![Tour helicopters are common along the scenic but inaccessible Na Pali Coast in Hawaii. [Adobe Stock]](https://flyingmag1.b-cdn.net/wp-content/uploads/sites/2/2025/09/FLY0925_1.jpeg?width=1163&height=793)
I learned to fly helicopters long ago, in a Bell 47, the little bug-headed two-seater familiar to viewers of M*A*S*H. Once I had the rating I seldom flew them again.
I did visit the nearby Robinson Helicopters factory from time to time, however, and the amiable Frank Robinson would send one of his pilots up with me to scuff some of the thick rust off my skills. On one occasion Robinson himself flew with me in an R22, and I remember his telling me that fixed-wing pilots transitioning into helicopters faced a special hazard because their instinctive reaction in certain circumstances, like suddenly noticing traffic on a collision course, was to pitch down. Pitching down was fine in an airplane but could have disastrous effects in a helicopter.
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When my friend Micco Godinez, who operates an ocean kayaking business at Hanalei on the Hawaiian island of Kauai, recently sent me a clipping about the fatal crash of an R44 off the Na Pali Coast, I recalled what Robinson had said. By now I had learned a bit more about helicopter aerodynamics, and I thought this might be an instance of the phenomenon that he had warned me about.
The R44, carrying the pilot and two passengers, broke up in flight in July 2024. Its wreckage was recovered from 77 feet of water. Examining the gearbox and main rotor mast, which had separated from the engine and airframe, accident investigators found evidence of mast bumping. That, I’m sure, was exactly what they expected to find, because mast bumping is a known susceptibility of Robinson and Bell helicopters with two-bladed main rotors.
Mast bumping—the term is a misleading confusion of the symptom with the disease, like calling pneumonia a cough—is associated with unloading the main rotor, that is, putting the helicopter into a reduced or zero-G trajectory. Without lift pulling the blades upward, the rotor flails. Its central hub may strike the mast, and the blades may slice into the tail boom or cabin. This was what Frank Robinson meant, although he didn’t get quite that explicit about it, when he cautioned me against abruptly pitching down.
When a mast bumping incident occurs, the usual suspect is turbulence. But that’s just a guess because, as when an icicle is used as a murder weapon, the evidence is gone by the time the investigators arrive. The assumed turbulence may or may not have been abetted by some cyclic movement by the pilot.
In this case, however, there was something like concrete evidence. It happened that hikers on the Kalalau trail, which overlooks the ocean, witnessed the accident. They reported a strong wind was blowing, and a sudden and very powerful gust, which knocked over a 50-pound backpack they had set down, was followed by a loud boom. Turning toward the noise, they saw the helicopter fall in pieces into the ocean.
The Na Pali Coast is both scenic and inaccessible, and as a result it swarms with tour helicopters. Several other helicopter pilots who flew that day described the conditions as borderline, with 20- to 30-knot winds and some bumps, but nothing out of the ordinary. So it seemed that the accident may have been due to a freak jet of wind that just happened to hit one unlucky helicopter.
In 2019, another Hawaiian air tour R44 broke up, this one over Kailua, Oahu. Again, mast bumping was implicated, and, again, the supposed cause was turbulence arising from a “downdraft, outflow boundary, or gust front” associated with low clouds and showers. Significantly, radar showed the helicopter flying at 92 knots, far above the emphatically recommended turbulence penetration speed of 60-70 knots. What part the pilot’s control inputs may have played was unknown.
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If the name Robinson seems to be occurring with more than chance frequency here, it’s because mast bumping has come to be associated, fairly or not, with Robinson helicopters. Only Robinsons, certain Bell models, and a few homebuilts and antiques use the teetering, semi-rigid, two-bladed main rotors that are susceptible to mast bumping.
Since 2000, the National Transportation Safety Board has identified 15 instances of in-flight mast bumping, two involving an ancient Fairchild-Hiller type, two Bell 206s, and 11 Robinsons. All were fatal. Instances of mast bumping in military service, which would include a lot of two-blade Bells, do not appear in National Transportation Safety Board reports. Nor do ones outside the U.S. or, obviously, ones that do not precipitate an accident.
In New Zealand, where the Robinson fleet is smaller, 15 incidents of mast bumping were identified between 1991 and 2018. Of those, nine were linked to possible low-G conditions associated with mountainous terrain and strong winds.
In 2018, the Los Angeles Times published a purported exposé of the four-seat R44, which it found to have a fatal accident rate markedly higher than that of any other type of helicopter. The article presented a graphic arrestingly titled “America’s deadliest helicopters.” It encompassed all types in common civil use, deadly or not.
The fatal accident rate of the R44 was highest by a wide margin, while that of the two-seat R22 was close to the median. From an engineering and aerodynamic standpoint, the R22 and R44 are quite similar, and so, if the statistics presented were valid, the disparity could have been due to the kinds of pilots flying them or the types of operations for which the different models were being used, rather than to traits peculiar to the R44.
That Robinsons account for the majority of mast-bumping events could be due to several factors. The low mass of the rotor and the high control responsiveness may be one, but more commonly mentioned are the prevalence of nonprofessional and relatively low-time pilots flying them and the large numbers in use for training.
There is nothing fundamentally unsound about the helicopters themselves. If you’d like to see what Robinsons are capable of, search YouTube for “herding cattle with helicopter.” You’ll find some of the wildest helicopter flying you can imagine. No one would recommend imitating it. But those gymnastics are done with low speed and positive G, while mast bumping is associated with high speed and low G.
All flying machines come with built-in hazards. The ones besetting helicopters—settling with power, ground resonance, dynamic rollover, retreating blade stall, mast bumping—outnumber those affecting fixed-wing aircraft. The fact that the R22 has an average safety record, however, suggests its susceptibility to mast bumping should be regarded as one risk among many and not a prohibitive risk so long as pilots are aware of it and the helicopter is operated within its recommended limitations.
Still, the fact that mast bumping can have such disastrous effects—the flailing rotor blades slicing the helicopter apart—is something not to be taken lightly.
This column first appeared in the September Issue 962 of the FLYING print edition.
