The results of such errors include engine problems, shutdown, or even loss of thrust on multiple engines, and the related risks to safety of flight. Operators and maintenance personnel can reduce the likelihood of this type of error by understanding its causes and taking steps to prevent repeating the same error on multiple engines.

Maintenance of all engines on an airplane at the same time or by the same individual or team presents the potential for error and the possible loss of thrust from all engines. Boeing has long advocated that operators stagger scheduled maintenance on their airplane engines. When multi-engine maintenance cannot be avoided during a single occasion, Boeing recommends that different maintenance personnel work on each engine and exercise added scrutiny. This added scrutiny may be as simple as redundant checks of work performed or as demanding as a nonrevenue flight to ensure the integrity of all systems.

Preventing engine problems or loss of thrust related to multi-engine maintenance error is possible by learning about the following:

  1. Examples of multi-engine maintenance errors.
  2. Causes of errors.
  3. Strategies for avoiding multi-engine maintenance errors.

Examples of Multi-Engine Maintenance Errors
Several problems with engines have been reported that occurred after multi-engine maintenance. The following examples show the variety of problems. In each example, the airplane experienced an in-flight shutdown or developed problems on more than one engine.

  • An L-1011 descending into Nassau, The Bahamas, lost oil pressure in engine no. 2. The flight crew shut down the engine and diverted back to Miami, Florida. On the return to Miami, engines no. 1 and no. 3 shut down. At 3,800 ft above the ocean, the crew recovered engine no. 2. The flight crew then completed a single-engine landing at Miami. The O-ring seals on the master chip detectors for each engine were missing, causing engine oil loss.

  • A 747-200 approaching Rome made a two-engine landing after two of the four engines were shut down because of a lack of oil. Both oil filters were found to have been improperly installed.

  • The engines on a 747-100 taking off from San Francisco failed to develop sufficient r/min, prompting the flight crew to perform a refused takeoff. Subsequent investigation at the gate discovered power stops on the throttles used during a maintenance check. These power stops inhibited the engines from achieving their takeoff thrust.

  • On a BAe-146, a low-oil warning on one engine prompted the crew to shut down the engine. Low-oil warnings then occurred on two additional engines, which were reduced to idle. The crew increased thrust on the remaining engine and diverted to the nearest airport. Inspection of the engines discovered that three of the four engines did not have seals on their chip detectors and that oil leaked from the engines. The airplane had just completed a shop visit and was returning to its home airport.

  • A 737-400 made an emergency landing after low-oil warnings occurred on both engines during climb. Investigation revealed that the hand-crank cover assemblies on both engines were missing from the engine gearboxes, allowing oil to leak.

  • During descent on a 747-100, engine no. 2 was shut down because of oil loss, and engine no. 3 was shut down for the same reason eight minutes before landing. Investigation revealed the improper installation of the centrifugal oil filter retaining rings in both engines.

Causes Of Errors
Errors occur for several reasons, but these reasons rarely include individual carelessness or incompetence. The fact that even careful maintenance personnel make mistakes has led to the discovery of several factors common to mistakes. Examples include the following:

  • Information that is difficult to understand, such as work/task cards and manuals.

  • Interruptions during performance of a task. The interruption may cause the individual to miss key elements of the task, such as replacing the O-rings or oil cap covers.

  • Inadequate lighting. Personnel not seeing properly can be a contributor to mistakes.

  • Poor transfer of information at shift change. The next shift may not be properly informed of the degree of completion for a task, including work on critical systems.

  • Airplane design. Component design may cause difficulty for maintenance personnel.

(See "Are Mistakes Repeated?" and "Human Factors Process for Reducing Maintenance Errors" in Aero no. 3, July 1998, for additional information about maintenance error.)

Strategies for Avoiding Multi-Engine Maintenance Errors
After investigating several multi-engine maintenance events, Boeing identified a key strategy for avoiding multi-engine maintenance error: If possible, operators and repair stations should avoid performing maintenance on multiple engines using the same personnel during a single maintenance visit. Following this strategy should also minimize the potential that improper maintenance will occur on redundant or backup critical systems, such as flight controls, electrical generation and distribution, and hydraulics.

To incorporate this key strategy, some operators have extended their ETOPS approach to their non-ETOPS fleets (see "ETOPS-Based Maintenance Guidance"). By adopting some or all of the ETOPS strategies, operators have also increased the reliability of their fleets.

Additional strategies for avoiding multi-engine maintenance errors include

  • Staggering maintenance.

