# NTSB SEA95LA154: Air Tractor AT-301, N4326S, Roberts, ID, July 17, 1995 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: July 17, 1995, 08:15 local (14:15 UTC) - Place: Roberts, ID - Coordinates: 43.730209, -112.260932 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=43.7302&lon=-112.2609&mi=10 - Phase of flight: [Initial climb](https://openairdocs.org/ntsb-phases/350) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 1 uninjured - Status: Completed, report February 8, 1996 - Report: Approved by Office Director - NTSB case key (mKey): 42196 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=SEA95LA154 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/42196/pdf - Page: https://openairdocs.org/ntsb/SEA95LA154 ## Probable cause A LOSS OF ENGINE POWER DUE TO THE SEPARATION OF THE NUMBER 3 CYLINDER EXHAUST VALVE ROCKER ARM SHAFT FROM THE CYLINDER HEAD, WHICH DISABLED MOVEMENT OF THE EXHAUST VALVE. A FACTOR WAS THE LACK OF SUITABLE TERRAIN FOR THE FORCED LANDING. ## 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, takeoff - initial climb - engine assembly,rocker arm/tappet - separation (cause) - engine assembly,valve,exhaust - disabled (cause) - Forced landing, emergency landing after takeoff - On ground/water encounter with terrain/water, emergency landing after takeoff - terrain condition - none suitable (contributing factor) - terrain condition - crop ## Analysis THE PILOT REPORTED THAT SHORTLY AFTER TAKEOFF, AT 40 FEET ABOVE GROUND LEVEL, THE ENGINE LOST POWER AND BEGAN BACKFIRING. THE PILOT MADE A FORCED LANDING IN A BARLEY FIELD WITH 3-FOOT-TALL CROPS, AND THE AIRPLANE CARTWHEELED. POSTACCIDENT EXAMINATION OF THE ENGINE REVEALED THAT THE EXHAUST VALVE ROCKER ARM HOUSING ON THE NUMBER 3 CYLINDER SEPARATED, DISABLING THE NUMBER 3 CYLINDER EXHAUST VALVE. ## Factual narrative On July 17, 1995, at 0815 mountain daylight time, an Air Tractor 301, N4326S, owned by Lewis Flying Service, Inc., a 14 CFR 137 agricultural aircraft operator certificate holder, received substantial damage in a forced landing following a loss of engine power on takeoff from the pilot's private airstrip near Roberts, Idaho. The commercial pilot, a co-owner of the single-seat agricultural aircraft, was not injured. The flight was a 14 CFR 91 positioning flight to Rigby, Idaho. Visual meteorological conditions existed and no flight plan had been filed. The pilot stated in his report on the accident: "During climb- out at approx[imately] 40 foot altitude engine lost power and [began] backfiring. All terrain (360 deg) was cultivated fields with near mature crops...Established glide into barley field - 3 foot tall crop...attempted soft-field landing. Wheels contacted barley; aircraft immediately pitched over and cartwheeled on left wing...." A post-accident inspection of the Pratt & Whitney R-1340-AN1 radial engine revealed that the boss securing the exhaust valve rocker arm shaft to the number 3 cylinder head had separated from the cylinder head, allowing the entire rocker arm to move freely rather than pivot. This failure prevented the number 3 cylinder's exhaust valve from opening during engine operation. No other evidence of engine damage or malfunction was noted. According to the accident report and copies of the aircraft and engine logs submitted by the airplane's owner, the engine had been operated 201.3 hours since the airplane's last inspection (an annual inspection conducted on April 7, 1995), and 233.8 hours since its last overhaul. The engine was installed on the airplane at zero time since overhaul on August 7, 1994 according to the engine log. The FAA publication, "General Aviation Airworthiness Alerts" (AC 43-16), December 1995 issue, which the FAA distributes to FAA- certificated Inspection Authorization (IA) airframe and powerplant mechanics, contained an advisory for cylinder cracks on Pratt & Whitney model R-1340 engines. The advisory reported: "After receiving reports from three engine overhaul shops concerning cracks in the cylinder exhaust rocker shaft housing area, an investigation and interviews suggest this may be a systematic problem. Two repair stations stated they reject approximately 3 percent of the cylinders passing through their shops for this type defect. The defects in the exhaust rocker shaft housings range from slight cracking to complete separation of the housing above the rocker shaft....The Service Difficulty Reporting (SDR) data base revealed 42 similar reports, and 7 others which may have been related to this topic....the exact cause for cracking in this area has not been determined...Since the rocker shaft uses an 'O-ring' seal under each of the nuts, it is possible that when oil seepage is found, the nuts are tightened beyond the torque limits established by the manufacturer's maintenance manual....It is also possible that thermal expansion and contraction...may result in this type defect....It would be wise to pay close attention to this area during scheduled inspections." ## Aircraft - Registration: [N4326S](https://openairdocs.org/aircraft/N4326S), registered now, S/N 301-0006 (OpenAirDocs) - Reported make and model: AIR TRACTOR AT-301 - Registry model: [Air Tractor AT-301](https://openairdocs.org/model/air-tractor-at-301) (registry code 0390103) (OpenAirDocs) - Registry engine: [Pratt & Whitney R-985 series](https://openairdocs.org/engine/pratt-whitney-r-985-series) (registry code 52008, reciprocating) (OpenAirDocs) - Category: Airplane - Serial number: 301-0006 - Airframe hours: 7174 - Engines: P&W R-1340-AN1 600 horsepower reciprocating - Damage: Substantial damage - Flight: Positioning, Part 91 - Registered owner: Lewis, Orlin & Julia - Airworthiness certificate: Special, Restricted (Special) - Last inspection: Annual, April 7, 1995, 202 hours before - Destination: U56, RIGBY, ID ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 50; commercial; airplane ratings: single-engine land; instrument ratings: airplane; medical: class 2, valid medical--w/ waivers/lim; seat: center; flight hours: 8,554 total, 3,301 in this make and model, 8,433 as pilot in command, 220 in the last 90 days, 100 in the last 30 days, 8 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 350° at 5 knots - Visibility: 60 statute miles - Lowest ceiling: none - Temperature: 57 °F, dew point 48 °F - Altimeter: 30.00 inHg - Observed at: IDA, 12 nm away ## 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 19950717019149A](https://openairdocs.org/incident/19950717019149A), Jul 17, 1995: Airtrc AT301, Roberts, ID, Substantial damage. Lost power on takeoff. Flipped and carthweeled landing in barley. Exhaust valve rocker arm shaft had failed. ## Other investigations on N4326S (3) 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. - [NTSB CEN22LA354](https://openairdocs.org/ntsb/CEN22LA354), Aug 1, 2022: Air Tractor AT-301 (N4326S), Lamoni, IA, destroyed. Probable cause: A loss of airplane performance for reasons that could not be determined. - [NTSB SEA98LA079](https://openairdocs.org/ntsb/SEA98LA079), May 19, 1998: Air Tractor AT-301 (N4326S), Idaho Falls, ID, substantial damage. Probable cause: The pilot's inadequate inflight decision to land with a tailwind and his failure to maintain aircraft control. Also contributing was the pilot's improper setting of the fuel mixture which resulted in… - [NTSB LAX82DA265](https://openairdocs.org/ntsb/LAX82DA265), Jul 18, 1982: Air Tractor AT-301 (N4326S), San Joaquin, CA, substantial damage 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.