T2CAS poised to usher in new stage of flying safety

Aviation International News » February 2003
January 14, 2008, 10:15 AM

Two days of test flying are all that remain before the FAA can put its stamp of approval on T2CAS, the combined TCAS and TAWS product from Phoenix-based ACSS. Pilots from the FAA’s Long Beach, Calif. certification office plan to evaluate the system for the final time in ACSS’ King Air C90 test aircraft later this month. If all goes according to plan, TSO authorization should be in hand within the next few weeks, clearing the way for initial STCs and first installations later this year.

ACSS–Aviation Communication & Surveillance Systems, a joint-venture company owned by L-3 Communications and Thales–claims its new TAWS is “demonstrably better” than conventional terrain-alerting products on the market, specifically Honeywell’s EGPWS, the enhanced successor to the Ground Proximity Warning System (GPWS), mandated for inclusion in certain commercial transport airplanes since the 1970s. While it’s difficult to say that any one TAWS device has proven itself a safer or better system, ACSS’ concept has gained attention from a number of large-fleet operators, including FedEx, Northwest Airlines, Aeromexico and the U.S. Customs Service. Now, with certification so close, the company hopes to build on its early momentum.

Executives and engineers for ACSS back up their claims that T2CAS is a better terrain-alerting concept by pointing out ways in which new technologies applied in the device provide extra margins of safety. While the company’s assertions seem reasonable, there exists little real accident data or human-factors research to support the position that T2CAS is safer than EGPWS. There has never been a CFIT accident involving an airplane equipped with EGPWS. In a number of cases, in fact, flight crews have reported that the EGPWS may have helped prevent such dangerous situations from developing in the first place.

T2CAS versus EGPWS
With that said, it is important to point out several major differences between the way T2CAS and EGPWS calculate terrain and provide pilots with warnings, each of which makes T2CAS a notably different, if not superior, system. The first distinction is the inclusion in T2CAS of a caution alert line, a moving yellow contour (or in some cases several contours) on the terrain display that lets the pilot know at a glance where he or she would get an aural terrain warning if the aircraft proceeded toward terrain. It’s a useful feature, particularly as an aid to pilot situational awareness around airports where hills and mountains pose a threat. If the pilot notices a caution line on the terrain display where he normally would not expect to see one, it could be an early clue that the aircraft is too low or off course.

The next differentiator between T2CAS and conventional TAWS systems are ACSS’ predictive aircraft climb model and the use of actual aircraft performance in T2CAS, as compared with the look-ahead parameters and fixed climb angle used in EGPWS. Where T2CAS can change its look-ahead constraints based on real-world conditions and depending on aircraft model, EGPWS uses a more or less static protection envelope. The benefit of the former is that additional safety margins are built in depending on real-world factors.

When everything is going wrong–say, the loss of an engine at night in poor weather– EGPWS may not always provide adequate protection to clear terrain ahead because the device’s protection scales do not change to account for the loss of performance created by the engine failure. T2CAS, on the other hand, considers an array of factors and adjusts its protection envelope to ensure that the aircraft can climb above terrain–or turn away from the hill or mountain–with time to spare.

Hal Adams, ACSS vice president of marketing, explained that the calculation logic present in T2CAS is proactive rather than reactive (the result of using a system that was originally designed by Dassault as a terrain-following system in Mirage fighters). Recognizing that CFIT protection is not generic for all aircraft, T2CAS developed an aircraft personality module (APM) to store specific configuration information. The APM can be programmed to consider a variety of factors and then calculate aircraft weight, OAT, configuration and engine health to provide a constantly changing view of terrain hazards that adapts accordingly.

“The idea behind T2CAS was to design a measurably safer system,” said Adams. “If we were a ho-hum, me-too box we wouldn’t get anywhere in the market. With our system wherever you are going, or might go, T2CAS looks.”

Another major difference with T2CAS is that in those rare instances where an aircraft could not out-climb the terrain, the TAWS display shows black Xs over the dangerous red part of the screen and gives an aural “Avoid terrain!” warning. This lets the pilot know immediately that it is not likely the aircraft can clear the mountain ahead. In these cases it’s up to the pilot to take a look at the terrain display and determine which way to turn to avoid hitting the ground.

This situation could occur if an aircraft suddenly turned into terrain rising to either side. With conventional TAWS systems, the protection envelope generally looks only straight ahead. In the case of T2CAS, however, when the system senses an increase in bank angle of more than 1.5 degrees, it automatically opens its protection aperture to look in the direction the aircraft is turning.

