Undetermined Reasons

Estimated reading time 28 minutes, 48 seconds.

On June 29, 1992, a flight instructor and his pre-solo student took to the air in a Robinson R22 helicopter over northern California's San Francisco Bay Area. The instructor was relatively experienced, with about 2,000 hours of R22 flight time. The student had only four hours of flight time, all in the R22. She had brought along a microcassette voice recorder, which was set up to tape the cockpit and radio communications during her lesson.

The helicopter's low rotor r.p.m. warning horn was checked on the ground before takeoff; it operated normally. And nothing appeared to be out of the ordinary during the 17-minute flight to a local practice area near Richmond, where the student, at the instructor's request, executed a shallow left turn.

The U.S. National Transportation Safety Board (NTSB) described what happened next: "Seconds later, while cruising at 2,200 feet, the CFI [certified flight instructor] began talking. In mid-sentence an undetermined event occurred which interrupted his speech. A wind-like background noise started, and the student exclaimed, ‘Help.'"

Radar data confirmed witness reports that the helicopter's tail boom and main rotor had separated in level flight. The aircraft plunged into the San Pablo Bay, killing the instructor and student. Examination of the wreckage revealed that the aircraft had experienced "mast bumping" — severe contact of the rotor hubs with the mast, a phenomenon that is often associated with low-G maneuvering. The main rotor blades had diverged to strike the tail boom, which can occur as a result of mast bumping or low r.p.m., leading to rotor stall.

Before the Richmond crash, 23 other Robinson R22s had experienced similar losses of main rotor control — events which are almost always fatal and, in the absence of recording devices, leave few clues as to their causes. Investigators trying to piece together circumstances after the fact had chalked up many of these accidents to low-G maneuvering or low rotor r.p.m., implying mishandling by the pilot.

But the recorded evidence in the Richmond crash simply didn't support the usual explanations. Neither pilot had voiced any concern about the operation of the helicopter before the breakup. The low r.p.m. warning horn didn't activate before or during the breakup sequence, and spectral analysis of the audiotape indicated that the aircraft was being operated at a normal main rotor r.p.m. Radar data showed that the airspeed was normal for cruise flight, and there was nothing to suggest low-G maneuvering.

With no easy way to explain the Richmond crash, the NTSB launched a special investigation into R22 loss of main rotor control accidents. Meanwhile, the U.S. Federal Aviation Administration (FAA), which had already conducted two special certification reviews of the R22, initiated a third. It also convened a technical panel to study R22 loss of main rotor control accidents, and commissioned the Georgia Institute of Technology (Georgia Tech) to conduct simulation studies of the R22 main rotor system.

In addition to several airworthiness directives and bulletins, in February 1995, the FAA issued Special Federal Aviation Regulation (SFAR) 73, which created specific training and proficiency requirements for Robinson R22 and R44 helicopters. When the NTSB issued its own special investigation report the following year, it was still unable to explain the Richmond crash and many similar accidents, but it was encouraged by the fact that no loss of main rotor control accidents had occurred since the SFAR was enacted.

"Although the Safety Board cannot conclude that the operational changes will eliminate all in-flight rotor strikes, the absence of such accidents since these actions were implemented suggests that they have been effective," the NTSB wrote. "The absence of such accidents also supports the proposition that most of the accidents were caused by large, abrupt control inputs and the corrective actions taken should help prevent such accidents."

Since SFAR 73 was enacted, Robinson loss of main rotor control accidents have occurred less frequently in the U.S., but they haven't stopped entirely. And in at least one country, New Zealand, they have continued to occur at a high rate, with New Zealand's Transport Accident Investigation Commission (TAIC) citing at least 12 such accidents or incidents since 1996, despite the relatively low total number of Robinson helicopters in the country.

In a very small number of these accidents, there is eyewitness testimony or other direct evidence to suggest improper handling by the pilot. But in most cases, investigators are no closer to being able to explain these accidents than they were 20 years ago. For almost all of them, the probable cause statements are essentially the same: "the divergence of the main rotor from its normal plane of rotation for an undetermined reason."

Robinson Helicopter Company is not the first helicopter manufacturer to struggle with the problem of mast bumping. The potential for mast bumping is inherent in all two-bladed helicopter main rotor systems, which teeter like a seesaw around a central rotor mast. The main rotor blades on all helicopters flap up and down as a way of equalizing lift throughout their plane of rotation, but in models with more than two blades, each blade flaps individually at the same distance from the mast.

