Mil Std 1797a

MIL-STD-1797A Flying Qualities Assessment (flight-mechanics/handling-qualities/mil-std-1797a)

Use when the task is grading the dynamic stability and response modes of a piloted aircraft against the MIL-STD-1797A (and its predecessor MIL-F-8785C) flying qualities level criteria: classify the flight phase category and aircraft class, check each mode against the level tables, and report the Level 1, 2, or 3 verdict plus the overall limiting level and Cooper-Harper band. This is the level-criteria leaf; mode derivative extraction and eigenvalue computation live in the stability-control leaves.

Domain quick reference

• Flight phase categories: A = nonterminal phases requiring rapid maneuvering, precision tracking, or precise flight-path control (air-to-air combat, ground attack, in-flight refueling as receiver, terrain following); B = nonterminal phases accomplished gradually without precision tracking (climb, cruise, loiter, descent); C = terminal phases (takeoff, approach, landing, wave-off).
• Aircraft classes: I = small light (utility, primary trainer); II = medium weight, medium maneuverability (small transport, tactical bomber); III = large heavy, low maneuverability (heavy transport, tanker, bomber); IV = high maneuverability (fighter, attack).
• Flying qualities levels: 1 = clearly adequate, desired performance with minimal pilot compensation; 2 = adequate with increased pilot workload or degraded mission effectiveness; 3 = safe but marginal, controllable with excessive workload. The overall level is the worst (limiting) level across modes.
• Short period (damping bands are category independent): Level 1 damping 0.35 to 1.30, Level 2 0.25 to 2.00, Level 3 minimum 0.15. Minimum frequency (rad/s) is a function of category, class, and n/alpha (g/rad), interpolated linearly between n/alpha = 1.0 and 3.0 and clamped outside: category A classes I/II/III/IV start at 3.6/3.6/3.0/2.5 and rise to 6.0; categories B and C start at 1.0 and rise to 2.0. Level 1 requires both damping and frequency; a frequency shortfall with Level 1 damping grades Level 2.
• Phugoid: Level 1 damping >= 0.04; Level 2 damping >= 0 (stable); Level 3 time to double >= 55 s (slow divergence allowed).
• Dutch roll (damping ratio, minimum frequency rad/s, minimum damping-frequency product): Level 1 category A class IV: 0.19, 1.0, 0.35; category A classes I-III: 0.19, 0.4, 0.35; categories B and C: 0.08, 0.4, 0.15. Level 2: 0.02, 0.4, 0.05. Level 3: damping > 0 (stable oscillation).
• Spiral mode minimum time to double: Level 1 20 s (A and C), 12 s (B); Level 2 8 s; Level 3 4 s.
• Roll mode maximum time constant (s): Level 1 1.0 (A), 1.4 (B), 1.0 (C); Level 2 1.4 (A), 3.0 (B), 1.4 (C); Level 3 10 (all).
• Roll performance (category A only): time to a 60 deg bank angle change: Level 1 1.3 s (classes I, II, IV) or 1.7 s (III); Level 2 1.8 s (I, II, IV) or 2.5 s (III); Level 3 3.6 s (I, II, IV) or 5.0 s (III). The first-order roll response phi(t) = p_ss * (t - tau * (1 - exp(-t / tau))) converts a steady roll rate p_ss and roll mode time constant tau into the bank angle reached in 1 s, the time to 60 deg, and the time to 90 deg.
• Cooper-Harper tie-in: Level 1 maps to ratings 1-3 (satisfactory without improvement), Level 2 to 4-6 (deficiencies warrant improvement), Level 3 to 7-9 (deficiencies require improvement); rating 10 (uncontrollable) sits outside the level framework. The pilot-assigned rating itself is collected per the cooper-harper-rating leaf.

Workflow

1. Classify the flight phase category (A, B, or C) from the mission segment and the aircraft class (I to IV) from the vehicle type.
2. Gather the measured or simulated mode metrics: short period damping and frequency (and n/alpha), phugoid damping, dutch roll damping and frequency, spiral time to double (or stable), roll mode time constant, and steady roll rate with roll mode time constant for roll performance.
3. Run each mode assessment from scripts/flying_qualities_logic.py, e.g. assess_short_period({"zeta_sp": 0.7, "omega_sp": 3.0}, "A", "IV") returns {"level": 1, "verdict": "PASS", ...}. Every assess function validates category, class, and inputs and raises ValueError on invalid category, class, or non-physical values.
4. Combine with overall_flying_qualities_level(state, category, aircraft_class): it runs all six modes, returns the overall limiting level, the limiting modes, and the Cooper-Harper band (1 to 3 for Level 1, 4 to 6 for Level 2, 7 to 9 for Level 3). Roll performance reports level None for categories B and C (the criterion applies to category A only) and is skipped by combine_levels.
5. Interpret against the requirement: Level 1 is the usual procurement and safety requirement for the operational envelope; any Level 3 mode flags a safety-marginal condition; a divergent mode (negative damping) grades Level 3 and needs design action.

