Airports · runway debris, hold lines, service roads

Runway FOD watch

Simulated

What a line of units beside a runway can see of foreign object debris, and where to put them so it is worth it.

Everything in Demo is one synthetic scenario. Nothing in it is a measurement. A magnetometer sees ferrous objects only, and a dropped one only as a step in the field: within about a metre of a unit for a hand tool, a few metres for large steel. The FAA AC 150/5220-24 benchmark (9 of 10 test objects found anywhere on the runway, each within 5 m) is not met at any unit count.

What works: vehicle crossings at a watched entry with direction and dwell, large ferrous debris left within a few metres of a unit, and which side of the pavement edge a step is on from an inner/outer pair. It does not identify the object and does not see people.

Runway and hotspotsthe demo runway; edit freely, it replans

Runway presetRunway nameThreshold latitude (°)Threshold longitude (°)True heading (°)Length (m)Width (m)

Where ferrous traffic enters the pavement

LabelKindAlong (m)Edge
Traffic weight 0.70
LabelKindAlong (m)Edge
Traffic weight 1.00
LabelKindAlong (m)Edge
Traffic weight 0.90

Units available

Plan7 units · 3 hotspots

ObjectThresholdView

● inner unit (pavement edge)● outer unit● site reference◆ hotspot

Steel tool box with tools, 5 kg: a step above 10 nT within 2.1 m of a unit (best-case moment, best-case geometry). Outside the discs it is not seen.

Where the units goheaviest traffic first; a reference from three units up

Hotspot	Kind	Along m	Edge	Weight	Watched by	Stations
Service road crossing	Service-road crossing	900	left	1.00	Pair (side + direction)	svc-rd-in, svc-rd-out
Construction entry	Construction entry	1400	right	0.90	Pair (side + direction)	constr-in, constr-out
Taxiway A hold-short	Hold-short line (taxiway entry)	400	right	0.70	Pair (side + direction)	twy-a-in, twy-a-out

Reference 40 m off the quieter edge at 0 m along, the point farthest from every hotspot: what it sees is the site, and is subtracted from every other unit.

Units at the pavement edge sit inside the runway safety area: the installation must be flush or frangible (FAA AC 150/5300-13), which is a civil-works question, not a sensing one.
A magnetometer has no beam. Orientation does not point it at the runway; the inner/outer pair's amplitude ratio is what tells the runway side from the outside.

What this placement sees

FAA test set: 3 of 9 objects are ferrous enough to register at all, each within 0.67 m of a unit; the best of them is seen on under 0.05% of the 30 m wide pavement. The circular asks for 9 of 10 anywhere on it. Not met, at any unit count.
Large ferrous debris (Steel tool box with tools 2.1 m; Ground-equipment steel part 3.1 m; Aircraft tow bar 4.9 m; Baggage cart 7.4 m): a strip beside each unit, 0.35% of the pavement at best.
Vehicles and equipment (car 18 m, fuel 31 m): every crossing at a watched entry is logged with direction and dwell. This is the layer that works.

What each object is seen onrange = best-case moment; % = share of the pavement inside some unit's disc

Object	Group	Material	10 nT: range m · pavement		25 nT: range m · pavement		70 nT: range m · pavement		Verdict
FAA metal cylinder, 1.2 × 1.5 in, if steel	FAA test	steel	0.60	0%	0.44	0%	0	0%	Under 1 m of a unit
FAA metal cylinder, 1.2 × 1.5 in, if aluminium	FAA test	aluminium	0	0%	0	0%	0	0%	Invisible (not ferrous)
FAA 8 in adjustable wrench	FAA test	steel	0.67	0%	0.48	0%	0	0%	Under 1 m of a unit
FAA wheel lug nut	FAA test	steel	0	0%	0	0%	0	0%	Has to touch the unit
FAA 8 in metal strip	FAA test	mixed	0.41	0%	0	0%	0	0%	Under 1 m of a unit
FAA 4.3 cm sphere (white, grey or black)	FAA test	mixed	0	0%	0	0%	0	0%	Invisible (not ferrous)
FAA aircraft tyre rubber piece	FAA test	rubber	0	0%	0	0%	0	0%	Invisible (not ferrous)
FAA chunk of asphalt or concrete	FAA test	concrete	0	0%	0	0%	0	0%	Invisible (not ferrous)
FAA runway light fixture part	FAA test	mixed	0	0%	0	0%	0	0%	Has to touch the unit
Aircraft fastener (titanium or Inconel bolt)	Common find	titanium	0	0%	0	0%	0	0%	Invisible (not ferrous)
Steel bolt or nut, 20 g	Common find	steel	0	0%	0	0%	0	0%	Has to touch the unit
Hand tool (pliers, 300 g)	Common find	steel	0.60	0%	0.44	0%	0	0%	Under 1 m of a unit
Steel tool box with tools, 5 kg	Large ferrous	steel	2.1	<0.05%	1.6	<0.05%	1.1	0%	A strip beside each unit
Ground-equipment steel part, 15 kg	Large ferrous	steel	3.1	<0.05%	2.3	<0.05%	1.6	<0.05%	A strip beside each unit
Aircraft tow bar, 60 kg	Large ferrous	steel	4.9	0.13%	3.6	0.08%	2.6	<0.05%	A strip beside each unit
Baggage cart, 200 kg	Large ferrous	steel	7.4	0.35%	5.4	0.17%	3.8	0.09%	A strip beside each unit
Car or pickup (moving or parked)	Vehicle	steel	18	2.4%	13	1.2%	9.5	0.58%	Covers the width
Fuel truck or sweeper	Vehicle	steel	31	8.9%	23	5.0%	16	2.5%	Covers the width

Exportsynthetic data, labelled as such inside each file

For integrators: event and export schemathe event contract, and the JSON export that wraps the plan, coverage and eventsShow

nextmag.fod-watch-event/1

schema: : nextmag.fod-watch-event/1.
id, source, unitId, pairId: : event id; measured or synthetic (every consumer must show it); the unit that saw the step and its pair, if any.
tOnset, tOnsetUnix, tDetected, tClearedUnix: : when the level first moved (ISO-8601 UTC and unix s), when it had stayed shifted long enough to flag, and when it returned (null: still there).
stepNt, axisStepNt, referenced: : the total-field step after the site reference is subtracted, the largest per-axis step when the unit reports components, and whether a reference was subtracted.
partnerStepNt, side, distanceM, distanceIsBound: : the pair partner's step at the same time; runway, outside, between or unknown from the two steps' ratio; the distance from the nearer unit it implies, an upper bound when the partner saw nothing.
impliedMomentAm2, candidateIds: : the dipole moment at that distance (axial best case, a lower bound) and the catalogue objects that could make the step there; never an identification.
driftRatio, confidence: : the reference window's own drift over the step (above about 0.5 the step may be the site) and 0 to 1.

nextmag.fod-watch/1

schema, generatedAt, source: : nextmag.fod-watch/1, when the file was made, measured or synthetic.
runway: : name, threshold latitude/longitude, true heading, length and width in metres.
stations: : id, label, role (inner, outer, reference), pairId, alongM from the threshold, offsetM from the centreline (positive right of the heading), serial; never a network address.
plan: : per hotspot the pair, single unit or none that watches it; where the reference sits and why; the planner's notes.
coverage: : per catalogue object and threshold: detection range m, pavement area m² and fraction inside some unit's range.
events: : every nextmag.fod-watch-event/1 above.