In two-bladed, so-called "semi-rigid" rotor systems, when one blade flaps up, the other flaps down. The root of the down-flapping rotor blade moves closer to the rotor mast, and may contact and damage the mast if the separation between them is further reduced. When mast bumping occurs in flight, it is almost always catastrophic. Flapping becomes so severe that a blade may slice through the tail boom or cabin. The rotor mast may be completely severed, resulting in the entire rotor system detaching from the aircraft.

Pilots of two-bladed helicopters can induce mast bumping through low-G maneuvering. A cyclic pushover after a climb — such as might occur if a pilot is flying low over hilly terrain — can momentarily unload the weight of the fuselage from the rotor disc. The thrust from the tail rotor above the aircraft's longitudinal center of gravity may then induce a rapid roll of the fuselage (to the right, in Robinson helicopters). If the pilot instinctively applies cyclic in the opposite direction to counter the roll, the still-unloaded rotor system will tilt excessively relative to the rotor mast, resulting in mast bumping.

Turbulence can also lead to low-G situations and abrupt control inputs, increasing the risk of excessive blade flapping and mast bumping. The risk is greater at higher airspeeds, which is one of the reasons why pilots are instructed to slow down when they expect or encounter turbulence.

Mast bumping was first identified as a significant issue by the U.S. Army in the early 1970s, following a number of fatal crashes of Bell UH-1 Huey and AH-1 Cobra helicopters in which the main rotor system separated from the aircraft in flight. (Notably, the Bell OH-58 Kiowa never experienced the same problems, and the rate of mast bumping accidents in Bell civil helicopters has also been very low.)

A simulation study conducted by Bell Helicopter in 1975 and 1976 confirmed that excessive main rotor blade flapping could occur "at center of gravity extremes, under low or negative-G conditions, with large abrupt control inputs, and in conditions of significant retreating blade stall." Accordingly, early training material emphasized the importance of pilots operating within "recommended flight envelopes" and avoiding low-G conditions.

A 1974 article in U.S. Army Aviation Digest warned, "Mast bumping is real; it can occur if we operate teetering rotors incorrectly; and it must be prevented. The lesson to be learned from this discussion is this: operate your aircraft within its design envelope."

A U.S. Army training film developed in the late 1970s stressed, "The basic lesson here, and the single most important message that should have come through, is that you as a pilot can prevent mast bumping by the way you handle the aircraft."

According to a 1983 article in U.S. Army Aviation Digest, mast bumping was associated with 59 accidents and 213 fatalities in U.S. Army UH-1 and AH-1 aircraft between 1967 and 1982. However, in 42 percent of these cases, the initial step in the accident sequence was some kind of mechanical failure, such as tail rotor failure. Weather or turbulence was implicated in 17 percent of the crashes, while only 10 percent were directly linked to low-G maneuvers or crew error. In 29 percent of the cases, the initial step in the accident sequence was "unknown."

The Army contracted Bell to explore design solutions to the problem, and Bell had developed a retrofittable hub spring to reduce the risk of mast bumping by the late 1970s. But with new-generation utility and attack helicopters on the horizon, the Army chose not to buy the hub spring. Instead, the emphasis remained on training pilots to avoid flight outside the approved envelope, even though the 1983 Army Aviation Digest article pointed out that "operating conditions that are within the approved envelope may cause high flapping and mast bumping depending on the individual pilot's reaction to certain situations."

Several months after that article appeared, an extraordinary accident involving an AH-1S Cobra at the U.S. Naval Test Pilot School in Patuxent River, Maryland, raised new questions about the possibility of experiencing mast bumping within the approved flight envelope. According to an account published by the reporter Mark Thompson in the Fort Worth Star-Telegram in 1984, an instructor pilot at the school, Major Larry B. Higgins, was training Major James M. O’Brien on use of the Cobra's pedals when O’Brien, who was in the rear seat, input more pedal than expected. The aircraft immediately rolled right and entered a dive. A main rotor blade smashed through the cockpit, instantly killing O’Brien and flinging Higgins into the air.

Because he was wearing a parachute in accordance with school protocol, Higgins survived. Based on his testimony, Navy accident investigators concluded that the pedal input was well within allowable limits, and that "there are possible unknown factors that may cause mast bumping to occur in flight regimes under which a pilot would not normally expect this phenomenon to occur."

