Propulsion System Critical Parts
As noted in Section 6.6.2.16, the TCB should be the primary consideration for the definition of propulsion system critical parts. Note that with the introduction of DASR came the expansion in scope from ESI to PSI programs, and ‘engine critical parts’ to ‘propulsion system critical parts’. Existing engine critical part lists identified at type certification are not expected to be updated, however, all new platforms should use the ‘propulsion system critical part’ approach to identify critical parts.
However, in recognition that not all airworthiness codes are equivalent, and some are not explicit on definition of critical parts, DASA provides the following general definition for propulsion system critical parts in DASR AMC 21.A.41:
Rotating and major static structural parts, and sub-systems of the propulsion system whose primary failure is likely to result in a hazardous propulsion system effect. Typically, propulsion system critical parts include, but are not limited to disks, spacers, hubs, shafts, high-pressure casings, propellers and non-redundant mounts or non-redundant sub-system components. For the purposes of this section, a hazardous propulsion system effect is any of the following conditions:
Non-containment of high-energy debris, including release of the propeller or any major portion of the propeller
Concentration of toxic products in the engine bleed air intended for the cabin sufficient to incapacitate crew or passengers
Significant thrust in the opposite direction to that commanded by the pilot
Uncontrolled fire
Failure of the engine mount system leading to inadvertent engine separation
Complete inability to shut the engine down.
Propeller failure resulting in the development of excessive drag or excessive imbalance
Partial or complete loss of thrust or power for single engine aircraft.
The definition above is based on EASA and FAA safety analysis requirements for turbine engines and propellers11. Therefore, ‘critical parts’ under EASA CS E / CS P and ‘life-limited parts’ under US 14CFR Part 33 / Part 35 should inherently meet the general DASR definition above. However, CS-E and Part 33 consider an engine failure in which the only consequence is partial or complete loss of thrust or power as a Minor engine effect. These codes define engine-level failure classifications in isolation, as civil type-certified engines may be installed on various aircraft types. However, since Defence certifies aircraft and engines together, DASA’s position is that partial or complete loss of thrust or power for single engine aircraft should be considered a hazardous propulsion system effect (as per point h. in the definition above).
Note that in the case of multi-engine aircraft, discrete failures in which the only consequence is partial or complete loss of thrust or power (and associated engine services) from an engine is typically not considered a hazardous propulsion system effect. An example of when this may not be appropriate is when flight outside the one-engine-inoperative envelope is part of the approved flight profiles (normally limited to rotorcraft).
Safety critical engine parts under MIL-HDBK-516C (defined through reference to JSSG-2007A) should also be compatible with the general DASR definition above. However, tailoring of requirements must be understood for any TCB derived from MIL-HDBK-516C.
PSIP Ongoing Monitoring and Periodic Assessments (Mission Analysis)
Overview of Mission Analysis. This section provides additional propulsion systems-specific guidance in support of Section 6.6.3.
Term ‘mission analysis’ is used to describe the process of:
collecting the data needed to define the usage and environmental parameters that can impact propulsion system critical part AwLs
periodic assessment of this data to determine if the current critical part AwLs remain valid for the operator
as required, calculation of new critical part AwLs.
Requirements for mission analysis under DASR include:
Initial Mission Analysis: Applicants for a MTC who are leveraging prior CAA / MAA certification should undertake an initial mission analysis in order to satisfy the propulsion system aspects of the CREA requirement (refer DASR GM 21.A.20)
Periodic Mission Analysis: The MTCH must ensure periodic mission analyses are undertaken throughout the service life in order to satisfy the propulsion system aspects of the DASR 21.A.44(c) obligation.
Mission analyses should ensure that propulsion system critical part AwLs remain compliant with the TCB based on the intended and actual Defence CRE. As defined in DASR AMC 21.A.41, AwLs include not only mandatory life-limit (retirement) and inspection requirements22, but any algorithm, equation, factor(s) or other engineering data which must be used to calculate life accrual against the interval (refer Section 6.6.2.24.d.(ii)).
Mission analysis is also included in the DASDRM Section 3 Chapter 13 as an essential airworthiness design requirement, and the DASDRM list some key considerations for developing a mission analysis plan or statement of work.
The remainder of this section contains further guidance to support the DASR obligations outlined above and the DASDRM design requirement.
Organisational Considerations. Propulsion system critical part lifing involves complex and proprietary methods and tools. Mission analysis involves taking current usage and environment data and performing detailed and quantitative lifing analysis using these same methods and tools. Therefore, DASA expects that mission analyses will usually be conducted with the direct involvement of the relevant propulsion system OEM (not the airframe OEM). Other organisations may be able to conduct mission analysis provided they have suitable expertise and access to the necessary type design data. Note that propulsion system OEMs are especially protective of technical data for commercial advantage and to protect technical supremacy on their military programs. This can present a significant hurdle for other organisations obtaining the data required to conduct mission analysis.
Initial Mission Analysis. Initial mission analysis should follow the same requirements as outlined in DASR AMC 21.A.44(c) and the DASDRM for periodic mission analysis. However, actual usage data is rarely available during the acquisition phase. Therefore, the initial mission analysis should be conducted based on the intended usage (ie mission mix and mission profiles) and operating environment detailed in the initial SOIU, supplemented by legacy usage data if appropriate. Where a new propulsion system is of similar design and intended role within the ADF then it may be appropriate to use legacy usage data as an input for the initial mission analysis.
