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Walking Into Trouble Accidents involving human contact with propellers and rotor blades persist, despite safety efforts to prevent them.
by Clarence E. Rash
eople continue to come into of paint schemes to increase blade The primary risk presented by a contact with spinning propellers conspicuity — visibility — and have de- spinning blade is that it is difficult and rotor blades, often with fatal veloped safety programs to emphasize — often impossible — to see. In addi- consequences, even though the propeller and rotor-blade hazards. tion, noise from the engine and slip- Pdanger they represent is well known. “Constant care and vigilance [are] stream/rotor downwash may obscure Although not common, these accidents required to keep the number of … ac- noise from the blades. continue to claim flight crewmembers, cidents at a minimum,” the Australian “It is often difficult for a pilot to passengers and ground personnel. Most Civil Aviation Safety Authority (CASA) appreciate the level of confusion that a are preventable. said in a 2003 advisory circular that dis- non-aviator may suffer in the vicinity of Aircraft manufacturers and op- cussed the prevention of injuries caused an aeroplane with its engine(s) run- erators have implemented a variety by spinning propellers and rotor blades.1 ning,” CASA said. “The area to the rear
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of an aircraft is where slipstream and the airplane and — after being told that U.S., in which the pilot of a Beech C35 noise are most evident, while the front the wheel chocks had been installed on Bonanza was killed as he hand-propped of the propeller area may be relatively the left side of the airplane — feathered his airplane.4 quiet. … Pilots of high-wing singles or the right (no. 2) propeller. The captain None of the airplane’s wheels had twins have to be particularly aware of said that he heard “three or four quick been chocked in front, and the ground the risk of a passenger moving from the thumps” and was signaled to shut down was damp after heavy rain the previ- exit doors forward … toward the pro- the right engine.3 ous evening, the report said. The pilot pellers. Pilots of helicopters have to be “When he exited the aircraft, he tried unsuccessfully to start the engine alert to the possibility of people walking saw the ramp service employee lying with the electric starter, then hand- into the tail rotor while focusing on the beneath the aircraft,” the report said. propped it. engine and main-rotor disc when ap- “An employee on the ramp [later] “The engine fired, and the propeller proaching or leaving the cabin.” stated that the ramp employee involved spun, hitting the pilot,” the report said. Propeller tip speeds — which ap- walked outside of the left wing with two Spinning helicopter main-rotor proach the speed of sound during take- chocks toward the nose of the aircraft. blades have unique risk characteristics off — are dangerous even at the slower She reported that he walked to the nose because the space occupied by the rotor speeds involved in ground operations. landing gear, slowed down and then blades is large — with a typical 40-ft For example, at typical engine idling continued walking around to the other (12-m) rotor-disc span that extends as Walking Into Trouble speeds of 1,000 to 1,200 revolutions side of the aircraft. … She heard the much as 13 ft (four m) beyond the sides per minute (rpm), propeller tip speeds ‘sound of something hit the propeller’ of the helicopter — and can be easily are 200 to 300 mph (322 to 483 kph) or and she saw the individual ‘flip and accessible. Rotating helicopter blades more.2 land on the ground.’” are especially dangerous to people out- One common scenario for propeller- The injured employee had been hired side the aircraft when the helicopter is strike injuries and deaths involves main- about one month before the accident and being powered down and the centrip- tenance and ground personnel working had received one week of general train- etal force on the main-rotor blades is around airplanes. ing, followed by specific training that had reduced, allowing them to droop closer For example, a ramp service employ- included instructions to always “chock to the ground. Because of their height, ee received serious injuries when he was the main landing gear by approaching they are associated with head trauma struck by a rotating propeller blade on from the rear,” the report said. more often than with any other type of a Saab-Scania 340B at General Mitchell Another accident scenario in- injury. International Airport in Milwaukee volves hand-propping to start a small A typical helicopter main rotor on Nov. 13, 2004. A report by the U.S. airplane’s engine. turns about 450 rpm, with tip speeds of National Transportation Safety Board For example, an NTSB report cited as much as 500 mph (805 kph) — far (NTSB) said that the captain recalled the pilot’s “inadequate start procedure” exceeding the minimum force required that he had shut down the left (no. 1) as the probable cause of a Nov. 25, to cause serious injury or death if they
engine before arriving at the gate, parked 1998, accident in London, Kentucky, strike someone. Hartzell© Copyright Propeller
