Paul Phelan/CAIRNS
On the eve of Australia's 1994 cattle mustering season, 54 helicopter pilots attended a three-day, privately conducted helicopter safety-awareness refresher course. It was one of more than a dozen runs in the past two years, by two highly experienced helicopter pilots, doubling as flight safety consultants. Their activity, which has now reached about 360 attendees (about one-third of Australia's working helicopter pilots), is actively encouraged by helicopter operators and enthusiastically subsidised by insurers.
A year later, according to Australian safety statistics, at least one of those 54 pilots should have been dead; six more should have been in intensive medical care; another seven hospitalised; and at least 14 of their helicopters, most of them R22s, should have been destroyed. In fact, in the following year, only one pilot in the group suffered a destructive accident - a wire strike. That outcome has been repeatedly demonstrated by preventive training, and experienced Robinson operators are clamouring for the message to be heard, in the face of US NTSB actions that might well lead to regulation in Australia.
On Australian mustering operations, light helicopters had accrued, an appalling record with losses leading to spiraling insurance premiums. Owners were being told that mustering helicopters could soon become uninsurable, and Robinson, because its aircraft dominated the mustering business, received particular attention.
Rather than being commercial pilots trained in the unpredictable behaviour of semi-feral cattle, most Australian mustering pilots are quite literally "cowboys", taught to use a helicopter to replace the horse. The work is perhaps the most exacting operation demanded of light helicopters on a regular basis. Keeping a herd moving in the right direction through timbered country, or dislodging an angry bull backed up against the trunk of an overhanging tree, are routine operations. Flying is invariably at high ambient temperatures and density altitudes. Aircraft are operated all day at extremely low levels, often among the trees below canopy height, and the need for quick response to cattle breakaways calls for vigorous and radical manoeuvring of these tiny aircraft hour after hour, in just the way their pilots formerly operated on horseback.
Dust and grit, gusting winds, low-level obstacles, and "dust devils" - rapidly rotating mini-tornadoes carrying airborne debris and reaching over 10,000ft(3,000m) - guarantee that mustering helicopters will always operate at the outer extent of their capabilities. Pure economics preclude the use of turbine-powered aircraft. Robinsons have been operated in cyclonic winds, up to 55kt (100km/h) in flood emergencies - moving cattle to higher ground - and operations in average ambient 15-20kt winds, have not caused a single mast-bump or tail-boom strike. Yet most mustering pilots, almost by definition, have relatively little experience.
A survey conducted by Aerostar Aviation's Tony Carmody, (one of the two safety activists) last month, shows that eleven commercial operators canvassed, most with mustering as their major activity, had flown well over 300,000h in R22s without an accident attributable to what the regulators have come to call "Robinson-type" aerodynamic factors. One operator, the Australian Agricultural (AA) Company, had unwittingly achieved a record with over 50,000 accident-free hours in six R22s since 1986.
Carmody and his colleague Rob Rich, are adamant that numerous Robinson accidents, have been wrongly attributed in the past, to mast bumping and rotor tail-boom strikes as primary causes, when they are actually part of the outcome of mishandling resulting, in low rotor revolutions per minute (RRPM) situations and consequent main rotor divergence. The reduction in accidents and insurance premiums among pilots attending their remedial training, is testimony to their beliefs.
INSURANCE BENEFITS
Until 1993 the overall Australian accident rate for insured mustering helicopters was around 25 per 100,000h flown, in an industry that flies about 600h a day during the season. Recognising the value of the courses conducted by Carmody and Rich, one underwriting group reduced hull insurance premiums for graduates to about 14% and hull-excess to 7.5%, tied to attendance every two years.
The insurers, who paid for pilot attendance, then budgeted for a reduction in accident rate from one in four to one in five aircraft. Even then, the budget for the 37 aircraft under the scheme at that time represented five write-offs, four rebuilds, and some A$50,000 ($37,000) in other costs for a year. Since the courses began, however, the accident rate among their graduates has been reduced to one write-off and one re-build due to wire strikes in agricultural operations. Insurers have further rewarded participating pilots with an unsolicited "profit commission" rebate, effectively reducing the rate by another 1.75%. The accident rate among non-attendees remains appalling.
In the typical Robinson accident, a pilot allows RRPM to reduce to around 85% without adding engine power, usually because of distraction by something outside the aircraft, and commonly while reducing speed by applying aft cyclic. The resulting flare produces an increase in RRPM, so the pilot (perhaps subconsciously) elects not to increase engine power.
As indicated airspeed (IAS) decreases below 53 knots (the speed for minimum power in level flight), the pilot decides to reduce speed even further, thus increasing the power required to maintain level flight. This increases the area of the small permanently stalled inboard sector of the rotor disc, and the aircraft begins gradually to lose height and RRPM, often unnoticed. If the pilot observes the reduced RRPM and moves to correct it by lowering the collective, he may also inadvertently reduce engine power because of the mechanical link between the collective and the throttle, so that a RRPM increase is not achieved.
As the rate of descent increases the stall progresses outboard along the blades, the resulting increased drag further, lowering the RRPM. At this point the pilot becomes alarmed and lowers the collective further. Again the mechanical link reduces throttle and the descent steepens further. RRPM fails to increase or reduces, and the pilot panics, fully opening the throttle and raising the collective. The main rotor angle of attack increases in response to the raised collective, and drag on the already stalled rotor further increases. The engine cannot cope with the rapid increase in load, and the fully stalled main rotor slows rapidly or stops, completely stalling the engine. The aircraft pitches vertically down and a crash is inevitable. The entire process from the point of airspeed reduction below 53 knots will typically take as little as 9-10 seconds.
A video shown on the course shows an R-22 crashing vertically after that course of events. Yet the main rotor is intact almost to the point of the blades being re-useable; a classic illustration that it had completely stopped during the descent. In other similar accidents, insists Rich, mast-bumping symptoms and boom strikes have been the result of main rotor divergence during the sequence, rather than of turbulence or wind gusts as implied by the FAA's operational limitations, now adopted by the Australian Civil Aviation Authority. Reported sounds of engine sputtering are consistent with an engine labouring against a stalled rotor, he says.
Rich acknowledges that various combinations of the low-inertia rotor, the highly effective throttle coordinator linking collective to throttle, and unfamiliarity with the type, can cause rapid onset of the sequence, requiring equally rapid reaction time. He insists, however, that it is a training problem rather than one related to systems design or operating conditions.
He also notes that no Robinson to his knowledge has crashed in these circumstances when fitted with an RPM governor; governors are fitted to all new Robinsons and available as a retrofit on older models.
Carmody and Rich say they are not preaching new doctrine. Both attended the Robinson factory course at Torrance, California, of which their own course is a development. They are amazed at the poor level of training revealed in Australian Robinson accidents, believing the same problem exists in countries such as Japan, where about the same number of Robinsons fly, with an equally dismal safety record. They find the NTSB's analysis to be not credible.
Source: Flight International
