Two heavy checks land in the same week: the manpower math nobody does

Every maintenance planner has lived this week: the hangar schedule says both aircraft fit, the customer delivery dates are committed — and by Wednesday you realize the plan was never about hangar space. It was about the three certifying staff who now need to be in two places at once. Here is the arithmetic that should have happened in January, not in the crisis meeting.

01Why the hangar schedule lies to you

Hangar slot planning answers one question: does the aircraft physically fit? Two bays, two helicopters, no conflict. But check packages don't consume floor space — they consume licensed man-hours across specific license categories, and those are not interchangeable:

• A B1 (mechanical) engineer cannot certify avionics tasks, and a B2 cannot certify the airframe.
• Only a fraction of your workforce holds certification authorisation on the specific type — support staff can turn wrenches, but every task card still needs a certifying signature.
• The critical path through a check — inspection findings, rectification, re-inspection — is gated by certifying staff availability at each gate, not by total headcount.

02The overlap arithmetic

Worked example: an AW139 600 FH/24M check and an AW169 600 FH check scheduled the same week. Each package is roughly 1,500 man-hours of scheduled work:

Total mechanics look short by two. But the binding constraint is worse: certifying coverage is short by 20%, and it cannot be borrowed across types. The B1 on the AW139 cannot sign for the AW169 no matter how light his day is.

03Why it surprises you every year

• Checks drift. Utilisation varies, so two checks planned four months apart migrate toward each other all year. Nobody re-runs the manpower check when the dates move — only the hangar chart gets updated.
• Findings are unplannable but statistically predictable. A 1,500-hour package routinely grows 20–40% with rectification. The overlap plan that "just fits" at scheduled hours fails at actual hours.
• Leave and training don't stop. The week two checks overlap is statistically certain to also contain someone's approved vacation.

04What your man-hour plan should have caught

This is exactly what AMC1 145.A.30(d) planning is for — but only if the plan models peaks, not averages. An annual average man-hour plan says "we have enough staff this year." A schedulable plan asks: in the worst two-week window of the year, do certifying hours by category cover the committed input? Run the overlap scenario before accepting the second delivery date — that conversation in January is free; the same conversation in the hangar costs a slip.