Flight number record

DL 639

Delta Air Lines · Los Angeles to Boston

Route: LAX → BOS
Departs: 07:55
Flights: 365
Period: September 2025 to August 2026

Historical statistics for September 2025 to August 2026, not live status. For today’s DL 639, check with Delta Air Lines.

DL 639 lands a B−, 4 points better than the national rate of 77.3%. Roughly one flight in five arrives 15 minutes late or worse, and those flights are late by a little over an hour on average. That rests on 365 flights, with a median arrival 5 minutes early.

Set against the rest of the country, DL 639's record is 4 points better than the US average of 77.3%. That gap is worth roughly 43 fewer late arrivals for every thousand flights.

On time

81.6%

Arrivals under 15 minutes late, of 354 completed flights

Avg delay

80 min

Across delayed arrivals only, not all flights

Cancelled

3.01%

National rate is 1.89%

1 hour+ late

6.2%

National rate is 8.3%

These figures cover 365 flights between September 2025 and August 2026. The median arrival is 5 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 639 flies

DL 639 flies LAX to BOS without a single fixed slot. Its most common scheduled departure was 07:55, used on 132 of 365 flights, with 06:40 (96 flights) and 06:45 (51 flights) next.

Departs 07:55132 flights36.2%
Departs 06:4096 flights26.3%
Departs 06:4551 flights14.0%

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 07:55 departure sits in the 5am–8am window. Across every US flight scheduled in that window the on-time rate is 89.3%, and DL 639 runs 8 points worse at 81.6%. 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 639 next to LAX to BOS

Every carrier on LAX to BOS combined arrived on time 76.2% of the time across 4,797 flights, so DL 639 is 5 points ahead of it. Delta flew 1,240 flights on the same city pair under all its flight numbers, including this one, and those ran 75.9% on time. Across its whole network Delta Air Lines managed 81.1%, close to this flight's figure.

DL 639 against its route and carrier
Compared with	Flights	On time	Avg delay	Cancelled	Grade
DL 639	365	81.6%	80 min	3.01%	B−
All Delta Air Lines on LAX–BOS	1,240	75.9%	89 min	2.18%	B
All carriers on LAX–BOS	4,797	76.2%	80 min	1.81%	B
Delta Air Lines, whole network	1,039,517	81.1%	71 min	1.74%	B

Across the year

DL 639 quarter by quarter

DL 639 did not run the same all year. Its best quarter was Jul–Sep 2026 at 89.8% on time and its weakest Jul–Sep 2025 at 73.3%, a swing of 17 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 flights73.3%
Oct–Dec 202592 flights77.5%
Jan–Mar 202690 flights76.5%
Apr–Jun 202691 flights87.9%
Jul–Sep 202662 flights89.8%

Scale runs 65% to 95%.

By weekday, DL 639 did best on Friday at 90.4% on time and worst on Saturday at 70.6%. Each day rests on about 52 flights, so a gap of 20 points is worth noticing but could still narrow in another year.

Arrival by arrival

How late DL 639 runs when it is late

DL 639 often beats the schedule: 93 of 354 completed flights, 26%, arrived 15 minutes or more early. The 65 late arrivals split 24 under half an hour, 19 between 30 and 60 minutes, and 22 an hour or more.

15+ min early93 flights26.3%
Within 15 min196 flights55.4%
15–29 min late24 flights6.8%
30–59 min late19 flights5.4%
1–2 hours late8 flights2.3%
2+ hours late14 flights4.0%

Scale runs 0% to 100%.

Share of 354 completed flights by arrival against schedule.

Time lost at the gate mostly stays lost. Of 43 departures that left 15 minutes late or more, only 4 made it up before arrival. In the other direction, 26 flights left on time yet arrived 15 minutes late or more, so that delay built up after pushback: in the air, in a holding pattern or on the ground at BOS.

The worst days

Worst days for DL 639

The latest arrival of the period was on 16 August 2026, 6 hours behind schedule; most of its delay minutes were filed as carrier-caused (crew, maintenance, fuelling, baggage). The same cause led on four of the five worst days.

The latest arrivals of DL 639 in the period
Date	Arrived late by	Largest filed cause
16 August 2026	6 hours	Carrier
3 November 2025	5 hours 18 minutes	Carrier
12 March 2026	4 hours 46 minutes	Late-arriving aircraft
25 December 2025	4 hours 22 minutes	Carrier
8 June 2026	4 hours 13 minutes	Carrier

Delay causes

Why DL 639 runs late

Carrier accounts for 59% of the 5,185 delay minutes carriers reported here, and the remaining four split what is left.

Carrier
59%
National Airspace System
34%
Late-arriving aircraft
6%
Extreme weather
1%

The largest share of delay minutes, 59%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. It accounts for 3,055 of the 5,185 delay minutes reported here. The split is drawn from 5,185 delay minutes reported on DL 639.

Cancellations run at 3.01%, against 1.89% nationally. That is roughly one flight in 30 cancelled outright. A cancelled flight is invisible in every on-time figure on this page, so the punctuality numbers above describe only the flights that actually operated.

Connections and routes

What a delay on this flight costs

6.2% of arrivals land an hour or more late, close to the national 8.3%, so a 60 to 75 minute layover is reasonable here while anything under 45 minutes is a bet. Most delays on this route last a little over an hour, which a normal layover absorbs.

At 2,611 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 639 in the data flew LAX to BOS. 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 639

For a booking, the cancellation rate is the figure to plan around on DL 639: 3.01% of its 365 scheduled flights did not operate. A late arrival costs a little over an hour, but a cancellation costs the day, so leave a later option open.

Where to go next

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 22 October 2026; recomputed monthly. How this data is collected.

Key figures