How the Score Is Built

What This Page Is

This page describes how FlyNook turns a flight's data into a number and a set of findings, as the product does it on the date above. It is versioned: when the method changes, the version and the changelog at the foot of the page change with it. Where something is planned rather than built, it says so.

An Index, Not a Probability

The 0–100 number is a Relative Risk Index. Higher means more hazard than usual for flights like this one. It ranks; it does not measure likelihood. A 64 is not a 64% chance of anything, and FlyNook publishes no probability of its own.

Six factors — the flight's weather, its aircraft, terrain, the landing field, NOTAMs, and weight and balance — are each scored 0–100 from tables set by people, not fitted to outcomes. The index is the higher of the weighted mean of the factors that could be scored, using the published weights below, and 0.8 times the worst of them, so one severe factor is never averaged down: LIFR at one end and VFR at the other reads High, not Low. Outcome data does not yet establish the size of any weight.

Accident history near the route is shown beside the index as context, never weighted: a count is not a rate. Your pilot profile, personal minimums and fitness answers are checked as findings — a minimum exceeded sends the flight to review — and never weighted, so the index is the same for every pilot on the same flight. While a CFI reviews this change, no flight's band falls below the band the previous method gave it; the page and the record say when that holds a number.

A factor that could not be read is left out of the mean and its weight is shared among the rest — with one guard: a missing factor is never allowed to lower the index. If counting it at its not-assessed level — set above what a source that answered and found nothing scores — gives a higher index, the higher value stands.

Low0–29

No additional review required.

Elevated30–59

Review the flagged drivers before departure.

High60–79

Apply mitigations, or defer.

Review Required80–100

Requires authorisation before departure.

A band names what a school's policy asks for next. It is not a verdict, and the same cut points apply in every school and every account.

The Factors and Their Weights

Each share is that factor's weight in a fully informed index (engine v10). When a factor is missing, the others' shares grow in proportion.

Weather38.5%

METAR and TAF at each end over the departure window, and SIGMET and G-AIRMET advisories along the route.

Aircraft readiness18.5%

The aircraft you selected and its profile.

Terrain / route15.4%

The highest terrain along the route against your planned cruise altitude.

Airport / airspace12.3%

Runway length at the landing field (FAA NASR) and its elevation.

NOTAMs7.7%

NOTAMs at the departure and destination.

Weight & balance7.7%

The latest weight-and-balance calculation saved for the selected aircraft.

What Is Shown Ahead of the Number

Some findings matter more than any average, so they are shown before the index, at every screen width, and are never folded into it silently:

Critical findings

Below VFR weather minimums (14 CFR 91.155): a flight not filed IFR, with a ceiling below 1,000 ft or a visibility below 3 SM reported at either end when it leaves within three hours or no forecast covers its window, forecast for its window (by the field's TAF or, for a local flight's field with none, by NBM guidance at 50% or more or the nearest TAF), or read or forecast for its working area. A convective SIGMET in force during the flight within 20 NM of the route or the working area (AIM 7-1-27). Weight and balance outside limits (14 CFR 91.9) on the calculation on file.

Your minimums

Any personal minimum the flight exceeds.

Weather

IFR or LIFR in the current report at either end.

NOTAMs and TFRs

NOTAMs graded critical, and TFRs along the route or over the practice area.

Fuel and runway

Fuel below reserve, and a runway shorter than your minimum.

A critical finding is never averaged away: it requires review, names its rule, and holds the index at no lower than 60 (High). Until a source resolves the airspace, the stricter 1,000 ft and 3 SM apply even where Class G would allow 1 SM clear of clouds. A flight with no weight-and-balance calculation is not checked, never within limits: weight and balance is listed among the sources that could not be read.

Today these findings inform the pilot's decision; they do not lock it. A Go against a flight the engine flags for review is recorded with the pilot's reason. On a school flight, an exceeded minimum, a critical finding, a school threshold or an index in High sends the flight to an instructor's review, and an index in Review Required to the chief pilot's (a chief instructor or a school admin), whichever of the engine and the school's policy asks for more, and the school cannot release a flight whose review was rejected, or an aircraft with a grounding squawk or a maintenance hold, whoever asks. Planned, not yet built: a status that stops a flight whatever the index says, each cited to the regulation or the school rule it enforces.

When Data Is Missing

Every factor is reported as read, estimated or unavailable, and the result carries a data-completeness figure: how much of what the index depends on was actually read. A source that could not be read is shown as missing. It is never shown as clear.

A source that was not read, or whose read failed, is held at its not-assessed level and never scored as clear; it is named on the verdict, on the saved record and in the PDF.

When the weather at either end, the forecast the flight needs, the NOTAMs, the TFR check or the SIGMET and G-AIRMET advisories could not be confirmed, or the inputs have aged since the check ran, the result does not state its most favourable finding. It reads “Incomplete — verify manually” and names what was not confirmed.

