display their TAWS information on multifunction displays (MFDs). These MFDs become an information center, and are capable of displaying terrain, real-time weather, chart information, traffic, and lightning-strike activity. MFDs are most often mounted in the center radio stack instead of in front of the pilot.
Sandel decided to go another route and created a self-contained panel-mount display system.
Sandel's 3.2-pound TAWS doesn't require any more panel space than an RMI (radio magnetic indicator) display, although the RMI is deeper, at 10.25 inches, than these displays. According to Sandel, it's very simple to replace an RMI with the ST3400. RMI data can be displayed on the ST3400 — and can be toggled onto the display at any time by the pilot. The result is an easy-to-install unit that is positioned where it's most needed — directly in front of the pilot — providing an easy-to-understand terrain avoidance picture in the pilot's primary instrument scan.
The display and the TAWS unit comply with technical standard orders (TSO C151a for TAWS and TSO C113a for the display). Like Sandel's EHSI, the ST3400 utilizes a patented active-matrix rear projection display. While the SN3308 has a fairly narrow projection width, the ST3400 features a second-generation (2G) display that is easily visible from the copilot's position. The 2G projection lamp has a life limit of 2,000 hours, more than 10 times the life of the first-generation lamp in the SN3308.
Can I clear that ridge?
The ST3400 unit doesn't see the terrain like radar — it plots the airplane's location in three dimensions (by reference to GPS) and continuously evaluates and displays this position in relation to a digital library of terrain, obstacles, and airports. Airplane and terrain position tolerances are less than 20 meters.
Sandel's engineers trumped the basic requirements of the FAA's TSO by augmenting the unit's predictive function. This function extrapolates the current flight path (from GPS data) over the terrain and obstruction database. With the ST3400, if the airplane's current flight path is predicted to strike terrain within 13 miles of the airplane's position, the pilot will be alerted by audio and visual displays.
The predicted point of impact appears as a circle on the display screen. During the caution stage, the impact circle is yellow and the pilot hears a repeating "caution — terrain" message in his headset. If action isn't taken, the impact circle turns red, a red flashing message appears on the screen and a repeating "terrain — pull up" message is heard.
The projected impact circle is intended to help the pilot make an informed decision on the best flight path for escape. If the terrain to the left or right is lower than the predicted impact point, the pilot will easily see the best avoidance path depicted on the display.
Eleven colors — red is dead
The ST3400 has many display options. Press the TOPO button, one of four menu buttons on the left margin of the ST3400, to see a sectional-chart format.
Hit the TERR button once and relative terrain height is displayed — normal sectional chart colors are replaced by "terrain relative to the airplane" colors. Push the TERR button a second time to enable the predictive feature and see terrain colors in relation to the predicted flight path. If there's a lot of red ahead in the predictive mode, it's time to do something — now. Terrain on the screen is depicted in 500-foot increments by using different colors. For instance, terrain that's higher than the airplane is in shades of red, with bright red for all terrain that is more than 1,000 feet higher.
Terrain that is 1 to 500 feet below the airplane's altitude is yellow; terrain that is 501 to 1,000 feet below the aircraft is depicted by a yellowish-green color. Colors change to green, light green, very light green, and so on until terrain 3,000 feet below the airplane is black. This "Silent/Dark" concept is designed in to ease pilot workload by presenting only need-to-know terrain information.
Push the VUE button to jump back and forth between a 360-degree view of the surrounding terrain and an arc view, which shows terrain ahead of the airplane.
The Sandel engineering team designed the ST3400 to protect the pilot, even if he gets distracted or neglects to preset the display for his flight mode.
"We've created a hands-off box that just sits there looking over the pilot's shoulder," says Sandel President Gerry Block.
No false alarms
The hard part in building a TAWS is to create a system that does the relatively easy en route alerts but still provides high reliability (no false alarms) when maneuvering near the ground during the landing, go-around, and takeoff phases of flight.
The Sandel answer to this conundrum is Virtual Approach Path (VAP). The ST3400 creates a Safe Operating Area (SOA) for all runways that are 3,000 feet and longer. When the pilot maintains FAA terrain clearances, the system is guaranteed not to alert. If the pilot flies the airplane out of the SOA, both visual and aural warnings are generated.
Class A and Class B installations
In addition to terrain awareness, the warning part of the ST3400 has capabilities for both Class A and Class B service. Class B installations are the minimum TAWS requirements for Part 91 turbine-powered aircraft with six or more seats and Part 135 (air taxi) turbine-powered aircraft with six to nine seats. Class A installations are required in Part 135 air taxi turbine-powered airplanes with 10 or more passenger seats and all turbine-powered Part 121 (scheduled airline) airplanes.
The most basic Class B installation requires only GPS, heading, and flap position inputs. Class B systems are upgradeable to Class A status by adding localizer/glideslope, radar altitude, and airdata computer inputs. An airdata computer crunches raw air data (temperature and pressure) with barometric pressure, yielding vertical speed and true airspeed that is then converted into a digital format.
No air data needed for ST3400 installations
On March 7, Sandel announced that the ST3400 can comply with the requirements of the Class A and Class B TSO by substituting GPS altitude data in lieu of air-data computer-derived altitude data. Since many GA airplanes don't have airdata computers, this Sandel exclusive (Sandel is one of the first companies to gain this approval from the FAA) should make the Sandel TAWS the system of choice for many operators.
Class A and Class B units have alerts for excessive rate of descent (ERD), negative climb rate after takeoff (NCAT), flight into terrain not in landing configuration (FITNL), excessive downward glideslope deviation (EDGSD), and altitude callouts. The Class A unit also has an alert for excessive closure rate to terrain (ECRT).
Both Class A and Class B units have forward looking terrain avoidance (FLTA) and premature descent alert (PDA). The Class B version of the ST3400 sells for just under $20,000.
The free CD
Sandel offers pilots a free CD-ROM demonstration of the ST3400's capabilities. The CD shows two accidents that could have been prevented with Sandel TAWS. The first is an accident where the pilot kept descending until the airplane hit the ground. The second accident shows a flight where the pilot lost situational awareness and, flying level, hit a mountain. The VAP technology is displayed during a VOR circle-to-land approach to Runway 15 at the Aspen-Pitkin County airport in Aspen, Colorado. There is also an 11-minute flight from Prince William Sound in Alaska, up the Valdez Arm, and into the Valdez Pioneer Field Airport.
For the ST3400 CD-ROM, contact Sandel Avionics at 877/726-3357 or e-mail a request to [email protected]. More information is also available on the Web site ( www.sandel.com).
E-mail the author at [email protected].
Related Articles
