How aviation data goes stale

Every weather report, forecast, and NOTAM is a photograph of a moment, not a live feed. It is issued at a time, it is valid for a period, and after that it is history. Knowing how each product expires is the difference between briefing on current information and briefing on yesterday's.

This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.

Why the timestamp is the most important field

Pilots learn to decode the wind and the cloud, but the field that decides whether a report is worth anything is the time. A METAR with perfect weather is useless if it was observed three hours ago and a front has since come through. The time group, always in UTC, is the first thing to read, not the last. The validity rules for these products come from ICAO Annex 3 (Meteorological Service for International Air Navigation), the WMO Manual on Codes (WMO No. 306), and, for NOTAMs, ICAO Annex 15.

How long each product lasts

Each kind of data ages differently:

• METAR. An observation, usually issued hourly, and every half hour at busier airports, with a SPECI in between when conditions change significantly. It describes a single moment, so its useful life is short, perhaps an hour in steady weather and far less when the weather is moving.
• TAF. A forecast with a stated validity period, commonly 24 or 30 hours, normally reissued several times a day. When the forecast changes, an amended TAF AMD is published, which supersedes the earlier one. The validity window is written in the TAF, so read it.
• Winds and temperatures aloft. A forecast issued in cycles and valid for set periods, so the one you hold may have been replaced by a newer run.
• NOTAM. Carries its own validity in the B) and C) fields. It is in force only inside that window, and an EST end time means it may be reissued and run longer. A NOTAMR replaces an earlier one; a NOTAMC cancels it.
• SIGMET and AIRMET. Short-fuse hazard warnings with brief validity periods, often a few hours, precisely because the hazards they describe move and change.

The common thread is that none of these is open-ended. Every one has an issue time and an expiry, and the moment you read it you should be asking whether it is still inside that window.

A worked example

Suppose at 0500 UTC you pull this METAR and TAF for your destination:

METAR 0420Z 36015KT 9999 SCT040 12/05 Q1015TAF 0400Z 0406/0512 36012KT 9999 SCT040 TEMPO 0408/0412 6000 -RA BKN012

At 0500, the METAR is 40 minutes old, which is fine in this steady northerly flow. The TAF was issued at 0400 and is valid from the 4th at 0600 to the 5th at 1200, and it warns of a temporary deterioration to 6000 metres in light rain with a broken ceiling at 1200 feet between 0800 and 1200. Now imagine you save this, fly out of coverage, and arrive at 1100. Your saved METAR is now seven hours old and tells you nothing about the present. Your saved TAF, though, is still inside its validity and still warns you that the period you are arriving in could be the worse one. The two products aged completely differently, and only the time fields tell you which to trust.

Offline is a snapshot, not a feed

This is where the honest limits of any app matter. When you are offline, you are not looking at the weather; you are looking at a copy of the weather as it was when you last fetched it. That copy is genuinely useful, as a record of what you briefed on and as a fallback when you lose signal, but it is frozen. Treating a saved briefing as if it were current is the trap.

There is also a trust dimension. If you save a briefing to prove what information you had before a flight, that saved copy should be exactly what was issued, unchanged, so the record is reliable. A snapshot that can be quietly altered is no record at all.

How often to refresh, by product

Knowing that data expires is one thing; turning it into a refresh habit is another. A practical cadence falls out of how each product ages, and the underlying duty is the legal one to become familiar with all available information before a flight, set out for example in 14 CFR 91.103.

• METAR. Pull it close to departure, and again if anything delays you. On a longer flight, refresh the destination observation before the descent if you have a way to receive it, because the one you read at the gate may be hours old by the time you arrive.
• TAF. Check it during planning and look specifically for an amended TAF AMD near departure, since an amendment supersedes the forecast you were working from.
• NOTAMs. Refresh the route or aerodrome bulletin close to departure, because new NOTAMs are issued continuously and an overnight runway closure will not be in last night's package.
• SIGMET, AIRMET, and convective warnings. These have short fuses by design, so the relevant one is the current one; an expired hazard warning tells you nothing about now.
• Winds aloft. Use the latest forecast cycle for your planned level, and re-check if your departure slips by hours.

The simple rule of thumb is brief, then re-brief: do the full picture in planning, and take a deliberate last look at the fast-changing products, weather and NOTAMs, as close to departure as you can. There is also a record-keeping reason to capture what you saw. If you ever need to show what information you had before a flight, a saved, time-stamped, unaltered copy of the briefing is the evidence, which is why a snapshot that cannot be quietly changed is worth more than a screenshot that can.

A timeline of one flight's data

Tracking the age of each product across a single flight shows why a refresh discipline matters. Suppose you plan at 1800 the evening before, depart at 0700, and arrive at 0900.

• At planning (1800). You pull a METAR (fresh, minutes old), a TAF valid through the next day (current), and a NOTAM bulletin (current as generated). All are good.
• At departure (0700). The METAR you saved is now 13 hours old and tells you nothing about the present, so you pull a new one. The TAF is still inside its validity, but you check for a TAF AMD in case it was amended overnight. The NOTAM bulletin is 13 hours old, so you regenerate it, and this is exactly when an overnight runway closure would appear that was not in last night's package.
• At arrival (0900). The destination METAR you read at 0700 is now two hours old. If you can receive a fresh observation before the descent, you do, because two hours is enough for the weather to have moved.

The pattern is that the slow-changing products, such as a long-validity TAF, survive across the flight, while the fast-changing ones, the METAR and the NOTAMs, need refreshing at departure and, where possible, again near arrival. The age of each piece, read from its time field, is what tells you which is which. Planning once and never refreshing is the trap; brief, then re-brief is the habit.

Common pitfalls

• Reading the weather before the time. The time group decides whether the rest is worth reading.
• Assuming a clear METAR stays clear. It is one moment; the next observation may differ.
• Missing a TAF amendment. A TAF AMD supersedes the earlier forecast; the one in your hand may be old.
• Trusting offline data as live. Saved data is a snapshot at the fetch time, not a current feed.
• Forgetting NOTAM validity and EST. A NOTAM applies only in its window, and an estimated end can extend.

In Pilot EFB

Pilot EFB is built around this exact distinction. Every weather report and NOTAM is shown with its source and the time it was fetched, and when you are offline the app marks the data with a staleness indicator so you can see at a glance that you are looking at a saved copy, not a live one. When you save a briefing into a flight's folder, the snapshot is kept and integrity-checked, so the briefing you saved is provably the briefing you keep. This is what offline-first means here: saved and already-fetched data stays available with no signal, while pulling fresh weather, NOTAMs, or roster data needs a connection. Pilot EFB is a planning and reference aid, not a certified Electronic Flight Bag, and the live official source is always the record.