Known gaps, being closed: when terrain cannot be resolved, or when no pilot profile or aircraft is set, those factors are scored at a stated default rather than marked unavailable; the weight-and-balance factor and its finding read the latest calculation saved for the aircraft, which may belong to an earlier flight, and check its weight and its forward and aft CG limits only, not the envelope's sloped edges or the landing CG; no flight rules are recorded yet, so an IFR flight is held to the VFR minimums too.

Not read yet: thunderstorms reported or forecast in a METAR or TAF (TS, CB) or seen by lightning detection, where no convective SIGMET covers them; and the weather between the two fields of a cross-country, beyond the SIGMETs and G-AIRMETs along the route; and, for a cross-country field with no TAF, its NBM guidance or the nearest TAF (its current report decides instead). None of these is a finding today: check them in an official briefing.

Where the Data Comes From

Each saved briefing lists its sources. As of the date above, they are:

NOAA Aviation Weather Center

METAR, TAF, PIREP, SIGMET and G-AIRMET, read when the check runs. US Government work, public domain.

NOTAMs

SkyLink, a commercial NOTAM service used under its own terms, and the FAA's NOTAM service where it answers. Not a sole source: read NOTAMs in an official briefing.

FAA TFR list

Temporary flight restrictions and their details from tfr.faa.gov. Public domain; the FAA describes it as informational and not a sole source.

Terrain

USGS elevation, from AWS Terrain Tiles: 3DEP (formerly NED), GMTED2010 and SRTM terrain data courtesy of the U.S. Geological Survey. Public domain in the US.

Runways and airports

FAA NASR runway data on the 28-day cycle, and an airport index built from FAA and OurAirports records. Public domain.

NTSB accident data

The NTSB aviation accident database, ingested and searched near the route. Public domain. FlyNook's summaries are not NTSB findings.

NOAA National Blend of Models

On local flights, when a recent model run is on hand, the chance of a low ceiling or visibility over a practice area, shown as issued with its model run and forecast hour. NOAA open data.

OpenSky Network

Live ADS-B traffic near the route, read only through FlyNook's licensed access and shown as context with its read time. It is not part of the index.

What you enter

Your pilot profile, aircraft, personal minimums, weight and balance, and preflight answers.

Maps are drawn with Mapbox; no map data feeds the index. FlyNook is not affiliated with, or endorsed by, the FAA, the NTSB, NOAA or the National Weather Service.

How Often It Looks Again

A check reads its sources when you run it; it is not a live feed. Route Watch re-checks watched routes once a day, at 11:10 UTC, for departures in the next 72 hours, and emails you when the result crosses your threshold.

What FlyNook Will Never Claim

These are not claimable at FlyNook's scale, or not true of the product by design. The same list drives the check that every page and email in the product is held to.

Verification Before Any Probability

What may be said is unlocked in stages, and each stage needs published evidence before its claim is made. The only probabilities shown today are National Weather Service guidance, as issued and labelled.

Next, in this order: a public verification of NWS guidance against archived observations — how often LAMP and NBM ceiling and visibility forecasts were right, by lead time and station. Only after that, a FlyNook probability that a named weather condition at your arrival time exceeds your limit, for the stations and lead times the verification covers, and nowhere else.

Later, and only when the data exists — not on a calendar:

Who Decides

FlyNook records information and decisions; it does not make them. The pilot in command is directly responsible for, and is the final authority as to, the operation of the aircraft (14 CFR 91.3). On a school flight, the school decides whether its aircraft is released under its own policy; once released, the pilot in command is the final authority.

FlyNook is not an official weather briefing, a dispatch or operational control system, or a substitute for flight instruction. Verify weather, NOTAMs, airworthiness and your minimums with official sources before every flight: read the terms

Changelog

v1.0October 6, 2026

First publication. Engine v7 (traffic is context, not weighted), bands risk-taxonomy-v1.

v1.1October 13, 2026

Critical findings (engine 2026.10.2): below VFR weather minimums, a convective SIGMET on the route and weight and balance outside limits each require review and hold the index at no lower than 60 (High). Weight and balance with no calculation is stated as not checked; the page no longer says it is shown ahead of the number.

v1.2October 13, 2026

Engine v10 (2026.10.5): a source that was not read, or whose read failed, is held at its not-assessed level, never scored as clear, and named on the verdict, the saved record and the PDF. The missing-factor guard counts a missing factor at that level, no longer at its most benign reading.

v1.3October 14, 2026

Relative Risk Index v1 (engine 2026.10.10): the index weighs six factors, the flight's conditions and aircraft; accident history is context, and the pilot's profile, minimums and fitness act through findings. One severe factor holds the index (0.8 times the worst factor). High needs an instructor's review and Review Required the chief pilot's, from the band alone; the 75-point rule is gone.