Mast rocking

November 1, 2011 by Tim McAdams

On May 12, 2009, a Robinson R-44 helicopter was damaged during a hard landing about 57 miles northwest of Iliamna, Alaska. The purpose of the flight was game management patrol for the Alaska State Troopers, Fish and Wildlife Service. After take-off from a ridge, about 300 feet above the ground, the helicopter was flying about 90 knots when the pilot felt an unusual medium-frequency vibration in the controls. The pilot told the NTSB that the vibrations turned to oscillations in both yaw and pitch to the point he felt the helicopter was going to come apart. He decided to make an immediate precautionary landing. During the descent the vibration increased and the helicopter landed hard causing the main rotor blades struck the tail boom.

The NTSB discovered that operators of the Robinson R44 helicopter were aware of similar events and that the condition had been dubbed “chugging.” According to Robinson Helicopter, tests determined that a mast rocking oscillation may develop during operation of the helicopter at high gross weight and about 90 to 100 knots. The oscillation was more of a “bucking” motion due to the fore-and-aft movement of the rotor mast. Tests also showed the tendency to enter the oscillation was exacerbated by a forward CG (within the CG envelope) and a 30 degree banked turn to the left. The oscillation is not divergent (that is, the main rotor blades do not diverge from their normal plane of rotation) and can be reduced by adding power. The oscillation is due to the firmness, or lack of firmness, of the transmission mounts. At the time there were no service alerts/bulletins referencing the phenomena or the remedies to resolve it.

On August 22, 2011 the NTSB issued the following safety recommendations to the FAA:

  • Require Robinson Helicopter to resolve the root cause of the mast-rocking vibration in the main rotor assembly to ensure that all applicable R44 helicopters are free of excessive vibrations in all flight regimes, as required by 14 Code of Federal Regulations Section 27.251, “Vibration.” (A-11-82)
  • Require Robinson Helicopter to maintain a database of all reported incidents of mast rocking in the main rotor assembly of R44 helicopters. (A-11-83)
  • Require Robinson Helicopter to issue a service letter to all approved service centers describing the mast-rocking vibration that can occur in the main rotor assembly of R44 helicopters and instructing service centers to report all incidents of mast rocking to the manufacturer. (A-11-84)
  • Require Robinson Helicopter to amend the R44 helicopter flight manual to inform pilots of the potential for mast-rocking vibration in the main rotor assembly and how to safely exit the condition. (A-11-85)
  • Require that the Robinson Helicopter R44 pilot training program be revised to provide pilot instruction in the recognition and mitigation of in-flight mast-rocking vibrations in the main rotor assembly. (A-11-86)

Most helicopter pilots are aware of mast bumping in semi-rigid (two-bladed) rotor systems, but this issue is new and not as well-known and raising awareness is important to safe operations.

16 Responses to “Mast rocking”

  1. Restor8 Says:

    As a R44 owner I thank you!

  2. R22widow Says:

    They had problems with the R-22 doing the same thing. Made up all kinds of excuses, blamed pilots, weather and all sorts of things. Even have a special FAR for the R-22. Now the R-44 does it and lo and behold it’s the rotor system. I have 5000 hours in all kinds of airplanes, helicopters and gliders. But I will never fly in a Robinson.

  3. Russ Kinne Says:

    Why a RECOMMENDATION? Why not an AD??
    Lives are at stake here

  4. Doug Anderson Says:

    A potentially deadly condition while the helicopter is within airspeed and CG limits? This is unacceptable.

  5. Doug Anderson Says:

    A potentially deadly condition when the helicopter is within airspeed and CG limits? This is unacceptable.

  6. grumpy Says:

    Seems like Robinson, along with Cirrus, are AOPAs’ “golden boys”.

