Flight number record
DL 859
Delta Air Lines · Boston to Salt Lake City
- Route
- BOS → SLC
- Departs
- 06:10
- Flights
- 365
- Period
- September 2025 to August 2026
Historical statistics for September 2025 to August 2026, not live status. For today’s DL 859, check with Delta Air Lines.
DL 859 earns an A. Close to nine flights in ten arrive inside the 15-minute window the DOT uses to define an on-time arrival, which puts it 10 points better than the national rate of 77.3%. When something does go wrong the average delay runs roughly 45 minutes. That rests on 365 flights, with a median arrival 7 minutes early.
Set against the rest of the country, DL 859's record is 10 points better than the US average of 77.3%. That gap is worth roughly 99 fewer late arrivals for every thousand flights.
Key figures
On time
87.2%
Arrivals under 15 minutes late, of 359 completed flights
Avg delay
47 min
Across delayed arrivals only, not all flights
Cancelled
1.64%
National rate is 1.89%
1 hour+ late
2.2%
National rate is 8.3%
These figures cover 365 flights between September 2025 and August 2026. The median arrival is 7 minutes early, which is the part an average hides: most flights here are fine, and the on-time rate is decided by a tail of badly late ones. On-time means under 15 minutes late, the DOT's own threshold.
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Schedule
When DL 859 flies
DL 859 flies BOS to SLC without a single fixed slot. Its most common scheduled departure was 06:10, used on 184 of 365 flights, with 06:15 (92 flights) and 06:05 (86 flights) next.
- Departs 06:10184 flights50.4%
- Departs 06:1592 flights25.2%
- Departs 06:0586 flights23.6%
Scale runs 0% to 100%.
Share of scheduled departures at each local time. Times used on fewer than one flight in ten are not listed.
A 06:10 departure sits in the 5am–8am window. Across every US flight scheduled in that window the on-time rate is 89.3%, and DL 859 runs 2 points worse at 87.2%. Early departures usually start with an aircraft that has not flown yet that day, which is most of why the window does well.
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Against the route
DL 859 next to BOS to SLC
Every carrier on BOS to SLC combined arrived on time 73.4% of the time across 1,264 flights, so DL 859 is 14 points ahead of it. Delta flew 1,083 flights on the same city pair under all its flight numbers, including this one, and those ran 77.5% on time. Across its whole network Delta Air Lines managed 81.1%, which this flight beats by 6 points.
| Compared with | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| DL 859 | 365 | 87.2% | 47 min | 1.64% | A |
| All Delta Air Lines on BOS–SLC | 1,083 | 77.5% | 67 min | 1.20% | B |
| All carriers on BOS–SLC | 1,264 | 73.4% | 65 min | 1.82% | C |
| Delta Air Lines, whole network | 1,039,517 | 81.1% | 71 min | 1.74% | B |
Across the year
DL 859 quarter by quarter
DL 859 did not run the same all year. Its best quarter was Jul–Sep 2025 at 100.0% on time and its weakest Jan–Mar 2026 at 80.0%, a swing of 20 points on roughly 73 flights a quarter. The twelve-month rate averages over that, so if you are flying in a weak quarter the quarter's own figure is the better guide.
- Jul–Sep 202530 flights100.0%
- Oct–Dec 202592 flights85.9%
- Jan–Mar 202690 flights80.0%
- Apr–Jun 202691 flights86.7%
- Jul–Sep 202662 flights93.5%
Scale runs 70% to 100%.
By weekday, DL 859 ranged from 80.8% on Saturday to 92.2% on Monday. With about 52 flights behind each day, a spread of 11 points is inside normal variation, so the day you fly is not worth changing for this flight.
Arrival by arrival
How late DL 859 runs when it is late
DL 859 often beats the schedule: 107 of 359 completed flights, 30%, arrived 15 minutes or more early. Most of the lateness is short. 30 of the 46 late arrivals were under half an hour behind, and 8 reached an hour or more.
- 15+ min early107 flights29.8%
- Within 15 min206 flights57.4%
- 15–29 min late30 flights8.4%
- 30–59 min late8 flights2.2%
- 1–2 hours late2 flights0.6%
- 2+ hours late6 flights1.7%
Scale runs 0% to 100%.
Share of 359 completed flights by arrival against schedule.
Only 17 of 359 completed flights left 15 minutes late or more, too few to say how often the time is made up.
The worst days
Worst days for DL 859
The latest arrival of the period was on 2 August 2026, 5 hours 20 minutes behind schedule; most of its delay minutes were filed as carrier-caused (crew, maintenance, fuelling, baggage). The same cause led on five of the five worst days.
| Date | Arrived late by | Largest filed cause |
|---|---|---|
| 2 August 2026 | 5 hours 20 minutes | Carrier |
| 12 June 2026 | 3 hours 34 minutes | Carrier |
| 28 December 2025 | 2 hours 50 minutes | Carrier |
| 27 December 2025 | 2 hours 21 minutes | Carrier |
| 13 October 2025 | 2 hours 9 minutes | Carrier |
Delay causes
Why DL 859 runs late
Carrier accounts for 60% of the 2,182 delay minutes carriers reported here, and the remaining four split what is left.
- 60%Carrier
- 40%National Airspace System
- 0%Extreme weather
40% of delay minutes are charged to the National Airspace System: air traffic volume, runway configuration, ground stops. No carrier can price its way out of that, which is why airport choice moves the number here more than carrier choice does. That is 868 minutes out of 2,182, against a national NAS share of 21%. The split is drawn from 2,182 delay minutes reported on DL 859.
Cancellations sit at 1.64% against a national 1.89%, which is ordinary. Diversions add another 0.00%.
Connections and routes
What a delay on this flight costs
Badly late arrivals are rare here: only 2.2% land an hour or more behind schedule, against 8.3% nationally. Connections built on this flight are about as safe as US domestic connections get, though a cancelled flight still ignores every layover you give it.
At 2,105 miles this is transcontinental flying, and long routes get a quiet structural advantage. Scheduled block time on a flight this length carries more padding in absolute minutes, so a twenty-minute late departure often still becomes an on-time arrival. Read the on-time rate here as slightly flattering next to a short-haul route with the same operational quality.
Every one of the 365 flights under DL 859 in the data flew BOS to SLC. The number was not reused on any other city pair, which makes this a clean record of one route.
One caution: this is a thin sample. 365 flights over twelve months is enough to clear this site's floor but not enough to pin the rate down tightly, and a single bad week moves it by more than a point. Treat the grade as a rough reading.
What to do with this
Booking DL 859
For a booking, read DL 859 as 87.2% on time with 1.64% cancelled over 365 flights. Those are rates from the past twelve months, not a forecast for any particular day.
Related
Where to go next
- BOS to SLC, every carrier and departure
- SLC to BOS, the return direction
- Delta Air Lines 464 on the same route76.2%
- Delta Air Lines 445 on the same route69.4%
- Boston Logan International (BOS) delay statistics
- Salt Lake City International (SLC) delay statistics
- Delta Air Lines on-time record
- Every Delta Air Lines flight number with a record
- The most and least reliable flight numbers in the US
- Worst months to fly in the US, ranked by the data
A flight number here means Delta Air Lines’s own number as it reports to the DOT. How the figures are computed is on the methodology page.
Source: US DOT Bureau of Transportation Statistics, on-time performance data, September 2025 to August 2026. Last updated ; recomputed monthly. How this data is collected.