NOTAM filtering by leg and time window — cutting noise without missing what matters.

Practical filtering methodology. Not a substitute for a careful pre-flight NOTAM read; the filters are an aid to focus, not a license to skim. Always cross-check the published source for any NOTAM your filtering is about to drop.

A typical European NOTAM dump for a 200-NM cross-country trip can return 80–150 entries. Of those, the ones that actually change your decision are usually fewer than ten. The other 70 to 140 are either irrelevant to your altitude, irrelevant to your time, irrelevant to your operation, or already-resolved items that haven’t yet been cancelled. The art of NOTAM reading at scale is filtering well, fast, without dropping the one entry that matters.

The NOTAM signal-to-noise problem

NOTAMs were designed for an era of teletype and a workforce of dispatchers paid to read them. They have not aged well. Each NOTAM is a structured piece of information, but the structure is dense, the format is formal, and the volume scales with airspace complexity. A summer-Saturday Madrid–Palma de Mallorca route at 6,000 feet might cross 30 different NOTAM-issuing FIR/aerodrome combinations, each with a backlog of standing items.

The signal: items that, if you don’t act on them, will affect your safety or compliance. The noise: items that don’t apply to your altitude, your time of flight, your operation type, or your specific equipment.

The framing of the problem matters. We do not want a machine that decides which NOTAMs are unimportant — that’s the dispatcher’s judgment, and it’s regulatorially yours. We want a machine that organises the NOTAMs so the items most likely to matter for your specific flight surface first, and the items least likely to matter are visible-but-de-emphasised, and you can quickly inspect what was filtered to confirm nothing is being silently dropped.

Leg-based filtering: corridor width, altitude band

The first dimension of filtering is geographic. A NOTAM at an airfield 80 NM south of your great-circle route, at 12,000 feet AGL, on a VFR cruise at 4,500, almost certainly does not change your flight.

Corridor width. Define a corridor along your great-circle route. A reasonable default is ±10 NM either side. For a VFR flight at low altitude over terrain that might force a deviation, widen to ±20 NM. For a high-altitude IFR cruise, ±5 NM is often enough.

Altitude band. Define an altitude band around your planned cruise altitude. ±2,000 feet either side is a reasonable default. NOTAMs above your max-altitude-of-operation are almost always noise; NOTAMs at your altitude or below need a closer read.

NOTAMs that fall outside the corridor width or the altitude band are not deleted — they’re de-emphasised. You can still see them and you can still expand any one to read it. But they sit below the items inside your corridor, and they don’t trigger the “you have unread NOTAMs” red flag.

Time-window filtering: ETD ± window

The second dimension is temporal. A NOTAM that was active for a runway closure last Tuesday and ends at 18:00 today, when your ETD is 09:00 tomorrow, is noise. A NOTAM that activates at 14:00 today for a one-hour parachute jump, when your overflight time at that location is 14:30, is signal.

ETD ± window. Define a time window around your departure: ETD ± a couple of hours covers the typical real-world departure flexibility. For each NOTAM, compute whether its active period intersects your window. If it does, surface; if it doesn’t, de-emphasise.

Permanent NOTAMs and NOTAMs without a clean end time stay always-active for filtering purposes. The point of the time window is to drop NOTAMs that have a definite expiry before your trip or activate after your trip is over.

Categories worth always-showing (RWY, NAV, ILS, GPS)

Some NOTAM categories are always relevant if they touch your route. The ICAO Q-code classification gives us a workable taxonomy:

• RWY — runway items. Closed runways, runway works, runway condition. Always show; you can’t land on a closed runway.
• NAV — navigation aids. VOR out of service, NDB unreliable, DME inoperative. If your route or backup-plan depends on a navaid, you need to know.
• ILS — instrument landing systems. ILS out of service, ILS test, ILS withdrawn. If you might shoot an ILS, you need to know.
• GPS — GPS interference, GPS jamming exercises, GPS RAIM unavailable. Increasingly important; both military and civilian sources of disruption are growing.
• Airspace activations — temporary restricted areas, parachute jumps, airshow displays, military exercises within your corridor.
• Aerodrome operating hours — if the destination tower or AFIS is closing earlier than expected, you need to know before you commit fuel.

These should always surface if they touch your corridor or your time window, regardless of other filtering.

Categories worth always-hiding for VFR

For a VFR flight at typical GA altitudes, some categories are almost always noise:

• High-altitude (FL200+) en-route navaid maintenance if your max altitude is FL150 or below.
• Procedure NOTAMs for IFR approaches at aerodromes you’re flying VFR through (you’re not flying the procedure).
• Cat II/III ILS minima changes if you’re VFR.
• NOTAMs for Class A airspace activations if you’re not entering Class A.
• Long-permanent restricted-area listings that have been static for years (you should know about these from the chart, not relearn them each NOTAM read).

The “always-hide” categories are still visible if you ask for them — they’re not gone. They’re just below the fold of the surfaced list.

Real example: a Madrid-Mallorca leg

Consider a typical summer-Saturday VFR flight, LEMD–LEPA, 4,500 ft cruise altitude, ETD 09:00 local, ETE about 1:40.

A raw NOTAM dump for this leg, including every aerodrome along the way and every airspace activation, can return 100+ items. After leg-based filtering with a ±10 NM corridor and ±2,000 ft altitude band: maybe 35. After time-window filtering with ETD ±2 hours: maybe 22. After always-show / always-hide category filtering: maybe 15 items in the surfaced list, with the other 85 visible-but-de-emphasised.

Of the 15 surfaced: probably 5 about LEMD departure procedures and runway operations, 3 about en-route restricted-area activations (military training), 2 about LEPA arrival procedures and parking, 2 about navaids along the route, 2 about GPS-interference exercises in the Western Mediterranean, and 1 about a temporary fuel availability change at LEPA general aviation. Each of those 15 deserves a careful read.

The 85 de-emphasised: high-altitude items irrelevant to a 4,500-ft VFR; standing restricted-area listings already on the chart; IFR-procedure changes at LEMD that don’t apply to a VFR departure; and so on. They’re not gone; they’re just below the fold.

This is the workflow Airworth Flights implements: leg corridor and ETD time window are the primary filters, with always-show / always-hide category overlays. Filter preferences persist in localStorage. Per-trip de-storming so a NOTAM you’ve read and judged doesn’t keep flagging across days.

For Spain-specific NOTAM sources (ENAIRE NOTAM Office, the Spanish AIS), see VFR flight planning in Spain. For where this fits in the broader trip workflow, see The owner-pilot trip-planning checklist.