Terrain Tactics

Steering Clear: TAWS Laws

November 1, 2004

"Whoop, whoop! Pull up..." to your avionics shop

March 29, 2005, will be a red-letter day for most owners and pilots of turbine-powered airplanes. That's the day an FAA mandate kicks in, requiring all turboprops or jet aircraft with six or more passenger seats to be outfitted with Class B TAWS avionics. TAWS stands for terrain awareness warning system. The Class B refers to the TAWS hardware and features applicable to Federal Aviation Regulations (FAR) Part 91 turbine aircraft and to FAR Part 135 turbine aircraft with six to nine passenger seats (inclusive). Because most of us fly under Part 91, this article emphasizes TAWS-B. (TAWS-A is for Part 135 with 10 or more passenger seats.)

But first let's set the stage for these new regulations. The need for some sort of terrain warning system had been debated ever since the 1970s, when accident studies began to identify the occurrence of a number of cases where perfectly good airplanes were flown into the ground. CFIT (controlled flight into terrain) became a new acronym in the aviation lexicon, and experts asserted that many CFIT accidents could have been prevented if there had been a terrain warning system aboard.

In 1974 the first ground proximity warning systems (GPWSs) were ordered to be installed on all FAR Part 121 (scheduled airline) aircraft, and some FAR Part 135 (charter and air taxi operations) aircraft. In 1992, the rule was expanded to include GPWS on all turbine-powered Part 135 airplanes with 10 or more passenger seats.

Over the following 25 years, GPS navigation came into its own. By the late 1990s worldwide GPS terrain and obstacle databases reached a sophistication unimaginable a decade earlier. Suddenly, the GPWSs — which base their warnings on information from radar altimeters — were outdated. Radar altimeters measure the altitude from the airplane to points directly below. This has obvious shortcomings, the biggest being that altitude measurements are stale by the time a fast-moving airplane passes by.

GPS databases let avionics engineers design equipment that is forward-looking (able to scan the terrain and obstacles ahead of an airplane), factors in the aircraft's groundspeed, altitude, and flight path, then calculates any dangerous closure rates and collision threats. This is the technology behind TAWS.

In 2000, the FAA put out the latest TAWS rules, the ones that go into effect in March. The first deadline required all new turbine-powered, six-plus-passenger-seat airplanes built after March 29, 2002, to be equipped with TAWS-B. The second requires that all such aircraft have the equipment retrofitted by the March 2005 deadline.


At this writing, there are six manufacturers of TAWS-B equipment. These include Honeywell Bendix/King, L-3 Communications, Garmin, Chelton Flight Systems, Universal Avionics, and Sandel Avionics.

All use the same basic hardware components: a GPS receiver, for database and navigation information; a TAWS processor, to calculate ground proximity, closure rates, and warning thresholds; an audio output, to generate aural warnings; and a display system, to provide visual warning cues.

Some systems are designed to work only with integrated displays made by the same manufacturer. Garmin's TAWS-B only works with Garmin 500-series GPS displays and the G1000; Sandel's system only plays on that company's ST3400 display; and Chelton's TAWS-B is included as part of its FlightLogic synthetic vision primary flight display (PFD).

TAWS-B pricing ranges from $6,495 (to retrofit the Garmin GNS 530), to $20,950 (for the Sandel ST3400 system), to some $75,000 for the full-blown Chelton system, which includes TAWS-B. The L-3 LandMark TAWS 8100 system will cost $12,500 and be retrofittable to the Bendix/King EFIS 40/50 electronic flight information system, and several Rockwell Collins electronic displays, including the Pro Line 21, Pro Line IV, and FDS-2000 units. Installation costs vary with the airplane, and depend on the amount of time required to attach antennas and fit internal components.

Finally, it's important to note that some systems had yet to be certified at the time this article was being written, and manufacturers stressed that pricing had yet to be finalized.


All TAWS-B units must provide the same warnings and adhere to the same warning thresholds. These are set down in an FAA technical standard order (TSO) named TSO-C151b. Display system details may vary, but the warning technology must meet the same standards. Some of the warnings are holdovers from the GPWS days, bolted on to the new GPS-driven forward-looking terrain awareness functions.

The five functions provided by TAWS-B, along with the appropriate aural alerts, include:

  • Reduced required terrain clearance, or imminent terrain impact. This is the forward-looking terrain alert function. This warning is generated when an aircraft is above the altitude of upcoming terrain along the projected flight path, but the projected terrain clearance is less than the required terrain clearance. The warnings depend on the phase of flight, and whether the aircraft is in level or descending flight. There are 60-second and 30-second warnings.

    60-second aural warning: "Caution, terrain; caution, terrain" (or "terrain ahead; terrain ahead") and "caution, obstacle; caution, obstacle."

    30-second aural warning: "Whoop, whoop. Terrain, terrain; pull up, pull up!" or "Whoop, whoop. Terrain ahead, pull up; terrain ahead, pull up." The "whoop, whoop" sweep tones are optional.
  • Premature descent alert. This alerts the pilot if there's a descent well below the normal approach glidepath on the final approach segment of an instrument approach procedure.

    Aural warning: "Too low, terrain!"
  • Excessive descent rate. This is a carryover from GPWS, and alerts you if the rate of descent is dangerously high compared to the aircraft's height above terrain — and, for example, if flying level over rising terrain.

    Caution alert: "Sink rate!"

    Warning alert: "Whoop, whoop! Pull up!"
  • Negative climb rate or altitude loss after takeoff. Another GPWS function, this is to assure a positive climb rate after takeoff or a missed approach.

    Caution alert: "Don't sink!" or "Too low, terrain!"
  • The 500-foot "wake-up call." This occurs whenever terrain rises to within 500 feet of the aircraft, or when you descend within 500 feet of the nearest runway threshold elevation during an approach to landing. It's intended as an aid to situational awareness, and doesn't constitute a caution or warning.

    Call-out: "Five hundred."

Bottom line

While terrain displays are available with all the TAWS-B systems on the market, they're not required by the TSO. You could theoretically get by with a simple annunciator panel. But fancy TAWS displays might as well be mandated, given the popularity of large full-featured primary flight displays, navigation, or other dedicated displays. They provide easy-to-interpret safe routes of escape from encroaching terrain, and have symbology representing projected impact points with terrain or obstacles.

If you own or fly an affected airplane and don't have TAWS-B installed by the end of March 2005, you're grounded. There will be no extension of the deadline, the FAA says. So for the turbine pilots, the options are pretty clear.

For those of us flying piston-powered aircraft, TAWS' arrival serves as a reminder of the value of terrain awareness equipment. There are a number of much-lower-cost alternatives in this department, ranging from the terrain features in handheld GPS units such as the Garmin 296 ($1,695) to the basic topography displays of such displays as the Avidyne FlightMax EX500 ($11,995), L-3 Communications I-linc ($8,850), and Garmin's MX20 ($14,995), GNS 430 and 530 ($9,250 and $14,995, respectively), and the G1000, which is priced by airframe manufacturers. These may not issue voice commands, but they go a long way toward keeping you away from terra firma.

E-mail the author at tom.horne@aopa.org.