Flight control systems

The Kawasaki 47G3B helicopter had undergone major servicing, part of which required the removal and re-installation of the tail rotor cables, gearbox and extension tube. The pilot had assumed the ground-running of the helicopter from another company pilot, who had conducted the daily inspection. Following the rectification of some problems with the engine, the ground runs were completed and the tail rotor successfully balanced. The rotor track and balance equipment was then transferred from the tail to the main rotor to conduct main rotor blade tracking and balancing ground runs. The licensed aircraft maintenance engineer (LAME) who had conducted the maintenance work accompanied the pilot during the ground runs.

After the flat pitch track had been checked and found to be within limits on the ground, the LAME requested the pilot to fly the helicopter into a hover to check the main rotor blade tracking. The pilot complied with the LAME's request. However, the maintenance documentation had not been completed in that the independent inspection required on the flying controls had not been certified.

As the helicopter lifted to a 3-ft hover, it began to yaw to the right. The pilot attempted to correct the movement through the application of left tail rotor pedal, but the yaw accelerated to the right. Because the yaw was accelerating despite the pedal input, the pilot concluded that the helicopter had sustained a tail rotor failure. As the helicopter completed the second or third turn, the pilot lowered the collective to place the helicopter back on the ground. The helicopter landed heavily, damaging the landing gear, main transmission system and components of the tail rotor system. Neither of the helicopter's occupants received any injuries. No bystanders were injured.

The tail rotor control cables were subsequently found to have been incorrectly routed onto the tail rotor pitch change drum, causing the tail rotor control to work in the opposite sense to the tail rotor pedals.

The LAME had previously completed similar tasks on the helicopter type. He could not explain why he had incorrectly installed the cables on this occasion, other than to comment that he may have become too familiar with the procedure.

Before disconnecting the cables to conduct the required servicing, the LAME pinned the tail rotor cable pulley assembly to avoid a later requirement to re-rig the tail rotor controls. After the tail rotor gearbox and extension tube had been fitted, the LAME reinstalled the two tail rotor control cables to the tail rotor control drum. The cables ran in parallel along the helicopter's tail boom to the rotary drum that adjusts the tail rotor pitch control. When installed, the cable lengths were similar and both of the cable ends were alike. Instructions detailing the installation of the cables to the drum were in the maintenance and overhaul instruction manual. Although these instructions referred the reader to a diagram that displayed the routing of the cables, the diagram did not highlight the need for the cables to cross over before being connected to the drum. The cables should have been installed such that the inboard cable was connected to the outboard section of the drum and the outboard cable to the inboard section of the drum. However, the cables were not installed such that they crossed over and were, therefore, attached to their respective sections of the drum. Despite the incorrect installation of the cables, the tail rotor pitch change drum still worked, albeit in the reverse sense. Because the LAME had pinned the tail rotor pulley assembly, he decided that the check rigging of the tail rotor detailed in the maintenance manual was not required. As a result, a procedural step that may have detected the incorrect routing of the control cables was missed.

After the LAME had completed the work, he certified the appropriate sections of the maintenance work package even though the secondary inspection of the flying controls had not been conducted or certified. The independent secondary inspection should have detected the incorrect routing of the tail rotor pitch cables. Civil Aviation Regulation 42G detailed the qualifications required to conduct secondary inspections. The regulation permitted a pilot who held a licence that was valid for the aircraft to conduct a secondary inspection. However, the pilot was not required to undertake any relevant specific training on the conduct of such inspections. There was no regulatory requirement for the pilot to check the maintenance documentation other than the maintenance release before flying the helicopter, so he was not aware that the secondary inspection of the flying controls had not been completed.

The LAME's circumvention of the procedures, probably because of his familiarity with the task, was a factor in the accident. Although the maintenance manual specified procedures that should have ensured the correct operation of the tail rotor system, the design of the cables did not preclude their incorrect routing and attachment to the drum. Therefore, the design of the cables was also a factor in the accident. The investigation also found that the maintenance manual description of how to route the tail rotor cables was deficient in that it did not adequately highlight the cable crossover to the drum.

SAFETY ACTION

As a result of this occurrence, the Bureau of Air Safety Investigation is currently analysing two possible safety deficiencies. The deficiencies identified involve tail rotor cable marking and routing instructions in the Kawasaki Bell 47G and the qualifications of people permitted to carry out secondary inspections.

Any recommendation issued as a result of this analysis will be published in the Bureau's Quarterly Safety Deficiency Report.

The pilot reported that he was ferrying the helicopter back to the company base after maintenance via a helicopter landing site near Jacobs Well. At approximately 200 ft AGL during the approach, the cyclic control became restricted in that the pilot was unable to introduce any left cyclic control. The pilot said there was insufficient control available to conduct a safe landing. Eventually, he was able to force the cyclic control free and land safely.

After landing, the pilot consulted the maintenance organisation and confirmed that work had been carried out under the front seats. The pilot then removed the front seats and discovered that the cyclic control had been jammed by a radio control box. The box had not been re-installed correctly after maintenance. The matter has been referred to CASA airworthiness at Brisbane for further investigation.

On approach to land, a slat fault indication was displayed when the flaps were selected. The crew discontinued the approach and obtained clearance to another area so that the appropriate checklist actions could be carried out. A normal landing followed. The crew indicated that the aircraft had a recent history of slat problems.

Maintenance staff conducted the specified maintenance checks, and no fault was found after the checks. On the following day a different flap problem was displayed, and the system was again reset, achieving normal operations. The aircraft had operated normally since then.

Soon after departure, the crew advised that the flight would be returning to Brisbane due to a hydraulic problem and requested that the fire services be placed on standby for the landing. The aircraft landed uneventfully a short time later and cleared the runway unassisted.

