There are certain flight conditions and control inputs that can overstress the main rotor mast components of a semi-rigid system, resulting in catastrophic failure. Consequently, it's vital that pilots fully understand the limitations of this type of system.

Inflight turbulence is one of the stresses and ultimately cost the life of a well-known Australian media personality. Graeme “Shirley” Strachan was piloting an Agusta 47G in the mountainous terrain of Queensland on Aug. 29, 2001, but never returned from the training flight. The helicopter wreckage was eventually located on the slopes of Mount Archer.

“The extensive damage to the helicopter, severed tail boom and the location of parts on the ground, led transport safety investigators to conclude that the main rotor blade may have contacted the tail boom in flight,” said Air Safety Deputy Director Alan Stray.

According to the Australian Transport Safety Bureau's report, “This type of damage was consistent with flying into mountain wave turbulence, and may have occurred from one of two events: blade flapping [divergence of the main rotor blade from its normal plane of rotation encountered during severe turbulence] or the pilot's instinctive reaction to pull up after a sudden nose-down pitch from a change in the helicopter control input. . . .”

Mast bumping, another critical condition that occurs when the rotor hub contacts the mast, can result when excessive main rotor flapping or abrupt control input produces a load factor below 1 g. The unloading of the rotor disc typically occurs after a sudden cyclic pushover in forward flight, or during high forward airspeed, turbulence and/or excessive sideslip. Once unloaded, main rotor thrust is reduced, and since the tail rotor is above the helicopter's center of mass, its thrust applies a right rolling moment to the helicopter (assuming the main rotors turn counterclockwise) . A pilot's instinctive reaction to the right roll is to counter it with left cyclic. However, with little or no rotor thrust, there is no lateral control available to stop the roll, and the hub makes contact. If that contact is severe, it can cause mast failure and/or blade contact with the fuselage or tail boom.

Should a helicopter begin to roll right under those conditions, the rotor must be reloaded before left cyclic will counter the roll. To reload the rotor, the pilot should immediately apply gentle aft cyclic and only after the weightless feeling stops is it safe to use lateral cyclic to correct the right roll.

The mast bumping danger became a major concern during the Vietnam war since both the Bell UH-1 Huey and AH-1 Cobra, two helicopters closely identified with that conflict, featured teetering rotor systems. Subsequent designs used by the military have been far less susceptible to mast bumping problems, and thus there are generations of former military aviators that weren't trained in this phenomena.

The Robinson R22, a favorite among civilian training outfits, features a teetering rotor system and its manual is quite specific on the procedures to avoid mast bumping. The document recommends maintaining cruise airspeeds within the middle of its operating envelope (specifically between 60 KIAS and less that 0.9 Vne, but no lower than 57 KIAS), using maximum “power-on” rpm at all times during powered flight, avoiding sideslip during flight, maintaining “in-trim” flight at all times, avoiding large, rapid forward cyclic inputs in forward flight, and avoiding abrupt control inputs in turbulence. That's good advice when operating any teetering system helicopter.

According to the FAA's Rotorcraft Flying Handbook, “Since a low g condition could have disastrous results, the best way to prevent it from happening is to avoid the conditions where it might occur. This means avoiding turbulence as much as possible. If you do encounter turbulence, slow your forward airspeed and make small control inputs. If turbulence becomes excessive, consider making a precautionary landing. To help prevent turbulence-induced inputs, make sure your cyclic arm is properly supported. One way to accomplish this is to brace your arm against your leg.”

The R22 manual contains these additional recommendations: “Do not over-control. Avoid large or abrupt control movements. Allow the aircraft to go with the turbulence, then restore level flight with smooth gentle control inputs. Leave the governor on and do not chase rpm or airspeed. Momentary rpm or airspeed excursions are to be expected. Avoid flying on the downwind side of hills, ridges or tall buildings where the turbulence will likely be most severe. Never fly into a blind or box canyon during high winds.”

One R22 accident further illustrates the type of error chain that can lead to catastrophic mast bumping. On Feb. 26, 1998, a pilot departed a private airport near Adelanto, Calif., on a solo FAR Part 91 personal flight. Witnesses reported that winds at the time of the accident were westerly at 20 to 25 kt. A short time later, a person near Littlerock, a town 40 mi. to the west, reported he heard a loud “schwapp-like” noise followed shortly by engine silence, and then saw the helicopter disappear from view in a near-vertical descent. The pilot was killed when the helicopter crashed to the ground.

A replay of recorded radar data at High Desert TRACON showed the helicopter flying from east to west at 3,800 ft. when it disappeared from radar at the accident location. Approximately 5 min. earlier, the radar displayed the aircraft approximately 7 mi. east of the accident location at 3,800 ft. MSL, at 80 kt. ground speed. A few minutes later the helicopter's ground speed had increased to 90 kt., then shortly thereafter to 130 kt. In the two subsequent radar returns, the ground speeds displayed were 140 kt. and 120 kt. Ten seconds later the aircraft had climbed 100 ft. and slowed to 80 kt. Radar returns of the aircraft were lost at this point.

