Human Factors in Aircraft Maintenance


Maintenance error is becoming a significant area of concern to the aviation industry. Of 14 major accidents investigated by the US National Transportation Safety board, 7 involved maintenance deficiencies. In addition to the human cost of accidents, maintenance errors impose a significant burden on airlines, as they are a major cause of flight delays and cancellations. (Marx & Graeber, 1993).[1]

According to a UK Civil Aviation Authority Study, as technology improved, aviation accidents attributed to mechanical failures alone have decreased, yet those attributed to human error have not shown the same reduction (Hawkins, 1993)[2]

Accidents Attributed to Maintenance Errors

Robinson R22 ZK-HVN

Date : 26 August 2005
Location : Murchison, New Zealand
Aircraft : Robinson R22 Helicopter
Accident Type : Maintenance error due to incorrect assembly

A safety investigation by the Civil Aviation Authority has concluded that the failure of an incorrectly assembled tail rotor drive shaft had caused a Robinson R22 helicopter ZK-HVN, to crash inverted in a paddock killing the pilot and seriously injuring the passenger.

In summary, the safety investigation concluded that:

  • The certifying licensed aircraft maintenance engineer did not directly supervise the unlicensed personnel as he was required to do during the final assembly of the tail rotor drive shaft.
  • Without supervision, the aft coupling on the tail rotor drive shaft was assembled incorrectly.
  • The certifying licensed aircraft maintenance engineer should not have signed the release to service statement without ensuring that the duplicate inspection had been completed and correctly certified as required by Civil Aviation Rules, and an adequate physical check had been carried out by a second person.
  • The CEO of the maintenance organisation had been made aware of the requirements for direct supervision and should have halted the critical maintenance tasks until the certifying licensed aircraft maintenance engineer was present to directly supervise the maintenance being performed.[3]
(Image embedded from NZHerald on 16 Sep 2009)

Continental Express Flight 2574

Date : 11 September 1991
Location : Colorado County, Texas, USA
Aircraft : Embraer EMB 120RT Brasilia
Accident Type : Maintenance error

Continental Express Flight 2574 crashed as it was approaching the runway for landing. Investigations by the National Transportation Safety Board (NTSB) revealed that bolts had been removed from the horizontal stabilizer during maintenance the night before the accident and, following a shift change, the screws were not replaced.

NTSB cited the failure of airline maintenance and inspection personnel to adhere to proper maintenance and quality assurance procedures. Contributing to the cause of the accident was the failure of the Continental Express management to ensure compliance with the approved maintenance procedures, and the failure of FAA surveillance to detect and verify compliance with approved procedures. [4]

(Image embedded from Tripod on 16 Sep 2009)

American Airlines Flight 191

Date : 25 May 1979
Location : O'Hare International Airport, Chicago, Illinois
Aircraft : DC-10
Accident Type : Maintenance error causing loss of one engine

The aircraft crashed on takeoff from Chicago after the #1 engine and pylon separated from the wing during rotation. Eight weeks before the crash, American Airlines, without the approval of the aircraft manufacturer McDonnell Douglas remove the engine-pylon as one unit during an engine change using a forklift. The original procedure called for removal of the engine prior to the removal of the engine pylon.To save time and costs, the maintenance crews of American Airlines performed the engine removal procedure despite the differences with the manufacturer's specifications on how the procedure was supposed to be performed. This has resulted in a damage to the pylon. After the accident, cracks were found in pylon bulkheads in both fleets of United and Continental Airlines using the same one-step procedure. [5]

(Image embedded from Ytimg on 12 Sep 2009)

Japan Airlines Flight 123

Date : 12 August 1985
Location : Mount Osutaka-no-one, Ueno, Gunma Prefecture, Japan
Aircraft : Boeing 747-SR46
Accident Type : Maintenance error causing structural failure and loss of hydraulic controls

According to the report by Japan's Aircraft Accidents Investigation Commission on the official cause of the crash, the aircraft was involved in a tailstrike incident at Osaka International Airport on 2 June 1978, which damaged the aircraft's rear pressure bulkhead.The subsequent repair of the bulkhead did not conform to Boeing's approved repair methods. Their procedure calls for one continuous doubler plate with three rows of rivets to reinforce the damaged bulkhead, but the Boeing technicians fixing the aircraft used two separate doubler plates, one with two rows of rivets and one with only one row. This reduced the part's resistance to metal fatigue by 70%. When the bulkhead gave way, the resulting explosive decompression ruptured the lines of all four hydraulic systems. With the aircraft's control surfaces disabled, the aircraft became uncontrollable.[6]

(Image embedded from Airport Webcam on 12 Sep 2009)

Human Factors in Aircraft Maintenance

A survey designed to identify safety issues in maintenance, with a particular emphasis on human factors, were distributed to Licensed Aircraft Maintenance Engineers (LAMES) in Australia.

