The aircraft was being operated as a scheduled passenger service from Sydney to Seoul, with the co-pilot as the handling pilot. The crew reported that the pre-departure flight control checks were normal. Shortly after becoming airborne from runway 34L, the co-pilot advised the pilot in command (PIC) that his control wheel had become jammed when attempting to make right wing down aileron inputs. The PIC took control of the aircraft and confirmed that his control wheel also had become jammed. He retained control of the aircraft and the co-pilot advised Air Traffic Services (ATS) that the aircraft was unable to turn to the right. He requested left turns and radar vectors to the south for fuel dumping prior to returning to land. ATS initiated a distress phase. The crew actioned the emergency/abnormal checklist for jammed or restricted flight controls, which includes the statement "use maximum force, including a combined effort by both pilots, if required", but they reported that their attempts made no change to the system. After fuel dumping was completed, the aircraft was vectored, using left turns only, to the runway 34L localiser and configured for the landing. At about 400 ft on final approach, the aileron controls became free and an uneventful landing was carried out.
Inspection by ground engineers determined that a plastic cable guard in the left aileron control cable system had broken. Pieces of shattered plastic were found in the vicinity of the left lower cable pulley system in the vertical cable run behind the cabin sidewall, forward of door 1L. The debris and all the remaining guards were removed from both left and right side vertical cable runs. The lateral control system, including the load limiter system, could not be faulted during full system testing. As there were no replacement cable guards available, the aircraft was approved to return to service with the guards removed.
The lateral controls on the aircraft consist of hydraulically powered inboard and outboard ailerons and flight spoilers on each wing. The controls are connected to the cockpit control wheels by cables, for pilot input. The cable runs are duplicated on each side of the aircraft. The left and right cable runs terminate at quadrants at the bases of the left and right control columns respectively. The control columns are interconnected by a cable loop connected to separate quadrants at the bases of the columns. The right quadrant includes a load limiter which consists of a detent and spring loaded cam assembly. The load limiter is designed to "break away" under applied force by the crew to enable one control wheel to provide lateral control input should the other side jam for any reason. Roll control is then available, but considerable force is required to overcome the detent cam in the load limiter. Other Boeing aircraft types utilise similar systems.
The aircraft manufacturer issued a Service Letter, 747-SL-27-134, in December 1993, advising that broken cable guards could result in high control wheel forces and suggesting that operators should replace the guards with improved parts when replacement is required. The guards on the right control system on the incident aircraft showed evidence of deterioration, as one guard had been previously repaired with adhesive tape.
The aircraft was leased from an overseas operator. Under the terms of the lease agreement, all major maintenance was conducted by the lessor. The last major maintenance inspection was completed on 25 August 1995. At the time of the incident the aircraft total time in service time was 50,400 hours.
The crew remained at the aircraft whilst the defect was rectified. Both crewmembers remarked that they were surprised at the force required to overcome the load limiter when the system was tested. Though they were aware of the load limiting system from ground training instruction, they had never been physically exposed to the forces required to operate the system.
The deteriorated condition of the plastic cable guards, and the use of tape to effect a "repair", suggests that the manufacturer's advice regarding replacement of the guards had not been heeded during major maintenance inspections.
It is likely that, when the plastic cable guard failed, a piece or pieces of plastic lodged in the left side cable run aileron control pulley, restricting the cable movement in one direction. The debris probably dislodged when the aircraft was at about 400 ft on final approach.
- The aircraft maintenance organisation had not replaced deteriorated parts with improved parts as suggested by the aircraft manufacturer.
- A cable guard had deteriorated to the extent that it failed and resulted in high control forces in the lateral control system.
- The operating crew were not aware of the high control inputs required to overcome the load limiter in the lateral control system.
As a result of the investigation, the Bureau of Air Safety Investigation issued recommendation R970128, to Qantas and Ansett on 29 September 1997. The recommendation stated:
"The Bureau of Air Safety Investigation recommends that Australian operators of aircraft manufactured by the Boeing Commercial Airplane Company:
- develop a simulator training procedure to ensure that aircrew are familiar with the procedures to be used in the event of lateral control jamming; and
- ensure that aircrew are aware of the control wheel forces required when the override mechanism is being operated in the event of jammed lateral controls".
A similar recommendation (R970145) was issued to the Boeing Commercial Airplane Company on 29 September 1997.
The following response was received from Qantas on 26 November 1997:
"I refer to your letter reference B97/099 which detailed a recommendation that a simulator training procedure be developed to ensure that all aircrew are aware of the procedure to be used, and control forces required, in the event of aileron control jamming.
Qantas simulators (with the exception of the B767-200 simulator) are equipped to simulate aileron control jamming and the control wheel forces required to override and regain control.
This scenario will be made a subject, both for discussion and demonstration, in the first available recurrent training simulator session. This will apply to the Boeing 747-400, 747-200/300, 767, 737 and Airbus A300 fleets".
Response classification: CLOSED - ACCEPTED.
The following response was received from Ansett on 24 June 1998:
"I refer to the above recommendation, which resulted from an incident involving a Boeing 747 aircraft at Sydney on 2 May 1997, and provide the following response to that recommendation.
The company conducts ground training for technical crews that includes instruction on aileron control jamming procedures. Additionally, simulator training is presently conducted for Boeing 737 aircraft and will be conducted in the Boeing 767 simulator when that simulator is upgraded to allow such training. For the Boeing 747, training is conducted in the aircraft, whilst on the ground, during type endorsement".
Response classification: CLOSED - ACCEPTED.
The following response was received from the Boeing Commercial Aeroplane Company on 13 February 1998:
"We have not yet committed any changes in our simulator training procedures or manuals. We are reviewing the reported event and looking at possible training and manual changes which would be implemented for all applicable Boeing models, not just 747.
However, additional time is necessary for this review before we can come to any conclusion. I anticipate that this review may take three more months. We plan to keep your office advised of the progress of our review".
A further response was received on 27 May 1998, and stated:
"Earlier this month I reviewed proposed changes to our operational documentation concerning flight control jams across all our various model airplanes. This has been a slow process trying to get agreement on. I anticipate that we will have some changes to be released in a couple of months. These changes would affect the Flight Manual, the Flight Crew Training Manual, the Operations Manual and the QRH".
Response classification: OPEN.
Local safety action
Boeing have also advised that Service Letter 747-SL-27-134, which addresses the need to replace deteriorated cable guards, is to be upgraded to service bulletin status in the near future to add more emphasis to this discrepancy.
|Date:||02 May 1997||Investigation status:||Completed|
|Time:||1045 hours EST|
|Location:||5 km N Sydney, Aero.|
|State:||New South Wales||Occurrence type:||Control - Other|
|Release date:||01 February 1999||Occurrence category:||Incident|
|Report status:||Final||Highest injury level:||None|
|Aircraft manufacturer||The Boeing Company|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Nil|
|Departure point||Sydney, NSW|
|Departure time||1045 hours EST|
|Role||Class of licence||Hours on type||Hours total|