# AvPlot Known Limitations

## Professional Use Notice

All AvPlot outputs require professional engineering
review by a licensed airport engineer before use in
any FAA submittal, construction document, grant
application, or regulatory filing. AvPlot is not
affiliated with or endorsed by the Federal Aviation
Administration.

## Airport Data Source

US airport data is sourced from the FAA NASR 28-day
subscription (authoritative; every response carries
the NASR cycle_date for the citation trail). Non-US
airports, and US lookups when NASR is unavailable or
misses a facility, fall back to OurAirports, a
community-maintained database — the active source is
always reported in `data_source.name` on the
/airport-lookup response. For stamped deliverables
and submittal drawings, always verify airport and
runway data against the current FAA Chart Supplement,
5010-1 form, or NASR data directly.

## Coordinate Accuracy

**Imported threshold coordinates**
US airports import FAA NASR runway-end coordinates.
When a lookup falls back to OurAirports (non-US
airports, NASR misses/outages), threshold coordinates
are derived from public aviation chart data, not
FAA-surveyed positions — accuracy is typically
±10–20 ft, and runway lengths computed from them may
differ from published values by up to ~20 ft. The
data-source badge identifies fallback responses.
Always verify against current FAA sources before
official submittals.

**Declared distances**
US airports import NASR-published declared distances
("As published"). When not published and not manually
entered, TORA, TODA, ASDA, and LDA are estimated from
physical runway length and displaced threshold.
Verify against the FAA Chart Supplement or 5010-1 form.

**State Plane projection**
For zones without a configured EPSG code, coordinate
conversion uses linear approximation (±16 ft accuracy).
X-AvPlot-Warnings header indicates when fallback is used.

## Surface Generation

**Part 77 Transitional Surface**
Lateral extent uses conservative maximum of 1,050 ft
(assumes primary surface at airport elevation).
For runways with significant grade, actual extent
on uphill end will be less. Grade-adjusted
computation planned for future release.

**Part 77 Conical Surface**
Represented as concentric circles per runway end.
At multi-runway airports, circles may overlap.
Actual boundary is the outer envelope of all arcs.

**Part 77 Horizontal Surface**
Arc centers at runway end coordinates (end of
primary surface). Radius: 5,000 ft utility,
10,000 ft standard per 14 CFR §77.19.

**Part 77 Approach Surface (historical exports)**
DXF/KMZ files generated before 2026-06-11 (commit
d715872) show the approach surface footprint anchored
at the threshold instead of 200 ft beyond it (the
primary surface end per 14 CFR 77.19(c)/(d)) — i.e.,
200 ft too close to the runway. Regenerate any
pre-fix exports before reuse.

**OCS7 Instrument Departure Surface**
Applied only to runway ends with instrument
departure procedures. Visual runway ends use
Part 77 approach surface from departure direction.

**IT-OFZ dimensions**
Uses empirical formulas from AC 150/5300-13B §3.11.4.
Specific equation numbers not cited in code.

**APV approach classification**
APV with vis ≥ 3/4 SM and HAT ≥ 250 ft receives
NPI-level Part 77 dimensions. Case-by-case
engineering judgement may be required.

## Analysis Tools

**ATCT Line-of-Sight (/analyze/atct-los)**
Object discrimination analysis is deferred to the
FAA towervistool by design. AvPlot covers angle of
incidence, lateral discrimination, minimum cab eye
height, and obstacle shadowing per Order 6480.4C.

**Exhibit A property analysis (/analyze/exhibit-a)**
Analysis cites ARP SOP 3.00, AC 150/5300-13B, and
14 CFR Part 77 only. AC 150/5100-17 (Exhibit A
drafting standards) is now in the reference library
(added 2026-06-11) but its citations are not yet
wired into the analysis — the checklist's manual
drafting-standards items remain to be deepened.
Property-interest indications are advisory, not a
legal determination of required interest.

**Aircraft classification (/classify-aircraft)**
Uses wheelbase as a CMG proxy when CMG is not in the
database. Known minor approximation.

## Taxiway Tools

**Fillet geometry**
Circular arc geometry per FAA Fillet Tool V3.
Cross-validated for TDG-4 at delta=90°. Full
validation across all TDG/delta combinations
not yet complete. Arc absolute coordinates differ
from the FAA Excel tool's coordinate system —
expected; the arc is geometrically correct.

**Exit geometry**
Centerline geometry only. Full pavement width
and edge geometry requires additional design work.

## Known Deferred Items

| Item | Impact | Priority |
|---|---|---|
| Transitional surface grade correction | Conservative for uphill ends | Post-beta |
| Fillet full cross-validation | Not validated all TDG/delta | Pre-production |
| FastAPI on_event deprecated | Cosmetic, works until FastAPI 1.x | Low |
| Rate limits in-memory | Resets on restart | Pre-scale |
| AC 150/5100-17 citations not wired into Exhibit A (PDF now in docs/reference) | Exhibit A citation depth limited until drafting items deepened | Pre-production |