# Bird Strike > Use when the task is bird strike impact analysis and certification of transport aeroplane structure per FAR 25.631 and CS-25: estimate the impact kinetic energy of a 4 pound or 8 pound bird at cruise velocity with impact_energy, convert the bird mass to kilograms with bird_mass_kg, compute the specific energy with specific_energy, and rate the strike against the component damage threshold with damage_severity_ratio, penetration_verdict, and residual_strength_fraction for the residual strength after impact. Covers leading edge, windshield, radome, and engine inlet damage modes, energy absorption, soft-body impact behavior, and test versus analysis compliance. Trigger: bird strike, birdstrike, bird-strike, soft body impact, soft-body-impact, impact energy, impact-energy, leading edge, leading-edge, 4 pound bird, 8 pound bird, FAR 25.631, windshield, radome, engine inlet. - Skill: `ashfordeou/bird-strike` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add ashfordeou/bird-strike` - Raw SKILL.md: https://api.skillmd.com/api/skills/ashfordeou/bird-strike/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Web & Frontend - License: Apache-2.0 - Author: ashfordeOU (https://skillmd.com/u/ashfordeou) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/ashfordeou/bird-strike --- # Bird Strike (structures/damage-tolerance/bird-strike) Use when the task is bird strike impact analysis for damage-tolerant structure: impact kinetic energy, soft-body impact damage severity, and residual strength after impact. ## Domain quick reference - The impact kinetic energy E = 0.5 * m * v^2 is the governing parameter for soft-body impact damage; m is the bird mass and v is the velocity of the aeroplane relative to the bird along the flight path. - Bird mass classes: the 4 pound bird (about 1.81 kg) is the FAR 25.631 empennage impact case; the 8 pound bird (about 3.63 kg) is the heavier class used for windshield, radome, and engine inlet assessments. Convert with bird_mass_kg. - Impact velocity: cruise speed of the aeroplane relative to the bird, about 250 m/s for a transport at Mach 0.8 cruise. Anchors at 250 m/s: the 4 pound bird gives 56.6 kJ, the 8 pound bird gives 113.4 kJ (twice the energy, mass enters linearly), and the specific energy is 31.25 kJ/kg. - Soft-body impact behavior: the bird deforms on impact, so the struck component sees an impact pulse rather than a rigid-body collision; a softer, more compliant target stretches the pulse and lowers the peak pressure, and the kinetic energy governs the damage extent. - Damage modes by component: leading edge skin and substructure denting, tearing, and penetration; windshield glazing fracture and crew protection; radome skin fracture and radar degradation; engine inlet lip damage and fan blade ingestion risk. - Energy absorption: stringers, frames, and crush zones absorb part of the strike energy by deformation, so leading edge damage resistance scales with skin gauge, stringer pitch, and absorbed energy; the residual strength fraction after impact is rated by comparing the strike energy with the component damage threshold. - Certification compliance is normally test-based (a bird gun fires a gel bird at representative components) with analysis used to show equivalence for other masses, velocities, and impact positions. The threshold comparison below is an analysis aid, not a test substitute. - Units (single convention, consistent across the logic module): mass in kg, velocity in m/s, energy and threshold in joules. Anchor: an 8 pound bird at 250 m/s against a 60 kJ leading edge threshold gives severity ratio 1.89 (penetration) and residual strength fraction about 0.05. ## Workflow 1. Identify the certification case: the component (leading edge, windshield, radome, or engine inlet) and the bird class (4 or 8 pound). 2. Convert the bird mass with bird_mass_kg; take the impact velocity as the aeroplane cruise speed relative to the bird. 3. Compute the impact kinetic energy with impact_energy and the specific energy with specific_energy. 4. Compare the strike energy with the component damage threshold with damage_severity_ratio and penetration_verdict. 5. Estimate the residual strength after impact with residual_strength_fraction; feed the result into the continued safe flight and landing assessment. 6. Position the result in the compliance argument: test evidence for the critical case, analysis for the remaining conditions. ## Pitfalls - Using the aeroplane ground speed instead of the velocity relative to the bird along the flight path. - Mixing units: a mass in pounds with a velocity in m/s shifts the energy by the kg per pound factor; keep kg and m/s. - Dropping the 0.5 factor in 0.5 * m * v^2. - Sizing the component to the 4 pound bird when the strike case is the 8 pound windshield, radome, or engine inlet case. - Using momentum instead of kinetic energy: energy scales with v^2 and governs penetration damage, momentum does not. - Treating the threshold comparison as certification evidence: the severity and residual strength models are analysis aids, the compliance basis is test plus similarity. - Routing event probability questions here: bird strike probability per flight and hazard classification belong to particular-risk-analysis (ARP4761A). - Routing fatigue crack questions here: crack growth and Paris law belong to crack-growth, residual strength after a crack belongs to residual-strength, and rainflow counting of the load spectrum belongs to load-spectrum-counting. ## Behavior contract (gate 3) The impact energy, severity, and residual strength logic is exercised by the gate 3 contract test: scripts/test_bird_strike.py against scripts/bird_strike_logic.py (stdlib unittest, offline). Run: python3 scripts/test_bird_strike.py ## Compliance - Standards referenced, not reproduced: FAR-25 is US government work (public domain) and CS-25 is a free EASA download; bird strike methodology is common soft-body impact knowledge, summary-only per standards-map.yaml. - compliance: STANDARDS-REF, gated: false.