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Sunday, November 1, 2009

Everything You Ever Wanted to Know About TAWS

Basics of flying: 1: Try to stay in the middle of the air. 2: Do not go near the edges of it. 3: The edges of the air can be recognized by the appearance of ground, buildings, sea, trees and interstellar space. It is much more difficult to fly there.—Anonymous

By Dale Smith


Helicopter accidents, especially those involving emergency medical services (EMS) flights, have been getting a lot of bad, but well-deserved press recently. The increasing number of helicopter EMS (HEMS) fatalities has also caught the eye of the National Transportation Safety Board (NTSB), which early this September issued a revised recommendation letter outlining 19 various safety issues relating to HEMS operators.

While every fatal accident is truly tragic, the ones that stand out as being really sad are those that involve controlled flight into terrain (CFIT). Recent industry statistics show that more than half of helicopter EMS fatalities are due to these types of accidents.

According to the NTSB, in 2008 alone, there were 12 HEMS accidents resulting in 28 fatalities. While it’s not clear if, or how many, were CFIT related, even if it was only one, it was one too many.

To the Rescue

To quote baseball great Yogi Berra; “It’s like deja-vu, all over again…” Commercial and HEMS helicopter operators are finding themselves in the same position that the major airlines faced back in the 1960s when a series of CFIT accidents killed hundreds of people.

As we all know, the solution to virtually eliminating airline CFIT accidents was the FAA’s mandating installation of ground proximity warning systems (GPWS) and subsequent enhanced GPWS (EGPWS) or terrain awareness warning systems (TAWS) in all U.S.-registered turbine-powered airplanes with six or more passenger seats.

The proof of how well the technology works is that no aircraft outfitted with a second-generation EGPWS or TAWS system has been involved in a CFIT accident in the U.S. And it works just as well for helicopters. According to Honeywell Aerospace vice president of product management, T.K. Kallenbach, “Honeywell’s helicopter EGPWS provides superior and potentially life-saving information for flight crews, even in changing weather with poor visibility, in rough terrain, or at low altitudes. More than 1,300 helicopters are already flying with our EGPWS and there is no evidence of any controlled flight into terrain incident with an operational Honeywell EGPWS on board.” FAA issued technical design and production approval for Honeywell’s Mark XXI and XXII EGPWS in early October.

While there’s no debate that EGPWS/TAWS systems enhance safety, there is some misunderstanding among pilots and operators about what the different systems are and how they work.


TAWS systems must be specifically designed for helicopter use. It’s not possible to take a fixed-wing TAWS unit and mount it in a helicopter. Elevations are color-coded on the display as depicted above.
Before getting into how the technology works, we have to begin by eliminating a point of much confusion: as spelled out in the FAA’s technical standard order (TSO), there is basically no technical difference between EGPWS and TAWS. Honeywell is using EGPWS as the brand name for its systems, while other manufacturers including Cobham Avionics and Sandel Avionics have chosen to use TAWS, which is the broader term used in the latest FAA TSO for a “terrain awareness and warning system.”

“I’ll say there are differences between manufacturers on they way they do their TAWS systems,” said Gerry Block, president of Sandel Avionics. “Like a lot of things in [FAA] TSOs, how you do what they want you to do is up to the manufacturer.”

“If you step back to the beginning of terrain awareness, whether you call it EGPWS or something else, the concept is the same,” Kallenbach explained. “You have an aircraft in flight, you have information on the terrain you are flying over and you have your current position and speed. Any of these systems basically give you advisories on how to avoid hitting the ground while you are in controlled flight.”

“The difference between an early GPWS system and a TAWS or EGPWS system is that we add the forward looking terrain awareness (FLTA) function, which uses a database to alert the pilot to hazardous terrain or obstructions that are ahead of the aircraft,” explained Gordon Pratt, vice president of business development for Cobham Avionics. “The TAWS/EGPWS units also have the premature descent alert (PDA) mode.”

Before getting too far, there is another player to introduce: terrain awareness systems (TAS). “We make a little product called the AV8OR handheld GPS aircraft navigation aid,” Kallenbach added. “On this piece of equipment, which is not certified, is a set of maps so you can basically look at a topographical map and see the terrain in your area. Now that could basically be called a terrain awareness system.” (With awareness being the key word.) Kallenbach said that while the system will make the pilot aware of the changing terrain by varying the colors, it has no way of actively alerting the pilot of the rising elevation around him. Basically it’s just telling the pilot there’s a potential problem based on its understanding of position and GPS altitude. While it has no FLTA capabilities, it’s certainly better than nothing.

“An awareness system can break the chain of events that lead to CFIT if you are paying attention to it,” Kallenbach said. “But a warning system will tell you if you don’t change your flight path you’re in big trouble.” He continued: “When you start to look at the distinctions between a terrain awareness system—a generic terrain system and EGPWS—what you will get into is really the fidelity of the database and the sophistication of the warning algorithms. The fidelity of the detailed knowledge of the ground and the knowledge of the obstacles and how the aircraft understands its position relative to that information.”


