ashfordeOU Flight Test Instrumentation (flight-test-operations/planning/flight-test-instrumentation)
Use when the task is designing the flight test instrumentation chain: choosing sensors and their ranges for the measurement parameters, sizing the data acquisition sample rate against the Nyquist and anti-aliasing limits, checking the ADC resolution, and confirming recording, telemetry, calibration, and uncertainty before the test.
Domain quick reference
- Measurement parameters: air data (static and pitot pressure, airspeed, angle of attack and sideslip), accelerations (load factors in g), angular rates (deg/s), strain (microstrain or N via calibrated bridges), control positions (deg), engine data (rpm, fuel flow, pressures, temperatures). Every parameter needs a sensor with a defined range, accuracy, and bandwidth.
- Sensor selection: the sensor range must cover the full expected signal plus margin, the accuracy must be better than the measurement requirement, and the bandwidth must extend beyond the highest frequency of interest. Units stay SI (Hz, V, g, N).
- Nyquist criterion: a signal of maximum frequency fmax can be reconstructed from samples only when the sample rate fs satisfies fs >= 2 * fmax. Below the Nyquist rate, aliasing folds high frequencies into the passband.
- Required sample rate: practical chains sample above Nyquist to leave headroom for the anti-aliasing filter rolloff: fs_req = margin * 2 * fmax, default margin 2.5 (5 times fmax).
- ADC quantization: an N-bit converter over a full-scale range R resolves steps of R / 2^N (1 LSB); the worst-case quantization error is half the step.
- Anti-aliasing: an analog low-pass filter ahead of the sampler must attenuate everything above half the sample rate; sample rate and filter cut-off are chosen together, never separately.
- Calibration: every channel is calibrated before the test with traceable standards; a channel is usable only when it is calibrated and recalibration is not due.
- Measurement uncertainty: the combined standard uncertainty is the root-sum-square of the sensor, conditioning, ADC, and calibration components; the expanded uncertainty uses the coverage factor.
Workflow
- List the measurement parameters for the test and their expected ranges, accuracies, and maximum frequencies (SI units).
- Select each sensor and check the range with sensor_range_verdict(measured_value, sensor_range); re-select when the value is over-range.
- Size the acquisition chain: compute the required rate with required_sample_rate(max_freq, margin) and confirm it with nyquist_ok(sample_rate, max_freq).
- Choose the ADC width and check the resolution with quantization_error(bits, range); the step must be small enough for the accuracy requirement.
- Confirm the recording and telemetry path covers every channel at the chosen rate, with time correlation across channels.
- Verify pre-test calibration with calibration_verdict(calibrated, due); release the channel only on a True verdict.
- Estimate the measurement uncertainty from the component errors and record it with the data.
Pitfalls
- Sampling below the Nyquist rate: aliased data is unrecoverable, no post-processing can remove it.
- Over-range sensors clip: an over-range value is not a measurement, it is a saturated channel.
- Choosing the ADC without the range: the resolution depends on both the bit count and the full-scale range.
- Sampling exactly at Nyquist: the criterion is a lower bound, not a target; real chains add margin for the anti-alias filter rolloff.
- Anti-aliasing without a filter, or a filter without the rate: the two are sized together.
- Flying on a channel whose calibration is due: the data cannot be traced to standards and the point is invalid.
- Mixing units: g, N, and V are not interchangeable without the documented sensitivity of each channel.
- Reporting uncertainty from one component only: the combined uncertainty sums all contributions in quadrature.
Behavior contract (gate 3)
The Nyquist check, sensor range verdict, ADC quantization resolution, required sample rate, and calibration verdict logic is exercised by the gate 3 contract test: scripts/test_flight_test_instrumentation.py against scripts/flight_test_instrumentation_logic.py (stdlib unittest, offline). Run: python3 scripts/test_flight_test_instrumentation.py
Compliance
- Standards referenced, not reproduced: FAR-25 and CS-25 set the flight test and certification context; instrumentation design practice (Nyquist sampling, sensor selection, quantization, calibration, uncertainty) is common measurement methodology, summary-only per standards-map.yaml.
- compliance: STANDARDS-REF, gated: false.