ACSS has run a number of flight scenarios for potential customers that the company said illustrates the circumstances in which T2CAS would permit escape from a dangerous situation while EGPWS would not. In one of the scenarios, developed for Air Wisconsin, an aircraft flying into Aspen, Colo., begins the missed approach procedure–a tight, climbing left turn–but it is assumed that for some reason the crew begins a shallower turn instead.

For the purposes of the demonstration, ACSS used actual T2CAS and EGPWS hardware running side by side in the lab and simulation software. The results are chilling. While the aircraft equipped with T2CAS gets a “Pull up!” warning just a few seconds into its turn toward a nearby mountain, the aircraft fitted with EGPWS continues to turn until its nose is pointed almost directly at the mountain. Unfortunately, by this time it is too late to pull up to avoid terrain, and through the audio system the EGPWS is heard to give continuous “Pull up!” commands until impact with terrain.

“Here’s a scenario where the pilot is pulling up with confidence, but he can’t out-climb the mountain and he doesn’t even know it,” said Adams. “Our TAWS would never let the pilot get into that position in the first place, and if he did he would get a second alert telling him, ‘Whoa, you can’t get over that hill, so you had better do something else.’” The something else in this case would be a tighter turn to the left to avoid terrain, not the pull-up maneuver commanded by EGPWS.

In a bid to drive home the point that T2CAS represents a new level of terrain awareness and safety, the company decided to hire well-known aviation safety consultant and former senior NTSB accident investigator Gregory Feith, who is a frequent television commentator and an air-safety advocate with more than 20 years’ experience. Feith said he decided to come on board with ACSS after demonstrations of T2CAS proved to him this was a “dynamic approach” to safety.

“The conventional corrective action pilots have long been taught–applying power and pulling back on the yoke–may not always be appropriate,” Feith said. “My personal opinion is that pilots have become too reliant on the technology in the cockpit. They will continue into a dangerous situation until the box squawks at them. That’s the wrong approach. The pilot should always be number-one in the loop, and the boxes should serve as the tools.”

Feith said ACSS’ philosophy–keeping pilots from getting into a dangerous situation in the first place–is a good approach. But training needs to be improved, he said, so that pilots better understand all of the warnings provided by the TAWS and know what they should be doing for each given scenario.

“It’s really kind of unfair that NTSB accident investigators get a year or 18 months to piece together a crash so that at the end they can point their finger at the pilot and say, ‘Aha, you messed up,’” he said. “The pilot has so little time to react when he finds himself in a bad situation.” Feith credited T2CAS for its ability to provide, on average, about four minutes of warning time before impact with terrain, compared with the one minute that is standard with EGPWS. He said it may never be possible to eliminate CFIT, but the chances of such a crash are greatly reduced with this type of system on board.

Flying the System

The T2CAS platform ($107,000 list price) consists of a partitioned avionics LRU with processors for both TCAS and TAWS functions. On one side are all the components that make up the ground collision avoidance module, the brains behind the device’s Class-A TAWS. Options include a GPS receiver card and reactive wind-shear alerter.

During a demonstration flight last month from Phoenix Deer Valley Airport, ACSS test pilot Pam Clements and staff engineer Rick Ridenour put T2CAS through its paces, first with a trip through the busy Class-B airspace around Phoenix Sky Harbor Airport, where the TCAS 2000 portion of the system dutifully pointed out a dozen or so nearby targets, and then on to Casa Grande and Newman Peak about 30 nm south of Phoenix.

After making several low passes toward the jagged, sandy Arizona mountains it became clear that T2CAS is capable of keeping an aircraft out of harm’s way without burdening the pilots with nuisance alerts that can be akin to the little boy who cried wolf. At no time during the demo flight did the ACSS King Air appear to be uncomfortably close to terrain, and yet T2CAS consistently provided warnings at a point where getting any closer probably would give the pilot an uneasy feeling.

During a few of the maneuvers T2CAS provided an “Avoid terrain!” alert rather than the more common pull-up command. It was easy to glance at the terrain display to determine what type of action was required to avoid the portion of the screen where black Xs on red terrain warned that a climb was not advisable. The system is designed so that a standard-rate turn away from the red portion of the display is sufficient to avoid the terrain.

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