Alternate and Interchangeable Part Numbers in Aviation

Why one part can have several numbers

Part numbers change as designs evolve. A manufacturer may introduce a modified version, replace a vendor, or release a new dash number for a revised configuration. Older numbers may remain installable, become superseded, or be restricted to certain serial numbers or modification states.

Different manufacturers can also hold approvals for parts that serve the same function, such as an original equipment part and an FAA-PMA replacement. Function alone does not make them interchangeable for a given aircraft.

Where approved alternates come from

The illustrated parts catalog for the aircraft or component is the usual starting point. It typically shows interchangeability between part numbers and any effectivity limits. Service bulletins, engineering orders, and type design data can also establish or restrict alternates.

FAA AC 20-62E places the eligibility determination on the installer, who must establish that an article is eligible before the aircraft is returned to service. It also notes that distributors who are not production approval holders cannot certify the airworthiness of the parts they sell. A supplier can surface options and share the data behind them, but should not declare a part eligible for someone else’s aircraft.

One-way versus two-way interchangeability

Some alternates are fully interchangeable in both directions. Others are one-way: the newer part may replace the older one, but not the reverse. Mixed configurations, such as pairs of parts that must match, add another constraint.

Ask the question explicitly in the RFQ. A quote for a one-way alternate that only works in the wrong direction wastes time for everyone.

How to write the RFQ

Give the sourcing desk room to search without giving away the eligibility decision.

• List the requested part number first, then every alternate your data already approves
• Note serial-number, modification, or effectivity limits for the aircraft
• State whether PMA or other approved replacement parts are acceptable
• Ask suppliers to quote the exact part number offered, not a family or description