Base vs line maintenance staffing: one headcount, two different jobs

Ask a maintenance manager how many engineers he needs and you'll get one number. That number is the problem. Line maintenance consumes engineers by the shift; base maintenance consumes them by the man-hour. They are two different demand curves wearing the same uniform, and if your man-hour plan adds them into a single pot, both sides will quietly rob each other until an AOG or a slipped check exposes it.

01Line staffing is coverage math

Line maintenance demand is driven by the flying programme and the shift pattern, not by total workload. A line station running two shifts with pre-flight, daily and turnaround work needs a fixed number of licensed people present — whether the day brings two defects or twenty. The unit of planning is the post: a certifying seat that must be filled every shift, every day the fleet flies.

Worked example — an operator running AW139s and AW189s on a two-shift line, 360 flying days a year:

Note what is not in that formula: the number of defects. If the fleet doubles its unscheduled work, you still need 8 people on roster — you just burn more overtime inside the same coverage.

02Base staffing is man-hour bank math

Base maintenance is the opposite shape. A 600 FH check doesn't care what day of the week it is — it cares that roughly 1,500 man-hours of work, plus rectification, are delivered inside a two-week input window. The unit of planning is the man-hour bank: scheduled hours, plus a findings allowance, divided by productive hours per engineer:

Two different answers: 8 for line, 5 for base — 13 total. And here is where the single-headcount plan goes wrong.

03The trap: borrowing across the fence

Every combined pool I've audited "balances" the same way: when a check comes in, line staff get pulled into the hangar. Take 3 line engineers off roster for a two-week input and the arithmetic above collapses — your 8-person line roster just became 5.5 against a 7.2 requirement, which means uncovered shifts, and the uncovered shift is always the one where the returning offshore AW189 lands with a chip light. The reverse raid is subtler: base engineers holding line duties between inputs look efficient on paper, but their check programme silently absorbs the disruption as slip. Borrowing works only if the plan prices it — every borrowed shift must show up as a cost against the donor side.

04What belongs in the man-hour plan

Under AMC1 145.A.30(d), your plan must show you can staff the work you commit to — and that means presenting line and base as separate budgets with an explicit crossover rule:

• Line budget: posts per shift × flying days ÷ workable days, by licence category. This is your non-negotiable floor.
• Base budget: committed inputs × package hours × findings allowance ÷ productive hours, peaked by month — not averaged across the year.
• Crossover rule: a written limit (for example, no more than one line post diverted to base, and never during peak flying weeks) so the borrowing is planned, not discovered.
• Quarterly re-run: utilisation drifts the check dates; re-run both budgets every quarter, not once a year.

The organisations that never have a staffing crisis are not the ones with the most engineers. They're the ones whose plan admits that "13 engineers" is actually two promises — 8 seats filled every day, and 4.7 FTE of man-hours banked for the hangar — and refuses to let one promise quietly pay for the other.