The Boeing 737 Standard Fault Isolation Practices refer to the procedures and guidelines used to identify and correct faults in the Boeing 737 aircraft. This is often documented in a Fault Isolation Manual (FIM).

The FIM is an essential tool for maintenance personnel, providing a systematic method to troubleshoot and isolate faults in the aircraft’s systems and components. It is designed to guide the technician from the general problem to the specific faulty component by following a logical troubleshooting process.

Boeing has developed an Interactive Fault Isolation Manual (IFIM) that makes it easier to identify and correct faults. This interactive manual is part of Boeing’s ongoing efforts to provide efficient and effective tools to support aircraft troubleshooting and maintenance. The IFIM is a web-based application that makes it easier to find the right fault isolation task for a fault and then shows the task in a form that is simple to follow.

Faster troubleshooting is now possible with Boeing’s Interactive Fault Isolation Manual (IFIM).

Please note that the specific practices and procedures can vary depending on the exact model of the Boeing 737 and the specific systems or components involved. Always refer to the official Boeing documentation and follow the approved maintenance procedures for the specific aircraft model.

Fault Reporting

Airplanes can display a wide range of fault symptoms, usually in the form of visual indications. Some indications are meant for the flight crew, some for the cabin crew, and others for maintenance personnel. Each fault indication must be clear in its meaning, and there must be a way to correct the underlying cause for each indication, to return the airplane to a serviceable condition.

Boeing assigns a unique eight-digit fault code and a textual description for each flight deck fault, cabin fault, and ground maintenance fault.

The Fault Reporting Manual (FRM) is a complementary document to the IFIM. It contains all the fault descriptions and their respective fault codes, which helps the flight crew effectively communicate the exact fault condition to maintenance personnel. The fault code is logged by specific airplane systems, serving as a direct reference to the IFIM. Maintenance messages are not intended for use by flight crews and do not have fault codes. They utilize a different numbering format than fault codes.

Types of Faults

Many types of fault conditions can occur in an airplane, and there are many ways in which the fault condition can be detected.

1. Status Messages

Status messages show on the flight deck displays. Status messages give information to the maintenance crew about the dispatch status of the airplane. The maintenance crew uses the status messages together with the Master Minimum Equipment List (MMEL).

2. Observed Faults

Observed faults are the problem symptoms (other than status messages) sensed by the flight crew or the maintenance crew. Observed faults that show on the pilots’ displays are called non-message flight deck effects (FDE).

These are the types of observed faults:

  • Non-Message Flight Deck Effects
    • Fault annunciations
    • Failure flags
    • Engine exceedances
  • Flight crew observations in the flight deck or during walk-around
  • Servicing crew observations
  • Ground maintenance crew observations.

3. Cabin Faults

  • Cabin faults are the problems symptoms which can occur with the systems and equipment in the passenger cabin.
  • Each cabin fault has an 8-digit fault code.
    • The last 3 digits of the fault code represent the location in the cabin where the fault occurred. The last 3 digits are the “location code”.
    • The airline operator assigns the location code.
  • Suggested location codes:
    • For passenger seats and amenities, use the closest seat number.
    • For lavatories and galleys, use the identifying number of the lavatory or galley.

Maintenance Messages

There is a special type of fault indication known as a maintenance message which is intended only for use in airplane maintenance. This message can be displayed as text by the airplane’s onboard maintenance system (OMS) or can be a diagnostic result from the built-in test equipment of certain airplane systems.

Maintenance messages help you find the cause of a status message or other fault. These are the types of maintenance messages:

1. Maintenance messages that show on the Onboard Maintenance Function (OMF).

  • Each maintenance message has a message number and a description of the problem.
  • The OMF has logic that correlates maintenance messages with status messages and Flight Deck Effects (FDE). When an airplane has a FDE, the OMF may show the related maintenance messages (not all FDEs have maintenance messages).
  • The OMF will also show “uncorrelated” maintenance messages that are not associated with a FDE. Uncorrelated maintenance messages should be used for troubleshooting and are not intended to affect airplane dispatch.

2. Maintenance messages that show on the Control Display Unit (CDU).

  • Maintenance messages can be fault indications that you get from the built-in-test (BITE) feature of a system or the individual component.
  • The CDUs are in the flight deck, you can access the BITE for several airplane systems through the CDUs.

3. Maintenance messages that show on the front panel of a Line Replaceable Unit (LRU).

  • These maintenance messages can help you find the root cause of an observed fault.
  • Most BITE tests are performed at the front panel of the component (LRU) in the electronic equipment bay. Some BITE tests are performed in the flight deck.
  • These are examples of front-panel maintenance messages:
    • A specific light or lights
    • An alphanumeric code
    • A group of words or text with or without an associated numeric code

Find Correlated Maintenance Message

How to find Correlated Maintenance Message for Status Message and Observed Fault

  • IFIM Task 31-00-00-810-801 : Status Message Standard Procedure – Fault Isolation
  • IFIM Task 31-00-00-810-803 : Standard Procedure for Observed Fault with Correlated Maintenance Message – Fault Isolation

It is not necessary to do this task if you already have the correlated maintenance message.

