Air Tractor AT-401 accident near Calexico, California, July 10, 1991
On July 10, 1991 at about 6:30 am local time, a Air Tractor AT-401, registered N4538K, was destroyed in an accident near Calexico, California. It was an aerial application (crop spraying) flight under agricultural flying rules (Part 137). 1 person had minor injuries. The weather was visual conditions (good weather).
The NTSB's probable cause their words, unchanged
THE ERECTION OF A CONSTRUCTION SIGN ON AN ACTIVE RUNWAY BY COUNTY WORKERS WITHOUT NOTIFYING THE AIRPORT OPERATORS, AND, THE INABILITY OF THE PILOT TO DETECT THE PRESENCE OF THE SIGN DURING THE DARK NIGHT TAKEOFF.
Source: NTSB aviation accident database, copy made October 5, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- July 10, 1991 · about 6:30 am local time
- Place
- Calexico, California · map
- Type
- Accident
- Injuries
- 1 person had minor injuries.
- Weather
- visual conditions (good weather)
- Aircraft
- Air Tractor AT-401
- Registration
- N4538K · no longer on the register · serial 401-0770
- Damage
- Destroyed
- Flight
- Aerial application (crop spraying) flight · agricultural flying rules (Part 137)
The NTSB's narrative final · quoted from the NTSB record
THE INVESTIGATION REVEALED THAT DURING THE DAY ON JULY 9TH, WORKERS FROM THE IMPERIAL COUNTY IRRIGATION DISTRICT ERECTED A LARGE CAUTION SIGN ON THE DEPARTURE RUNWAY ABOUT ONE THIRD OF THE WAY FROM THE START. THAT NIGHT THE PILOT ARRIVED AT THE AIRPORT FOR HIS NIGHT SPRAY RUN AND HE WAS UNAWARE OF THE NEWLY PLACED SIGN. AFTER LOADING THE CHEMICALS TO BE SPRAYED ON BOARD THE AIRCRAFT, THE PILOT TOOK OFF AND THE AIRCRAFT STRUCK THE SIGN WITH THE LEFT SPRAY BOOM. THE BOOM WAS TORN LOOSE FROM THE STRUCTURE, HOWEVER, DID NOT SEPARATE TOTALLY AND BEGAN TO FLAP IN THE AIRSTREAM AS CHEMICALS POURED FROM THE HOPPER. THE PILOT TURNED THE AIRCRAFT TO RETURN TO THE AIRSTRIP AS HE REACHED FOR THE EMERGENCY SHUTOFF VALVE TO STOP THE FLOW OF CHEMICALS. AS THE AIRCRAFT NEARED THE AIRSTRIP, IT COLLIDED WITH A STACK OF HAY BALES RECENTLY ERECTED NEAR THE RUNWAY.
The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.
The factual record from the NTSB's investigation tables, in plain English
Pilot
- Certificate: commercial pilot
- Ratings: multi-engine land; single-engine land; rotorcraft: helicopter
- Flight time: 8,100 hours in all; 1,400 in this make and model; 160 in the last 90 days
- Medical certificate: Class 2 (valid medical--w/ waivers/lim.)
- Seat: unk
The aircraft
- Last inspection: inspection type not recorded, January 2, 2000
- Maximum gross weight: 6,000 lb
- Seats: 1
- Landing gear: fixed
- Engine: P&W R-1340 (piston); 0 hours total
The flight
- Flight plan: none
- Runway 0
Weather at the time
- Light: night, dark
- Wind: from 250° at 6 knots
- Visibility: 30 statute miles
- Sky: clear
- Temperature: 81°F (27°C), dew point 0°F (-18°C)
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Crew | 1 |
Documents from the investigation the NTSB's docket: the evidence folder behind the report
The docket, the folder of records gathered during an investigation (maintenance records, photographs, witness statements, examinations), has not been read from the NTSB for this case yet; the list appears here once it has. Open the docket at the NTSB.
Everything on this page comes from the NTSB's public records. The narrative, probable cause and findings are the NTSB's own words; the coded tables behind the report are written out in plain English, with pilots' ages, home towns and medical details left out. The documents and photographs are the NTSB's docket, shown as the NTSB released them. This site's own text never names anyone involved. A preliminary report can change; the final report usually follows one to two years later, and the page is refreshed when it does.