Bell disputed both Higgins’ testimony and the Navy's conclusion. However, the attention associated with the case — and a lawsuit brought by the widows of pilots killed in an earlier mast-bumping accident — prompted the Army to make some modifications to its Bell Helicopters. Eventually, it moved away from two-bladed rotor systems altogether. Today, the Huey and Cobra live on in the U.S. military's inventory as the UH-1Y Venom and the AH-1Z Viper, respectively, but these modern variants have main rotor systems with four blades, not two.

The U.S. Army may have largely eliminated its mast bumping problems, but the training film it created on the subject endures. When I began my primary helicopter flight training in the Robinson R22 in late 2004 — nearly a decade after the enactment of SFAR 73 — I dutifully sat through the U.S. Army Safety Center's Mast Bumping: Causes and Prevention as part of my required education on mast bumping and low-G hazards.

"It may be necessary to view the film more than once," its narrators told me, and by the time I became a certified flight instructor in Robinsons, I had. I taught my students what I had been taught — that mast bumping will not occur in the absence of an incorrect action by the pilot. As the film had drilled into me, "you as a pilot can prevent mast bumping by the way you handle the aircraft."

By the time I started my flight training, demonstrations of low-G recoveries in Robinson helicopters had been prohibited in the U.S. However, the film also drilled into me the official recovery technique for a low-G right roll: first aft cyclic to reload the rotor disc, and only then left cyclic to correct the roll. I recited this mantra on my stage checks and checkrides, and my students recited it on theirs.

After accumulating around 1,200 flight hours in Robinson and Bell helicopters with two-bladed main rotor systems, I assumed that mast bumping was a well understood, relatively noncontroversial phenomenon. So I was surprised to learn, earlier this year, that New Zealand's Transport Accident Investigation Commission holds a somewhat different view.

In May, the TAIC released its final report on a fatal R66 crash that occurred in the Kaweka Range of New Zealand's North Island on March 9, 2013. The helicopter was being used to ferry hunters and fishermen to and from remote sites in the mountains when it experienced mast bumping and broke apart in flight, killing the pilot, who was the only person on board at the time.

The TAIC determined that "the mast bump very likely occurred when the helicopter encountered moderate or greater turbulence, which likely resulted in a condition of low G." The helicopter was calculated to have been within its weight and center of gravity limits, although at 756 kilograms (1,667 pounds), it was on the light side of the allowable range of 635 to 1,225 kg (1,400 to 2,700 lbs.). Likewise, while its estimated airspeed of 115 knots, as calculated from satellite tracking data, was below the never-exceed speed of 123 knots for the prevailing density altitude, it was much faster than the 60 to 70 knots that Robinson recommends in conditions of "significant" turbulence. Both the light weight and high airspeed would have "exacerbated" the effect of any turbulence, the commission noted.

The TAIC acknowledged that "the possibility that an intentional or inadvertent control input by the pilot contributed to the mast bump cannot be excluded." But it also identified some additional safety issues related to the aircraft's certification.

Like the R22 and R44, the R66 has a main rotor system that differs from other semi-rigid rotor systems in that it incorporates coning hinges for each blade, as well as a central teeter hinge. The R66 certification program took advantage of these similarities; for example, the FAA accepted the results of an earlier R44 rotor flapping survey as evidence that the R66 met the certification blade clearance requirement.

However, the TAIC called attention to the fact that the R66 certification program "was not required to test the helicopter's response to low G, in spite of low G being known to present a serious risk of mast bump for the R22 and R44." And the FAA's Flight Standardization Board specifically declined to apply the training requirements of SFAR 73 to the R66, declaring that "the R66 performance and flight characteristics were typical and unremarkable compared to other part 27 helicopters of similar rotor design, therefore the R66 does not require specific training for unique flight characteristics."

In May 2014, about a year after the Kaweka crash, Robinson did finally conduct an R66 main rotor flapping angle survey, testing push-over maneuvers to a minimum G of +0.33 (in each case, the test pilot initiated recovery action as soon as the anticipated roll began). According to the TAIC, "the survey showed that the R66 responds to low G in the same way as the R22 and the R44."

The TAIC expressed the concern that because the R66 was certified without any special training requirements, pilots with no previous Robinson experience might start flying the R66 without fully appreciating the hazards of low G. Meanwhile, pilots familiar with the R22 or R44 — including the pilot in the Kaweka crash — "could infer from the lack of any special training for the R66 that the R66 does not require the same careful handling as the smaller types."