  7. Drew Clark Says:

    Russ,

    The NTSB is charged with investigating accidents and delivering probable causes and recomendations only. They have no power to enact legislation or force anyone to do anything.
    It’s up to the FAA to take the recommendations and decide to pursue a course of action (such as an AD). Of course, the FAA has to open a comment period (Notice of Proposed Rule Making) before any AD becomes effective.

  8. Steve R Says:

    Aircraft of all types have short comings that may require special attention by the operator, (the Bell tail-rotor is prone to LTE, any Cessna single can loose elevator authority while slipping with full flaps). This is why insurance companies require training and time in type. If you want a fool-proof aircraft, stay in the simulator. The R44 is a great machine when used properly, no aircraft will ever do everything well, learn your limitations, the limitations of your equipment, and act accordingly.

  9. Jim Borger Says:

    “No aircraft will do everything well” Compared to what? All aircraft have certain limitations and some do certain things better than others. As long as an aircraft is operated within the limitations in the RFM or POH you shouldn’t have to worry about it trying to kill you or/and itself.

  10. Les Says:

    As an operator of an R22 and R44 this is great infomation.

    Is the FAA just going to wait until there are more reports and or fatal accidents of mast rocking until they have an AD.

    I agree with the comments posted that if the aircraft is operated within the limits of the POH you SHOULD NOT have to worry about killing yourself.

    Come on Robinson, fix this

    Totally ridiculous.

  11. Maria Says:

    I’d heard rumors about something called Mast Rocking in R44s but hadn’t really looked into it until someone pointed me (indirectly) this post. Oddly, just yesterday I was flying with this kind of profile: a bit heavy, forward CG, and 90-100 knots. I definitely noticed more vibration in the rotor system than usual. Certainly not enough to set down, but now it’s got me wondering if it had anything to do with this.

    As an owner, I fear ADs because of the costs involved. The recent Robinson fuel bladder SB will cost me about $12K to comply with; an AD that affects the transmission mount or anything under the hood could cost far more than that and will be REQUIRED. I’d much rather see a change in the CG envelope and recommendations for avoidance coupled with a new design that can be implemented at overhaul.

    Also, if my memory serves me right, when I flew Long Rangers at the Grand Canyon (back in 2004), a hard set down on the pad would often cause a thump-thump-thump of the main rotor system. We called that chugging. I’m wondering if its somehow related. No issues in flight though; Long Rangers with 4-7 on board fly remarkably level no matter how you load them.

  12. Dangerous Flying: Abrupt Control Inputs | An Eclectic Mind Says:

    [...] January 31, 2012 note: Since writing this, a friend on the Rotorspace site has brought the topic of Mast Rocking to my attention. Apparently, some folks think that this accident may have been caused by Mast [...]

  13. S Miller Says:

    All I can say is this…..how many modern, commercial, successful and proven helicopter designs have such an extensive and TALL mast for the main rotors on their copters, I think nil…..Bell, Aerospatiele, Sikorksy ( The BIG russian giants ), ALL of their rotor masts are short comparitevely……come on,…I am a fixed wing pilot, 44 years old and have been an enthusiast of aviation for most of my life and even though I am NOT a helicopter specialist, even I can see and appreciate the shortcomings of this design.( And yes, I have flown in helicopters before ). It is SIMPLE maths and physics….so much mass spinning around creates a huge centrifical force, the shorter the central locating shaft, the less movement and vibration no matter HOW you try to change it.

  14. S Miller Says:

    I am Australian, and we have had several Robinson fatal accidents before, and I believe that many of them are just simply NOT pilot error….some of these pilots were experienced NZ and Aussie helicopter pilots that have flown other types without incident.

  15. S Miller Says:

    Why is the mast SO high in the first place?,….have a look back to the late 40′s and early 50′s and you will see that the greats of aeronautical design and funding had exhausted and tested many of these theories before,( Notably the Russians, French, Americans, British and Italians)…..with HUGE financial backing, intelligence and hindsight…..why have most of the exsisting designs come up with the short mast versus horsepower….especially with the development of the gas turbine…..

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