Bell 412SP and Piper AEROSTAR 601 mid-air collision near Merion, Pennsylvania, April 4, 1991
On April 4, 1991 at about 5:10 pm local time, 2 aircraft, Bell 412SP (N78S) and Piper AEROSTAR 601 (N3645D), were involved in a mid-air collision near Merion, Pennsylvania. 5 people were killed. The weather was visual conditions (good weather).
The NTSB's probable cause their words, unchanged
THE POOR JUDGEMENT BY THE CAPTAIN OF THE AIRPLANE TO PERMIT THE IN-FLIGHT INSPECTION AFTER HE HAD DETERMINED TO THE BEST OF HIS ABILITY THAT THE NOSE LANDING GEAR WAS FULLY EXTENDED, THE POOR JUDGMENT OF THE CAPTAIN OF THE HELICOPTER TO CONDUCT THE INSPECTION, AND THE FAILURE OF THE FLIGHTCREW OF THE HELICOPTER TO MAINTAIN SAFE SEPARATION.
THE POOR JUDGEMENT BY THE CAPTAIN OF THE AIRPLANE TO PERMIT THE IN-FLIGHT INSPECTION AFTER HE HAD DETERMINED TO THE BEST OF HIS ABILITY THAT THE NOSE LANDING GEAR WAS FULLY EXTENDED, THE POOR JUDGMENT OF THE CAPTAIN OF THE HELICOPTER TO CONDUCT THE INSPECTION, AND THE FAILURE OF THE FLIGHTCREW OF THE HELICOPTER TO MAINTAIN SAFE SEPARATION. CONTRIBUTINGTO THE ACCIDENT WAS THE INCOMPLETE TRAINING AND CHECKING THAT THE FLIGHTCREW OF N3645D RECEIVED FROM LYCOMING AIR SERVICE AND THE FAA PRINCIPAL OPERATIONS INSPECTOR ASSIGNED TO THE OPERATOR.
Source: NTSB aviation accident database, copy made October 5, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- April 4, 1991 · about 5:10 pm local time
- Place
- Merion, Pennsylvania · map
- Type
- Accident · mid-air collision
- Injuries
- 5 people were killed.
- Weather
- visual conditions (good weather)
- Aircraft 1
- Bell 412SP · all 412SPs on the register
- Registration
- N78S · registry record · serial 33114
- Damage
- Destroyed
- Flight
- Executive or corporate flight · general aviation rules (Part 91)
- Aircraft 2
- Piper AEROSTAR 601 AEROSTAR 6 · all AEROSTAR 601s on the register
- Registration
- N3645D · no longer on the register · serial 61-0844
- Damage
- Destroyed
- Flight
- Flight of an unrecorded kind · charter and air-taxi rules (Part 135)
The NTSB's narrative for the Bell 412SP final · quoted from the NTSB record
PIPER AEROSTAR 601, N3645D, WAS ARRG AS BELL 412 (HELICOPTER), N78S, WAS DEPG. N3645D RPRTD THE ACFT'S NOSE GEAR INDCR LGT DID NOT ILLUMINATE TO VERIFY THE NOSE GEAR WAS DWN & LOCKED. CREW OF N78S HEARD THE COM & SAW N3645D AS THEY CROSSEDBENEATH N3645D'S APCH PATH. THEY RPRTD N3645D'S NOSE GEAR APPEARED TO BE EXTDD. N3645D FLEW PAST THE TWR & TWR PSNL ALSORPRTD THE NOSE GEAR APPEARED TO BE EXTDD. AS N3645D WAS CLRD TO TURN DWNWND, N78S OFFERED TO JOIN FOR A CLOSER LOOK. CAPT OF N3645D ACCEPTED THE OFFER. DRG JOINUP, N78S CONVERGED FM LEFT REAR, THEN RPRTD THEY WOULD PASS ARND THE RGT SIDE& LOOK AT EVERYTHING AS THEY WENT BY. THEY RPRTD THAT EVERYTHING LOOKED OK. MOMENTS LTR, THE 2 ACFT COLLIDED & CRASHED TO THE GND. NO MECH RSN WAS FND THAT WOULD HAVE RESULTED IN THE ACDNT. NONE OF THE PLTS HAD TRNG FOR FLT IN CLOSE PROX- IMITY TO ANOTHER ACFT. THERE WERE INDCNS THE CAPT OF N3645D LACKED TRNG IN ACFT SYSTEMS & THAT HIS EMPLOYER (THE OPER) LACKED SURVEILLANCE BY THE FAA. THE EMERG PROC SXN OF THE AEROSTAR FLT MANUAL LACKED INFO ON EMERG GEAR EXTN.
