Short answer
There is no universal number. Set the minimum by reasoning from your own history, your equipment and how long you can afford to wait for a replacement. This article gives the method. I do not give a number, because it would be invented.
Step 1: List what can fail and where
For each plow, broom or other unit, list the edge or wear item, its size and its position. Two units with the same edge share one spare pool; different sizes do not.
Step 2: Estimate how often you change edges
The estimate only needs to be good enough to size stock, not precise to the hour.
- Use your own change log from the last two or three seasons.
- Separate planned changes (wear) from unplanned ones (strikes, damage).
- If you have no log, start one now and use a cautious guess for the first season, labelled as a guess.
Step 3: Decide how long you can wait
Spare stock is a calculation, not a habit. Two answers define it.
- Ask your supplier for the real time to deliver an urgent single set, in writing. The number to use is the bad case, not the average.
- Decide how many hours a unit can be out of service before the clearing plan suffers.
- The gap between those two answers is your exposure. Stock should cover that gap, not the whole season.
Step 4: Turn exposure into a spare
| Situation | Leaning |
|---|---|
| Critical unit, no backup unit, supplier delivery slower than your tolerance | Keep a full spare set on the shelf |
| Critical unit with a backup unit that uses the same edge | One shared spare may do |
| Non-critical unit, fast local supply | Order when needed |
| Unplanned strike history on a unit | Add stock for the damage-prone position |
Step 5: Review
The review is what keeps the spare level honest after the first season.
- Check stock each month during winter and after every storm.
- Reorder when stock falls to the minimum, not when it hits zero.
- After the season, compare actual use with the plan and adjust.
What to keep with the spares
Store bolts, nuts and any shoes with each set, label the unit it fits and keep the parts dry. A spare that cannot be fitted quickly is not much of a spare.
Boundary: this is a reasoning method, not an FAA or manufacturer requirement. Apply your airport’s own rules and plans.
