Misrigged Flying Controls: Fatal Maintenance Check Flight Accident
Time pressures, a maintenance error rigging flying control cables and a potentially non-compliant design conspired to cause a fatal accident in the US in 2015.
The 6 seat Piper PA-46-350P Malibu Mirage N962DA had undergone an annual inspection at Rocket Engineering in Spokane, WA and was due for a post-maintenance check flight on 7 May 2015. The US National Transportation Safety Board (NTSB) say in their report (and the associated public docket) that:
….routine maintenance was performed, along with the replacement of the four aileron cables in the wings, and an aft elevator cable.
An added complexity was that the owner had contracted another maintenance organisation to embody avionic modifications while the aircraft was at Rocket Engineering. The avionics maintenance organisation:
…stated that as the upgrade progressed, the owner made multiple requests to add additional items to the work scope, and due to time constraints, not all of his requests could be accommodated.
The owner reported that he had made arrangements to pick up the airplane on May 5th, however as the work progressed, he was informed that the airplane would not be ready in time, and the date was pushed back to May 7 and then May 8.
The mechanic who performed and signed off on the annual inspection and who did the control cable replacement said:
….he was called multiple times by the airplanes owner for update checks during the weeks leading up to the accident. Each time additional items were discovered which needed to be repaired, further pushing back the completion date. He eventually referred the owner to the sales representative…because he felt he could be more, “diplomatic” with the owner.
The mechanic…held an FAA airframe and powerplant certificate (A&P), with inspection authorization (IA). He had been an A&P mechanic for 22 years, and attained his IA rating 17 years prior. He reported replacing aileron cables in the PA-46 series about five prior times in his career.
He stated that he worked exclusively on the accident airplane during the weeks leading up to the accident, and that he replaced the left and right aileron balance cables, along with the two aft aileron drive cables on May 5, 6 and 7. He reported replacing the cables in accordance with the procedures outlined in the maintenance manual, and that he removed and replaced each cable one-at-a-time to prevent inadvertent misrouting. Following completion, he checked aileron operation from both inside and outside the airplane, confirming smooth and full deflection. As part of the test procedures, he checked the neutral position on both ailerons, and then he used a protractor for angular aileron deflection measurements.
[He] stated that there was no unusual rush or pressure to get the airplane finished that day [and] he was not aware of any outside pressure from the pilot to have it finished that day.
The NTSB do however comment that the pilot due to do the post-maintenance check flight:
…had an appointment with an FAA medical examiner the next morning (Friday), and he typically did not work on Fridays. It is likely that the mechanic and pilot felt some pressure to be finished that day so the owner could depart in the morning and the pilot could attend his appointment.
The NTSB say:
At the time of the accident, Rocket Engineering had a set of inspection criteria in place for aircraft that had undergone heavy modifications…. However, no formal procedures were established requiring that the work performed by a mechanic following an annual inspection be independently inspected. Furthermore, although 14 Code of Federal Regulations Part 121 and 135 [only for aircraft with 10 passenger seats or more] (air carrier, commuter, or on-demand operations) state, in part that, “No person may perform a required inspection if that person performed the item of work required to be inspected,” there is no equivalent requirement for aircraft operated under Part 91 regulations.
The installing mechanic reported that once the work was completed, he asked another mechanic to check his work, asking him specifically to confirm the operation of the ailerons.
In a subsequent interview, the other mechanic stated that he assisted with reattaching the ailerons, along with checking security and installation of safety wire, but he was never asked to, nor did he, confirm the correct operation of the ailerons.
The Piper Maintenance Manual includes a functional check and caution:
The Accident Flight
The pilot was a retired Air Force Lieutenant Colonel and former test pilot who was employed byRocket Engineering and typically performed post-conversion, post-maintenance and customer familiarisation flights. He was accompanied by a customer service and sales representative, who held a private pilot certificate, “as a second set of eyes”.
The preflight checklist outlined in the normal procedures section of the pilot’s operating handbook, includes a check to confirm “proper operation” of the primary flight controls from within the cockpit, along with a check of the ailerons and hinges during the walk-around.
The location of the pilot’s seat within the cockpit allows for a clear view of both ailerons through the cabin windows.
In a another sign of possible haste:
According to the mechanic, the pilot performed the preflight inspection while the mechanic was still reinstalling the seats and readying the cabin.
Shortly after takeoff, the airplane began to roll right. As the climb progressed, the roll became more pronounced, and the airplane entered a spiraling dive. The pilot was able to maintain partial control after losing about 700 ft of altitude; he guided the airplane away from the airport and then gradually back for a landing approach. During this period, he reported to air traffic control personnel that the airplane had a “heavy right aileron.” As the airplane passed over the runway threshold, it rolled right and crashed into a river adjacent to the runway.