  • Varying maintenance personnel.

  • Developing specific processes for multi-engine maintenance.

  • Employing existing maintenance personnel expertise.

  • Educating maintenance personnel about critical systems.

  • Using tools provided by airframe manufacturers and engine companies.

Staggering maintenance.
If possible, all scheduled maintenance on engines should be staggered to avoid multi-engine maintenance by the same personnel during a single shop visit.

Varying maintenance personnel.
Multi-engine maintenance during a single visit may be unavoidable. For example, multiple bird strikes that affect all engines may require both inspections and repair to be performed. To reduce errors during both tasks, different personnel or teams should work on each engine of a multi-engine airplane. This multiple-personnel approach increases the number of mental models (see "Mental Models: How People View the World") available during maintenance that could help prevent redundant errors.

Developing specific processes for multi-engine maintenance.
At those times when multi-engine maintenance may be required during one visit, operators or repair stations should consider the following to help prevent error:

  • Develop particular checklists that require additional scrutiny.

  • Require additional functional checks to ensure the integrity of the maintained systems.

  • Incorporate a nonrevenue flight into the check process after some maintenance actions.

  • Encourage process thinking by the mechanics. Usually if a mechanic develops a pattern of action associated with an activity, it is less likely that an error will occur. This pattern of action must be correct to prevent errors from becoming incorporated into normal processes (see "Same Process, Same Outcome").

  • Provide a kit of parts for specific maintenance activities. For instance, if the engine requires an oil filter replacement, all the parts that must be replaced can be included in a kit. Then, if O-rings remain in the kit after the task is complete, it should be obvious to maintenance personnel that the O-rings were not installed.

Employing existing maintenance personnel expertise.
An operator's maintenance planning department, shop supervision, and mechanics will have excellent ideas on how to avoid errors. For example, Boeing worked with one of its engine suppliers to reduce the in-flight shutdown rate for a particular engine. By working with several operators' mechanics, Boeing and the supplier learned about a variety of ways to reduce error.

Educating maintenance personnel about critical systems.
Though all airplane components are important, some components have greater influence on airplane flight safety than others. For instance, oil is necessary for any airplane engine to run. Thus, all the places where oil can leak become critical, especially those locations that are likely to require handling, such as oil fill covers, gearbox access covers, oil filters, chip detectors, and plugs.

Using tools provided by airframe manufacturers and engine companies.
As part of the Boeing initiative to reduce maintenance-related error, the company is developing the following error-prevention tools for operators:

  • Multi-engine maintenance workbook. In joint development with engine manufacturers, this book will provide pictures of problem areas (engine locations where mistakes have occurred in the past), cues for avoiding problems, and rationale for why multi-engine maintenance events cause concern about safety of flight.

  • Wallet cards. These cards will list potential problem areas and strategies to reduce errors.

  • Multi-engine maintenance decal. This decal will highlight the theme of reducing error and is designed for use in repair areas.

  • Service letter. Boeing sent out the following service letters on this topic in September 1998: 707-SL-05-005-A, 727-SL-05-006-A, 737-SL-05-004-A, 747-SL-05-004-A, 757-SL-05-001-A, 767-SL-05-002-A, 777-SL-05-001-A.

  • Boeing task cards and maintenance planning document supplement. (These changes will occur as the new documents are developed for Boeing operators.)

The U.S. Federal Aviation Administration (FAA) and other agencies have also developed tools to help operators reduce and manage error. For example, the FAA has published a poster entitled "Maintenance Personal Minimums Checklist." This poster suggests some error-reduction ideas for maintenance personnel to consider both before and after a task.

Aviation industry professionals involved with multi-engine maintenance, including trained maintenance personnel and operators' planning departments, work together to avoid maintenance errors. However, because mistakes continue to occur and are repeated on multiple engines, Boeing has developed strategies for operators and repair stations to follow to recognize the possibility of error and reduce the effect of these errors. These strategies include staggering scheduled maintenance on engines, using different personnel, and developing special processes for multi-engine maintenance during a single shop visit. Operators may also consider applying some aspects of their ETOPS approach to their non-ETOPS fleets. The ultimate result of combining these actions may be to reduce multi-engine maintenance errors and improve flight safety.

Thomas Murray
Safety Engineering
Propulsion Technology
Boeing Commercial

Vince Robel
777 GE/PW/RR Engine
Buildup and Strut
Systems Engineering

Boeing Commercial Airplanes

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