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For example, published reports which people were struck by propel- described an accident in March 2006 lers or rotor blades; of these, 137 (83 in Nibong Tebal, Malaysia, in which an percent) involved airplanes and 29 official of a housing development com- (17 percent) involved helicopters. The pany was killed when he was struck on accidents were more than three times the head by a rotor blade of a Eurocopter more frequent during the first five years © Copyright 2006 Chris Photography © Copyright Sorensen AS 365 Dauphin. The accident occurred of this period, when 56 accidents oc- as the official walked away from the heli- curred, than during 2001–2005, when copter after checking to ensure that one 16 accidents were recorded.7 of its doors was latched securely before The causes and circumstances of what was to have been an aerial tour for propeller and rotor-blade accidents several schoolchildren.5 have been recognized for many years. Tail-rotor blades — designed to In the early 1970s, as part of a nation- counteract the torque of the main rotor wide accident-prevention program, the by generating a sideways force to con- U.S. Federal Aviation Administration trol yaw and maintain the direction of (FAA) initiated an effort to educate flight — rotate up to seven times faster pilots about the hazards.8 than the main-rotor blades, with blade- As part of that effort, in June 1975, tip speeds of about 900 mph (1,448 FAA published the first of several ad- kph; see “Unconventional Tail Rotors”). visory circulars that discussed fatalities Tail-rotor blades have been cited in and serious injuries caused by spinning nearly three-quarters of U.S. helicopter propellers and rotor blades, and issued rotor-blade accidents. recommendations to prevent these ac- For example, an NTSB report cited cidents, which “with proper education a hospital security guard’s failure “to and discipline … could be reduced to maintain clearance with the operating zero.”9 tail rotor” as the probable cause of a Jan. The FAA effort was credited with 22, 2001, accident at a hospital helipad helping to achieve a substantial de- in Quincy, Illinois, U.S., in which the crease in propeller/rotor blade acci- security guard received serious injuries.6 dents. A 1993 analysis of NTSB reports The security guard — who was on propeller/rotor blade accidents by responsible for keeping unauthorized the FAA Civil Aeromedical Institute people away from the Bell 206L-1 (CAMI; now the Civil Aerospace Medi- LongRanger — walked into the tail cal Institute) showed that the annual rotor while crewmembers were prepar- average number of accidents peaked Paint bands on ing for departure on an emergency in 1970–1974 at 25.6. The subsequent propellers and rotor medical services flight. The security decline resulted in an annual aver- blades become visible guard had attended a training session age of 15.8 accidents from 1975–1979 concentric circles when about one year before the accident that — about 40 percent fewer than the in motion, helping discussed “how to approach the aircraft previous half decade.10 ground personnel in a safe manner while the rotors are in That decline “seems attributable identify and avoid their operation,” the report said. to several actions taken by the FAA dangerous arcs. in the mid-1970s,” the CAMI report Well-Known Hazard said. “The methods included safety A review of the NTSB accident data- seminars, handouts, posters, a film base found that, from 1982 through depicting an actual accident resulting early 2006, there were 166 accidents in from improper hand-propping and the
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Unconventional Tail Rotors
onventional tail-rotor designs in or enclosure, that forms part of the flying in the downwash of the rotor helicopters have from two to five tail fin of the helicopter. This also is system, producing up to 60 percent of Cblades and rotate in the verti- known as a “fan tail” design. In addi- the anti-torque required in a hover.”1 cal plane. The length of the tail-rotor tion, a fenestron has between eight The jets of air change the direction of blades typically is in proportion to blades and 13 blades, compared with the airflow in the tail boom to create the length of the main-rotor blades; conventional tail rotors, which seldom an aerodynamic force that opposes the typically, a tail-rotor blade is about 4.0 have more than four blades. Fenestron main-rotor torque. ft (1.2 m) long, and the tail-rotor disc blades are smaller than conventional Andy Logan, chief technology span is about 8.0 ft (2.5 m). The bottom tail-rotor blades and rotate at faster officer for MD Helicopters, said that by edge of this span can be 5.0 ft to 6.0 ft speeds. eliminating the tail rotor, the NOTAR (1.5 m to 1.8 m) above the ground. Because of the fenestron’s enclo- system has eliminated the cause of Two alternative designs that sure within the shroud, its blades are more than 25 percent of helicopter significantly reduce the risk of injuries far less likely to come in contact with accidents worldwide. He said that the associated with tail-rotor blades are the people — or with trees, electric power system also has secondary benefits fenestron and the NOTAR — or NO TAil lines and other obstructions. that are “subtle and many,” including Rotor. The NOTAR design, in use on the improved access to confined spaces, an The fenestron, patented by more recent models of MD Helicopters, ability to operate closer to obstructions Aérospatiale and in use on helicopters eliminates the tail-rotor apparatus and and quieter operations. manufactured by Eurocopter, resem- in its place uses jets of compressed — CER bles a conventional tail rotor in that air that are forced out of two slots on both systems have spinning blades the right side of the tail boom. MD Note that generate an aerodynamic force to Helicopters says that the result is the counteract the torque of the main- creation of “a boundary-layer control 1. MD Helicopters. NOTAR Technology. rotor blades. However, a fenestron’s called the Coanda effect. The result is
