July 1, 2005
By Thomas B Haines
It was a dark and not-at-all-stormy night. A relatively inexperienced pilot was about six miles from home, descending for the airport, which was in sight. During the descent, the pilot suddenly felt uncomfortable, as if something was about to go terribly wrong on what had otherwise been an uneventful flight. Terrain! Passing through about 2,300 feet the pilot suddenly realized he didn't know where he was relative to the ridgelines just west of the airport. It was pure black just ahead and below. Was he too low? How high were the ridges and the towers atop them? He cobbed the throttle, pulled the nose up, and zoomed the Piper Warrior up a thousand feet.
Upon later investigation, he determined that at 2,300 feet he was only about 350 feet above the towers on the ridge, but still about 1,000 feet above the ground. Had he continued his rate of descent, he would have been fine because he was, in fact, almost a mile from the towers.
I was that pilot on that dark night more than 15 years ago. Now, every time I fly over those hills at night I find myself remembering that sudden near panic as I discovered I was unsure of my position relative to the terrain and towers. These days, though, I have two tools working in my favor — tools we only dreamed about when the most sophisticated device in the panel was a VOR receiver. One tool is the vertical navigation (VNAV) planning function built into the Garmin GNS 530 that now dominates my panel. With the VNAV function, I can set up a descent profile that will have me arrive at a point in space just above pattern altitude and within a couple of miles of the airport — a point well away from the ridgelines.
The newest tool, though, is the recent terrain-awareness-warning-system upgrade to the GNS 530 that will alert me if terrain or obstructions will affect my flight. Garmin certified its TAWS-B system last December as an upgrade to the GPS 500 and GNS 530/530A units. For those who don't need the sophistication of a full-up TAWS-B, Garmin will offer a simpler Terrain Interface System by the end of the summer that will provide many of the features of the TAWS.
Controlled flight into terrain (CFIT) represents a causal factor in a significant number of accidents each year, especially in turbine aircraft. In fact, more aircraft and lives are lost in this category of accident than any other in commercial aviation. Lighter general aviation aircraft are not immune to CFIT accidents, but the CFIT accident rate is lower for them because such airplanes tend to fly less often at night and less frequently in instrument conditions than do turbine airplanes on commercial missions. Light airplanes also carry less momentum and have engines that respond immediately to throttle changes, allowing for an easier escape if the pilot recognizes terrain or an obstruction as a problem. In turbine airplanes, which are heavier and have engines that take a couple of seconds to respond when the pilot commands more thrust, it takes longer to arrest a descent if terrain or an obstruction is spotted at the last second.
Recognizing the CFIT threat, the FAA, starting March 29 of this year, required all turbine aircraft with six or more seats and operating under Part 91 of the federal aviation regulations to carry TAWS-B. Part 135 turbine operators, basically those flying charter, with six to nine passenger seats also must install TAWS-B. Larger Part 135 aircraft must install the more sophisticated TAWS-A.
While TAWS was conceived for the jet set, it brings a great deal of safety benefit to your average light airplane, especially if you often fly at low altitudes, in areas of mountainous terrain or towers, or at night. On a recent flight in my Beechcraft A36 Bonanza, shortly after the TAWS upgrade, the system alerted me to numerous tall towers near my flight path. We were flying at about 1,000 feet agl over the Piedmont just south of Norfolk, Virginia. My airplane was the photo platform as we were shooting pictures for the magazine. I was flying lead, looking for traffic, and trying to find interesting terrain features and good light for the photos. It was a busy time, but the big yellow "Caution-Obstacle" message on the Garmin display caught my attention, allowing me to steer well clear of some tall towers.
Likewise, on a recent flight in Florida, not exactly a place with noteworthy terrain, the system showed a series of very tall towers near my flight path just west of Stuart.
To compute terrain and obstacle threats, the Garmin system compares your 3-D GPS position to a worldwide terrain and obstacle database stored in the unit. The database occupies the right slot on the face of the Garmin 530. The left slot is for navigation data. Using a dedicated navigation page on the display, the system presents a 2-D picture of the surrounding terrain relative to the altitude and position of the aircraft. In cruise flight — assuming you're more than 1,000 feet above any terrain or obstructions — there's not much to see; the screen will be black with only the flight path depicted. The rocker button changes the range from one nautical mile up to 100 nm. The pilot can also choose whether to overlay aviation data on the terrain page. A 120-degree option places the aircraft at the bottom of the display, providing a wider view of terrain. In the 360-degree mode, the aircraft is in the center of the display surrounded by a compass rose.
For the most part, terrain-warning systems earn their keep during the takeoff and landing phases of flight. But first a couple of comments about the Terrain Interface versus the TAWS-B. As can be seen in the accompanying table (below), the two systems are similar except that the TAWS-B features aural warnings — voice messages that definitely get your attention. In addition, the TAWS-B also provides more sophisticated alerting during approaches and immediately after takeoff (see " On Display: Steering Clear: TAWS Laws," November 2004 Pilot). One nice feature of the TAWS-B is the 500-foot call-out. Any time you descend to within 500 feet of the ground, the system calls out "five hundred." It's a helpful reminder on an approach — VFR or IFR. If by then the gear isn't down, correct airspeed established, and the needles nicely centered, it's time to go around.
