|Bristow Group’s Eurocopter EC225s fly over water on a regular basis, making ditching a necessary part of pilot and crew training. Bristow|
Most ditching events and water impacts would be survived if on the ground. Civil aviation authorities, helicopter makers and equipment manufacturers are struggling to improve the survivability of these accidents, as the number of fatalities due to drowning of conscious occupants is still unacceptable. Improving floatability is a major focus. Just as important, avoiding helicopter capsizing would save many lives.
Emergency breathers can help, but can be challenging to use. Underwater evacuation training helps, too, but only happens once every four years. At December’s European Aviation Safety Agency (EASA) helicopter ditching workshop in Cologne, Germany, about 65 attendees—a lot of them representing North Sea oil-and-gas offshore operators—heard how difficult it will be to make this safety picture somewhat rosier.
Let’s first define what they are dealing with. A ditching event is an emergency landing on water. It is performed so that it enables a “safe and orderly egress” of the occupants. A water impact is uncontrolled or partially controlled. An example is a Sikorsky S-76C+ accident in 2005 near Tallin, Estonia. All 14 passengers and crewmembers drowned (they inhaled water), although there was enough survivable volume after the impact. Statistics show that what is probably the most strictly run helicopter activity, offshore transportation, lags behind in terms of safety. Commercial airlines have 0.9 fatal accidents per million flight hours. Offshore helicopter transportation has 5.7 (2010 numbers).
Why focus on ditching and water impacts? According to a review of world civil water impacts, 98 survivable water impacts happened between 1971 and 1992. They caused 338 fatalities, including 192 caused by drowning. An underwater escape (from a flooded helicopter cabin) too often mismatches breath-hold time.
Shell Aircraft senior aviation advisor Alan Ward insisted his company has “a strong belief in using more modern aircraft.” Oil companies wanting to buy safer aircraft is one more incentive for the manufacturers to carry on with their efforts in this direction. First, they may want to have manufacturers better understand ditching dynamics. Russia-based Kazan Helicopter is thus endeavoring to properly model these forces. “This will help determine proper piloting,” said Dmitry Nedelko, chief of Kazan’s calculation bureau.
Operators may want to have their helicopters floating longer. As John Franklin, an EASA safety analysis coordinator, noted, in 26 of 184 accidents (since 1970) that involved ditches, the helicopter sank much too early. This was immediately or during the evacuation.
Sometimes floats did not inflate. Sometimes they inflated, then deflated. So what about emergency floatation system (EFS) crashworthiness? “Statistics indicate improving floatation is the most important factor for better survivability,” noted Dave Howson, UK civil aviation authority (CAA) flight operations research manager. Indeed, the major drowning cause is the inability to escape an inverted helicopter.
Surprisingly, computation has shown that a 100-percent increase of EFS design strength translates into a modest crashworthiness improvement. A designer should think of a greater number of floats rather than stronger floats. In simulations, the helicopter stayed afloat in all impacts, providing it had two high-mounted floats—in addition to the usual four floats at the bottom of the airframe. The two upper floats bring redundancy and a side-floatation capability.
Other ways to save lives by improving floatation include, for example, automatic activation. This can be done with immersion switches. However, to automatically activate the EFS, it has to be armed at all times.
This has a cost. “You either have to demonstrate inadvertent deployment does not jeopardize the flight or certify a system, using speed or altitude switches for example, to prevent such inadvertent deployment,” Howson explained. As a result, the estimated cost per life saved would be about $380,000. This is considered highly cost effective. “Oil & Gas UK uses a figure around $7 million per life saved,” Howson reminded.
All of the North Sea helicopters already have automatic floats. They proved their value in a Super Puma (G-REDU) accident there in February 2009. “Most of us believe that the helicopter would have capsized and drowned at least some of the occupants if it hadn’t been for the automatic activation system,” Howson told Rotor & Wing.
|This model shows an example of the “side floating” concept.|
AgustaWestland claims to have demonstrated its AW139 medium twin is seaworthy up to sea state 6 (very rough sea). Initially, it was just complying with the regulation—sea state 4 (moderate). The flight manual now refers to sea state 6.
The Anglo-Italian manufacturer recalculated float and structural loads. It then performed tests in a water tank on a 1/12th model, said Daniele Robustelli, marine and general airframe systems specialist. On a video, the AW139 model appears not to capsize, despite the simulated waves and wind.
Robustelli was challenged, though. The audience expressed surprise at seeing smooth water surface on the video. It turned out that the rules that govern such demonstrations are quite relaxed. “I agree irregular waves may be a more severe environment, which we have not tested,” Robustelli added.
A Eurocopter representative pointed out that irregular wave testing would require a huge water tank. This would be to accommodate the drift caused by the simulated wind over the irregular wave scheme period. EASA rotorcraft certification manager Massimo Mazzoletti wondered, “should we mandate sea state 6?”
As for post-ditch stability, adding float scoops would be a significant improvement. The EFS would then just cost 10 percent more or so. A float scoop is a small, flexible bucket, attached to the exterior of the float. It naturally fills up with water when the float immerses. The added weight of the water acts to increase the aircraft’s righting moment when above the water. The drag of the pocket also dampens rolling when below the surface. As a result, the helicopter can withstand one more sea state—sea state 5 instead of sea state 4, for example.