As a result the Australian Transport Safety Bureau has provided the following analysis, conclusions and recommendations.[7]

Table 1. Outcome of Safety Occurrences

Airline Non-airline
System operated unsafely during maintenance 18% 7%
Towing event 9% 3%
Incomplete installation, all parts present 8% 9%
Person contacted hazard 7% 9%
Vehicle or equipment contacted aircraft 7% 1%
Incorrect assembly or orientation 6% 11%
Material left in aircraft 4% 5%
Part damaged during repair 4% 2%
Panel or cap not closed 3% 3%
Incorrect equipment/part installed 3% 4%
Part not installed 3% 6%
Required servicing not performed 3% 4%
Degradation not found 3% 5%
Other 24% 31%

Table 2. Occurrences Factors

Unsafe Acts in Occurrences Airline Non-airline
Memory lapse 21% 20%
Procedure shortcut 16% 21%
Knowledge-based error 11% 18%
Trip or fumble 9% 11%
Failure to check 6% 2%
Unintended action 3% 6%
Failure to see 5% 6%
Occurrence factors Airline Non-airline
Pressure 21% 23%
Fatigue 13% 14%
Coordination 10% 11%
Training 10% 16%
Supervision 9% 10%
Lack of equipment 8% 3%
Environment 5% 1%
Poor documentation 5% 4%
Poor procedure 4% 4%



1.The original Human Factor Analysis Classification System (HFACS), designed by Wiegmann and Shappell, was intended to be use as an analysis and classification tool for any Human Factor induced incident. The HFACS model has a long list of about 250 factors of which several can be vague or overlapping. The design of HFACS only allowed either 'MAN' or 'MANAGEMENT' to be selected as causal factor, not considering MISSION,MEDIUM and MACHINE in the analysis trail as a holistic analysis of HF should. The design of HFACS as a supplementary report also resulted in repetition and gaps in reporting.

2. The HFACS design was reviewed and revamped to the Human Factor Analysis Model (HFAM) customized to the current situation and adapted to Reason's Swiss Cheese Model. The HFAM is an intelligent-flow system designed to analyses all occurrences in a holistic manner. The aim of HFAM is to faciliate a sharper analysis of all incidents/accidents to seek out HF issues in line with the Swiss Cheese model which is the basis of the HFACS. In essence, HFAM introduces the "4L" (Individual, Team, Management and Organizational Management level factors) into the 5M model, which would ensure a smooth reporting flow while triggering a deeper analysis of the events.

3. HFAM adapts the HFACS's 4 level of failure (Unsafe Acts, Preconditions for Unsafe Acts, Unsafe Supervision and Organizational Influences) to cover 4 levels of analysis within the organization, ranging from the individual to Organization Managements. This is know as the 4Ls factors, namely:

a. Individual Factors The level closest to incident at the individual level which are broadly categorized into physical effects/discomfort, cognitive/thoughts process, emotional state and personal readiness that can potentially cause an individual to commit an error.

b. Team Factors This is 1 level up from the individual which is the immediate Team performing the task/mission together. It looks at gaps in the skills required of a team to function effectively and gaps in how the mission/tasks was managed.

c. Departments Management/Section Factors This level covers any group beyond the team up to a Section level entity where it covers Resource Management and Planning. It cover areas where the Individual and Team are being influenced within their immediate Section.

D. Group Managements Factors This is the highest level to address issues that are out of the 3 lower levels purview/control. It addresses issues at Higher Management. i.e CEO level.

Definitions of Occurrence Outcomes

System operated unsafely during maintenance
Activating an aircraft system such as flaps or thrust reversers when it was not safe to do so, either because personnel or equipment were in the vicinity, or the system was not properly prepared for activation.
Towing event
A safety occurrence which occurred while an aircraft was under tow.
Incomplete installation, all parts present
Although all necessary parts were present, the installation procedure had not been completed. For example, a connection may have been left ‘finger tight’ rather than torqued.
Person contacted hazard
A worker came into contact with a hazard which caused, or had the potential to cause injury. Includes electric shocks, falls and exposure to aircraft fluids or other chemicals.
Vehicle or equipment contacted aircraft
A stationary aircraft was contacted by a vehicle or maintenance equipment such as stairs or moveable stands.
Incorrect assembly or orientation
A component was installed or assembled incorrectly.
Material left in aircraft
A maintenance related item such as a tool was inadvertently left behind by a maintenance worker.


The Australian Transport Safety Bureau considers that the issues identified in the survey are not specific to Australia but will be of use to safety agencies around the world, and has identified a number of recommendations: [8]

  • the need for refresher training for aircraft maintenance engineers
  • the need to remove barriers which discourage aircraft maintenance engineers from reporting incidents
  • the need for fatigue management programs
  • human factors training for management and engineers, and
  • program to address aircraft maintenance engineers concerning memory lapses, pressure, fatigue and coordination difficulties.
1. Marx, D.A. and Graeber, R.C. (1993). Human Error in Aircraft Maintenance. Boeing Commercial Airplane Group. Seattle, Washington.
2. Hawkins, F.H. (1993) Human factors in flight. Aldershot, England: Ashgate
3. Robinson R22 ZK-HVN Aircraft Accident Report Retrieved on 14 Aug 2012 from Civil Aviation Authority of New Zealand
4. Continental Express Flight 2574 Retrieved on 16 Sep 2009 from Wikipedia
5. American Airlines Flight 191 Retrieved on 16 Sep 2009 from Wikipedia
6. Japan Airlines Flight 123 Retrieved on 16 Sep 2009 from Wikipedia
7. Aircraft Maintenance Safety Survey Retrieved on 16 Sep 2009 from Australian Transport Safety Bureau
8. ATSB Recommendation Retrieved on 16 Sep 2009 from Australian Transport Safety Bureau

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