#ssim

Departures on each weekday of an illustrative route drawn as bars, Friday highlighted, on an Active Flights blog card.

SSIM & Schedules27 Oct 20262 min read

Why counting flights and seats from SSIM is harder than it looks

Weekly frequencies and seats are the first figures anyone wants from a schedule — and the ones most often wrong. The traps behind the numbers, and why they matter.

A validation checklist with one item flagged, on an Active Flights blog card.

SSIM & Schedules27 Oct 20263 min read

Why SSIM files fail quietly — and why validation matters

An SSIM schedule can be wrong without ever raising an error. What goes wrong in real files, what it costs downstream, and why validation has to come first.

Flight XX 123 drawn twice: variation 01 as two legs through a middle station, variation 02 as one non-stop leg, on an Active Flights blog card.

SSIM & Schedules27 Oct 20267 min read

Itinerary variation and leg sequence in SSIM

How SSIM names a flight leg: airline, flight number, itinerary variation and leg sequence. How variations are numbered, the overflow byte, and multi-leg dates.

Four one-letter service type codes — J, C, F and P — each with its meaning, on an Active Flights blog card.

SSIM & Schedules27 Oct 20264 min read

SSIM service type codes: what J, C, F and P mean

The one-letter service type on every SSIM flight leg says what kind of flight it is. What the codes mean, how to group them, and the traps when filtering.

Aircraft type codes shown in both IATA three-character and ICAO four-character forms mapping to the same aircraft, on an Active Flights blog card.

SSIM & Schedules26 May 20262 min read

Reading aircraft type codes: IATA vs ICAO in the schedule

The 'equipment' field in a schedule is a short code for an aircraft type — and there are two competing code systems for it. Knowing which is which, and mapping between them, is a small skill that prevents big mistakes.

A single departure time interpreted three ways — local, UTC, and across a DST boundary — diverging into different answers, on an Active Flights blog card.

SSIM & Schedules28 Apr 20262 min read

Time zones, UTC and DST: the schedule's quiet minefield

Almost every serious schedule-data bug is, at bottom, a time bug. Local vs UTC, per-station offsets, and daylight-saving transitions turn a simple 'departure time' into one of the trickiest fields in the file.

A single schedule rule expanding into a calendar of concrete operating dates, with day-of-week markers, on an Active Flights blog card.

SSIM & Schedules14 Apr 20263 min read

Days of operation and period expansion: the date math that breaks parsers

A single SSIM flight-leg line isn't one flight — it's a rule that expands into many. Getting that expansion exactly right, across days-of-operation, frequency, and season edges, is where a lot of quiet bugs live.

SSIM records 1 to 5 stacked with their serial numbers 000001 to 000005; the trailer is highlighted and its check reference names 000004, the record before it, on an Active Flights blog card.

SSIM & Schedules2 Apr 20266 min read

The trailer record: the checksum you shouldn't skip

The last line of an SSIM file — the record type 5 trailer — is easy to ignore, and it's how you know the file arrived whole. Here's what it carries and why checking it is basic hygiene.

Two cost curves over time — an in-house parser climbing steadily while a bought tool stays flat, on an Active Flights blog card.

Data & Engineering31 Mar 20263 min read

Build vs buy: the true cost of an in-house SSIM parser

Writing your own schedule parser looks cheap — one engineer, a week, done. The real cost is the maintenance treadmill that follows. Here's the total-cost-of-ownership case, honestly.

A departure and arrival converted through their UTC variations into a correct block time, on an Active Flights blog card.

SSIM & Schedules5 Mar 20265 min read

Block time, done right: computing it UTC-correct

Departure and arrival times in a SSIM leg are local. Subtract them directly and a cross-timezone flight looks impossibly short, long, or negative. The correct block time goes through each end's UTC variation — here's the method, with a worked example.

An aircraft configuration string decoding into labelled cabin compartments and a derived total seat count, on an Active Flights blog card.

SSIM & Schedules19 Feb 20264 min read

Seats from a string: aircraft configuration in SSIM

SSIM doesn't hand you a seat count. It hands you an aircraft configuration string describing each cabin compartment and its seats — and you derive the capacity from that. Here's how to read it, and why the equipment code isn't the same thing.

An overnight flight leg spanning two calendar days with a +1 arrival-day offset highlighted, on an Active Flights blog card.

SSIM & Schedules5 Feb 20265 min read

Overnight flights and the day that changes

A red-eye departs one day and lands the next. SSIM encodes that with a day offset — and the 2400-vs-0000 midnight edge — which naive time handling turns into negative block times and arrivals before departure.

A validation panel listing schedule issues, some flagged as real errors and some acknowledged as known conventions, on an Active Flights blog card.

SSIM & Schedules28 Jan 20263 min read

The data-quality edge cases that break schedule parsers

A field guide to the real-world quirks in SSIM feeds — time modes, overnight arrivals, DST, operational suffixes, season edges — that turn a 'simple' fixed-width parser into a maintenance treadmill.

A flight-leg record fanning out into labelled segment-data elements tagged by DEI numbers, on an Active Flights blog card.

SSIM & Schedules22 Jan 20266 min read

Inside RT4: the data element identifiers SSIM carries

The flight-leg record says who, where, and when. The segment-data record (RT4) carries the extras — codeshares, aircraft configuration, meal service, traffic restrictions — per segment, keyed by Data Element Identifiers. A spec-level walk through what RT4 holds and why it matters.

Schedule-change action-code chips — NEW, CNL, TIM, EQT — flowing from a schedule to downstream systems, on an Active Flights blog card.

SSIM & Schedules18 Dec 20253 min read

SSM and ASM: how schedule changes actually travel

A schedule is published once, then changed constantly. SSM and ASM are the IATA messages that carry those changes — bulk and ad-hoc — with action codes like NEW, CNL, TIM and EQT. Here's how they work.

A stack of five labelled SSIM record-type rows — a header, a carrier record, a flight leg, a segment record, and a trailer, on an Active Flights blog card.

SSIM & Schedules3 Dec 20254 min read

SSIM Chapter 7 record types, explained: reading a schedule file end to end

A field-level walk through the SSIM record types — header, carrier, flight leg, segment data, and trailer — with a sample file, the fields that matter, and the gotchas that bite parsers.

A raw fixed-width SSIM record line decoding into labelled structured fields, on an Active Flights blog card.

SSIM & Schedules12 Nov 20254 min read

What is IATA SSIM? A plain-English guide to the format that runs airline schedules

SSIM is how the airline industry exchanges its schedules — a fixed-width, 200-byte text format that's deceptively simple and genuinely hard to get right. Here's what it is and why it matters.

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