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release of FAA advisory circulars on the from 1972 through 1991.11 Eleven of the variety of paint schemes on propellers hazard of propellers.” injuries (46 percent) resulted in fatali- and rotor blades. Another notable decline occurred ties, mostly from head trauma. Of the 24 Today, for example, many airplanes from 1985–1989, when the annual people injured, half were crewmembers, and helicopters have stripes of high average was 7.2 accidents — down 48 seven (29 percent) were passengers, visibility paint on their propellers and percent from the annual average of 14 three (13 percent) were ground crew- rotor blades, and many helicopter tail accidents in 1980–1984. The decrease members, and two (eight percent) were rotors still have a black-and-white paint resulted partly from the “steady im- bystanders. During the years included in scheme — which was designated as provement in general aviation accident the study, one rotor-blade-strike fatality “most conspicuous” in a 1978 report statistics,” as well as from a decrease in occurred approximately every 1.7 years. by CAMI.13 Among the most recent the number of hours flown, the report When data were analyzed in five-year recommendations is one from Defence said. periods from 1972 through 1991, a Research and Development Canada, The CAMI report said that, of the downward trend in accident frequency which calls for two brightly colored 104 propeller/rotor blade accidents was similar to the trend identified in stripes, discontinuous from one blade to that occurred from 1980 through 1989, the 1993 study of U.S. civil helicopter the next, on each of the four main rotor 81 accidents (78 percent) involved accident data. blades of the CH-146 Griffon combat airplanes; 21 accidents (20 percent) in- support helicopter. This configuration is volved helicopter rotor blades, includ- designed to produce a circular flickering ing 15 accidents that involved tail-rotor effect as the blades rotate.14 blades; and two accidents (2 percent) Among the factors that affect the involved seaplanes. Nearly half of the conspicuity of propellers and rotor rotor-blade accidents were fatal. blades are color contrast between ele- The 104 accidents resulted in 106 ments of the blade color scheme and deaths and injuries; of those killed and color contrast between the blade color injured, 66 percent were passengers, 16 scheme and backgrounds. Brightness percent were ground crewmembers, 14 contrast between elements of the blade percent were pilots, and 3 percent were color scheme and brightness contrast spectators. One-third of all deaths and between the blade color scheme and injuries occurred during deplaning, backgrounds also are considered, as are 25 percent occurred as the victim was Paint Schemes the patterns of colors on the blades, the assisting the pilot, 18 percent occurred For decades, manufacturers and opera- rotational speed of the blades, and the during hand-propping a propeller, and tors have used various methods to size and number of blades. 14 percent occurred during enplaning. help prevent injuries caused by rotor Hartzell Propeller paints contrast- Twenty-seven percent of the blades and propellers. One involves ing blade-tip markings on the forward accidents occurred during dusk or the use of different paint schemes to surfaces of its propellers — and on both darkness, “when ordinary propeller increase the conspicuity of rotating ro- forward and aft surfaces of pusher-type conspicuity, even at a well-lighted air- tor blades and propellers. This strategy propeller installations. The company port, would be considerably reduced,” was suggested as early as 1954, when encourages its customers — aircraft the report said. About 44 percent of the U.S. Navy began using a black, red manufacturers and aircraft modifiers those accidents involved people provid- and white pattern on propeller blades; — to choose blade-tip stripes in con- ing assistance to pilots, 29 percent were when the propeller is in motion, this trasting colors, such as white tip stripes deplaning accidents, and 13 percent pattern results in the visual effect of on a black blade, or red and black were enplaning accidents. concentric circles.12 stripes on a white blade, said Richard A similar study of U.S. Army heli- In the years that followed, blade Edinger, vice president for certification copter accident records found that 24 conspicuity was studied repeatedly; and flight safety at Hartzell. rotor-blade-strike injuries, half of which researchers often reached different “We have observed propellers (more
© Copyright 2006 Chris Photography © Copyright Sorensen involved tail-rotor blades, occurred conclusions, and manufacturers used a commonly those in military use) where
