The 25% deviation rule: when your man-hour plan stops being a plan

Most organisations treat the AMC1 145.A.30(d) man-hour plan as a January ritual: derive the hours, size the workforce, file the PDF, done. But the AMC expects something harder — a plan that is monitored against reality and revised when the deviation grows teeth. The working threshold the industry audits against is 25%. Cross it, and the document in your MOE no longer describes your operation.

01What the rule actually asks

AMC1 145.A.30(d) has two moving parts that get confused:

• The plan itself: anticipated maintenance workload converted into man-hours, by trade and by license category, matched against available staff.
• The review: the plan must be reviewed — in practice at least every three months — and updated when actual workload or staffing deviates significantly from what was planned. The commonly applied materiality trigger, and the one auditors quote, is ±25%.

A plan that is 26% wrong is not "mostly right." It is a staffing commitment you cannot honour, signed off in your exposition.

02The workload-side deviation, in numbers

Worked example: an operator with five AW139s. The annual plan assumed 1,100 flight hours per aircraft — 5,500 FH total — converted at the fleet's historical factor of 1.5 maintenance man-hours per flight hour into roughly 8,250 scheduled man-hours per year. Q1 was planned at 1,375 FH. Here is what the quarterly review actually found:

Two aircraft individually crossed 25%, and the fleet as a whole sits at +29.5%. Translate that into manpower: 405 extra flight hours × 1.5 MH/FH = 607 unplanned man-hours in one quarter — about 1.2 additional full-time mechanics for thirteen weeks that the plan never provided.

03The staffing side deviates just as fast

Deviation runs both directions. The same quarter, the plan's assumption of six type-rated certifying staff became four: one resignation, one engineer pulled into a six-month type course. That is −33% certifying coverage — past the trigger on its own, before workload is even discussed. Watch the classic quiet drifters:

• Utilisation drift: a new charter contract or firefighting season adds hours that never get fed back into the plan.
• Findings rate: an ageing fleet's rectification hours creep from +20% to +35% over scheduled package hours.
• Authorisation erosion: headcount stays flat, but the number of people who can actually certify shrinks through expired recurrent training and leavers.

04The quarterly review that catches it early

A defensible review is a one-page exercise, not a project. Every quarter, for each line of the plan: pull actual flight hours and actual maintenance man-hours, compute the deviation per aircraft and per staff category, flag anything beyond ±25%, and either revise the plan or write down — with numbers — why the deviation is temporary. Do the same for certifying staff count versus plan. The auditor's question is never "is your plan perfect?" It is "show me where you compared it to reality, and what you did." A deviation log with three dated entries beats a beautiful plan with none.