MIL-STD-1798 Mechanical Equipment And Subsystems Integrity ProgramThe purpose of this standard is to describe the general process to achieve and maintain the physical and functional integrity of the mechanical elements of aircraft systems. The goal of this integrity program is to achieve the desired level of safety and aircraft availability at the most economic cost across the life cycle of the weapon system. This standard allows the process to be tailored in a competitive environment to meet specific equipment, subsystem, and/or system requirements. The Mechanical Equipment and Subsystems Integrity Program MECSIP is implemented in the planning process and continued until retirement of the system.

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MIL-STD-1798C

3.38  Mean Time Between Failures (MTBF).

MTBF is a parameter that historically has been used to define the reliability of components.  It is usually expressed as:

MTBF  =  [(FLYING HOURS or ACTIVE TIME or POSSESSED HOURS)  * (QUANTITY PER AIRCRAFT) * (USAGE FACTOR)] / (INHERENT FAILURES)

3.39  Not Mission Capable (NMC) (M), (S) (B).

Not Mission Capable is used by the MECSIP Program to indicate the discrepancy that prevents the aircraft from being able to fly the required mission.  The aircraft may be Partially Mission Capable (PMC), wherein it can fly some, but not all, required missions.  The letters "M", "S" and "B" indicate the aircraft is in NMC status due to maintenance, supply or both maintenance and supply respectively.  NMC = NMCM + NMCS + NMCB.

3.40  Other/expendable components.

Other/expendable components includes all components of a system not classified as safety- critical, mission-critical, durability-critical, or durability-noncritical.  The failure of these components could be handled during routine maintenance and would not impact the mission, safety, or operational readiness.

3.41 Probability of detection (POD).

A POD is a statistical measurement of the likelihood, with a specified confidence level, of finding a flaw of a defined size using a specific inspection technique. (Reference MIL-HDBK-1823.)

3.42  Production Quality Deficiency Report (PQDR).

The PQDR is a tool which can identify internal quality problems. A Reliability Analysis Team (RAT) member should lead the PQDR program; Maintenance should be encouraged to PQDR every defective parts meeting the criteria of 5.5.2.14 herein; and results should be analyzed until a satisfactory answer is provided.  The RAT will also review troubleshooting techniques during their PQDR investigation.

3.43  Proof testing.

A test is performed on each production component that can effectively demonstrate that the part is damage-tolerant. The proof test must be supported by analysis.

3.44  Required operational service life.

The required operational service life is that operational life specified for the specific system, subsystem, or component-usually in terms of service or operation time.

3.45  Redundancy.

Redundancy in design incorporates dual/multiple components or duplicates function to provide operational capability (without degradation) upon failure of a single component or function. Failure of a single component or function must be detectable (i.e., system is both fail operational and fail evident). Detectability may be through system Built-in Test (BIT), Prognostic Health Monitoring (PHM),  or inspection. Redundancy also may describe a component that has redundant features.

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