Worked example

Fighter in air-to-air combat (category A, class IV) with measured mode metrics: short period zeta 0.7, omega 3.0 rad/s, n/alpha 1.0; phugoid zeta 0.05; dutch roll zeta 0.25, omega 1.5 rad/s; spiral time to double 25 s; roll mode tau 0.8 s; steady roll rate 100 deg/s with tau 0.5 s. Run:

import sys
sys.path.insert(0, "scripts")
from flying_qualities_logic import overall_flying_qualities_level
state = {"zeta_sp": 0.7, "omega_sp": 3.0, "n_over_alpha": 1.0,
         "zeta_ph": 0.05, "zeta_dr": 0.25, "omega_dr": 1.5,
         "t2_spiral": 25.0, "tau_roll": 0.8,
         "roll_rate_ss": 100.0, "roll_mode_tau": 0.5}
result = overall_flying_qualities_level(state, "A", "IV")
print(result["level"], result["limiting_modes"],
      result["cooper_harper_band"])

Every mode is Level 1: short period frequency 3.0 >= 2.5 rad/s minimum and damping 0.7 in band; dutch roll product 0.375 >= 0.35; spiral 25 s >= 20 s; roll mode 0.8 s <= 1.0 s; roll performance time to 60 deg about 1.04 s <= 1.3 s. Result: level 1, no limiting mode, Cooper-Harper band 1-3. Now degrade the spiral to 10 s: overall drops to level 2 with limiting mode "spiral" and band 4-6. A short period damping of 0.15 drops the short period verdict to level 3 and the overall level to 3.

Verification checklist

• Category, class, and level definitions match the standard's framing (A/B/C, I-IV, levels 1-3).
• Short period: damping 0.35-1.30 Level 1; frequency table by category/class with n/alpha interpolation.
• Phugoid: 0.04 Level 1 minimum damping; 55 s Level 3 time to double for divergence.
• Dutch roll: 0.19 / 1.0 / 0.35 category A class IV Level 1; product criterion enforced (a high frequency alone does not pass a low damping product).
• Spiral: 20 s (A, C) and 12 s (B) Level 1; 8 s Level 2; 4 s Level 3.
• Roll mode: 1.0 s category A Level 1 maximum.
• Roll performance: 60 deg bank criterion per class, first-order response model, category A only.
• Overall level is the worst limiting level; Cooper-Harper band reported.
• Contract test passes: python3 scripts/test_flying_qualities.py (offline, deterministic, all known-good values asserted).

Pitfalls

• Misclassifying category or class: the short-period frequency floor and the dutch roll tables are category- and class-dependent (category A class IV starts at 2.5 rad/s, category B at 1.0), so a Level 1 verdict computed for A/IV is not transferable to another category/class.
• Checking dutch roll on damping alone: the product criterion (damping times frequency) is enforced, so a high frequency does not pass a low damping product; report the limiting of damping, frequency and product.
• Applying roll performance outside category A: the time-to-60-deg criterion applies to category A only; assess_roll_performance reports level None for B and C and combine_levels skips it, so do not force a category-A roll rate check in cruise.
• Comparing spiral and roll mode in the wrong units: spiral is a minimum time to double (s) and roll mode a maximum time constant (s); they grade on separate tables and are not interchangeable thresholds.
• Reading the overall level from one mode: the overall level is the worst (limiting) level across all six modes, so a single Level 3 mode (for example divergent spiral time to double below 4 s) drops the whole assessment even when every other mode is Level 1.
• Passing invalid category, class or non-physical metrics: every assess function validates and raises ValueError on invalid category/class strings and non-physical values such as negative damping.

Behavior contract (gate 3)

scripts/test_flying_qualities.py (stdlib unittest, offline) is the correct-answer oracle: it asserts the known-good Level 1 case (short period damping 0.7, frequency 3.0 rad/s, category A, class IV), the low damping case (0.15 grades Level 3, not Level 1), the spiral Level 2 case (time to double 10 s, category A), the dutch roll product gating, the first-order roll response values (bank angle in 1 s about 56.77 deg and time to 90 deg about 1.37 s for p_ss 100 deg/s and tau 0.5 s), the overall limiting level, the Cooper-Harper band tie-in, and ValueError on invalid category, class, and non-physical inputs. Run: python3 scripts/test_flying_qualities.py.

Related skills

• cooper-harper-rating: collects the pilot-assigned rating that the level bands map to; use together for piloted evaluation results.
• dynamic-stability: computes eigenvalues and mode metrics from derivatives; feed its outputs into this leaf's assess functions.
• short-period-mode-analysis: derives short period frequency and damping from stability derivatives; this leaf grades the result.
• lateral-directional-stability: derives dutch roll, roll, and spiral mode metrics; this leaf grades them.
• pilot-induced-oscillation: PIO susceptibility is a separate pilot-in-the-loop coupling assessment, not a mode level.

Compliance

MIL-STD-1797A (Flying Qualities of Piloted Aircraft, successor to MIL-F-8785C) is referenced, not reproduced: the criteria tables in scripts/flying_qualities_logic.py are a summary paraphrase of the standard's level framework for assessment use (compliance STANDARDS-REF, reference-only: true, gated: false).