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 NTSB's narrative for the Piper AEROSTAR 601 final · quoted from the NTSB record
PIPER AEROSTAR 601, N3645D, WAS ARRG AS BELL 412 (HELICOPTER), N78S, WAS DEPG. N3645D RPRTD THE ACFT'S NOSE GEAR INDCR LGT DID NOT ILLUMINATE TO VERIFY THE NOSE GEAR WAS DWN & LOCKED. CREW OF N78S HEARD THE COM & SAW N3645D AS THEY CROSSEDBENEATH N3645D'S APCH PATH. THEY RPRTD N3546D'S NOSE GEAR APPEARED TO BE EXTDD. N3645D FLEW PAST THE TWR & TWR PSNL ALSORPRTD THE NOSE GEAR APPEARED TO BE EXTDD. AS N3645D WAS CLRD TO TURN DWNWND, N78S OFFERED TO JOIN FOR A CLOSER LOOK. CAPT OF N3645D ACCEPTED THE OFFER. DRG JOINUP, N78S CONVERGED FM LEFT REAR, THEN RPRTD THEY WOULD PASS ARND THE RGT SIDE& LOOK AT EVERYTHING AS THEY WENT BY. THEY RPRTD THAT EVERYTHING LOOKED OK. MOMENTS LTR, THE 2 ACFT COLLIDED & CRASHED TO THE GND. NO MECH RSN WAS FND THAT WOULD HAVE RESULTED IN THE ACDNT. NONE OF THE PLTS HAD TRNG FOR FLT IN CLOSE PROX- IMITY TO ANOTHER ACFT. THERE WERE INDCNS THE CAPT OF N3645D LACKED TRNG IN ACFT SYSTEMS & THAT HIS EMPLOYER (THE OPER) LACKED SURVEILLANCE BY THE FAA. THE EMERG PROC SXN OF THE AEROSTAR FLT MANUAL LACKED INFO ON EMERG GEAR EXTN.
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 for the Bell 412SP from the NTSB's investigation tables, in plain English
Pilot
- Certificate: airline transport pilot, flight instructor, commercial pilot
- Ratings: multi-engine land; single-engine land; instructor: airplane multi-engine; instructor: airplane single-engine; instructor: helicopter; instructor: instrument airplane; instructor: instrument helicopter; instrument: airplane; instrument: helicopter; rotorcraft: helicopter
- Flight time: 8,000 hours in all; 17 in the last 90 days; 6 in the last 30 days
- Medical certificate: Class 2 (valid medical--no waivers/lim.)
- Seat: unk
The aircraft
- Airframe total time: 1,841 hours
- Last inspection: annual inspection, February 12, 1991; 6 hours since
- Maximum gross weight: 11,900 lb
- Seats: 7
- Landing gear: fixed
- Engine: P&W PT6T-3B (turboshaft); 0 hours total
- Fire on the ground
- Operator: Sun Company INC.
The flight
- Departed from: PHL Philadelphia PA at 5:00 pm
- Destination: Radnor PA
- Flight plan: VFR
- Runway 0
- A second pilot was aboard
Weather at the time
- Light: daylight
- Wind: from 240° at 10 knots
- Visibility: 10 statute miles
- Sky: scat at 25,000 ft
- Temperature: 63°F (17°C), dew point 41°F (5°C)
- Altimeter: 30.00 inHg
- Observation at 4:55 pm from PHL, 8 miles away
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Crew | 2 |
The factual record for the Piper AEROSTAR 601 from the NTSB's investigation tables, in plain English
Pilot
- Certificate: airline transport pilot, flight instructor, commercial pilot
- Ratings: multi-engine land; single-engine land; instructor: airplane multi-engine; instructor: airplane single-engine; instructor: instrument airplane; instrument: airplane
- Flight time: 1,876 hours in all
- Medical certificate: Class 2 (valid medical--w/ waivers/lim.)
- Seat: left
The aircraft
- Airframe total time: 2,739 hours
- Last inspection: annual inspection, February 25, 1991; 25 hours since
- Maximum gross weight: 6,000 lb
- Seats: 6
- Landing gear: retractable
- Engine: Lycoming IO-540-S1A5 (piston); 0 hours total
- Fire on the ground
- Operator: Lycoming Air Service
The flight
- Departed from: IPT Williamsport PA
- Destination: PHL Philadelphia PA
- Flight plan: IFR
- Runway 0
- A second pilot was aboard
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Crew | 2 | |||
| Passengers | 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.