It remains that capsizing is a non-linear process. “Therefore, it is difficult to relate it to measurable helicopter parameters such as dimensions,” Howson noted. Fundamentally, the reason why helicopters roll over is that a lot of weight is located on the top—engines and main rotor. “Don’t stay in the aircraft if it seems to keep upright, as it may capsize,” warned UK CAA’s Tony Eagles, a former Royal Navy pilot.
Other ideas have been pitched to avoid capsizing and sinking. Mazzoletti pleaded for the “jettison fuel” item to be removed from the ditching checklist. “It does not improve floatability and makes the cg higher,” he said.
Separately, the suggestion to let some water in (the “wet floor” concept) to stabilize the aircraft has been ruled out. It brings a risk of capsizing when the water rolls—this is called the “free surface effect.” The side-floating concept brought a lot of attention and debate. Howson supports it. Human subject trials showed it is much easier to escape from a helicopter that is floating on its side, rather than inverted. An asymmetric configuration, with only one upper float, is preferred, he said. It provides a single rather than two-side floating position. This avoids the helicopter switching from one position to the other. Moreover, it is cheaper, lighter and generates less aerodynamic drag (even when stored, emergency floats make the helicopter’s exterior less smooth).
Eurocopter had quite a different view. Louis Delorme, in charge of EFS design, does not quite like seeing a float near the main rotor, in case of inadvertent deployment. Moreover, the proximity of gas exhaust requires new fabrics or thermal protection. Then, the weight of the extra floats can be close to 200 lbs.
On the asymmetric configuration, Delorme said it does not provide as much air volume in the cabin as a symmetrical one. “Today, we are not sure of the risk-benefit ratio of side floating,” he summarized. Eurocopter is therefore focusing on upright stability.
What about a sea anchor? Howson said it is good because it helps keep the helicopter facing waves—hence much more stability. Therefore, the helicopter has a better chance to avoid rolling over. “But the time it takes to deploy a sea anchor is an issue,” he added. It doesn’t work until the connecting line is tight and the helicopter could capsize before that is achieved.
François Hochart, an investigator at the French BEA (the equivalent of the NTSB), noted additional equipment to prevent helicopter capsizing appears to be difficult to design. “Were other ways investigated in terms of design or training to improve safety?” he asked. He suggested to find other locations for emergency exits and to enhance passenger training.
|AgustaWestland AW139 flying over water. Photo courtesy AgustaWestland.|
As things stand, if the efforts to keep some exits “dry” fail and the helicopter actually inverts, occupant disorientation is immediate. According to Paul Sparkes, a UK CAA flight operations inspector, the occupants’ allies will be training, equipment “and a lot of good luck.” Seats should be aligned with windows. “Otherwise, once inverted, you’ll never find your way,” Sparkes stressed. Not all modern helicopters used today in offshore operations have such alignment.
Once the occupant has pushed the window out, he or she often needs something to hang on the fuselage to help egress. But recent aircraft have smooth, aerodynamic surfaces. This does not make egress easier, several speakers noted. The Sikorsky S-61 Sea King had a lifeline rope. It even had a hull properly designed for landing on water, like the Sud Aviation SA321 Super Frelon.
Although the fuselage offers little to hang on, it happens that it damages liferafts. “We have to delethalize the fuselage,” Sparkes said. Also, as Paul Hannant, a UK Aircraft Accident Investigation Branch (AAIB) senior inspector emphasized, survival equipment should not be mounted on a door that can be jettisoned.
Then, an occupant has to wear a well-sized immersion suit. “They are sometimes chosen too large for comfort reasons,” Michael Cunningham, investigator in charge of the 2009 Cougar flight 91 (a Sikorsky S-92) crash, said. There is also a risk of snagging. Involved can be emergency breathers or, for the military, survival equipment (such as a dinghy) attached to the back of the pilot.
Water temperature can add one challenge. To keep dexterity, occupants are encouraged to put gloves on only after egress. But, in one instance, a survivor got his fingers so cold that he could not don his goggles. This affected his vision.
Typical escape time is 45 to 60 seconds in a real accident. Meanwhile, breath-hold time can be as short as 20 seconds in cold water.
To help solve the mismatch between underwater escape and breath-hold time, three types of emergency breathing systems (EBS) are available. The first one uses a small bottle of compressed air. The second one is a rebreather. The third type is a hybrid of the first two.
The idea is to have at least one minute of breathing time. “It helps overcome panic and disorientation,” Howson added. Training is needed, though. In addition to the snagging hazard, the added buoyancy can impede egress from an inverted aircraft. Research is going on to ensure there is a net safety benefit in using EBS. Howson hopes the EASA will adopt the resulting specification. It could then issue it as a European Technical Standard Order (ETSO).
Consultant Sue Coleshaw is developing a technical standard for EBS. She pointed out that using an EBS should be easy. For example, the mouthpiece should not be hidden in the folds of the stored air pocket. Also, the emergency breather should not prevent harness release. Designing an EBS with a deployment time clearly below breath-hold time is still a challenge.
Underwater escape training is obviously very useful but it must be properly done. Regulations only call for one such training session every four years. In a presentation by Michael Taber, director of research and development with Survival Systems Training, it appeared this brings somewhat contradictory requirements. The simulated cabin has to be as realistic as possible with accurate window and handle locations etc. However, as training sessions do not occur often, the simulator also should be representative for several types. One passenger may be carried in several different helicopters over the four-year interval.
Taber also insisted the stress level in underwater escape training should be high enough for knowledge and skills to be retained.