AW139 vs AW189 man-hour planning: why one plan can't crew two fleets

I teach both types, so I get this question a lot: "We added two AW189s to our four AW139s — can we just scale the man-hour plan by flight hours?" The honest answer is no. The two airframes look like siblings, fly similar offshore profiles, and share a type-course classroom for a week — but they generate different man-hour loads per flight hour, different check packages, and non-transferable certifying authorisations. A fleet-average plan hides all three.

01The three ways the types diverge

• Workload per flight hour. The AW189 is a bigger, heavier aircraft (roughly 8.3–8.6 t vs ~7 t MTOW), with more cabin, more systems and more components to inspect and service. In real offshore operations, line maintenance typically runs around 2.1–2.4 man-hours per flight hour on the AW139 versus 2.5–2.9 on the AW189. Your own CAMO reliability data should pin these down — but never assume they're equal.
• Check package size. A 600 FH / 24M-type package on an AW139 commonly lands around 1,400–1,600 man-hours; the equivalent structural and systems depth on an AW189 runs 1,800–2,200. Same interval philosophy, ~30% more content.
• Certifying authorisations. An AW139 type rating (P&WC PT6C) does not certify an AW189 (GE CT7-2E1). Cross-coverage requires a full type course or differences training plus company authorisation — a 1–2 week course at best, 4–5 weeks for a full type rating. Until then, that engineer is not AW189 capacity, whatever the roster says.

02Worked example: six-aircraft offshore fleet

Take a typical mixed offshore operator: 4 × AW139 flying 1,200 FH/year each, 2 × AW189 flying 1,400 FH/year each. Build the annual demand per type:

Now run the shortcut your spreadsheet wanted to take: fleet average of 2.38 MH/FH applied to 7,600 hours gives 18,100 line man-hours — looks fine. But apply that average to the AW189 fleet alone and you've budgeted 6,660 hours against a real need of 7,560. You've under-crewed the AW189 by ~900 man-hours a year (~12%) — before a single base check is counted.

03The certifying-staff side of the same problem

Convert those hours into people per license category, per type. At roughly 1,600 productive hours per engineer-year, 39,520 man-hours implies about 24–25 engineers overall. But the split matters: if only four of your B1s hold the AW189 rating, the AW189's ~17,000 annual hours have to flow through those four authorisations — and two of them are on rotation at any given time under offshore rosters. The moment one goes on leave or type recurrent, the AW189 line is one sick day from an AOG with nobody legal to sign. This is why the man-hour plan under AMC1 145.A.30(d) must show staff by qualification, not just headcount.

04The practical fix: plan per type, pool the slack deliberately

• Keep separate MH/FH ratios per type, refreshed quarterly from your actual work-order data — the same discipline as the quarterly deviation review.
• Track certifying coverage as a matrix: engineers × types × categories (B1/B2). Any type with fewer than three authorisations per category is a single-point failure.
• Book AW189 differences training ahead of the second aircraft's delivery, not after — course slots and OJT consolidation take months, the same pipeline problem as new hires.
• Let mechanics be pooled, but plan certifying staff per type. Shared hands, dedicated signatures.

Do the per-type arithmetic once and the mixed fleet stops being mysterious: you can see exactly which type is driving the next staffing conversation.