Ground resonance

July 13, 2012 by Tim McAdams

According to the NTSB a certificated flight instructor and student pilot were conducting a hover taxi in a Schweizer 269C helicopter from the hangar area to a fuel pump. The student was initially at the controls. The flight instructor took the controls from the student upon reaching the fuel pump, after the student stated he was uncomfortable landing on a raised platform in the confined area. The flight instructor landed the helicopter on the platform, where it then entered into ground resonance. The flight instructor rolled off the throttle immediately, but the ground resonance intensified, resulting in substantial damage to the helicopter.

Ground resonance happens in helicopters with fully-articulated rotor systems (rotor systems with three or more blades), or more specifically rotor systems with lead-lag hinges. These hinges permit the blades to independently move slightly forward and aft in the plane-of-rotation allowing them to speed up and slow down at different points as they spin around the mast. Known as a drag hinge they are necessary to relieve the stress that might otherwise damage the blades from the acceleration and deceleration of the rotor system. To prevent this back-and-forth movement from creating a serious vibration, hydraulic dampers are used to slow down the movement. Ground resonance cannot occur in a two bladed semi-rigid rotor system because the blades do not lead and lag.

Ground resonance typically occurs during a hard landing when the pilot sets the helicopter down on one corner of a skid or on one tire of a wheel equipped helicopter. The jolt transmits a shock through the fuselage to the main rotor system causing the blades to move out-of-phase with each other. In this condition the weight of the rotor system becomes concentrated on one side of the rotor disk causing the rotor system to become unbalanced. As long as the helicopter stays in contact with the ground the out-of-balance condition in the rotor system rapidly increases in frequency until the helicopter shakes itself apart.

If ground resonance starts, the best option is to lift the helicopter into the air allowing the blades to realign. If flight is not achievable then some improvement might be possible by reducing blade pitch and shutting down the engine. However, since the out-of-phase condition can cause major damage in a matter of seconds this approach is only sometimes successful. Helicopters with fully-articulated rotor systems can have shock-absorbing landing gear that will absorb the energy that feeds ground resonance. When ground resonance happens in these helicopters, it is usually because dampeners or shock absorbers have been improperly serviced.

10 Responses to “Ground resonance”

  1. Erik Says:

    As mentioned at the end of the article, improper servicing of the shock absorbers is a contributing factor to ground resonance. If you have a Aerospatial / Eurocopter AS350 series helicopter, you should be familiar with these. There are always more than 1 way to skin a cat, but I found that doing a visual inspection of the shocks will show you if there is a leak or broken seal, it won’t tell you if there is enough hydraulic fluid in the shock. Make sure you or your mechanic are regularly filling the shock with fluid if you are noticing vibrations on the ground. Yes it can be the resonance hammers or the track and balance, but if they disappear in flight, the ground resonance dampening shocks (absorbers) can be the likely culprit.

    And don’t forget the skid springs on the AStars either. The bolts that hold them on the skid tubes can get loose and cause similar problems as you very first touch the ground with that right skid low.

  2. Harold Says:

    Tim,

    I am curious if this was the HU-269 accident in Mount Olive, Alabama back in May 2008? It sounds a lot like it.

    Ironically, if it is the accident from 27 May 2008 in Mount Olive, it was a scandal invilving not only the unfortunate ground resonance event, but the discovery that the “student” was really an FAA Inspector who was supposed to be the “PIC”, and who had been illegally using some 35K dollars worth of FAA FSDO training dollars to get his CFI Rotorcraft-Helicopter license by renting this helicopter from a flight school that was under his local FSDO’s jurisdiction (makes it kind of tough for the DPE to fail him, if the FAA Inspector can then come next week and evaluate the DPE and his school???). The poor CFI that was aboard was not really even supposed to be there, since the aircraft was being rented under the FAA’s “4040/EBC Rental program” for FAA Inspectors to stay proficient in their “assigned aircraft” of responsibility. Since this Inspector was not qualified as a CFI in helicopters, he had not been assigned responsibilities to work with helicopters, which is why he was trying so desperately to get his helicopter CFI, even if it meant stealing FAA funds to do it!
    Sadly, if this Inspector is “uncomfortable” landing on a platform, he has no business wanting to be an FAA Inspector which will then have to fly with Part 135 helicopter pilots giving them their checkrides, landing on rooftops and hospital heliports!
    Another sad example of a needless accident, and even worse, if it was really this same case of an FAA Inspector stealing FAA funds to get additional ratings at the expense of the taxpayers.