Examination of the Aircraft
The NTSB say:
Examination of the right aileron control system revealed that the aileron balance and control cable routing had been crossed over in the area of the cabin floor, between the center pulleys and wing root pressure vessel seals. As such, the balance cable continued through the wing on a route to the drive side of the aileron sector wheel, and the drive cable conversely continued on a route to the balance cable side.
Under this condition, both the left and right ailerons would have deflected in the same direction rather than differentially. Therefore, once airborne, the pilot was effectively operating with minimal and most likely unpredictable lateral control, which would have been exacerbated by wind gusts and propeller torque and airflow effects.
While the NTSB note that the maintenance manual contained a caution on cable reversal, we also observe that the applicable certification requirement of Part 23.685 Control System Details is:
(d) Each element of the flight control system must have design features, or must be distinctively and permanently marked, to minimize the possibility of incorrect assembly that could result in malfunctioning of the control system.
The NTSB make no direct comment on the aircraft type’s compliance with that requirement (originally dating from 1964), but they do say:
The sections of the two interchanged cables within the wing were about equal lengths, used the same style and size of termination swages, and were installed into two same-shape and -size receptacles in the aileron sector wheel. In combination, this design most likely permitted the inadvertent interchange of the cables, without any obvious visual cues to maintenance personnel to suggest a misrouting.
This implies the design was non-compliant with the applicable certification requirements. Identical requirements also exist in Part 25.671(b) Control Systems General for large aeroplanes, Part 29.671(b) for large rotorcraft and Part 27.671(b) for smaller rotorcraft.
The NTSB go on:
Although the misrouting error should have been obvious during the required post-maintenance aileron rigging or function checks, the error was not detected by the installing mechanic [or by the pilot].
NTSB Probable Cause
The mechanic’s incorrect installation of two aileron cables and the subsequent inadequate functional checks of the aileron system before flight by both the mechanic and the pilot, which prevented proper roll control from the cockpit, resulting in the pilot’s subsequent loss of control during flight.
Contributing to the accident was the mechanic’s and the pilot’s self-induced pressure to complete the work that day.
It is disappointing that no comment is made by the NTSB on the lack of a human centred error tolerant design or the certification compliance, though they do say that:
Its not clear how many misrigged cables have occurred but been caught by functional checks in the past, though reporting of near misses is not nearly as effective as an initial error proof design. We note that in a 22 year career the mechanic had only done this task just 5 times, further highlighting the importance of error tolerant human centred design (HCD).
Aerossurance has previous looked at these associated topics:
- First Eleven: Guidance for Designers on Maintenance Human Performance
- James Reason’s 12 Principles of Error Management
- Back to the Future: Error Management
- Maintenance Human Factors: The Next Generation
- Aircraft Maintenance: Going for Gold?
- Rockets Sleds, Steamships and Human Factors: Murphy’s Law or Holt’s Law?
- Critical Maintenance Tasks: EASA Part-M & -145 Change
- Misassembled Anti-Torque Pedals Cause EC135 Accident
- EC130B4 Accident: Incorrect TRDS Bearing Installation
- Fatal $16 Million Maintenance Errors
- C-130J Control Restriction Accident, Jalalabad
- Business Aviation Compliance With Pre Take-off Flight Control Checks
- Maintenance Check Flights: Safety Lessons
- UPDATE 16 October 2016: B747 Landing Gear Failure Due to Omission of Rig Pin During Maintenance
- UPDATE 12 February 2017: Flying Control FOD: Screwdriver Found in C208 Controls
- UPDATE 26 March 2017: Cessna 208 Forced Landing: Engine Failure Due To Re-Assembly Error
- UPDATE 16 April 2017: Insecure Pitch Link Fatal R44 Accident
Also see our review of The Field Guide to Understanding Human Error by Sidney Dekker presented to the Royal Aeronautical Society (RAeS): The Field Guide to Understanding Human Error – A Review
UPDATE 25 May 2017: Nearly 6 months after article Business and Commercial Aviation make this article available online: Two Chances Lost: Pilot And Mechanic Both Miss Aileron Hook-Up Error
Aerossurance is pleased to again be supporting and presenting at the annual Chartered Institute of Ergonomics & Human Factors’ (CIEHF) Human Factors in Aviation Safety Conference. The conference takes place at the Radison Blu Hotel, East Midlands Airport, 7-8 November 2016.