In fact, the TAIC argued, "the emerging accident trend and instructional experience to date suggest that the R66 does require the same careful handling." The commission's report pointed out that three of the seven fatal accidents that had occurred in the R66 to date had involved an in-flight breakup and main rotor separation. (Another accident that occurred during unauthorized flight instruction in Colombia was due to low r.p.m. rotor stall, which is also an SFAR 73 training requirement.) In June of this year, another fatal R66 crash occurred near Wikieup, Arizona; the NTSB's preliminary report for that accident also suggests an in-flight breakup.

The TAIC recommended to the FAA that it extend the knowledge and training requirements of SFAR 73 to pilots of the R66 helicopter (at the time of this writing, the FAA was still composing its official response). But the commission made another recommendation, too — that the FAA "reinstate research into the dynamic behavior of two-bladed, teetering, underslung rotor systems." Because while I was taught, and many people still maintain, that mast bumping can only occur a result of mechanical failure or improper handling by the pilot, the TAIC is not so sure.

In 1995, as part of the FAA and NTSB's investigation into Robinson loss of main rotor control accidents, Robinson conducted a series of flight tests with an R44 at its facilities in Torrance, California. The aircraft was instrumented to record information from the main rotor system, performance information, and flight control positions.

As was the case for similar testing performed with the R22 in 1982, the tests indicated that the aircraft could safely perform a full range of normal maneuvers — including engine power reductions and other flight training maneuvers — without any main rotor divergence tendencies.

However, the flight tests could not safely evaluate the aircraft's response to large, abrupt cyclic inputs in normal high-speed forward flight, a condition in which the cyclic is already displaced forward and to the right. So the FAA awarded a grant to the Georgia Tech School of Aerospace Engineering to develop a high-fidelity computer simulation model to investigate the response of the R22 to selected control inputs and wind gusts.

The school used a blade element approach to develop the model through its Flight Simulation Lab. But as the NTSB explained in its 1996 special investigation report, "modeling of such a complex system required more resources than had been allotted for the project." With its limited funds, the FAA elected to investigate the model's response to only a small set of cases involving large control inputs.

The results did indeed suggest that large and abrupt control inputs could lead directly to mast bumping and loss of main rotor control. However, the NTSB noted, because typical pilot control inputs while reacting to flight dynamics were not modeled, "it is unknown if smaller control inputs would have produced mast bumping."

The Georgia Tech report "strongly recommended" additional development of the model, finding it "clear that some additional investigations are warranted in this area." The NTSB endorsed this conclusion in a recommendation of its own, which called for the FAA and NASA to work together to continue the development of the simulator model of lightweight helicopters.

In 1998, however, the FAA determined that such a simulation tool would have "limited application," and that "subsequent validation of the math model would involve extensive testing with significant risk to flight safety." The NTSB accepted that the FAA had reached the limits of its technical involvement, and the effort was dropped.

Fifteen years later, New Zealand's TAIC revisited the subject during its investigation of the Kaweka R66 crash. According to the commission, "Robinson submitted that turbulence alone cannot lead to low G mast bumping, adding that an improper input or reaction by the pilot was also required."

That, of course, is what I had been taught as a student on the R22. But unlike me, the commission didn't accept it as proven fact. The TAIC acknowledged that Robinson helicopters can be operated safely in some degree of turbulence, as demonstrated by their extensive operational history. However, it argued that the behavior of the Robinson rotor system in turbulence has not been fully tested, and that the low-G conditions that have been tested were planned maneuvers in which the test pilot initiated an immediate recovery, thus avoiding any subsequent dynamic response.

The commission also called attention to Robinson's own report on its R66 rotor flapping survey, which stated, "Although low-G flight characteristics may be similar between Robinson models, the exact boundary between ‘safe recovery can be performed’ and catastrophic mast bumping cannot be predicted. Small changes in entry speed and pilot technique may produce large changes in roll rates."

In recommending that the FAA and NASA reinstate research into two-bladed, teetering, underslung rotor systems, the TAIC pointed out that computational sciences and aerospace engineering have advanced greatly since 1995. It suggested that remotely controlled helicopters could provide data on rotor behavior under conditions that are too dangerous for test pilots, thus circumventing one of the FAA's original concerns.

