Tests being carried out in the USA are proving the worth of ground-proximity warning systems for helicopters.
Paul Seidenman/SAN FRANCISCO
THE US NAVY AND a civil-helicopter operator have begun evaluation of the world's first ground-proximity warning system (GPWS) to be tested successfully in a helicopter application. The GPWS, which warns pilots of impending ground collisions, often because of controlled flight into terrain (CFIT), have been used in fixed wing aircraft since the 1970s.
The helicopter GPWS, developed by Cubic Defense Systems of San Diego, California, is the first such system produced to address, and virtually eliminate, the false and nuisance alarms which have long plagued conventional GPWS when used in dynamic flight regimes. This capability has been proven in a fixed-wing version of the system, used since 1992 on seven Lockheed Martin C-130s operated by the Canadian Forces.
The false alarms occur because the GPWS is usually based upon a so-called "envelope approach", using a pre-selected minimum-altitude setting, which, if penetrated by the aircraft, will set off an aural or visual warning to the pilot that closure with terrain is imminent. Warnings are triggered, if barometric and radar-altimeter information fed to the GPWS, indicate that the edge of the envelope has been penetrated.
An envelope-based GPWS may issue warnings when they are not warranted, such as during a normal approach and landing, or, in the case of a helicopter, during low-level manoeuvring. Under these circumstances, a pilot might consider the alert to be a false, or nuisance, alarm and choose either to ignore or de-activate the system. This makes for an obvious and lethal trap, should the helicopter pilot become distracted and the aircraft descends below a safe, recovery threshold.
To reduce the possibility of false alarms, the software for the Cubic GPWS eliminates the envelope in favour of a predictive approach. The predictive method measures the aircraft's actual dynamic state every 0.1s, and continuously computes the point at which a warning must be given to avoid penetration of a previously selected minimum recovery altitude (MRA).
As the aircraft approaches its pre-programmed safe MRA, the GPWS takes four altitude-loss factors into account - pilot response time, roll recovery, rate-of-descent recovery and terrain closure. If the sum of the four factors, is equal to, or less than, the MRA, only then will the warning be sounded. The calculations of those four factors take place in real time and the CFIT solution is updated every 0.1s. Because it is a "just-in-time" warning, the pilot must act promptly.
The Cubic GPWS is also the first to be "terrain-predictive", in that it can warn of an impending collision with level ground, or a hill. This is based upon real-time historical tracking of terrain picked up by the aircraft's sensors. To predict changes in terrain, the system compares the altitude information provided by the aircraft's radar and barometric altimeters, and stores the difference between the two in memory. When it has enough stored information to create a terrain trend, it projects that trend along the aircraft's flight path.
Cubic Defense Systems saw an opportunity in 1993 to configure the system's software for use on helicopters. According to Chris Wilson, manager of business development, avionics, the need for a helicopter GPWS was obvious.
"In the US military, between 1987 and 1993, some 30 aircraft accidents a year were caused by CFIT, and helicopters were involved in about half of those accidents," Wilson says. "Estimates are that between 70% and 85% of those CFIT incidents could have been prevented if an operable GPWS had been on board. We have found that, even in the low-altitude environment of helicopter operations, our system's predictive algorithm approach could provide CFIT warnings without the false alarms."
Cubic had identified several factors which justified the installation of a GPWS in helicopters, but which did not necessarily apply to fixed-wing aircraft, as Wilson explains.
"Helicopters normally spend a lot of time close to the ground - under 1,000ft [300m]. In a flight regime in which a high percentage of time can be spent in a hover, or relatively small area of airspace, pilot fatigue and boredom become an issue, which could lead to loss of alertness. It's not hard for a helicopter pilot to get into a situation that could cause a 2,000ft/min [102m/s] rate of descent. At 500ft, this means the pilot would have only 15s to recognise the danger and react," he says.
According to Capt Bob Smith, director of safety systems in the US Naval Air Systems Command's GPWS programme office, the USN's interest was generated because of the system's ability to aid pilot-situational awareness to prevent CFIT.
"There is no question that we have lost some helicopters to CFIT, which takes place mainly in low-altitude cruise situations, as well as during take-off and landing, day and night," Smith reports. "The Navy now has no helicopter GPWS in an operational status," he adds.
Smith explains that the US Navy started developmental flight testing of the Cubic GPWS in August 1995 at the Naval Air Test Center at Patuxent River, Maryland. The technical evaluation, which was carried out by the USN's Aircraft Test Division, used 15 system sets, and consumed 25h of flight time, using a single Sikorsky CH-53E Sea Stallion.
By April, the USN expects to begin a further 40h of flight tests for operational evaluation, using the same aircraft, but this time at MCAS Yuma, Arizona. The operational evaluation will probably be completed within three months.
Concurrent with the USN's technical and operational evaluations of the Cubic GPWS, Era Aviation of Anchorage, Alaska, has been carrying out its own trials of a similar system (see box). The charter operator, with a fleet of 80 turbine-powered craft, began to test the system in November 1995 on a single Bell 212.
According to Dennis Martin, Era's avionics manager, the evaluation of the system has taken place during regular missions in support of oilfield operations at Prudoe Bay and Dead Horse in the Alaskan Arctic, and also on flights to offshore oil platforms in the Cook Inlet, off the Kenai Peninsula
"Systems of this kind have the potential to make a difference in helicopter safety," says Martin. "For example, in the 15 years since fixed-wing commuter-aircraft operations in the USA have been required to have ground-proximity warning systems, there have been virtually no losses because of CFIT. With that kind of record, there is no reason that the same thing could not be done for helicopters."
The Cubic GPWS consists of a computer using an Intel 960 32-bit processor. The processor unit weighs just 3.4kg, is contained within a 190 x 60 x 315mm box, and is fully interchangeable between different aircraft types, without costly retrofits. The processor has built-in diagnostics and has been designed for a mean time between failures of more than 10,000h.
The GPWS, which is compatible with ARINC 429 or military-standard 1553B databuses, uses standard aircraft electrical power and does not require forced-air cooling.
The type of warnings required can be programmed into the system. Wilson indicates that most customers will probably specify a standard synthesised-voice warning, but the system can also generate a message via a cockpit display - as text and in colour. The aural warning can be configured for either a male or female voice, and by choice of language, or inflection.
Wilson believes that a major selling point of the Cubic GPWS involves the fact that the software can be embedded in the computer box as a stand-alone unit, or directly into the helicopter's mission or flight-control computer, in those aircraft with a digital databus.
"With a direct installation into the flight-control computer, there will be no extra weight," he says.
He also points out that the GPWS has a built-in data-recording capability, which will allow storage and replay of up to 51 different flight parameters. Those parameters include roll angle, rate of descent, airspeed, or height above ground.
These data are retrieved via lap-top computer, which draws the information directly from the GPWS unit and the lap-top transfers the data on to a floppy disk, which can then be installed on any standard personal computer, with diagnostic software for analysis.
"The advantage of this is that a pilot or evaluator can analyse every warning that the pilot receives. This allows technicians or pilots to analyse every parameter associated with the GPWS, and to show the circumstances under which the warnings were issued," Wilson says.
Wilson says that the software's architecture has been designed to accommodate any new-technology sensors, including those for wind-shear detection, forward-looking obstacle detection, and global positioning, in conjunction with an embedded digital terrain database.
Source: Flight International