ANDREW HEALEY / LONDON
With safety now a major selling point in rotorcraft design, developments such as air bags and GPS tracking have placed the focus on maximising survivability rather than minimising risk
A conventional helicopter can only fly so fast and hover so high. While performance can be tweaked, and hybrids like the tiltrotor can expand the envelope substantially, designers of new-generation rotorcraft have to look elsewhere for that elusive edge. There is still room for significant improvement in helicopter safety, both in preventing accidents and making accidents survivable. Crashworthiness, occupant protection and the location of downed aircraft are all areas where improvements are being made.
It is no longer a question of simply minimising the risk of an accident, but of maximising the chances of surviving one. The latest helicopters incorporate new levels of crashworthiness made possible by advances in computer modelling. Air bags promise to protect both crew and passengers in a crash, and satellite-based position monitoring can help locate a downed aircraft quickly and accurately. Work is also under way to make safety-critical systems, such as flight controls, more survivable.
Design of the Bell/Agusta Aerospace AB139 commercial helicopter, scheduled for certification later this year, incorporates lessons learned on three key European military rotorcraft programmes: the Agusta A129 Mangusta attack machine, the EH Industries EH101 large helicopter and the NH90 Industries NH90 tactical transport. Each represents a different structural approach, and the combined knowledge base contributed significantly to AgustaWestland's research into how structures deform during impacts.
Working with the department of engineering at Italy's Politecnico di Milano and the Centro Italiano Richerche Aerospaziali, Agusta carried out a series of drop tests using full-scale fuselages during the late 1990s. The last one, in 1997, involved an A129 but was used to predict AB139 characteristics.
Krash testing
Telemetry and high-speed cameras followed both events, and data was then analysed to develop a mathematical model known as Krash.
In the world of Krash, numbers represent masses and the internal "beams" that connect them together at "nodes". Non-linear "springs" simulate deformable underfloor structures. The A129 Krash model, which was also used to predict the AB139's characteristics, simulates 30 masses, 28 springs, 64 internal beams and nine non-linear beams. Measuring the positions of the nodes before, during and after an impact shows how the structure deforms.
The result of Krash analysis determined that the AB139 fuselage, without taking the landing gear into account, can withstand US Federal Aviation Administration and military recommended standard 8m/s vertical impact velocity. The controlled deformation process is designed to preserve the passenger compartment's vital space and prevent its penetration by high-mass components. The attachment points for these components, which are all located above the cabin, can withstand the load factors prescribed in European Joint Airworthiness Requirements (JAR).
Cockpit and cabin seats meet the same standards. Energy absorption capabilities limit the impact load on an occupant to survivable levels at 8m/s for vertical and 12.8m/s horizontal. The landing gear, ideally the first part of the machine to hit the ground, can take 10.5m/s, while the fuel system satisfies JAR requirements and military standards for preventing fuel leaks, after impacts at rates exceeding 8m/s.
BAAC says the AB139's "unmatched" crashworthiness characteristics are not the result of any single item or feature, rather a "multi-path approach employed in a sequential and progressive manner". This includes landing-gear components; the distribution of the high-mass components; the overall fuselage with its anti-plough shape; the availability of large emergency exits with push-out emergency windows for each seat row; crashworthy seats with modern restraints; and human protection from fuel and other hazardous elements.
The newer the design, the better a crew's chances of surviving if a helicopter ends up hitting the ground. However, even if the machine becomes involved in a "survivable" accident - defined as one where 85% of the aircraft is intact when it stops - 30% of crews will not survive.
This is nothing to do with any inherent weakness in the airframe or the lack of a "stroking" seat to cushion the impact. The most prevalent cause of death, in US Army helicopter accidents at least, comes as the gruesome result of becoming impaled on the cyclic stick.
Wearing a helmet and keeping the harness tight may not help the pilot avoid this grisly fate, according to Stan Desjardins of Simula Safety Systems. "Under crash forces, the surprisingly malleable human body can contort around the firmest of restraints. But a simple system of air bags, deployed from under the cockpit glare shield or alongside the side window, can counteract these complex forces."
Simula, under a $11 million contract, is delivering hundreds of cockpit air bag systems (CABS) to the US Army to fit to its Sikorsky UH-60A/L Black Hawks and Bell OH-58D Kiowas.
With the technology adaptable to most cockpit layouts, the potential for Simula in the military market alone is huge.
Cushioning the blow
The Black Hawk system consists of forward and lateral air bag modules, which are controlled by a programmable electronic crash sensor unit (ECSU) under the pilot's seat. The three-axis ECSU, which is essentially a series of accelerometers, triggers the device and records the forces present during those final seconds of flight. The data can be digitally downloaded into the army's crash investigation system and a Windows interface allows the user to tailor the crash discrimination algorithm to specific aircraft.
The more open cockpit structure of the OH-58D Kiowa dictated a slightly different approach by the Tempe, Arizona-based company. Simula's solution was to integrate an inflatable tubular structure (ITS) into the cabin doors. Similar bags are now standard equipment in BMW cars in the USA. The company is working on an inflatable tubular torso restraint or harness strap.