The TAIC's recommendation sounded reasonable to me, but I’m not an expert. So I paid a visit to two people who are: Daniel Schrage, director of the Center for Aerospace Systems Engineering and the Vertical Lift Center of Excellence at Georgia Tech, and Marilyn Smith, associate director of the school's Vertical Lift Center of Excellence.

Schrage was the lead investigator on the original research conducted for the FAA in 1995. "We had a really talented team working on it," he recalled. "I think we did the most creditable job we could have done on it at the time."

But he and Smith agreed that modeling techniques and computing technology are far ahead of where they were 20 years ago. "The development of aeroelastic [modeling] tools are now coming into their own," said Smith, explaining that researchers today have the ability to model much more complex aerodynamic phenomena, including transient conditions that could lead to mast bumping, and ways in which the fuselage interacts with airflow.

Smith suggested that modeling tools already developed by the U.S. Army and Georgia Tech in its capacity as a Vertical Lift Center of Excellence could be readily applied to Robinson helicopters. And Schrage echoed the TAIC's suggestion that an instrumented, remotely piloted aircraft could be used to validate the simulation model without putting a test pilot at risk.

"This is a civil safety problem for helicopters that really needs to be addressed," Schrage said. "Someone needs to go back and close the loop."

When I spoke with Robinson Helicopter Company president Kurt Robinson about the TAIC's recommendations, he told me that it's hard to argue against more research, and that he had no objections to further modeling and simulation efforts. But he emphasized that the company has flight tested its helicopters repeatedly over the years, looking for any indications of main rotor divergence. It simply hasn't found any within the helicopters’ normal operating envelope, which he takes as strong evidence that mast bumping really is a pilot handling problem.

Instead of searching for hypothetical unknown causes of mast bumping, he said the company is focusing its efforts on addressing known causes — namely, deliberate or inadvertent flight into low-G conditions. "Our position has always been that you avoid the feeling of weightlessness in the helicopter," he said. (An exception to this is the "weightless" feeling upon entry into autorotation, which is not associated with mast bumping, despite a common misconception that it is.)

In its report on the Kaweka R66 crash, the TAIC found that at the time of the accident, Robinson helicopter flight manuals did not adequately warn pilots of the hazards associated with turbulence. In response, the company added a new "caution" notice to the normal procedures section of the R44 and R66 flight manuals, advising pilots to reduce power and use a slower than normal cruise speed if turbulence is expected. The company also revised its safety notice regarding flight in high winds or turbulence to emphasize that helicopters are more susceptible to turbulence at light weight, and advising pilots to "reduce speed and use caution when flying solo or lightly loaded."

Meanwhile, New Zealand's Civil Aviation Authority (CAA) has taken steps to strengthen its training requirements for pilots of Robinson helicopters. Although the CAA enacted some new training requirements in 1995, they were never as stringent as the provisions of SFAR 73, and they were not enforced consistently. Effective July 1, 2016, the CAA has adopted more of the requirements of SFAR 73, such as the requirement that students receive 20 hours of dual flight instruction before flying solo in the R22 or R44. The CAA has also spelled out who can conduct Robinson safety awareness training, and how that training should be conducted.

Notably, the CAA has also now mandated that "low-G hazard training must not be demonstrated or practiced in flight." Robinson submitted to the TAIC, and Kurt Robinson reiterated to me, the company's belief that New Zealand's high rate of mast-bumping accidents is related to the fact that instructors there have continued to demonstrate low G to their students, when such demonstrations have long been prohibited in the rest of the world. Besides the fact that some mast-bumping accidents have occurred during low-G demonstrations, Robinson has suggested that such demos could engender "a false sense of security" in the pilot because they do not accurately replicate a sudden low-G roll. As the TAIC explained, "in fact, if it happens, the roll is very rapid, leaving the pilot, no matter how experienced, virtually no time to react before a mast bump occurs."

Because it has been impossible to safely demonstrate this kind of realistic low-G roll in flight — including during certification flight testing — it has also been impossible to say under what conditions an average pilot could be expected to recover from one. And this has led to some differing opinions on the best and most effective recovery technique.

The technique officially endorsed by Robinson is the same one that was drilled into me during my own flight training: "first aft cyclic to reload the rotor disc, and only then left cyclic to correct the roll." However, Robinson's own Safety Notice 11 cautions against too abrupt an application of aft cyclic during the recovery from a low-G condition, explaining that the resulting main rotor torque reaction, combined with tail rotor thrust, can also lead to a rapid right roll. The safety notice calls for "gentle aft cyclic," but this might be hard for a pilot to gauge in an emergency, especially without previous practice.

