Flight profile · Delta Air Lines
DL400
San Francisco (SFO) to Boston (BOS) · scheduled 7:10 AM – 4:10 PM
How DL400 performs
784 operations, June 2024 – July 2026.
DL400 arrived on time 90.4% of the time, against 77.9% for all SFO → BOS flights, so this departure is 12.5 percentage points better than the route as a whole. Against Delta Air Lines’ own flights on this route it is 13.1 points better.
Source: US DOT Bureau of Transportation Statistics · 784 operations, June 2024 – July 2026 · If DL400 is cancelled or significantly changed, DOT’s automatic refund rule may entitle you to your money back. The refund checker walks through eligibility.
What stands out
How DL400 compares to other flight numbers, not just to itself.
This flight runs unusually far ahead of the carrier's own average on this route, better than 96% of 11,589 comparable flight numbers.
90.4% on time for this specific departure, against 77.3% for the same carrier's other flights on this route.
Source: FlyerIntel analysis of US DOT BTS data · June 2024 – July 2026 · The one thing about this specific departure that ranks most extreme against every other published flight number, out of everything FlyerIntel checks — shown only when something clears a real statistical bar, not for every flight.
Where DL400 is right now
A live reading, only if you ask for it. It is not part of the reliability record above.
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Against the rest of the route
| Flight or route | On time | Cancelled | Median delay | Severe | Flights |
|---|---|---|---|---|---|
| DL400 | 90.4% | 1.40% | -13 min | 2.5% | 784 |
| Delta Air Lines on SFO → BOS | 77.3% | 1.03% | -5 min | 7.8% | 2,529 |
| All SFO → BOS flights | 77.9% | 1.04% | -7 min | 8.5% | 14,227 |
Source: FlyerIntel analysis of US DOT BTS data · June 2024 – July 2026
Why DL400 runs late
Where US DOT attributed the delay minutes on this flight.
airspace and airport capacity, including weather-driven flow control
crew, maintenance, boarding and fueling, all within the carrier’s control
the inbound aircraft was late, so this flight started late
weather severe enough to stop this flight specifically
Share of the delay minutes US DOT attributed to each cause for DL400 on SFO → BOS. Only delays of 15 minutes or more are attributed.
Source: US DOT Bureau of Transportation Statistics · Data covering June 2024 – July 2026
Does a late aircraft delay this flight?
How often the plane that flies DL400 comes in late, and what that does to the departure.
DL400 is usually the aircraft’s first flight of the day: the schedule gives it a median of 7.8 hours on the ground at SFO, so its “inbound” is mostly the aircraft’s arrival the evening before.
When the aircraft came in 15 or more minutes late, DL400 pushed back an average of 11 minutes late; when it came in on time or early, an average of 0 minutes early.
Each departure is matched to the flight the same aircraft flew into SFO just before it, using the tail number in the DOT file, 745 of 773 departures in all. A departure is left out when that flight is unknown, was cancelled or diverted, or the aircraft had sat for more than 12 hours. A flight the airline cancelled never left and is not in the count, and one flown by a substitute aircraft is matched to the aircraft that flew it, not to the one first planned.
Against the network at the same turn length
| Share of departures | Inbound landed 15+ late | Left 15+ late after a late inbound | Left 15+ late after an on-time inbound |
|---|---|---|---|
| DL400 | 29% | 9% | 3% |
| Network, same turn | 29% | 12% | 9% |
A night on the ground absorbs most of a late arrival, so a late inbound matters far less on a turn this long than on a same-day one.
The network row is every flight with a scheduled turn of 6 hours or more, usually overnight. How often DL400’s aircraft landed 15 or more minutes late is about the same as the network’s and, once the aircraft was late, how often DL400 left 15 or more minutes late is lower than the network’s.
More: Which Flight Numbers Pay for a Late Aircraft lists the flight numbers that pass a late aircraft on.
Source: US DOT Bureau of Transportation Statistics · Data covering June 2024 – July 2026
From pushback to gate
When it leaves SFO late, does that still matter by the time it reaches BOS?
DL400 pushes back an average of 3.6 minutes late and reaches the gate at BOS an average of 6.3 minutes early. The 9.9 minutes between those two figures are slack the published schedule already carries, which is why a late pushback on this flight does not reliably become a late arrival. Its 18.2 minutes on the taxiways at SFO is about what any departure from the airport in that hour spends.
