Great question! 'Zombie parts' are those sneaky inventory items that look available on paper but have dangerously long lead times that could bring production to a halt. Here's how to spot them and the key metrics that expose these supply chain time bombs:

First, you need to track Lead Time Variability - this is your #1 indicator. Look for components where the average lead time has suddenly spiked from, say, 30 days to 90+ days. The consistency (or lack thereof) in supplier delivery times tells you which parts are becoming unreliable.

Next, monitor Inventory Turnover Rate for specific components. If certain parts have extremely low turnover but you're still ordering them regularly, that's a red flag. These are likely 'zombie parts' - technically in your system but with lead times that don't match your production needs.

Also watch for Supplier Performance Metrics. Track on-time delivery rates, quality issues, and communication responsiveness. Suppliers who consistently miss deadlines or become unresponsive are often the source of zombie parts.

The key is creating a dashboard that flags components when: 1) Lead times exceed 90 days, 2) Lead time variability increases by more than 50%, 3) Inventory turnover drops below a critical threshold, and 4) Supplier performance scores decline.

Regularly review your 'critical parts list' - those components that would stop production if unavailable. For each, track actual vs. promised lead times, and set up alerts when discrepancies exceed your risk tolerance.

Remember, zombie parts often hide in plain sight because they're technically 'available' - but that availability is an illusion when lead times stretch beyond your production planning horizon. The metrics that reveal them are all about timing, consistency, and supplier reliability.