# Structural Coupling Test > Use when you must assess the structural coupling of the flight control system with the airframe structural modes from measured frequency response data: compute the gain margin from the amplitude response at the phase crossing and the phase margin from the phase response at the gain crossing, plan the frequency response testing of the closed-loop control system with swept sine, chirp, or impulse excitation across the flight envelope, and judge the margins against the typical 6 dB gain margin and 45 degree phase margin criteria of the flutter and coupling guidance. Produces the gain margin, the phase margin, the PASS or FAIL margin verdict, and the excitation test point set that gate the structural-coupling-test assessment. Trigger: structural-coupling-test, structural-coupling, gain-margin, phase-margin, frequency-response, swept sine, chirp, impulse excitation, airframe modes, flight envelope test points. - Skill: `ashfordeou/structural-coupling-test` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add ashfordeou/structural-coupling-test` - Raw SKILL.md: https://api.skillmd.com/api/skills/ashfordeou/structural-coupling-test/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - License: Apache-2.0 - Author: ashfordeOU (https://skillmd.com/u/ashfordeou) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/ashfordeou/structural-coupling-test --- # Structural Coupling Test (flight-test-operations/envelope/structural-coupling-test) Use when the task is the structural coupling test (SCT) of a flight control system: measuring the closed-loop frequency response of the control system over the band that brackets the airframe structural modes, computing the gain margin and the phase margin from the measured amplitude and phase response, and judging the margins against the typical criteria of the flutter and coupling guidance. The output is the margin verdict and the excitation sweep plan, not the flutter clearance itself or the ground vibration modal data. ## Domain quick reference - Structural coupling is the closed-loop coupling of the flight control system with the airframe structural modes: the control surface motion excites a structural mode and the sensor feedback reinforces it. The SCT demonstrates that the coupled loop stays stable with the required stability margins at every test point. - Frequency response testing: measure the amplitude and phase response of the control loop over the frequency band of interest, normally up to several times the first elastic mode frequency. Excitation methods: swept sine (slow continuous sine sweep with dwell near the resonances for coherence), chirp (fast linear or logarithmic sweep over the band), and impulse (broadband excitation with low energy density, best for on-line checks). - Gain margin: from the amplitude response at the frequency where the open-loop phase crosses -180 degrees, gain margin in dB = -(amplitude in dB at that crossing). A negative amplitude there means the gain can grow by that amount before the loop goes unstable. - Phase margin: from the phase response at the frequency where the amplitude crosses 0 dB (unity gain), phase margin in degrees = 180 + phase at that crossing. - Typical margin criteria: 6 dB gain margin and 45 degrees phase margin, per the flutter and coupling guidance; a margin at or above the criterion passes, anything below fails. Worked example: amplitude -9 dB at the phase crossing gives a 9 dB gain margin PASS, phase -135 degrees at the gain crossing gives a 45 degree phase margin PASS. - Test points across the flight envelope: cover the altitude, speed, weight, center of gravity, and configuration range, plus the flight control system states that change the loop dynamics (control law gains, feel system, gain scheduling); the critical points are the conditions with the least damped structural modes or the highest control gains. ## Workflow 1. Define the frequency band and the test points across the flight envelope: bracket the airframe structural modes of interest and cover the altitude, speed, weight, cg, and configuration range. 2. Excite the closed-loop control system with swept sine, chirp, or impulse excitation and measure the amplitude and phase response over the band; check coherence before trusting the points. 3. Find the phase crossing frequency with phase_crossing_frequency(freqs, phase_deg) and interpolate the amplitude there with interpolate_response, or get the margin directly with gain_margin_from_response(freqs, mag_db, phase_deg). 4. Find the gain crossing frequency with gain_crossing_frequency(freqs, mag_db) and interpolate the phase there, or get the margin directly with phase_margin_from_response(freqs, mag_db, phase_deg). 5. Judge the margins with margin_verdict(gain_db, phase_deg) against the default 6 dB and 45 degree criteria; PASS requires both margins at or above their criteria. 6. Plan the excitation sweep with excitation_frequencies(f_min, f_max, points_per_decade) so the sweep covers the band with enough points per decade, and repeat the margin computation at every envelope test point. ## Pitfalls - Routing flutter stability questions here: the required flutter speed from the design dive speed factor, the damping trend extrapolation, and the frequency separation check belong to flutter-testing; the SCT covers the servo-elastic closed-loop coupling with the flight control system, not the aeroelastic divergence analysis. - Routing ground vibration questions here: modal damping from the half-power bandwidth, mode candidates from FRF peaks, and shaker or impact hammer mode extraction belong to ground-vibration-testing; the SCT works from the in-flight closed-loop frequency response. - Routing generic flight test planning here: the build-up test point ordering, instrumentation coverage, and go/no-go gates belong to flight-test-planning and test-point-matrix-design. - Routing controller design margins here: gain and phase margins from a designed open-loop transfer function belong to the control design leaves in gnc-autonomy; the SCT measures the installed closed-loop response from flight test data. - Swapping the crossings: the gain margin uses the amplitude at the phase crossing (-180 degrees) and the phase margin uses the phase at the gain crossing (0 dB); mixing the two gives meaningless margins. - Extrapolating past the measured band: interpolate only inside the band, the interpolation helpers raise ValueError outside it; a missing crossing (no -180 degree phase crossing or no 0 dB gain crossing in the measured band) returns None and must be reviewed before claiming a margin. - Trusting low coherence data: a fast chirp or an impulse can give low coherence near the resonances; dwell with the swept sine or repeat the runs before computing the margins. - Checking only one flight condition: the loop margins change with altitude, speed, weight, and control gains, so the verdict must be repeated across the flight envelope test points, not taken from a single condition. ## Behavior contract (gate 3) The gain margin, phase margin, margin verdict, frequency response interpolation and crossing helpers, the response-derived margins, and the excitation frequency sweep logic is exercised by the gate 3 contract test: scripts/test_structural_coupling_test_logic.py against scripts/structural_coupling_test_logic.py (stdlib unittest, offline). Run: python3 scripts/test_structural_coupling_test_logic.py ## Compliance - Standards referenced, not reproduced: FAR-25 and CS-25 frame the stability clearance context for the aeroplane including the flight control system and the structural coupling; the 6 dB gain margin and 45 degree phase margin criteria are common certification practice in the flutter and coupling guidance, summary-only per standards-map.yaml. - compliance: STANDARDS-REF, gated: false.