MSG-3 Level 1 – Question 1: Evident or Hidden Functional Failure

In the MSG-3 decision logic for Systems and Powerplant items, the first question at Level 1 is precise and must be applied without interpretation beyond its intent:

Is the occurrence of a functional failure evident to the operating crew during the performance of normal duties?

This question must be asked separately for each functional failure of the item being analyzed. Its purpose is to clearly segregate evident functional failures from hidden functional failures. The outcome of this step determines the direction of the remaining analysis and the type of maintenance task that may be required.

Definition of the Operating Crew

For the purpose of this evaluation, the operating crew consists of:

• Qualified flight compartment personnel on duty

• Qualified cabin attendant personnel on duty

Ground crew is not part of the operating crew in this context. Maintenance personnel, ramp agents, and other ground-based staff are excluded from the assessment of whether a failure is evident.

The focus of Question 1 is strictly on awareness during aircraft operation.

Meaning of “Normal Duties”

Normal duties are those associated with the routine operation of the aircraft on a daily basis. For flight crew, these duties are described, at least in part, in the Airplane Flight Manual (AFM) and must be accomplished by the flight crew.

When determining whether a failure is evident, working groups may consider required flight crew checks described in the AFM as part of the operating crew’s normal duties. However, whenever credit is taken for such checks in categorizing a failure as evident, this must be documented in the analysis.

If a system failure is indicated to the operating crew while they are performing these normal duties, it is considered an evident functional failure.

If the failure could occur without the crew becoming aware of it during those duties, it is classified as a hidden functional failure.

The distinction is based solely on whether the crew will be aware of the loss of function—not on system importance, redundancy, or design intent.

Use of Assumptions During Initial Analysis

During the initial MSG-3 analysis, the AFM may not yet be available. In such cases, working groups may assume that certain flight crew checks will be included in the AFM and form part of normal duties.

When assumptions are made, they must be clearly recorded in the analysis for later verification. This requirement applies equally to:

• Assumptions regarding flight crew checks

• Assumptions concerning the frequency of system use

• Assumptions related to automatic tests performed by electronic equipment

If there is uncertainty about how frequently certain systems are used during normal operations, and assumptions are necessary to proceed with the analysis, those assumptions must be documented.

The same discipline applies to automatic built-in tests. If it is assumed that electronic equipment will automatically perform tests and provide indications to the crew during normal duties, that assumption must be recorded.

Assumptions cannot remain implicit.

Verification Once the AFM Is Available

When the AFM becomes available, all Level 1 failure analyses based on assumed flight crew checks must be verified to ensure those checks are actually included in the AFM.

If the assumed checks are confirmed, the classification of the failure as evident remains valid.

If any assumed flight crew check is not included in the AFM, the Level 1 analysis must be redone for that functional failure. In that case, the failure can no longer be considered evident based on that assumption.

Furthermore, evidence of AFM tasks that were assumed in the MSG-3 Level 1 analysis and submitted to the Maintenance Review Board (MRB) must be available prior to MRB Report approval. If such evidence is not available, the Level 1 analysis submitted to the MRB must be based on the assumption that these tasks are not part of the crew’s normal duties.

This requirement ensures that the classification of failures is aligned with documented operational practice, not projected or informal procedures.

System Failures Indicated to the Crew

The rule remains direct:

System failures that are indicated to the operating crew while they are performing their normal duties shall be considered evident.

There is no additional interpretation required. If the failure is annunciated, displayed, or otherwise made known to the operating crew during routine operations, it is evident.

If it is not, it is hidden.

Consideration Under Operator Reliability Programs

Although the evident or hidden classification is established during the MSG-3 analysis and reflected in the MRB Report, aircraft operators must monitor the overall effectiveness of the approved maintenance program through their Reliability Programs in accordance with continuing airworthiness requirements. In service, operators analyze failure data and task performance to ensure that maintenance tasks — including those intended to detect hidden failures — remain effective at the approved intervals. Reliability data supports adjustments to task content or intervals where necessary.

Importance of Accurate Classification

The separation between evident and hidden functional failures is fundamental to the MSG-3 methodology. It ensures that:

• Failures detectable during normal operation are treated differently from those that are not.

• Hidden failures receive appropriate consideration in subsequent decision logic steps.

• The maintenance program reflects actual operational exposure and awareness.

An incorrect classification at this stage affects the remainder of the analysis. For this reason, documentation of assumptions and later verification against the AFM are mandatory elements of the process.

Question 1 is not a general inquiry about system visibility. It is a structured determination tied directly to crew awareness during normal duties as defined in operational documentation.

Its disciplined application preserves the integrity of the MSG-3 analysis and the maintenance program that follows.

If you want to fully understand how MSG-3 logic translates into real Aircraft Maintenance Program decisions, the Aircraft Maintenance Programs (Advanced Expert) course is designed for you.