  • ONBD MAINT >
    • LINE MAINT >
      • INBOUND FLIGHT DECK EFFECTS
      • EXISTING FLIGHT DECK EFFECTS
    • EXTENDED MAINT >
      • FAULT HISTORY > SELECT FDE BY LEG > Select a Flight Leg

The flight deck display system can latch this status message when it occurs. After you find the cause of the fault and correct it, it is possible that the status message will continue to show.

  • IFIM TASK 31-00-00-810-804: Status Message Latched by Flight Deck Display System – Fault Isolation

If you already have the correlated maintenance message, then do these steps:

  • Go to the fault isolation task for the maintenance message and do the steps in the task.
  • After you do the actions in the task that correct the fault, select MAINT CTRL PGS and then LATCHED MESSAGE ERASE.
  • If the status message shows on the ERASABLE STATUS MESSAGES display, then do these steps:
    • Select ERASE to remove the message from the status display.
    • If the status message does not show on the display, then you corrected the fault.

FIM General Note

The FIM (Fault Isolation Manual) is a tool to help you operate the airplane economically. The tasks in the FIM help you to quickly isolate the cause of each airplane fault.

To isolate the cause of a fault, you can also use your knowledge from:

  • Your past experience with airplane faults
  • The conditions when the fault occurred
  • The history of faults on your airplane or your fleet

It is not a requirement to do the steps in the FIM task in the order shown.

  • If you do not plan to do the steps in the order shown, then make sure that you read the task before you start to isolate the fault.
  • Some FIM tasks have important steps that have an effect on other steps later in the task.

If you do a corrective action that is not in the FIM task, then Boeing recommends that you do the repair confirmation steps of the FIM task after your action. This helps you know if your action corrected the fault, or if it did not correct the fault.

Assumed Conditions at Start of Task

  • External electrical power is connected to the forward receptacle.
  • Hydraulic power is off.
  • Engines are shut down.
  • All circuit breakers for the system are closed.
  • Airplane systems are in the ground mode.
  • No equipment in the system is deactivated.
  • The fault was caused by a single failure, not multiple simultaneous failures.
  • Electrical power for the system is available.

Initial Evaluations

At the beginning of most fault isolation tasks there is an Initial Evaluation paragraph. The purpose of the Initial Evaluation is to verify that the fault condition still exists before you start the fault isolation procedure.

When a specific fault occurs, the steps in the Initial Evaluation will usually verify that the fault exists. Then they will instruct you to do the fault isolation procedure that follows the Initial Evaluation.

When the Initial Evaluation cannot verify that the fault exists at the present time, then the steps in the task cannot isolate the fault. In this case, the Initial Evaluation will tell you that there was an intermittent fault.

  • If you have an intermittent fault, then follow your airline’s policy for intermittent faults.
  • This task can help you with intermittent faults in the airplane wiring:
    • Intermittent Wiring Special Detailed Inspection, AMM TASK 20-10-80-280-801.

Look at all of the corrective actions in the fault isolation task.

Use your judgement to decide which corrective action to do first.

Use the WDM to identify the connectors in the failed system. Inspect the connectors for contamination, loose connections, or damage.

Monitor the airplane to see if the fault occurs on subsequent flights.

If the fault occurs on subsequent flights, then use your judgement to determine which corrective action to do next.

There is no Initial Evaluation paragraph for these faults:

  • Faults which have a single possible cause.
  • Observed faults whose existence is readily apparent (for example, wiper blade missing or ceiling panel wet).
  • Maintenance messages that can only occur as a result of a ground test on the OMF.
  • Maintenance messages that are for internal display system software problems.

Fault Isolation Paths

Each fault isolation tasks is a tree structure in the form of text paragraphs. To follow the correct path through the tree, remember these rules:

  • Do the steps of the task in the specified order.
  • Obey the “If … then” statements that you see. These will guide you along the correct path.
  • When you are at the endpoint of the path, the step says “You corrected the fault.” When you see these words, complete the current step (if necessary) and exit the procedure.

Replacing Components

The steps of the fault isolation make a clear distinction between the installation of a new component and installation of the same component you removed.

REPLACE: When a step says “Replace the component”, then remove the specified component and install a new unit from your airline’s stock of spare parts.

RE-INSTALL: In some procedures, a step tells you to remove a component so that you can do a wiring or other check. If the subsequent steps say “Re-install the component”, then install the same unit that you removed previously.

INSTALL A NEW: In some procedures, a step tells you to remove a component so that you can do a wiring or other check. If the subsequent steps say “Install a new component”, then install a new or certified serviceable component from your airline’s stock of spare parts.

Repair Confirmation

After each corrective action in a fault isolation procedure, there are steps that will verify that you corrected the fault.