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with the procedure should be at one or more blades are painted with pilots and other crewmembers, ground the controls; alternating stripes of black and white the personnel, passengers and airport/ full length of the blade, and the remain- heliport managers. • People who must walk beneath ing blades are painted with the reverse For example: a helicopter’s main-rotor blades arrangement,” Edinger said. “This gives should crouch low well before ap- a noticeable strobing effect, although • Airport managers should provide proaching the blades. If they are the appearance would probably be very safety barriers and related mark- suddenly blinded by dust or de- unappealing to many.” ings to ensure that unauthor- bris, they should stop moving and Recommendations to aircraft own- ized people do not loiter among crouch lower; a better alternative ers and operators are that they maintain parked aircraft; is to sit and wait for help. They or adopt a paint scheme to enhance should not try to feel their way • Operators should ensure that all propeller/rotor blade conspicuity. How- to or from the helicopter. No one personnel receive recurrent safety ever, an existing paint scheme should should reach up for any object training in the risks of work- not be changed unless a specialist has that might be blowing away or ing around propellers and rotor determined that the new paint scheme chase after the object; will not interfere with pilot vision, blades. Warning signs displayed induce flicker vertigo or result in an within aircraft cabins and in pas- • Whenever possible, engines unbalanced blade condition. senger pre-boarding areas should should be shut down before Other engineering strategies also describe the risks presented by enplaning or deplaning passen- are recommended to aid in prevent- propellers and rotor blades and gers. When engines must be kept ing deaths and injuries associated with the enplaning and deplaning operating, flight crewmembers propeller/rotor blade strikes. methods developed to minimize should tell passengers — before For example, audible or visual those risks; they exit the aircraft — which path to follow to avoid propellers warning signals sometimes are used • Airport personnel should direct and rotor blades. A helicopter to alert helicopter pilots if doors are passengers from terminal doors pilot should orient the helicopter opened while the engine(s) are being to their aircraft, or rope stan- with its tail rotor away from the operated. This lets pilots know if pas- chions should be provided to passengers’ route to or from the sengers unexpectedly attempt to exit designate appropriate walkways. helicopter; and, the helicopter. Helicopter passengers should Additional lighting of the rotor blades be told always to approach and • Pilots, maintenance person- — with wing lights or tail boom lights depart the helicopter from the nel and others working in and aimed at the blades, for example — can front — never from the rear; if around aircraft should behave increase conspicuity at dusk, in darkness a helicopter landing area is on at all times as though ignition or in other low-light conditions. Other or adjacent to a hill, passengers switches are “on.” If they are solutions include blade markings that are should not approach or depart carrying long tool rods or other visible only at idling speeds and flashing the helicopter on the upslope equipment as they approach a he- strobe lighting to direct attention to the side so that they avoid the area of licopter, the equipment should be tail-rotor blades.15 lowest rotor clearance; positioned horizontally to avoid possible contact with the main- Safety Programs • Before starting an engine, flight rotor blades. Although engineering solutions are crewmembers should ensure vital to accident prevention, aviation that all personnel are clear of all Aircraft manufacturers and operators safety specialists also recommend propellers or rotor blades. Only have tried for years to prevent ac- well-designed and well-implemented individuals with experience cidents in which people are struck by safety programs that address human in hand-propping an airplane spinning propellers and rotor blades. factors. Programs to reduce propeller/ should perform the procedure; Today, although these accidents rotor-blade accidents should involve when they do, a person familiar are infrequent, they often are fatal. www.flightsafety.org | AviationSafetyWorld | august 2006 | 33 Groundsafety
Nevertheless, authorities say, with at- 3. U.S. National Transportation Safety Board 10. Collins, W.E. A Review of Civil Aviation tention to blade conspicuity and safety (NTSB). Report CHI05LA028. Nov. 13, Propeller-to-Person Accidents: 1980–1989. programs, most of these accidents can 2004. FAA Office of Aviation Medicine Report AM-93-2 (1993). be avoided. ● 4. NTSB. Report no. NYC99LA027. Nov. 25, 1998. 11. Crowley, J.S.; Geyer, S.G. “Helicopter Clarence E. Rash is a research physicist at the Rotor Blade Injury: A Persistent Safety U.S. Army Aeromedical Research Laboratory 5. “GM Dies in Freak Accident.” The Star Hazard in the U.S. Army.” Aviation, Space, (USAARL) in Fort Rucker, Alabama, U.S. He Online.
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