  3. Ed Says:

    Tim,
    This sounds as if Harold has pinpointed the event exactly. The “student” was a helicopter rated pilot and the flight was not a training flight even though that is how it was shown on the NTSB accident report. The inspector was attempting to gain enough proficiency to be able to obtain his helicopter CFI. The process in the FSDO, in all FSDO’s, is to send an inspector to a formal school, determined by the FAA, paid for by the FAA, and then be tested as part of the additional ratings program. This inspector was not attending a “formal” course and was using money from a different account to prepare himself for the CFI test. All simply outside of the normal process within the FAA. An inspector, having multiple type ratings, can’t test or check in these aircraft even though they were issued by the FAA until he attends one of these formal courses with a successful check at the end. The inspector in this case was using funds that were supposed to be used for keeping the appropriately rated inspectors current in the equipment that they would be giving authorized tests and checks. The helicopter was wrecked and to the best of my knowledge the legal battle between the owner and the FAA is ongoing as they don’t want to pay for the totaled equipment. This is a snake pit and it involves the Alabama Northwest Florida FSDO, BHM FSDO, and its management and some of its inspectors.

  4. Paul Says:

    It has been close to 25 years since last flying a helicopter and that was my commercial rotorcraft check ride at FlightSafety when they were at Vero Beach, Florida. To say I am rusty on everything I have learned would be an understatement. I remember vividly our talks on phugoid oscillation in fully articulated rotor systems. How is this different from ground resonance mentioned above or are they one in the same?

  5. Alan Barnes Says:

    Completely different things. I’ve never heard of Phugoid oscillations talked about in reference to rotorcraft. That’s not saying they don’t exist – just that I’ve never heard of it in my limited experience.

    Phugoid oscillations are aerodynamic in nature. Ground resonance is mechanical in nature. Maybe mechanical is the wrong term but it’s certainly not aerodynamic. It requires some sort of shock to the system (ground contact) to knock the blades out of phase with each other.

    The way I explain a phugoid, hopefully not too incorrectly, is it’s like a paper airplane. It will climb until it loses too much speed and then will start to descend. Which will increased speed, increase lift and it starts to climb again. Just not as far since it’s an un-powered aircraft. Phugoid oscillations were a big problem with UA232 in Sioux City back in 89 when they were flying the plane with throttles alone after an engine failure severed the triple redundant hydraulic system. Oops.

  6. Alan Barnes Says:

    gues i should stand corrected in one sense; the rotorcraft flying handbook actually says “ground resonance is an aerodynamic phenomenon…” but I still say it’s more mechanical in nature :)

  7. Ivan Says:

    Well, here is an aerodynamicist’s somewhat simplified explanation of helicopter phugoid oscillation and ground resonance:

    In any aircraft, phugoid oscillation is an exchange between potential energy (mass x absolute altitude) and kinetic energy (mass x square of speed). For fixed wing aircraft, and a helicopter beyond transition speed, it shows up exactly as Alan explains it – when the a.c. is at the top of the phugoid, its speed is the lowest and at the bottom, where the height is lowest the speed is the highest. It is petty slow for a fixed wing aircraft – takes about 15 to30 sec to complete a cycle.

    Helicopter ground resonance is still a phugoid that happens at zero forward speed. It is now much faster – and of course it is “aerodynamic in nature” in the sense that it would not happen without aerodynamics (that is at zero rotor speed). In this case the phugoid oscillation is an exchange between the rotor’s kinetic energy (goes up with the square of the true airspeed of the blades and proportional to their inertia about the rotor axis) and their potential energy. (Aerodynamic in nature” or “mechanical in nature” is not really much of a distinction – it is “just words”).

    Finaly, damping “takes away” from this energy continuously, and in case of ground resonance it is partly mechanical (taken out by the shock absorbers, roughly proportionally to the relative speed of the fixed and movable parts), and partly aerodynamic (due to the variation of the up-down speed of the rotor plane relative to the air mass propelled downwards).

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