Calculations

How to Calculate Buy-to-Fly, Inspection Burden, and Yield in Aerospace Manufacturing

Work through the core aerospace and defense manufacturing formulas with real units and numbers, from buy-to-fly ratio to inspection burden and disposition time.

Start with the buy-to-fly ratio, the single number that governs machined aerostructure economics. It equals purchased stock weight divided by finished part weight. A titanium bulkhead that ships at 18 lb but starts from a 142 lb Ti-6Al-4V forged block gives 142 / 18 = 7.9:1. For 5-axis machined 7075-T7351 aluminum ribs the ratio typically lands between 3:1 and 8:1; open-die titanium forgings routinely hit 12:1 to 20:1. The Buy-to-Fly Ratio calculator takes stock and finished mass in the same unit, so convert everything to kilograms or pounds first before dividing, or the ratio is meaningless.

Convert that ratio into removed material to plan chip volume and cycle time. Removed mass equals stock mass minus finished mass, so 142 - 18 = 124 lb of titanium becoming chips. At a titanium metal removal rate near 3 cubic inches per minute on a rigid 5-axis machine and a Ti-6Al-4V density of 0.16 lb per cubic inch, 124 lb is 775 cubic inches, giving roughly 258 minutes, about 4.3 hours of pure cutting before you add tool changes, probing, and setup. That single derived number drives both your machine-hour estimate and your scrap-value recovery on returned chips.

Next compute inspection burden, which is inspection labor as a share of total touch labor. The formula is inspection hours divided by total production hours, times 100. A part with 6.5 inspection hours against 34 production hours carries 6.5 / 34 = 19.1 percent burden. Aerospace machined detail parts commonly run 12 to 25 percent; flight-critical fracture-critical hardware can exceed 40 percent. The Aerospace Inspection Burden calculator lets you split first article, in-process, and final inspection so you can see which stage dominates. Track dimensional characteristics inspected per hour, typically 8 to 15 for CMM work, to sanity check the hours.

First article inspection load scales with characteristic count, not part size. Estimate FAI hours as balloon count times minutes per characteristic, divided by 60. An AS9102 package with 240 ballooned characteristics at 4.5 minutes each is 240 x 4.5 / 60 = 18 hours for measurement alone, before you add form documentation and the FAIR write-up, which typically adds 30 to 50 percent. The First Article Inspection Load calculator turns your drawing balloon count into planned hours. A useful rule: budget 0.06 to 0.09 FAI hours per ballooned characteristic including paperwork on a first-time part.

Traceability cost per lot is a fixed administrative charge spread across the units in a lot, so the per-unit figure is entirely lot-size driven. Divide total lot documentation cost by units per lot. If material certs, heat-lot records, serialization, and the C of C package cost 380 dollars of labor and system time per lot, a lot of 25 units carries 380 / 25 = 15.20 dollars per unit, while a lot of 4 units carries 95 dollars per unit. The Traceability Cost per Lot calculator makes this visible; the lesson is that small defense lots amortize fixed paperwork over almost nothing.

Nonconformance disposition time is throughput planning, not just quality overhead. Average disposition time equals total disposition hours divided by number of nonconformances closed. If your MRB closed 46 NCRs last month using 138 engineering and quality hours, that is 138 / 46 = 3.0 hours per NCR. Multiply by expected NCR arrival rate to size the queue: 46 NCRs per month at 3.0 hours is 138 hours, roughly one full-time disposition engineer. The Nonconformance Disposition Time calculator helps you convert NCR volume into staffing so material does not sit in the MRB cage for weeks.

Rework hour capacity closes the loop between defect rate and available skilled labor. Required rework hours equal units produced times defect rate times average rework hours per defect. A run of 500 units at a 4 percent rework rate and 2.2 hours per reworked unit needs 500 x 0.04 x 2.2 = 44 hours. Compare that against available rework capacity, say two technicians at 150 productive hours each, or 300 hours, and you have a 15 percent utilization on rework alone. The Aerospace Rework Hour Capacity calculator sizes this so a defect spike does not silently blow your delivery date.

Published 2026-07-01.