Arc Skills / Standards and development assurance
Review development assurance level allocation rationale
This task reviews whether development-assurance allocations follow the approved safety basis and stated architecture. Arc Skills returns an allocation trace with separate function, item, software and hardware decisions and unresolved rationale.
Use this skill
Use Arc Skills to review development-assurance allocations for the named aircraft or system baseline. Follow each failure condition into functions, implementing items and software or hardware.
Inputs:
- Approved safety assessment and failure-condition definitions.
- Function/item architecture and allocation record.
- Independence, monitor and common-cause evidence; authority agreements.
Return a trace table with FDAL, IDAL, software level and hardware DAL only where actually assigned, plus rationale and open decisions. Do not derive a letter from severity or substitute an AI allocation for an approved one.What you provide and what you get
| What you have | How it is used | What you get |
|---|---|---|
| Approved safety basis | Defines failure conditions and function decisions | Source trace |
| Function/item architecture | Shows shared implementation and monitor path | Allocation chain |
| Independence evidence | Tests any claimed reduction or decomposition | Unsupported rationale |
A monitored pair needs evidence for the claimed reduction
Illustrative engineering example.
A synthetic flight-control function FC-1 has an approved function allocation recorded as FDAL A. Items I-1 and I-2 implement a monitored pair. The allocation sheet proposes a reduced item level for I-1 because I-2 monitors it, but the monitor’s independence and effectiveness records are absent.
| Scope | Recorded assignment | Rationale supplied | Review question |
|---|---|---|---|
| FC-1 function | FDAL A in approved safety assessment | Severe failure condition FC-F1 | Retain approved function assignment |
| I-1 implementing item | Proposed reduced IDAL; not approved | I-2 detects erroneous I-1 output | Provide monitor coverage and independence analysis |
| I-2 monitor and its SW/HW | Item/software/hardware levels not established | Shared supply and interface shown | Assign through approved process; assess common cause |
FDAL A is an illustrative recorded project assignment, not a severity-to-letter rule. The row for I-1 is only a proposal. The same monitor that motivates reduced assurance could fail silently or share a resource with the primary path, so its effectiveness and independence matter to the allocation argument.
The review does not automatically raise or lower any level. It identifies the missing evidence and leaves the item, software and hardware assignments to the controlled allocation process and authority. Shared supplies, multiple-function items and design changes must be reflected in the trace before the rationale can be judged.
Keep each assurance assignment in its own scope
- Trace failure condition to function and the approved function allocation.
- Trace function to every implementing and monitoring item, including shared resources.
- Inspect independence/decomposition credit and evidence at the failure-condition boundary.
- Record approved versus proposed IDAL, software level and hardware DAL separately.
Questions about this task
Does FDAL A make every item Level A?
No automatic inheritance follows from the label alone. Item allocation requires the applicable development-assurance process and architecture rationale.
Can a monitor justify reduced assurance?
Potentially, but only with an approved argument and evidence for detection, independence, common causes and integration.
Sources and further reading
- FAA AC 20-174: Official recognition of ARP4754A as an acceptable civil-aircraft development process.
- FAA AC 20-115D: Official guidance on airborne software assurance using DO-178C and applicable supplements.
- FAA AC 20-152A: Official guidance on airborne electronic hardware development assurance.
FAA AC 20-174 recognizes ARP4754A for aircraft/system development; controlled project documents govern detailed allocations and authority decisions.