The EASA is considering increasing the time in which a pilot is expected to respond to engine failure in a single-engine helicopter, to align certification standards with real-world human performance. The new standard, if adopted, would require helicopters to be designed so that the pilot has more time to respond before a decay in rotor rpm takes the machine into hazardous dynamic territory. A study by Dutch aerospace research center NLR shows that this would add weight and cost.
Establishing autorotative flight after loss of power requires immediate recognition and response. Current certification requirements assume the pilot can react in one second in cruise flight and in 0.3 second in other flight phases, explained Jos Stevens, a senior scientist at NLR’s helicopters and aeroacoustics department.
Actual response times are between two and three seconds, he said, well outside the 0.5- to 1.2-second window of allowable time if the engine fails in a hover, according to an NLR study that evaluated the HeliSport CH-7 Kompress, the Robinson R44 Raven I, the Bell 206B-3 JetRanger III and the Airbus Helicopters AS350B2 Ecureuil. The real-life bottom line: in most cases, recovery after a complete power loss in the hover is unlikely.
The number of accidents attributable to a late entry into autorotation is significant. Between 2000 and 2011, there were 151 accidents related to engine failure in a single-engine helicopter in the 32 EASA member states, and 22 of them were caused by tardy initiation of autorotation. NLR estimates that if the fallen helicopters had been certified to a pilot response time of two seconds, 18 of the crashes could have been averted, saving 10 lives.
NLR studied two ways of increasing the time available for the pilot to react: providing a separate source of emergency power and adding rotor inertia. Both potential fixes are intended to keep rotor rpm above its allowable minimum while the pilot responds.
The first solution could be a backup electric motor, similar to the 160-hp (120-kW) one Airbus Helicopters tested in 2011. NLR estimates such as system would have to provide 160 shp on the AS350 and Bell 206. On the downside, the added weight is estimated to be 209 pounds for the AS350 and 242 pounds for the 206. The second solution, boosting rotor inertia, would add 207 pounds to the AS350 and 264 pounds to the 206. These weight penalties include not only the electric motor or heftier rotor but also everything needed to keep the payload and range unchanged–a larger fuel tank, extra fuel and a more powerful engine.
Both solutions would be expensive too. Across the European helicopter fleet, the extra fuel burn to move the added weight would cost operators €6.7 million ($9 million) annually. NLR deems this to be out of balance with the annual estimated safety benefit of €3.9 million ($5.3 million) in averted accidents, fatalities and injuries.
Finally, NLR suggests a third option: recommending, rather than mandating, that manufacturers voluntarily increase the available response time to two seconds. Assuming that 12 percent of buyers would opt for helicopters equipped to provide the added response time, NLR places the safety benefit at €470,000 ($630,000) per year and the additional fuel cost at €800,000 ($1.1 million) per year.