Landing Distance Determination (flight-test-operations/performance/landing-distance-determination)
Use when the task is landing distance determination for a flight
test: approach speed Vref, the airborne flare segment, the braking
ground roll, the 1.67 certification field length factor, and the
runway fits verdict.
Domain quick reference
- Speeds in m/s, time in s, distances in m, deceleration in m/s^2.
- Vref = factor * vs1g is the reference approach speed, with factor
defaulting to 1.23 per FAR 25.125 landing approach practice
(configurable, e.g. 1.3 for a higher margin).
- Airborne flare segment: s_air = Vref * t_air, from the 15.24 m
(50 ft) threshold crossing to touchdown; t_air defaults to 5 s.
- Braking ground roll: s_ground = Vref^2 / (2 * a_brake), with
a_brake the braking deceleration magnitude; a_brake = mu * g from
the braking friction coefficient mu and g = 9.80665 m/s^2.
- Demonstrated landing distance: s_demo = s_air + s_ground.
- Certification field length: s_field = 1.67 * s_demo by default,
the multiplier applied to the demonstrated distance per FAR 25.125
landing field length practice.
- Runway verdict: margin = runway_m - required_m; verdict is fits
when margin >= 0, else too short.
- Analytic check: vs1g = 50 m/s gives Vref = 61.5 m/s; t_air = 5 s
gives s_air = 307.5 m; mu = 0.45 gives a_brake = 4.4130 m/s^2 and
s_ground = 428.536 m; total = 736.036 m; the 1.67 factor gives a
certified field length of 1229.180 m (3 dp).
Workflow
- Collect the reference stall speed vs1g, the flare time, the
braking friction coefficient (or the deceleration directly), and
the available runway length.
- Derive the approach speed Vref with approach_speed.
- Compute the airborne flare segment with airborne_distance and the
braking ground roll with ground_roll.
- Combine both legs with total_landing_distance.
- Apply the certification factor with certified_field_length.
- Check the certified field length against the available runway
with runway_verdict and gate the landing on the verdict.
Pitfalls
- Using the stall speed instead of Vref for the ground roll: the
ground roll starts at the touchdown speed, which is the approach
speed Vref in this model.
- Counting the flare leg twice or not at all: the demonstrated
distance is the airborne leg plus the ground roll, not either
alone.
- Confusing demonstrated and certified distances: the 1.67 factor
applies to the demonstrated distance, and the certified field
length is the larger number that must fit the runway.
- Mixing units: m/s with m/s^2 gives distances in m; knots or ft/s
need conversion first.
- Reading a negative margin as an error: runway_verdict returns too
short, which is a real, reportable landing outcome.
- Applying the 1.23 approach factor to the wrong base: the factor
scales the 1g reference stall speed in the landing configuration
context, not a takeoff configuration stall speed.
Behavior contract (gate 3)
The landing distance determination logic is exercised by the gate 3
contract test: scripts/test_landing_distance.py against
scripts/landing_distance_logic.py (stdlib unittest, offline). Run:
python3 scripts/test_landing_distance.py
Compliance
- Standards referenced, not reproduced: FAR-25 (14 CFR Part 25) is
US government work (public domain) and CS-25 is a free EASA
download; the landing distance method with the 1.23 approach
factor and the 1.67 field length factor is common flight-test
methodology in the FAR 25.125 context, summary-only per
standards-map.yaml.
- compliance: STANDARDS-REF, gated: false.
1---2name: landing-distance-determination3description: Use when you must determine the landing distance for a flight test: derive the approach speed Vref from the reference stall speed with the 1.23 factor, add the airborne flare segment and the braking ground roll, apply the 1.67 certification field length factor, and check the result against the available runway length. Produces the airborne and ground roll legs, the total demonstrated landing distance, the certified field length, and the runway fits verdict that gate the FAR 25.125 landing performance assessment. Trigger: landing distance, Vref approach speed, flare distance, braking ground roll, field length factor, runway length.4license: Apache-2.05---67# Landing Distance Determination (flight-test-operations/performance/landing-distance-determination)89Use when the task is landing distance determination for a flight10test: approach speed Vref, the airborne flare segment, the braking11ground roll, the 1.67 certification field length factor, and the12runway fits verdict.1314## Domain quick reference1516- Speeds in m/s, time in s, distances in m, deceleration in m/s^2.17- Vref = factor * vs1g is the reference approach speed, with factor18 defaulting to 1.23 per FAR 25.125 landing approach practice19 (configurable, e.g. 1.3 for a higher margin).20- Airborne flare segment: s_air = Vref * t_air, from the 15.24 m21 (50 ft) threshold crossing to touchdown; t_air defaults to 5 s.22- Braking ground roll: s_ground = Vref^2 / (2 * a_brake), with23 a_brake the braking deceleration magnitude; a_brake = mu * g from24 the braking friction coefficient mu and g = 9.80665 m/s^2.25- Demonstrated landing distance: s_demo = s_air + s_ground.26- Certification field length: s_field = 1.67 * s_demo by default,27 the multiplier applied to the demonstrated distance per FAR 25.12528 landing field length practice.29- Runway verdict: margin = runway_m - required_m; verdict is fits30 when margin >= 0, else too short.31- Analytic check: vs1g = 50 m/s gives Vref = 61.5 m/s; t_air = 5 s32 gives s_air = 307.5 m; mu = 0.45 gives a_brake = 4.4130 m/s^2 and33 s_ground = 428.536 m; total = 736.036 m; the 1.67 factor gives a34 certified field length of 1229.180 m (3 dp).3536## Workflow37381. Collect the reference stall speed vs1g, the flare time, the39 braking friction coefficient (or the deceleration directly), and40 the available runway length.412. Derive the approach speed Vref with approach_speed.423. Compute the airborne flare segment with airborne_distance and the43 braking ground roll with ground_roll.444. Combine both legs with total_landing_distance.455. Apply the certification factor with certified_field_length.466. Check the certified field length against the available runway47 with runway_verdict and gate the landing on the verdict.4849## Pitfalls5051- Using the stall speed instead of Vref for the ground roll: the52 ground roll starts at the touchdown speed, which is the approach53 speed Vref in this model.54- Counting the flare leg twice or not at all: the demonstrated55 distance is the airborne leg plus the ground roll, not either56 alone.57- Confusing demonstrated and certified distances: the 1.67 factor58 applies to the demonstrated distance, and the certified field59 length is the larger number that must fit the runway.60- Mixing units: m/s with m/s^2 gives distances in m; knots or ft/s61 need conversion first.62- Reading a negative margin as an error: runway_verdict returns too63 short, which is a real, reportable landing outcome.64- Applying the 1.23 approach factor to the wrong base: the factor65 scales the 1g reference stall speed in the landing configuration66 context, not a takeoff configuration stall speed.6768## Behavior contract (gate 3)6970The landing distance determination logic is exercised by the gate 371contract test: scripts/test_landing_distance.py against72scripts/landing_distance_logic.py (stdlib unittest, offline). Run:73`python3 scripts/test_landing_distance.py`7475## Compliance7677- Standards referenced, not reproduced: FAR-25 (14 CFR Part 25) is78 US government work (public domain) and CS-25 is a free EASA79 download; the landing distance method with the 1.23 approach80 factor and the 1.67 field length factor is common flight-test81 methodology in the FAR 25.125 context, summary-only per82 standards-map.yaml.83- compliance: STANDARDS-REF, gated: false.
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