  • If the steps of the repair confirmation tell you that the fault is corrected, then you know that the fault is gone.
  • If the steps of the repair confirmation tell you that the fault still exists, then return to the subsequent step of the fault isolation procedure (the step after the one that referred you to the repair confirmation).

Unlatching Maintenance Messages

Each maintenance message is either “non-latchable” or “latchable”.

  • A non-latchable message is a message that does not stay after the fault condition stops.
  • A latchable message is a message that stays after the fault condition stops. It lets you see a fault that only occurs when the airplane is in a specific flight phase.
  • If a message latches, the OMF will show LATCHED for the message. The OMF will continue to show LATCHED after the fault condition goes away, until you do the steps necessary to unlatch the fault.

Each maintenance message is either “mode independent” or “mode dependent”.

  • If the airplane system monitors at all times (continuously) for the fault, the message is mode independent.
  • If the airplane system can only sense the fault when the system is in a specific mode of operation, the message is mode dependent.

Each fault isolation procedure contains steps that will unlatch the maintenance message.

  • For some messages, there is a ground test that you do on the OMF to unlatch the fault.
  • For some messages, there is no ground test. To unlatch the fault, the procedure has steps to simulate the applicable mode of operation of the system.

These are the different methods used in fault isolation procedures to unlatch the fault (or to tell that the fault is gone):

  • Non-latchable, mode independent messages
    • The FIM procedure tells you to look at the OMF to see if it shows ACTIVE or NOT ACTIVE with the maintenance message.
    • If the OMF shows NOT ACTIVE, then the fault is gone.
    • If the OMF shows ACTIVE, then the fault still exists.
  • Latchable, mode dependent messages without ground test
    • The FIM procedure tells you to put the system in a specific mode of operation and then look at the OMF.
    • If the OMF shows NOT ACTIVE for the message while the system is in operation, then the fault is gone.
    • If the OMF shows ACTIVE for the message while the system is in operation, then the fault still exists.
  • All messages with ground test
    • The FIM procedure tells you to do a ground test on the OMF.
    • When the ground test is completed, if the specific maintenance message does not show on the ground test display, then the fault is gone.
    • When the ground test is completed, if the specific maintenance message shows on the ground test display, then the fault still exists.

Erasing Latched Status Messages

When a status message occurs during flight, it can be latched by the airplane system that detected the fault. When you do the fault isolation procedure for the correlated maintenance message, the system will usually erase the status message when you correct the fault.

Some status messages are latched by the airplane central display system instead of the specific airplane system. In this case, you must erase the status message after you complete the fault isolation procedure for the correlated maintenance message. Reference this task:

  • Fault Interrogation for a Maintenance Message, AMM TASK 31-65-00-750-801.

Electrical Checks

When a step says “Do a wiring check”, do these four types of electrical checks for the specified contacts (pins):

  • Continuity from contact to contact
  • Short circuits between the contacts
  • Short circuits from each contact to structure ground.
  • Connector damage and corrosion (refer to the Standard Wiring Practices Manual)

ARINC 429 Wiring Checks

  • An ARINC 429 wiring circuit connects a transmitting LRU to one or more receiving LRUs.
  • To check the resistance between two pins of an ARINC 429 wiring circuit, first remove all LRUs which are connected to the circuit (refer to the applicable wiring diagram to see which LRUs are connected to the circuit).
    • This will prevent effects on the measurement from the resistance of the ARINC 429 receivers and transmitters in the LRUs.
    • This will prevent damage to connected LRUs by test equipment that operates at a high voltage (for example, when you use an ohmmeter which measures very high resistances to check for insulation problems).
  • After you complete the wiring check (and the subsequent wiring repair, if necessary), reinstall the LRUs you removed.
  • Refer to the Standard Wiring Practices Manual when you connect all connectors.

Circuit Breaker Panels

These panels and modules contain thermal circuit breakers:

  • E90005 Circuit Breaker Panel
  • J11 Aft Equip Center Misc Relay & Circuit Breaker Panel
  • J40 Junction Box, SDS
  • J9 Battery Shield
  • J9000 Circuit Breaker Panel
  • M01720 Standby Power Control Unit
  • P18 CAPT Electrical System Panel
  • P6 F/O Electrical System Panel
  • P60 Misc Telephone Equipment Panel
  • P8 Circuit Breaker Panel 5
  • P91 Power Distribution Panel Number 1
  • P92 Power Distribution Panel Number 2
  • P94 Frequency Conv Panel

Numbering Scheme for Wheels, Tires, and Brakes

  • Wheel number 01 = main gear left outboard
  • Wheel number 02 = main gear left inboard
  • Wheel number 03 = main gear right inboard
  • Wheel number 04 = main gear right outboard
  • Wheel number 05 = nose gear left
  • Wheel number 06 = nose gear right
AviationHunt Team

Our team at AviationHunt is a group of aviation experts and enthusiasts. We aim to provide aviation insights, technical notes, best aircraft maintenance practices, and aviation safety tips.

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