In both configurations a gas generator inflates the cockpit bags in the same way as it would in a car. However, CABS uses a proprietary hot gas which cannot vent, whereas car manufacturers use a cold gas which gradually escapes. As the gas cools the bag slowly deflates, allowing a longer period of inflation - 3s as opposed to milliseconds in a car - to cope with the longer train of events inherent in helicopter accidents.
Allaying fears
To get where it is today, the system has had to face its share of challenges. And the biggest problem in promoting helicopter air bags to the commercial community is a perceptual one. "We've got it down to a reasonable weight," says Desjardins, "and we've tried to overcome the persistent concern that, somehow, it might go off in flight. It won't - it can't - but that gave us a major marketing headache. We've even set if off while airborne, in a real Black Hawk in a 150ft hover, with a safety pilot in the left-hand seat."
Experts from the US armed forces are not so worried about the risk of inadvertent inflation. In a 2000 poll of test pilots and scientists serving at various test and research establishments, on whether they thought CABS was a worthwhile piece of equipment to have in their cockpits, 95% answered 'yes'.
"You see the thing open up and you hear a muted 'bang'," says Simula product manager Jack Cress, "but the helicopter doesn't move." The pilot noted a small flash and heard the bang, but before he knew what had happened, the bag had deflated and was lying out of sight behind the instrument panel. "A non-event," says Cress. They tried it underwater as well. The pilot just pushed the deflating bag out of the way and stepped out of the window.
The chances of air bags making it into the civil helicopter community are not high, however. Current FAA rules call for safety features - stroking seats, strengthened structures, reduction of protrusions and effective restraints - in all new aircraft, but the rules are not so insistent, for example, on the specifications of crashworthy fuel cells. They also fail to address the potential dagger-like effects of the cyclic stick, recommending merely that harnesses are designed to steer the fast-decelerating pilot away from it.
The overriding factor governing the survivability of any incapacitating accident is how quickly you can be located. Helicopters spend most of their working lives outside controlled airspace, below radar cover and often in remote areas. Regular check-calls are often the only way of staying in touch, or if missed, of triggering an alert. Even then the monitor may only have a rough idea of last-known position.
Monitoring the position of North Sea helicopters used to be done manually by specialist plotters. Now it is achieved by a combination of radar watch and computerised tracking, based on crew reports and calculations. It is reliable, but still involves an element of guesswork.
Over the Gulf of Mexico, however, Chevron keeps an eye on its helicopter fleet with a system that draws together the PC, GPS and the internet. The OuterLink automatic flight following (AFF) package derives its position data from either its own GPS or the aircraft's, and transmits it to a geostationary satellite as often as once every 10s. Chevron has it set to report once a minute.
The satellite relays the data to a ground station, which relays it to OuterLink's base in Concord, Massachusetts. From there it is placed on the internet and made available to Chevron's dispatch centre in Louisiana. Using proprietary software, the dispatcher logs on to see the helicopter's real-time positions. The aircraft's satellite transceiver/modem can transmit additional flight information, such as latitude/longitude position and speed with inputs from up to eight sensors.
OuterLink AFF equipment on the helicopter is lightweight, weighing less than 4kg (9lb), and has found a home on air ambulance helicopters across the USA such as those operated by Aerocare, and with Ontario's Ministry of Natural Resources, which has 65 systems on both helicopters and firefighting aircraft operating over the remote Great Lakes area. The system has proven its worth on several occasions.
Satellite tracking
Over a 12-day period at the end of August last year, four air ambulances crashed in the USA. Three of them had AFF. Two of those three depended on OuterLink equipment. The one that did not have AFF was not found during the search: instead a farmer discovered the wreckage the next day, as he worked his field in South Dakota. The two aircraft with OuterLink, on the other hand, were located within 1min of going down.
OuterLink's Mike Sullivan says: "We can see anything between New Mexico and the Arctic Ocean, Hawaii and the Bahamas, and are working to achieve worldwide coverage during 2003. The challenge is to get frequencies allocated by the satellite vendors. GPS is only part of it; it'll tell you where you are, but we need to tell someone else where we are as well."
There is a health and usage monitoring system (HUMS) link to AFF, with Altair Avionics using it to transmit HUMS data, and the US Navy using Draper Laboratories to flight test the transmission of HUMS data from a Sikorsky SH-60B. "ERA Aviation sends an engineer from Alaska to Mexico every three months to clear HUMS data from a contract aircraft. Using this system, if a parameter became overloaded, the engineer would know within seconds. There's also the potential for two-way communications," says Sullivan.The US Border Patrol, which is taking delivery of 13 new Eurocopter AS350B3s, likes AFF for another reason. "Every so often the crews get a call from a senator, complaining they're making incursions into Mexico. Now they can show exactly where they've been flying."
Source: Flight International
