Pilots experience main landing gear shimmy as o shudder or low-frequency vibration that con cause extensive damage to wheel assemblies and related mounting components. Instances of landing gear heavy vibration occurred during MD-80 production flights prior customer delivery, leading Douglas to launch on investigation that has led to effective corrective action.

 Wheel shimmy is a widely known dynamic phenomenon that has been a subject of engineering study for more than 60 years.

On aircraft landing gear, wheel shimmy is a dynamic instability resulting from a coupling of torsional and lateral deflections Of the landing gear. In the textbook case, wheel shimmy results when the phase lag between lateral and torsional deflections is 90 degrees. Since many tire bending properties are dependent on speed and applied load, the existence of wheel shimmy is also dependent on a specific range of aircraft velocities. Once induced, the vibration level achieves a steady-state limit and continues until the aircraft slows below the speed at which the shimmy occurred.

During initial certification of the DC-9 Series 10, Douglas discovered that a free-rolling shimmy condition existed on the aircraft at speeds above 90 knots. Design and installation of a main landing gear (MLG) shimmy damper corrected the problem. Douglas incorporated the shimmy damper design within oil models of the DC-9 and MD-80.

In early 1989 Douglas implemented an improved hydraulic bleed procedure for production MD-80 aircraft. Following implementation of the new bleed procedure, MD-80 aircraft experienced several instances of heavy MLG vibration while performing maximum braking evaluations prior to delivery. As a result, Douglas initiated on extensive investigation of possible brake-induced wheel shimmy on MD-80 landing gear. The investigation included a detailed system analysis coupled with laboratory testing of various aircraft components, laboratory simulation of MD-80 landing gear and braking systems and flight testing of MD-80 aircraft.

HYDRAULIC RESPONSE, HYDRAULIC CROSS TALK AND SERVO-CONTROL VALVES. An understanding of hydraulic response, hydraulic cross talk and hydraulic servo-control valve assemblies
serves to aid in understanding the specific causes of MD-80 landing gear vibration.

The dynamic response of a specific hydraulic system is defined as the relationship between oscillatory valve signal input and output broke pressures. The relationship is described in terms of response amplitude and phase with respect to the oscillatory input and is referred to as "system frequency response" Phase lag is defined as the time delay between oscillatory input and output pressures. In general, as the frequency of input oscillation increases, the output amplitude of oscillation attenuates and phase lag Increases.

The MD-80 braking system utilizes four anti-skid control valve assemblies. Incorporated into each control valve ore two hydraulic servo valves that regulate brake pressure during anti-skid operation. Note that as operational demand on the anti-skid control valve assembly increases, the possibility of a single servo valve affecting operation of another servo valve in the same assembly also increases. These effects are referred to as hydraulic cross talk.

Two types of hydraulic cross talk - supply line hydraulic cross talk and return line hydraulic cross talk - are applicable to the MD-80 brake and antiskid system.

Supply line hydraulic cross talk results when the demand for hydraulic fluid through the control valve assembly exceeds the hydraulic supply capability of the system. Under these conditions, the inlet pressure of both servo valves is reduced, thus effecting the output of the other servo valve. An imbalance in outlet pressure occurs between the independent servo control valves and produces torsional loading of the landing gear.

Return line hydraulic cross talk results when demand for release of hydraulic fluid through one control valve assembly exceeds the hydraulic return capability-
 of the system. This "overflow" of return pressure causes a hydraulic transient from one servo valve into an adjacent servo valve, producing a buildup in output pressure of the adjacent servo valve and additional torsional loading of the landing gear.

AIRCRAFT ANTI-SKID CONTROL SYSTEMS. The design logic of an aircraft anti-skid control system can allow for a reduction of brake pressure as the landing gear bends back and an increase in pressure as it returns forward during anti-skid operation. In this manner the control system adds damping and impedes vibration of the landing gear. However, if a sufficient amount of phase lag is introduced by the anti-skid/brake system, increased landing gear vibration is a potential result.

The MD-80 can be configured with either Aircraft Braking Systems (ABS) or Bendix brakes and hydraulic response is different for each configuration. The frequency response of the ABS brake system is significantly higher than that of the Bendix system.

Normally, the higher dynamic response of the ABS brakes aids in providing a more efficient brake/antiskid control system. However in this case the ABS brake/anti-skid control system results in increased susceptibility to MLG vibration due to two factors: the ability of ABS brakes to respond to the effects of phase lag and hydraulic cross-talk in the anti-skid control valve and reduced roll-off in the hydraulic system response at the gear natural frequency. Given a high-amplitude initial vibration and a sufficiently high aircraft speed, the vibration can degenerate into wheel shimmy.

CAUSES OF WHEEL VIBRATION. Analysis of data from the Douglas MD-80 MLG vibration investigation revealed the susceptibility of the airplane to wheel shimmy. Results of numerous flight, ground and laboratory test, coupled with a joint Douglas/ABS dynamic simulation effort, are summarized below:

· The MLG vibration problem was most repeatable in simulation with full anti-skid braking, light gross weight, full flaps and ground speed above 110 knots. However, high level vibration also was encountered with lower flap angles and speeds, these factors therefore cannot be ruled out as conditions that contribute to MLG vibration susceptibility.
· Investigators determined supply line and return line hydraulic cross talk between servo control valves in one anti-skid module to be primary contributors to the MD-80 vibration. The parking brake/shut-off valve configuration increases return line resistance and is an additional contributor to MLG vibration.
· The higher response capability of ABS brakes  increased aircraft susceptibility to shimmy/vibration by increasing brake response to hydraulic cross talk. The shimmy/vibration may be self-sustaining if the amplitude of oscillation exceeds the maximum stroke of the shimmy damper while at ground speeds greater than 90 knots .

CORRECTIVE ACTION. Test results indicated that installation of brake line restrictor valves in the brake lines increases hydraulic damping and thus reduces system gain at the strut natural frequency. This minimizes the possibility of gear vibration during heavy braking. Laboratory simulation and flight testing confirmed the effectiveness of the orifices in eliminating the MD-80 MLG vibration. Douglas received certification of the brake line restrictor valves from the FAA on March 5, 1991.

Douglas provides in-service modification information through MD-80 Service Bulletin 32-246. Until operators accomplish the service bulletin, Douglas recommends the following procedures to avoid MLG vibration:

· Plan the final approach so as to avoid landing long.
· Use the thrust reversers in accordance with the flight crew operating manual to reduce wheel braking energy during landings and aborted takeoffs. Ground spoilers should be used for all landings.
· Avoid unnecessary heavy braking during the landing. Extend the landing roll to a later turnoff point, if one is available, to reduce braking loads further. If equipped with the autobrake system, use the MINIMUM or MEDIUM positions when possible.
· Autobrakes, if installed, should be armed for ta takeoff and used per existing procedures in the event of an aborted takeoff. If manual brakes are used for takeoff and on abort is initiated at a speed above 80 knots, the pilot should use maximum braking.
· If a substantial MLG vibration is felt above approximately 50 knots during deceleration of the aircraft, momentary release the brakes and reapply them smoothly to a lower braking level as soon as the vibration stops. If emergency conditions warrant, use maximum braking.