Comparing Two Airlines: What Actually Predicts a Late Flight

You’ve narrowed it to two airlines on the same route at similar prices. Which one is the safer bet against a late arrival? The federal record can answer that, but only if you compare the right things — and the number people reach for first is usually the least useful.

Five figures, read together

A fair head-to-head puts the same five measures next to each other for both carriers, over the same window:

• On-time arrival rate — share of flights under 15 minutes late. The headline, and the easiest to over-trust.
• Cancellation rate — the flights that never ran. A carrier can lift its on-time rate by cancelling, so this has to sit beside it.
• Diversion rate — flights that landed somewhere else. Rare, but wholly disruptive.
• Median delay — the typical late flight’s lateness. What an ordinary bad day looks like.
• 90th-percentile delay — the tail. One flight in ten is at least this late.

Why the worst case matters more than the average

Two carriers can post the same on-time rate and the same median delay and still be very different to fly, because delays are right-skewed: most are short, a minority are severe, and the severe ones do the damage. The 90th-percentile delay is where that difference shows up. If Carrier A’s tail is 55 minutes and Carrier B’s is two hours, B is the riskier choice for anything with a downstream commitment — even if their averages are a dead heat.

Delay causes tell you whose fault it tends to be

The Department of Transportation also files why flights ran late, split into five causes (BTS delay-cause reporting). Two are within an airline’s control — air-carrier delay (crew, maintenance, fuelling) and, largely, late-arriving aircraft, which reflects how well the carrier keeps its own fleet rotating on schedule. Weather and National Aviation System delays hit every airline on a route roughly alike, so when one carrier is worse than another on the sameroute, the difference usually lives in the causes it actually controls.

Read it fairly

The one rule that makes a comparison honest: same route, same window, all five figures at once. The Airline Head-to-Head lays two carriers out exactly that way — on-time rate, cancellations, diversions, median and worst-case delay, and the delay-cause split — all from the DOT filings, all reproducible. For the full field rather than a pair, see the Airline On-Time Rankings.