Spin Testing

Spin Testing (flight-test-operations/envelope/spin-testing)

Use when the task is the spin flight test: the spin test point matrix across configuration, center of gravity, weight, and altitude, the entry, incipient, and developed spin phases, the recovery control procedure and its turn count and altitude loss criteria, the recovery parachute decision, and the spin resistance verdict with pro-spin controls held at the stall. This leaf covers the spin itself and its recovery; the stall boundary and warning onset are the stall-characteristics-testing leaf, and the post-stall and deep stall envelope with departure and spin entry resistance from the high angle of attack side is the high-angle-of-attack-testing leaf.

Domain quick reference

• Spin entry procedure: from a stabilized condition at or above the entry altitude, slow to the stall, then apply the pro-spin controls (full aft stick, full rudder in the spin direction, ailerons neutral or with the rotation) and hold them until autorotation develops; the entry point is recorded at the first sustained yaw rate.
• Incipient versus developed spin: the incipient phase covers the first 1 to 2 turns while the rotation rate builds and the motion is not yet steady; the developed spin is the steady autorotation with a stabilized yaw rate, typically from about 2 turns onward.
• Spin recovery controls: the standard recovery is opposite rudder against the rotation, stick forward to break the stall, and ailerons neutral, held until rotation stops, then the resulting dive is arrested; recovery is measured from the moment the recovery controls are applied.
• Recovery criteria: a program typically requires the recovery to be complete within a set turn count (for example 2 additional turns) and a set altitude loss (for example 3000 m) from recovery control application; exceeding either limit marks the spin unrecoverable by the criterion.
• Recovery parachute requirements: the spin chute is the last-resort recovery device, required when there is no prior recovery demonstration for the configuration, when developed spin testing is planned, when the recovery check predicts an unrecoverable spin, or on the first flight of a new configuration.
• Test point matrix: points combine configuration (gear and flap setting), CG position in percent MAC, gross weight, and pressure altitude; the critical points are the aft CG, the light weight, and the high altitude cases, and points outside the approved CG envelope are flagged for envelope approval or parachute coverage before they fly.
• Spin-resistant criteria (FAR 25.201 / CS-25.201 context, paraphrased): in the stall demonstration with pro-spin controls applied at the stall, the airplane must be resistant to spinning; if autorotation develops it must stay within the allowed turn count and altitude loss and stop promptly.
• Instrumentation and data reduction: rotation rate, bank angle, angle of attack, sideslip, control positions, altitude and altitude loss, and the turn count from entry are recorded and reduced per flight; the phase classification uses the turn count and the sustained yaw rate.
• Safety and abort criteria: a minimum entry altitude floor, the parachute arm and disarm sequence, and the abort rule that the spin is terminated the moment the recovery controls fail to stop the rotation within the limits.

Workflow

1. Build the spin test point matrix with spin_test_point_matrix(configs, cg_conditions, weights_kg, altitudes_m): every combination of configuration, CG, weight, and altitude is one point, and each point carries the cg_envelope_ok flag against the approved CG envelope.
2. Classify the rotation with spin_phase_classify(turns, yaw_rate_deg_s) to label the entry, incipient, and developed phases of each spin.
3. Check the recovery with spin_recovery_check(turns_to_recover, altitude_loss_m, turns_limit, altitude_loss_limit_m) for each spin flown.
4. Decide the recovery parachute with recovery_parachute_requirement(prior_recovery_demonstrated, developed_spin_planned, unrecoverable_predicted, first_flight) before the campaign starts and before any developed spin point.
5. Judge the spin resistance with spin_resistance_check(pro_spin_turns, pro_spin_altitude_loss_m, max_allowed_turns, max_allowed_altitude_loss_m) for the pro-spin control checks at the stall.
6. Confirm the deterministic behavior with the contract test scripts/test_spin_testing.py.

Worked example

A transport test program plans spin points over configurations [clean, takeoff-flaps], CG positions [18.0, 30.0, 36.0] percent MAC, weights [18000, 21000] kg, and altitudes [3000, 6000] m, with an approved CG envelope of 15 to 35 percent MAC:

• spin_test_point_matrix builds 2 * 3 * 2 * 2 = 24 points; the 8 points at 36.0 percent MAC carry cg_envelope_ok False and need envelope approval or parachute coverage before they fly.
• spin_phase_classify(0.5, 25.0) returns "entry"; spin_phase_classify(1.5, 30.0) returns "incipient"; spin_phase_classify(2.5, 40.0) returns "developed".
• spin_recovery_check(1.5, 1800.0) against the 2 turn and 3000 m limits returns recoverable; spin_recovery_check(3.5, 1500.0) exceeds the turn limit and returns unrecoverable.
• With no prior recovery demonstration and developed spins planned, recovery_parachute_requirement returns required with two reasons, so the spin chute is fitted before the developed spin points.
• spin_resistance_check(0.8, 900.0) against the 1 turn and 1500 m limits returns resistant; spin_resistance_check(2.1, 800.0) returns not-resistant.

Pitfalls

• Flying CG points outside the approved envelope without approval: the 8 points at 36.0 percent MAC against a 15-35 percent approved envelope carry cg_envelope_ok False and need envelope approval or parachute coverage before they fly.
• Classifying the phase from the turn count alone: the phase label uses the turn count with the sustained yaw rate - (0.5, 25.0) is entry, (1.5, 30.0) incipient, and (2.5, 40.0) developed.
• Judging recoverability against the wrong criteria: the recovery check compares turns and altitude loss against the program limits (2 turns, 3000 m here), so (1.5, 1800.0) recovers while (3.5, 1500.0) exceeds the turn limit and is unrecoverable.
• Flying developed spin points before the parachute decision: with no prior recovery demonstration and developed spins planned, recovery_parachute_requirement returns required with two reasons and the spin chute is fitted first.
• Confusing spin resistance with spin recovery: resistance is judged against its own limits (1 turn, 1500 m), so (0.8, 900.0) is resistant while (2.1, 800.0), past the turn limit, is not.
• Treating the parachute as optional whenever recovery is predicted: the chute is also required on the first flight of a new configuration and when an unrecoverable spin is predicted, per the requirement logic.

Related leaves

This leaf sits in the envelope pack beside stall-characteristics-testing (the stall boundary, warning onset, and stall recovery demonstration), high-angle-of-attack-testing (the post-stall and deep stall envelope with departure and spin entry resistance), envelope-expansion (clearing the envelope that spin testing probes), and v-speeds (the speed references for the entry maneuvers).

Behavior contract (gate 3)

The matrix builder, phase classification, recovery check, parachute requirement, and spin resistance logic is exercised by the gate 3 contract test: scripts/test_spin_testing.py against scripts/spin_testing_logic.py (stdlib unittest, offline). Run:

python3 scripts/test_spin_testing.py

Compliance

• Standards referenced, not reproduced: FAR-25 is US government work (public domain) and CS-25 is a free EASA download; the spin entry, phase, recovery, parachute, and spin resistance practice is common flight test methodology, summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.