Are spare parts inventory or fixed assets?

It depends on how the part is used, and the split matters because it changes both your balance sheet and who is accountable for the stock.

Under the International Financial Reporting Standards (IFRS), parts consumed in the ordinary course of maintenance are inventory under IAS 2, and their cost hits the income statement when they are used. Major spare parts and stand-by equipment are capitalized as property, plant and equipment under IAS 16 when the company expects to use them over more than one period, or when they can only be used in connection with a specific asset. A spare turbine held for one production line is typically property, plant and equipment. A stock of bearings and seals is inventory. US Generally Accepted Accounting Principles (GAAP) reach a comparable outcome through the same use-based logic.

In practice the frontier is drawn by the expected holding period and by whether the part is dedicated to a single asset. Insurance spares, held for years against a low-probability failure on a critical machine, are the classic capitalization case.

This describes common accounting treatment, not accounting advice: confirm the classification of your own spare parts with your auditor.