On arrival at BOS, DL400 spends 10.5 min taxiing from touchdown to the gate — ground time a connecting passenger sits through before they can even leave the aircraft, on top of however late the flight itself ran. That is 1.5 minutes longer than a typical BOS arrival (9.0 min).
Departure delay is averaged over every pushback and arrival delay over every completed arrival, so the two counts differ by the flights that never reached BOS. Taxi-out is gate to wheels up and sits inside the scheduled block rather than on top of it; the comparison is 23,713 departures from SFO scheduled in the clock hour DL400 most often leaves in. Taxi-in is landing to gate, compared against every arrival FlyerIntel has on file at BOS (440,886 flights), not just DL400’s own. More on taxi time.
Source: FlyerIntel analysis of US DOT BTS data · June 2024 – July 2026
The hour it leaves SFO
Part of any flight’s record belongs to the clock rather than to the flight. DL400 has one departure time, so this is what that time is worth.
On-time arrival rate. 23,713 departures from SFO are scheduled in the 7 AM–8 AM hour over the period, against 435,779 across the whole day.
DL400 is scheduled out of SFO at 7:10 AM, in the 7 AM–8 AM bank. Departures from SFO in that hour arrive on time 90.1% of the time against 78.7% across the airport’s whole day — the slot alone is worth 11.4 points better before anything about this particular flight is considered.
The two airport figures cover every departure from SFO to every destination, so they describe the slot rather than this journey — the like-for-like comparison against the route is above. Hours are scheduled local, and a number whose schedule moved between banks inside the period has no single hour and is not described here at all. Why the hour matters.
Source: FlyerIntel analysis of US DOT BTS data · June 2024 – July 2026 · Hours below 120 departures are not scored.
Other routes DL400 has flown
A flight number is a slot in a schedule, and carriers reassign it. These are the other city pairs this number served in the analysis period.
| Route | Operated | Flights | On time |
|---|---|---|---|
| SFO→DTW San Francisco to Detroit | July 1, 2023 – June 6, 2024 | 187 | 91.9% |
| SNA→SLC Santa Ana to Salt Lake City | April 9, 2024 – May 6, 2024 | 24 | — |
| DEN→ATL Denver to Atlanta | March 10, 2024 – April 8, 2024 | 30 | — |
| SLC→BOS Salt Lake City to Boston | January 8, 2024 – March 9, 2024 | 62 | — |
| DTW→SFO Detroit to San Francisco | September 6, 2023 – December 14, 2023 | 18 | — |
| ATL→LAS Atlanta to Las Vegas | October 28, 2023 – November 4, 2023 | 8 | — |
Because the same number has served different journeys, the statistics above describe only the route DL400 currently operates. Blending them would produce an average of two different flights.
When it goes wrong
The average describes a usual day. This is the day that ruins a connection.
14 min late
against a median of 13 min early
Percentiles over 784 operations; cancelled and diverted flights are counted separately.
Source: US DOT Bureau of Transportation Statistics · Data covering June 2024 – July 2026
Other flights on this route
Same airports, same journey, different odds of arriving on time.
| Flight | Airline | On time | Median arrival | Operations |
|---|---|---|---|---|
| DL400 this flight | Delta Air Lines | 90.4% | 13 min early | 784 |
| B6434 | JetBlue | 86.2% | 11 min early | 988 |
| AS356 | Alaska Airlines | 85.1% | 9 min early | 712 |
| UA2647 | United Airlines | 83.0% | 9 min early | 835 |
| UA1665 | United Airlines | 81.8% | 8 min early | 1,045 |
| B6734 | JetBlue | 76.9% | 6 min early | 810 |
| B61734 | JetBlue | 74.9% | 6 min early | 1,055 |
| DL977 | Delta Air Lines | 71.6% | 4 min early | 829 |
Every departure listed carries at least 600 operations, so the comparison is between figures that mean something. DL400 is placed in the same ordering rather than left out of it; the rows around it are the most punctual departures on the route, not all of them, so this is a comparison and not a rank. Schedules change; confirm the departure still operates before booking around this.
Data and methodology
Calculated by FlyerIntel from 784 records of DL400 operating SFO → BOS, published by the US DOT Bureau of Transportation Statistics. A flight counts as delayed when it arrives 15 or more minutes after schedule. Cancelled and diverted flights are excluded from the on-time rate and reported separately. Flight numbers are resolved as carrier plus number plus route, because the same number serves different routes over time.
Full methodology · Data sources · Report a problem
Related research: Most and Least Reliable Flight Numbers · The Delay You Inherit · Making Up Time in the Air
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