Building a flight-test plan

A disciplined, step-by-step method

Whether certifying a multi-engine commercial transport or proving an experimental airframe, structured flight testing relies on a disciplined, step-by-step methodology. FAA Advisory Circular AC 90-89C, issued in February 2023, provides guidance for amateur-built aircraft, ultralights and light-sport aircraft. Its build-up approach mirrors professional flight testing, which for transport-category airplanes under 14 CFR Part 25 is described in the FAA flight test guide, AC 25-7D.

Pre-flight preparation and risk mitigation

Before an aircraft leaves the ground, extensive ground validation reduces flight-test risk. Preparation covers three areas: weight and balance verification, static system checks of the flight controls and avionics, and ground engine and taxi runs.

• System static calibration: verify flight control deflections, cable and actuator tension, fuel flow calibration, and pitot-static system leak checks.
• Mass and balance: calculate gross weight and center of gravity (CG) limits using calibrated scales. Commercial flight test programs install movable water-ballast tanks to shift the CG during flight.
• Airport selection: choose an airfield with long, wide runways, clear emergency overrun areas, low traffic density and crash-fire-rescue support.
• Test crew briefing: establish emergency egress plans, chase-plane procedures, telemetry monitoring roles and strict go/no-go and abort criteria before every test block.

Taxi tests

Testing progresses from static ground runs to high-speed taxi and then initial airborne validation. Low-speed and high-speed taxi tests evaluate steering, brake effectiveness, engine thermal behavior and directional stability. High-speed taxi runs verify control surface authority, such as elevator and rudder response, near rotation speed (VR) without lifting off.

First flight

The primary goal of the first flight is confirming airframe controllability, propulsion stability and basic systems health, not exploring performance limits.

• Leave retractable landing gear down for the initial flight.
• Remain within gliding distance of the airport at a safe altitude.
• Perform basic controllability checks, such as 10° to 20° bank turns and pitch response.

Post-flight analysis and inspection

After landing, perform a complete physical inspection for fluid leaks, heat damage or fastener movement, and review the recorded data against the engineering predictions in a full debrief.

Expanding the flight envelope

Envelope expansion systematically moves the aircraft from known-safe operating points toward its design limits in small, incremental steps. Testing starts at low altitude, conservative weight and a forward CG. Airspeed and altitude are then increased incrementally, followed by moving the CG aft and increasing weight toward maximum takeoff weight, until the aircraft has been demonstrated at high speed (VDF/MDF), maximum altitude and heavy weight.

Key envelope expansion test phases

• Flutter and aeroservoelasticity: impulse inputs, such as control pulses and stick raps or dedicated flutter exciters, check structural damping at each speed increment up to VDF/MDF, the demonstrated flight diving speed and Mach number.
• Stall and low-speed handling: characterize stall warning speed (VSW) and stall speed (VS) across flap and slat configurations, power settings and sideslip angles.
• Performance and systems validation: measure climb rates, cruise fuel burn, takeoff and landing field lengths, the engine relight envelope and environmental control system (ECS) performance.

Phase I and Phase II

Under the experimental certification rules (14 CFR 21.191), an aircraft must complete a Phase I flight test period in an assigned flight test area, proving its systems are reliable and developing its flight envelope, before entering Phase II, the normal operating phase where it usually stays for the rest of its life.

Flight-hour method vs. task-based option

AC 90-89C introduced a task-based alternative to the traditional flight-hour method:

• Requirements: the traditional method requires at least 25 hours with a type-certificated engine and propeller combination, and at least 40 hours with a non-certificated or altered combination. The task-based option uses an individualized test plan with predefined task completion criteria in a matrix.
• Flexibility: the traditional method is a fixed number of hours regardless of progress. With the task-based option, the time needed depends on completing all test objectives safely, which may take fewer hours depending on the aircraft's complexity.
• Documentation: the traditional method ends with a logbook sign-off when the hours are complete. The task-based option is allowed through an operating limitation and requires a specified logbook entry, a flight test report, the completed task matrix and an aircraft operating handbook (AOH) kept on board.

Major modifications and re-testing

Whenever a major modification is embodied, such as an engine swap, propeller change, winglet installation or major avionics overhaul:

• Re-evaluate airworthiness: update weight and balance records, equipment lists and electrical load analyses.
• Re-test the change: execute a targeted flight-test plan focused on the modified system or envelope. For an experimental aircraft, this usually means returning to Phase I flight testing as its operating limitations require.
• Update operating documentation: amend the airplane flight manual (AFM), pilot's operating handbook (POH) or aircraft operating handbook (AOH) before returning to normal service.

Key takeaways

• Targeted objectives: every test flight must have clear, documented tasks and pre-briefed abort criteria.
• Incremental expansion: always move from known-safe conditions toward the envelope boundaries of speed, weight and CG in small, deliberate steps.
• AC 90-89C task-based option: allows Phase I to be completed by fulfilling a structured task plan rather than simply flying off 25 or 40 hours.
• Modifications demand re-testing: major structural or propulsion changes require a return to flight testing to re-validate safety and update the manual's limits.