Post flight examination found that a piston seal on the right outboard elevator power control actuator had failed. The actuator was replaced and the aircraft returned to service without further problems.

The pilot reported that the flaps became asymmetric when full flap was selected at about 300 ft on final for landing at Palm Island. The pilot immediately returned the flap selector to the previous position and carried out a "go around". The flight returned for an uneventful landing at Townsville. Post flight examination found the right flap extend cable had failed in the radius of a pulley. The cable was due for special inspection in approximately 20 hours flight time.

The maintenance organisation, Operator and CASA Airworthiness in Townsville were all contacted concerning this incident. The maintenance organisation is currently submitting a Major Defect Report detailing the failure. The maintenance organisation will be recommending to CASA that the affected cables be placed on a shorter special inspection period or be replaced at short time intervals.

Sequence of Events

Following an extended period of hovering practice and a circuit, the student remarked that the lateral trim was heavy. The instructor flew another circuit. He noticed that increasing left cyclic control pressure was required to prevent the helicopter from rolling right. At about 150 ft AGL a loud metallic "springing" noise was heard and a reaction felt through the controls and airframe. The cyclic pressures returned to normal. The helicopter was landed safely and closed down.

An after-flight inspection found that the lateral trim springs had parted, and right cyclic control was restricted to about 50% of the normal control authority.

Safety Action

Following this incident, the Civil Aviation Safety Authority was notified and made two submissions for the manufacturer to take urgent action. The manufacturer had been aware of a potential problem involving undue wear on a shaft in the trim assembly which could cause binding. A service letter, SL-13, which called for an inspection at each 100-hour inspection and annually was issued prior to this incident. Service Bulletin, SB-19, which called for a mandatory inspection of the trim shaft to determine the minimum diameter of the shaft was issued on 16 April 1997. Time of compliance was within the next 50 flight hours or by 31 May 1997, whichever came first.

The glider was nearing the end of a 190 km cross country flight. The weather was fine, with good visibility and light west to north westerly winds. At about five km from the airfield the pilot decided to descend from 3,500 ft by increasing airspeed to 116 kt, some 3 kt less than the maximum airspeed limit.

The pilot reported that initially the glider was flying normally at this speed, but suddenly the control stick was pulled from his grasp and vibrated back and forth, banging against its stops. He managed to regain his hold on the stick and began to raise the nose of the aircraft to reduce airspeed. At this point the glider rolled into an inverted attitude.

Believing that the aircraft had suffered a major structural failure, the pilot elected to escape from the cockpit and make a parachute descent. Initially, he experienced difficulty in releasing the canopy, but once open he fell freely from the glider. The pilot made a successful parachute descent, whilst the glider proceeded to land in bushes, sustaining minimal damage.

During a subsequent airworthiness inspection by the gliding authority, it was found that the mass balance of the tailplane was beyond limits. The out of balance condition had apparently occurred some three years previously when the glider had been rebuilt. As a result, elevator flutter had occurred as the aircraft reached its maximum airspeed limit. Under normal circumstances the critical airspeed for the onset of elevator flutter should have been well in excess of the maximum airspeed limit. 

The gliding authority has advised that it has taken action to prevent a recurrence by issuing an airworthiness directive, and conducting surveillance of the maintenance organisation concerned.

The pilot advised that shortly after the aircraft entered the first of what was to be a series of aerobatic manouvers, the aircraft's left rudder cable failed. After establishing what degree of flight control was still available, the pilot was able to make an uneventful landing.

Later examination found that the rudder cable had pulled through the rear swage fitting at the rudder horn end. The investigation found that both the left and right rudder cables were replaced at the last periodic maintenance inspection only three flight hours prior to the incident.

An examination of the manufacturers cable making processes revealed that the clamping tool used in the manufacture of the failed cable was not adjusted correctly. It was also found that there were significant wall thickness variations for the type of swage sleeves being used. All cables previously manufactured by the particular method used for the failed cable were identified and withdrawn from service. The manufactures cable making process for the failure type are currently under review by the CASA.

During climb the Boeing B737 crew noticed an un-commanded stabiliser movement. The stabiliser would continue to run on after a command had ceased. The aircraft returned to Melbourne where a normal landing was carried out.

Maintenance investigation determined that the fault was in the stabiliser clutch. The clutch was found to be sticking confirming the crew's observations.

Clutch sticking had previously been recognised by the manufacturer as a problem in B737-400 and 500 aircraft. The manufacturer devised a modification to prevent clutch sticking and an instruction to modify all actuators was notified to the industry via Alert Service Bulletin 737-27A1191 Rev 1 dated 3 November 1994.

On 10 February 1995, the operator acted on the manufacturer's advice, raising an internal engineering instruction EI-737-27-26 to expedite incorporation of the modification into its fleet. The actuator fitted to the incident aircraft was not one of those that had been modified.

The Civil Aviation Authority (now CASA) raised Airworthiness Directive AD/B737/88 on 12 June 1995. The AD was published in CAO Part 105 Amendment 8/95 (August 1995). The AD referenced the manufacturers Alert Service Bulletin and required that all affected actuators fitted to Australian registered aircraft be modified prior to 12 June 1996.

The operator has since amended the EI to conform with the requirements of the AD.

Significant factors

The following factors were considered relevant to the development of the incident:

1. The stabiliser actuator clutch contained a known fault that allowed the clutch to remain engaged.

2. The clutch had not been modified to eradicate the fault.

3. The clutch stayed engaged after a command was removed.

Safety actions

The following safety actions were taken:

1. The manufacturer raised an Alert Service Bulletin to notify the need to modify the actuators.

2. The operator raised an Engineering Instruction to expedite the incorporation of the modification into its fleet.

3. CASA raised an AD to require all Australian registered B737-400 and 500 aircraft to be modified before 26 June 1996.