LHR→SEA

Delta Air Lines is the most punctual carrier on the departing leg at 87.5% across 364 flights, and Alaska Airlines the least at 60.9% across 92. 27 points between them is a large gap for one city pair, large enough to prefer the better carrier at a modest fare difference. No carrier here cancels unusually often; the highest cancellation rate on the route is Virgin Atlantic's 0.7%.

Flights are scheduled departures in the CAA file, operated plus cancelled; on time is within 15 minutes of the planned off-block time. The DOT count below includes the return direction and may differ.

July was the weakest month for departures at 72.0% on time and February the strongest at 92.6%, a swing of 21 points across the year. That is a wide seasonal range, wider than the gap between the best and worst carriers, so the month you fly matters more than the airline you pick. The DOT's count puts the busiest month at 293 flights in June 2026 and the thinnest at 162 in February 2026, so capacity follows the summer peak.

Coming the other way, 78.5% of arrivals from Seattle-Tacoma International into London Heathrow reached the gate within 15 minutes of schedule, across 1,372 flights. 27% arrived more than 15 minutes early, which is normal for an eastbound crossing with the jet stream behind it and a schedule padded for the days it is not. 0.57% of those arrivals were three hours or more late, the figure that decides a compensation claim on the return trip, and 0.9% were cancelled.

EUROCONTROL's figures for London Heathrow as a whole, every airline and every destination: 80.3% of departures and 80.5% of arrivals within 15 minutes of schedule from 1 January to 21 October 2026, with 96.7% of scheduled flights operated. The last seven days ran at 80.8% for departures, in line with the year. Air traffic flow management delay attributed to the airport averaged 1.2 minutes per arriving flight this year, 1.0 over the last week.

Every US route from London Heathrow, and its full punctuality record

Seattle-Tacoma International is scored on this site from the DOT's domestic data: 79.5% of its 164,759 domestic departures arrived on time over the last twelve months, with delayed flights running about an hour late, and 0.93% were cancelled. Those are domestic flights, not this route, but an arriving transatlantic flight shares the same runways, the same weather and the same ground delay programmes, so a bad day at SEA reaches a flight from LHR just as surely. The airport's own page has the hour-by-hour and month-by-month detail, and its live FAA status appears above when the FAA has something to say.

Seattle-Tacoma International (SEA) delay statistics, hour by hour and month by month

Every flight that departs London Heathrow falls under UK261, the retained version of the EU regulation, whichever airline flies it and wherever it lands. If it arrives at Seattle-Tacoma International three hours or more late, or is cancelled with under 14 days' notice, and the cause was within the airline's control, each passenger is owed £520 (£260 if the airline got you there under four hours late), on top of rebooking and care. Weather, air traffic control restrictions and strikes outside the airline are the usual exemptions. The return flight from SEA is covered only when the carrier is UK or EU based. The sector is about 4,786 miles, 7,701 km, which puts it in the top distance band.

Delayed three hours or more? Check a claim with Compensair →

Booked blind, LHR to SEA is a B+ route by the departure record. Booked with the numbers, it is Delta Air Lines at 87.5%, outside July if the dates are flexible, and a connection at SEA with at least two hours in hand, because 4% of departures were an hour or more late and immigration comes before any onward flight.

• London Heathrow (LHR): every US route
• Seattle-Tacoma International (SEA) delay statistics
• LHR to JFK, New York (JFK)14,572 flights
• LHR to EWR, Newark (EWR)6,322 flights
• LHR to LAX, Los Angeles (LAX)5,981 flights
• LHR to ORD, Chicago (ORD)5,762 flights
• KEF to SEA, from Reykjavík1,413 flights
• AMS to SEA, from Amsterdam1,139 flights
• CDG to SEA, from Paris1,023 flights
• FRA to SEA, from Frankfurt856 flights
• Every Europe-to-US route and airport

The departure record is the CAA's, measured at London Heathrow; the arrival record for the return leg is also measured at London Heathrow. Nothing here is measured at SEA, where the DOT keeps no record of international flights.