The high-time Robinson flight instructor Simon Spencer-Bower, owner of Wanaka Helicopters in New Zealand, has recently argued for an alternative low-G recovery technique: coordinated application of down collective, aft cyclic, and right pedal (as in a quick stop maneuver) which will reduce the tail rotor thrust that is contributing to the right roll and reload the rotor disc at the same time. "I consider low-G demonstrations would increase the awareness of the dangers of low-G roll and show how important it is to reduce power, and it is sad that they are now forbidden to be demonstrated," Spencer-Bower said. (Kurt Robinson told me that the company "has always felt lowering the collective is helpful," but wants to emphasize aft cyclic because it directly addresses low-G loading. Note that some pilots have mistakenly interpreted "reload the rotor disc" to mean they should raise the collective, which will only make the situation worse.)

Further modeling of the mast bumping phenomenon could provide solid evidence for the relative merits of these various recovery techniques, leading to better and clearer guidance for all helicopter pilots. Validation of main rotor behavior outside the established flight envelope could also allow for realistic simulator training of low-G conditions. Even if one contends that low-G demonstrations in flight impart a false sense of security, I’m not sure that my own training gave me any better appreciation for the reality of a low-G roll. A visceral simulator demonstration probably would have done more for me than a dozen repeat viewings of Mast Bumping: Causes and Prevention.

For all that we might learn from further research, however, there's one thing we’ll never know — what really happened in the cockpits of those helicopters that broke apart in flight. When the victim of such an unexplained breakup is simply the anonymous "pilot" in an accident report, it's easy to imagine him or her doing something reckless or unauthorized; something that "you as a pilot can prevent by the way you handle the aircraft." When the victim is a friend, however, that assumption becomes a lot harder.

In August of this year, several months after I had started researching mast bumping, New Zealand's TAIC released its investigation report for the February 2015 Robinson R44 in-flight breakup that killed flight instructor Stephen Combe and his student, James Patterson-Gardner. I met Combe in 2008, when I flew with him for a story about mountain flying training in New Zealand. Like his former students who were interviewed for the investigation, I had found him to be "a very thorough and professional instructor and pilot" who had "good empathy with his students." By the time of the accident, he had accumulated around 4,500 flight hours in helicopters, including more than 2,400 hours of mountain flying and nearly 1,400 hours of flight instruction.

Investigators originally suspected that the accident might have been the result of a main rotor blade fatigue failure, and they issued an airworthiness directive that grounded R44s fitted with a certain model of rotor blades. However, the grounding was lifted after a metallurgist determined that the blade fracturing in the accident was due to impact, not fatigue. Investigators found that mast bumping initiated the in-flight break up of the helicopter, but they were unable to say what caused the mast bumping event. They did not find any mechanical defect or failure that could have contributed to the accident, and found it unlikely that the helicopter had experienced low main rotor r.p.m.

So what happened? The accident occurred when the aircraft was flying across mountainous terrain at a relatively high forward airspeed, estimated at about 102 knots. Although the weather was generally calm, investigators found it "about as likely as not" that the aircraft had hit a pocket of light to moderate turbulence, and "about as likely as not" that the student had been on the controls at the time of the event. Combe had used his mobile phone at an earlier point in the flight, so it's possible that he was distracted, or had simply let his guard down, when his student made an abrupt or improper control input. But this is speculation. Like so many similar accidents, this one occurred for "undetermined reasons."

"The uncertainty around the circumstances of this accident are not unique," the TAIC stated. "There have been many other fatal mast bump accidents involving Robinson helicopters in New Zealand and around the world that have gone largely unexplained."

Twenty years ago, the helicopter industry had to take such uncertainty as a given. Cockpit voice and flight data recorders existed, but they were strictly for transport-category aircraft, as they were far too heavy and bulky to install on a helicopter as small as a Robinson.

Today, that's no longer the case. As the TAIC pointed out, lightweight and affordable cockpit video and flight data recorders are now readily available, and are installed as standard equipment in some smaller helicopters. Widespread adoption of these devices could finally solve the mystery of mast bumping, supplanting theories and speculation with real evidence.

"It is difficult to identify the lessons from an accident and make meaningful recommendations to prevent similar accidents if the underlying causes cannot be determined," the TAIC concluded. "This is a serious safety issue that the industry will need to address."