Flight number record

DL 639

Delta Air Lines · Los Angeles to Boston

Route: LAX → BOS
Departs: 07:55
Flights: 365
Period: July 2025 to June 2026

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

DL 639 lands a B, 6 points better than the national rate of 77.6%. About one flight in six arrives 15 minutes late or worse, and those flights are late by a little over an hour on average. That is a workable record, not a spotless one. That rests on 365 flights, with a median arrival 5 minutes early.

Set against the rest of the country, DL 639's record is 6 points better than the US average of 77.6%. That gap is worth roughly 56 fewer late arrivals for every thousand flights.

On time

83.2%

Arrivals under 15 minutes late, of 357 completed flights

Avg delay

74 min

Across delayed arrivals only, not all flights

Cancelled

2.19%

National rate is 1.81%

1 hour+ late

5.3%

National rate is 8.1%

These figures cover 365 flights between July 2025 and June 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.

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 (77 flights) and 07:00 (53 flights) next. Carriers retime flights through the season, so the rates here blend slightly different positions in the aircraft's day.

Departs 07:55132 flights36.2%
Departs 06:4077 flights21.1%
Departs 07:0053 flights14.5%

Scale runs 0% to 100%. Rows with fewer than 30 flights show a dash.

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 6 points worse at 83.2%. Early departures usually start with an aircraft that has not flown yet that day, which is most of why the window does well.

Against the route

DL 639 next to LAX to BOS

Every carrier on LAX to BOS combined arrived on time 78.4% of the time across 4,718 flights, so DL 639 is 5 points ahead of it. Delta flew 1,230 flights on the same city pair under all its flight numbers, including this one, and those ran 78.6% on time. Across its whole network Delta Air Lines managed 81.6%, which this flight beats by 2 points.

DL 639 against its route and carrier
Compared with	Flights	On time	Avg delay	Cancelled	Grade
DL 639	365	83.2%	74 min	2.19%	B
All Delta Air Lines on LAX–BOS	1,230	78.6%	86 min	1.71%	B
All carriers on LAX–BOS	4,718	78.4%	81 min	1.42%	B
Delta Air Lines, whole network	1,038,570	81.6%	71 min	1.62%	B

Across the year

DL 639 quarter by quarter

DL 639 did not run the same all year. Its best quarter was Jul–Sep 2025 at 90.2% on time and its weakest Jan–Mar 2026 at 76.5%, a swing of 14 points on roughly 91 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 202592 flights90.2%
Oct–Dec 202592 flights77.5%
Jan–Mar 202690 flights76.5%
Apr–Jun 202691 flights87.9%

Scale runs 70% to 95%. Rows with fewer than 30 flights show a dash.

By weekday, DL 639 did best on Thursday at 88.5% on time and worst on Saturday at 72.5%. Each day rests on about 52 flights, so a gap of 16 points is worth noticing but could still narrow in another year.

Delay causes

Why DL 639 runs late

One category carries most of the weight here. Carrier accounts for 54% of the 4,431 delay minutes carriers reported, and the remaining four split what is left. The split is by minutes rather than by flights, so a few very long delays count for as much as many short ones.

Carrier
54%
National Airspace System
38%
Late-arriving aircraft
7%
Extreme weather
1%

The largest share of delay minutes, 54%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. Unlike weather, that is a management outcome, and it is the category where the gap between two carriers on the same schedule is genuinely earned. It accounts for 2,396 of the 4,431 delay minutes reported here. The split is drawn from 4,431 delay minutes reported on DL 639.

Cancellations sit at 2.19% against a national 1.81%, which is ordinary. Diversions add another 0.00%.

Connections and routes

What a delay on this flight costs

5.3% of arrivals land an hour or more late, close to the national 8.1%. That is the figure that decides a connection, and it says 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, read DL 639 as 83.2% on time with 2.19% cancelled over 365 flights. Those are rates from the past twelve months, not a forecast for any particular day.

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, July 2025 to June 2026. Last updated 15 September 2026; recomputed monthly. How this data is collected.

Key figures