A report prepared by an NTSB meteorologist stated that surface observations near the time of the accident from surrounding stations described westerly to northwesterly surface winds at 10 to 30 kt. Winds aloft from the accident elevation to 10,000 ft. were northwesterly from 6 kt. increasing to 40 kt., and several pilots reported moderate to severe turbulence below 8,000 ft. with winds aloft of up to 50 kt. and updrafts and downdrafts near the mountains. AIRMET TANGO 3 was valid at the time of the accident noting occasional moderate turbulence below 12,000 ft. MSL.

The accident investigation revealed that one main rotor blade had cut into the cockpit through the left windshield half and sliced the cabin floor aft to the left seat. The location of an indentation in the cyclic-control cross tube, made by the rotor blade, corresponded to the cyclic control having been about in the full left control position.

The NTSB determined the cause of the accident was the pilot's improper use of the rotorcraft cyclic control in response to encountering terrain-induced turbulence. A factor in the accident was the pilot's failure to obtain a preflight weather briefing, which contained precautionary notices for turbulence.

In addition to mast bumping, it appears that “chugging” is about to enter the rotary-wing lexicon as yet another form of excessive stress on a teetering rotor system's components. An NTSB investigation of a recent accident heightened awareness of this little-known threat.

On May 12, 2009, a Robinson R44 on a game management patrol for the Alaska State Troopers, Fish and Wildlife Service was flying at about 90 kt. and 300 ft. AGL when the pilot felt an unusual medium-frequency vibration transmitted primarily through the pedals. First, a slight yawing motion developed, then the vibrations turned to oscillations, in both yaw and pitch, to the point he felt the helicopter was going to come apart. With disaster in the offing, he picked a spot on the ground and made a controlled descent while fighting to maintain control, touching down with 5 to 10 kt. of forward airspeed.

The helicopter landed hard, and the main rotor blades struck the tail boom. According to the pilot, after the accident he recalculated the weight and balance for the helicopter, and determined that the helicopter was below its gross weight, but about 1.1 in. forward of the forward c.g. limit.

On May 15, the NTSB investigator in charge examined the helicopter at the State Trooper facility in Anchorage. No pre-accident mechanical anomalies were discovered, and a review of the helicopter's logbooks showed no discrepancies. Small dents were noted in the cabin top fore and aft of the main rotor mast fairings.

An Internet search of helicopter sites by the investigator revealed that R44 operators were aware of similar events, and that the condition had been dubbed “chugging.”

According to the search on Dec. 16, 2006, another R44 had a similar vibration resulting in an emergency landing and damage near Ballymena, Ireland.

On March 15, 2007, an R44 operating near Opa Locka, Fla., had a similar vibration, which resulted in the pilot making an emergency landing on unsuitable terrain, damaging the helicopter. And on Sept. 30, 2007, an R44 flying near Jackson Center, Ohio, had a similar vibration, which resulted in the pilot making an emergency landing and damaging the helicopter.

Robinson Helicopters reported conducting flight tests related to the phenomena and determined that an oscillation could develop when operating the helicopter at high gross weight, about 90 to 100 kt., and that the oscillation was more of a “bucking” motion due to the fore-and-aft movement of the rotor mast. Those tests showed the tendency to enter the oscillation regime was exacerbated by a forward c.g. (within the c.g. envelope) and a 30 deg. banked turn to the left. A company accident investigator said the helicopter could also begin to oscillate in a right turn but enters the oscillation regime more easily in a left turn. The tests also showed that chugging could occur within the normal c.g. range, and most typically at or near a gross weight condition.

However, according to the manufacturer, it was determined that the oscillation is not divergent (destructive to the helicopter), and that the helicopter can be landed safely. The manufacturer attributed the oscillation to the firmness, or lack of firmness, of the transmission mounts, and said that the OEM changes the mounts on helicopters that exhibit a tendency toward chugging during post-manufacturing test flights.

The NTSB determined the cause to be the main rotor transmission mount design, which resulted in an inflight vibration/oscillation and damage to the helicopter during an emergency descent and hard landing. Contributing to the accident was the lack of information from the manufacturer regarding this known flight oscillation, and loading the helicopter beyond the forward c.g. limit by the pilot.

The Safety Board recommended that the FAA require Robinson to resolve the root cause of the mast-rocking vibration in the main rotor assembly to ensure that all applicable R44s are free of excessive vibrations in all flight regimes, as well as to maintain a database of all reported incidents of mast rocking in the main rotor assembly of R44s. And to better understand the rate of occurrence of this phenomena, the NTSB wants the OEM to issue a service letter to all approved service centers describing the mast-rocking vibration and instructing them to report all such incidents to the manufacturer.

The NTSB also wants Robinson to amend the R44 manual to inform pilots of the potential for mast-rocking vibration and how to safely exit the condition, and to revise the R44 pilot training program to provide instruction in the recognition and mitigation of inflight mast-rocking.

A review of the NTSB accident reports revealed no other “chugging” references, but rotorcraft investigations can be complicated by the sheer number of moving parts in the aircraft. Once this phenomena becomes better understood it might be better recognized and diagnosed in other aircraft. It is definitely a subject worth following. BCA