# NTSB NYC91LA104: Piper J-4A, N23179, Mahopac, NY, April 7, 1991

- Type: NTSB accident investigation
- Classification: Accident · Minor injuries · Limited investigation · Final report
- Date: April 7, 1991, 12:48 local (16:48 UTC)
- Place: Mahopac, NY
- Coordinates: 41.359058, -73.739799 (estimated)
- Nearby events: https://openairdocs.org/map.md?lat=41.3591&lon=-73.7398&mi=10
- Phase of flight: [Approach](https://openairdocs.org/ntsb-phases/500) (OpenAirDocs' grouping)
- Highest injury: Minor injuries
- Injuries: 2 minor
- Status: Completed, report May 27, 1993
- Report: Approved by Office Director
- NTSB case key (mKey): 36901
- NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=NYC91LA104
- NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/36901/pdf
- Page: https://openairdocs.org/ntsb/NYC91LA104

## Probable cause

THE ERRATIC PERFORMANCE OF THE LEFT MAGNETO RESULTING IN INADEQUATE CLIMB CAPABILITIES AND IN FLIGHT COLLISION WITH TERRAIN.

## Sequence of events

In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf).

- Loss of engine power(partial) - mech failure/malf, go-around (vfr)
  - ignition system,ignition coil - deteriorated
  - maintenance - inadequate, pilot in command
  - ignition system,distributor - worn
  - ignition system,magneto - erratic
- In flight collision with object, maneuvering
  - object - tree(s)
  - terrain condition - rising
  - terrain condition - ground

## Analysis

DURING AN INITIATED GO-AROUND AT MAHOPAC AIRPORT, MAHOPAC, NEW YORK, THE ENGINE EXPERIENCED PARTIAL POWER LOSS.  FULL POWER COULD NOT BE DEVELOPED RESULTING IN DETERIORATED CLIMB CAPABILITIES.  THE AIRPLANE COULD NOT CLEAR RISING TERRAIN AND HIT TREETOPS, EVENTUALLY CONTACTING THE GROUND. POST ACCIDENT EXAMINATION OF THE ENGINE REVEALED THAT THE LEFT MAGNETO WAS EXCESSIVELY WORN AND DETERIORATED CAUSING THE SPARKS TO BE ERRATICALLY DISPLACED TO THE SPARK PLUGS.

## Aircraft

- Registration: N23179 (not matched to a registry record)
- Reported make and model: PIPER J-4A
- Category: Airplane
- Serial number: 4-546
- Airframe hours: 1853
- Engines: CONTINENTAL A-65 65 horsepower reciprocating
- Damage: Substantial damage
- Flight: Personal, Part 91
- Operator: Michael Demaio
- Registered owner: Michael Demaio
- Airworthiness certificate: Standard, Normal
- Last inspection: Annual, October 1, 1990, 25 hours before
- Departed: DXR, DANBURY, CT

## Flight crew

As NTSB recorded them. NTSB leaves out names.

- Pilot; age 49; private; airplane ratings: single-engine land; medical: class 3, valid medical--w/ waivers/lim; flight hours: 249 total, 20 in this make and model, 150 as pilot in command, 3 in the last 90 days, 1 in the last 30 days

## Weather

- Conditions: Visual (VMC), Day
- Visibility: 15 statute miles
- Lowest ceiling: none
- Temperature: 84 °F, dew point 48 °F

## FAA reports on this event (1)

The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs).

- [Accident 19910407009219A](https://openairdocs.org/incident/19910407009219A), Apr 7, 1991: Piper J4A, Mahopac, NY, Substantial damage. During go around. Partial power loss hit tree tops. Left magneto worn and deteriated. Caused erratic spark.

From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model.

---
OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.