In addition, TAWS-B calls out "pull up!" if the airplane enters an excessive rate of descent while near the ground. Excessive descent rates higher above the terrain will generate a cautionary "sink rate." Right after takeoff, if nearby terrain is a threat and the airplane is not climbing rapidly enough, the system issues a "negative climb rate after takeoff" alert and calls out "don't sink!" Each of the aural messages is accompanied by a similar text message on the display — yellow for alerts and red for warnings.
The system uses red, yellow, and black on the dedicated terrain page to depict nearby terrain and obstructions. Terrain or obstacles within 100 feet or above the aircraft's altitude are depicted in red. Terrain or obstacles more than 100 feet and within 1,000 feet of the aircraft's altitude are shown in yellow. Terrain or obstacles more than 1,000 feet below the airplane are shown in black.
While the terrain depictions are helpful, they are not detailed enough to allow the pilot to thread his way through narrow canyons, for example. That's not the intent of the system. The database presents the terrain in blocks, causing it to appear chunky on the display. Obstructions show up as either red or yellow chevrons or towers, depending on their elevation above ground level.
The heart of both the TAWS-B and the Terrain Interface is what Garmin calls the Forward-Looking Terrain Avoidance alert. As in all things aviation it has its own abbreviation, FLTA. The FLTA has two subfunctions, the Reduced Required Terrain Clearance (RTC) Avoidance alert and the ominous-sounding Imminent Terrain Impact (ITI) Avoidance alert.
The RTC alert comes when the aircraft is above terrain but is projected to come within minimum clearance limits. The ITI alerts occur when the aircraft is below the elevation of terrain in the aircraft's projected path. In the case of an ITI alert, the display helpfully plots the projected point of impact with a red X. Obviously, the objective of the moment is to make that X go away by climbing or turning away from the terrain.
The minimum clearance limits vary depending on the phase of flight. For example, in level flight, in what the GPS senses as the en route phase, the system will issue an alert if the projected flight path will come within 700 feet of terrain. If that terrain is within 60 seconds' flight time, a yellow "Caution — Terrain" Message pops up on the Garmin display, no matter which page the pilot happens to be using at the time. TAWS-B also issues a "caution terrain, caution terrain" voice message. If the terrain is within 30 seconds' flight time, the text message is instead red and says "Terrain Ahead — Pull Up." TAWS-B also chimes in with a "terrain, terrain; pull up, pull up" warning.
During a descent in the en route phase, the alerts won't issue unless the flight path is projected to come within 500 feet of the terrain or obstacle, instead of 700 feet. The vertical limits for alerts vary for other phases of flight, such as terminal, approach, and departure, and whether the aircraft is in level flight or a descent. Such alerts are automatically inhibited when the aircraft is below 200 feet agl within one-half nm of the approach runway.
One final type of advisory is the Premature Descent Alert (PDA) mode. This part of the system alerts the pilot with a "Too Low — Terrain" aural and text message if it senses the aircraft is lower than it should be. The alert criteria vary depending on what type of approach, if any, is loaded into the Garmin system. If an ILS approach is loaded, for example, the system will alert if the aircraft descends 0.7 degrees below the glideslope.
A GNS 530 with its IFR GPS, moving map, and full complement of VHF nav/com gear costs $14,995; the smaller GNS 430 costs $9,250. Adding the TAWS-B to a new 530 or any of the 500-series units costs an additional $10,000; upgrading an existing 530 costs $8,000. TAWS-B is not available for the 400-series units. The Terrain Interface upgrade for existing units (400- and 500-series) will cost $500 when it's available later this summer. For the TAWS upgrade, the unit must be returned to the Garmin factory. The typical turnaround time at the factory is 24 hours. Garmin's goal is to make the Terrain Interface upgrade a field installation. If it must be returned to the factory, the turnaround will be about three days. The terrain database will be updated about every six months for $295; the obstacle database will be updated every 56 days for $35. In both cases, customers can buy a USB programmer from Garmin to download the data from the company Web site.
The nice thing about this sort of safety gear is how useful it is every day. While weather detection and datalink systems are certainly useful and worth their weight in gold on bad-weather days, they are not all that valuable on most flights, which typically occur in reasonably good weather. The terrain-alerting system is at work on every flight, looking for obstacles and terrain that may reach out to smite a pilot.
E-mail the author at email@example.com.
Links to additional information about terrain warning systems and related accidents may be found on AOPA Online ( www.aopa.org/pilot/links.shtml).
AOPA Editor in Chief Tom Haines joined AOPA in 1988. He owns and flies a Beechcraft A36 Bonanza. Since soloing at 16 and earning a private pilot certificate at 17, he has flown more than 100 models of general aviation airplanes.
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