Sandel’s ST3400H helicopter terrain awareness warning system (HTAWS). Sandel
The “fidelity” of the system’s database is the true key to making helicopter TAWS (HTAWS) work. In fact, the lack of a dependable, high-resolution database was one of the factors that have kept HTAWS from being available sooner than it was. Since airplanes do their best to avoid flying close to the ground, they don’t need the detailed database that is critical to keeping a helicopter safe. And that difference is why you can’t just take a fixed-wing TAWS system and mount it in a helicopter.

“It’s really quite simple in theory,” Block said. “You have a data set, which holds all the terrain elevations to very high resolution and you also have a data set of obstacles. All of them have altitudes associated with them. The system knows the position and altitude of the aircraft so it basically takes a look at the trajectory of the helicopter against the points of data about the terrain and obstacles. Then it tries to determine whether or not, based on current conditions, the helicopter will hit anything.”

Where does the high-resolution database come from? “You can thank the Space Shuttle astronauts for that. We have a database that’s accurate down to 100 square feet,” he said. “It was all done on mapping flights by the shuttle and the data is now available from NASA.”

In fact, the manufacturers actually use a combination of the Shuttle Radar Topography Mission imagery (see image underlaying this story) combined with a database supplied by the U.S. Department of Defense as the foundation, then they “customize” the data to their particular needs. “The obstacles are a different story,” Block added. “The government has a database of obstacles around airports. And the FCC registers large antennas so they’re pretty much known. But there’s a lot of information on the obstacles that’s not complete. We’re putting specific emphasis on power lines because we know that it’s an important thing for helicopter operators. It’s not normally available so we’re working hard to get the data and include ways to alert it on our HTAWS.”

Because construction cranes, towers or whatever can literally spring up over night, each of the EGPWS/HTAWS system manufacturers have a way for its customers to go in to the software and add obstacles that they know about but may not be on the current database.

Maintaining an accurate database for obstacles is critical because unlike, say a radar or TCAS system, EGPWS/HTAWS cannot actively go out and “look” for trouble. As far as the system is concerned, if it’s not in the database, it doesn’t exist. Try telling that to the 300-foot crane they just erected across the street from the hospital’s helipad.

Danger Will Robinson

The hardest part about creating a truly effective EGPWS/TAWS system for helicopters is teaching the unit when it does and does not need to give the flight crew a warning or an alert. Because helicopters operate at much lower altitudes, the manufacturers have to use very sophisticated algorithms to ensure that they system knows when it’s needed.

“I think some people believe that nuisance alerts are an unavoidable part of an HTAWS system,” Block said. “Nothing could be further from the truth. Each manufacturer’s system will be different in its ability to suppress nuisance alerts. If poorly designed, nuisance alerts may actually diminish safety by causing unwanted distractions. It could get to the point where the pilot will either ignore the alerts or turn the system off.”

PFD image at bottom depicts the valley (top) where the helicopter is flying. Cobham combines forward-looking 3D synthetic vision with helicopter TAWS. Cobham

While it will change by the particular manufacturer and the helicopter’s configuration, the typical EGPWS/HTAWS system can have up to six GPWS alert/warning modes:

• Mode 1 is Excessive Sink Rate. The rate of descent is hazardously high as compared to the altitude. You are descending into terrain.

• Mode 2 is Excessive Terrain Closure Rate. The rate of change of height above ground is hazardously high. The terrain is rising up to meet your flight path.

• Mode 3 is Descent After Takeoff: You are descending after you take off or in the first leg of a missed approach.

• Mode 4 is Flight Into Terrain when not in landing configuration. You are descending but don’t have your gear down.

• Mode 5 is Excessive Downward Deviation from the glideslope. You are detected to be too low during the final segment of an ILS.

• Mode 6 includes three warnings: Altitude Callouts, Excessive Bank Angle and Tail Strike.

So what would a typical EGPWS/HTAWS warning scenario look and sound like? Well, it looks as you would expect it to: the terrain will change from green to yellow to red as it gets closer to your aircraft. There are also graphical indicators on the display that will match the audio cautions and warnings.

The whole time your display is changing color, there’s a complementary aural warning blasting in your headset. If you manage to ignore the visual and aural warnings then you’re on your own. (Keep in mind that this is for illustrative purposes only. Each manufacturer does it a bit differently. But the end goal is the same.)

Thirty seconds before you hit something you’d hear, “Glideslope … Glideslope …” If you just pull up enough to stop hearing the warning you’re okay. If you ignore it and continue within 15 seconds or so you’ll hear “Caution Terrain … Caution Terrain…” Time to take corrective action.

Ten seconds before impact you’ll hear, “Warning Terrain … Warning Terrain…” Then five seconds before the collision, you would hear: “Pull Up … Pull Up…” You really don’t want to hear what comes next.

“Hi, I’m from the FAA...”

While no one can question the safety enhancing value of EGPWS/HTAWS, there’s still considerable speculation on whether or not the FAA will mandate its installation on commercial helicopters.

“There’s certainly a lot of talk about a TAWS mandate for EMS helicopters.” Pratt said. “They are highly motivated pilots trying to get out and save injured people. They fly in darkness and all kinds of terrible weather. TAWS will prevent a lot of accidents. It’s a lifesaver. No pilot intends to run into the terrain,” he continued. “But people need it. While I would like to see people see the benefits and equip on their own, the FAA has demonstrated that if operators don’t use lifesaving technologies they will step in and make it happen.” It’s like deja-vu, all over again…

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