If the initial mission analysis is limited by the fidelity of the data available, or cannot conclude with sufficient confidence that the intended ADF CRE is within the bounds of that used for prior CAA / MAA certification, then a follow on mission analysis should be conducted as soon as a sufficient quantity of stable representative in-service data is available. In this case, safe interim propulsion system operations must be ensured; this may require interim life reductions, if sufficient safety margin cannot be demonstrated.
Periodic (In-service) Mission Analysis. Throughout the service life of a platform a number of operational and other changes can influence the validity of previous mission analyses. These can include but are not limited to:
changes to the platform role(s), mission mix or profiles
changes to relevant actual operational practices33
changes in operating environment
changes to the platform configuration44
changes to the usage monitoring system (ie changes to the parameters or fidelity of parameters collected)
refinement of OEM lifing models or design assumptions.
Therefore, mission analyses must be undertaken periodically throughout the service life for ADF aircraft (as required under DASR 21.A.44(c)).
The frequency at which mission analysis are conducted will depend upon a number of factors including the platform type / role, age, outcomes of previous mission analysis and stability of the factors outlined under Paragraph 15. Regardless, the frequency of mission analysis should ensure that the underlying basis of AwLs would not be exceeded, even in the case of a significant underestimation of actual usage severity.
Historical experience indicates that mission analysis should be conducted at least every five years. However, significant changes in Defence CRE may require the conduct of a mission analysis before this interval and should be the primary driver for a mission analysis as opposed to an arbitrary time period
Input Data. The type and quantity of data required to conduct a representative mission analysis should be based on advice from the relevant OEM or organisation conducting the mission analysis.
The data required to conduct a mission analysis will usually include:
data gathered by the health / usage monitoring system
aircrew interviews / surveys
environmental data
mission profiles and mission mix
the approved SOIU.
Usage and environment data should be validated (ie checked for accuracy) and confirmed as being representative of future ADF operations.
Outputs of Mission Analysis. Outputs should explicitly confirm (through formal written correspondence) that the propulsion system critical part AwLs remain valid for the Defence CRE. The mission analysis and outputs must sufficiently encompass all propulsion system critical parts and all aspects of the AwLs as defined in DASR AMC 21.A.41 (refer Section 6.6.2.24).
Tracking Propulsion System Life-Limited Components
This section supports Section 6.6.4 and provides additional guidance and examples related to tracking life accrual against propulsion system AwLs.
Propulsion system AwLs are typically defined in terms of equivalent or reference cycles, however, other metrics such as engine hours or calendar time may be used depending on the complexity of the lifing policy and the potential failure mode the AwL relates to.
The definition of ‘cycles’ for tracking purposes will vary between engine type, OEM and age of the engine design. The OEM should define the method(s), equation(s), algorithm(s), etc required to track life consumption, and this is considered part of the AwL (refer Section 6.6.2.24.d.(ii)).
Depending on the cycle tracking definition and the fidelity of the usage monitoring system used for the engine, life consumption for life-limited components may be tracked on a per part, per module or whole of engine basis.
Conversion Factor (CF). A simple means to track life consumption against cycle-based AwLs is to use a cycles/hours CF to translate the AwL intervals into hours-based limits. The system for tracking life accrual then only needs to record hours. Under this philosophy the CF are considered part of the AwL.
Under this method, CFs should be developed based on actual operational usage data, be suitably conservative to envelope potential variability in mission profiles and mission mix and should be periodically confirmed to remain valid for Defence CRE (all through the DASR 21.A.44(c) obligation).
Manual Cycle Tracking. Simple cycle definitions allow for manual recording to track life accrual (ie when parameters can be reliably identified and recalled post-flight by aircrew). This type of cycle definition and method of tracking is usually only appropriate for engines with a predominantly constant operating speed and / or well-defined missions with few excursions to full power (eg generally only on take-off). Under this method, the manually identified cycles are recorded directly into the system for life tracking (eg CAMM2).
Manual cycle tracking methods may also be used as a back-up when automated cycle tracking (refer below) is unavailable. This may potentially require additional factors (often referred to as fill-in factors) to account for the lower fidelity approach.
Simple cycle definitions are usually underpinned by complex engine performance and usage assumptions, and therefore do not negate the requirement for an understanding of these assumptions and the conduct of appropriate periodic mission analysis. As described at Paragraph 9, mission analysis should evaluate all aspects of the AwLs, which in this context includes the assessing the ongoing validity (or otherwise) of the simple cycle definition used for tracking life accrual.
Automated Cycle Tracking. More complex cycle definitions and critical parts with more advanced lifing methods will usually require automated systems that calculate cycle accrual based on recorded engine data and parameters. The tracking system will take the on-board recorded data and determine cycle accrual per-flight based on OEM defined algorithms / equations. These algorithms / equations can range from relatively simple monitoring of shaft speeds to advanced methods incorporating temperature and pressure data.
Whilst this approach is usually more accurate (ie includes less conservatism), it requires more extensive validation, infrastructure, resources and oversight.