# NTSB WPR20LA035: Zenith Stol CH701, N701WJ, Byron, CA, November 29, 2019 - Type: NTSB accident investigation - Classification: Accident · No injuries · Class 3 · Limited · Final report - Date: November 29, 2019, 15:45 local (23:45 UTC) - Place: Byron, CA - Coordinates: 37.830276, -121.626113 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=37.8303&lon=-121.6261&mi=10 - Phase of flight: [Approach](https://openairdocs.org/ntsb-phases/500) (OpenAirDocs' grouping) - Defining event: [Approach-VFR pattern base: Loss of engine power (partial)](https://openairdocs.org/ntsb-events/342) - Highest injury: No injuries - Injuries: 1 uninjured - Status: Completed - Report: Approved by Office Director - Investigation class: Class 3 - NTSB office: Western Pacific Region - Parties: Federal Aviation Administration - NTSB case key (mKey): 100634 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=WPR20LA035 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/100634/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=100634 - Page: https://openairdocs.org/ntsb/WPR20LA035 ## Probable cause The pilot/builder's improper construction of the cowling, which resulted in inadequate venting of the engine, its overheating, and a partial loss of engine power. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Power plant: engine cowling system](https://openairdocs.org/ntsb-findings/01057110): Aircraft › Aircraft power plant · Design (probable cause); cases with it: [Soto William J CH701, 1 case](https://openairdocs.org/ntsb-findings/01057110/cases.md?model=056375K), [All aircraft, 11 cases](https://openairdocs.org/ntsb-findings/01057110/cases.md) - [Power plant: engine cowling system](https://openairdocs.org/ntsb-findings/01057110): Aircraft › Aircraft power plant · Incorrect use/operation (probable cause); cases with it: [Soto William J CH701, 1 case](https://openairdocs.org/ntsb-findings/01057110/cases.md?model=056375K), [All aircraft, 11 cases](https://openairdocs.org/ntsb-findings/01057110/cases.md) - [Maintenance: fabrication](https://openairdocs.org/ntsb-findings/02062025): Personnel issues › Task performance · Owner/builder (probable cause); cases with it: [Soto William J CH701, 1 case](https://openairdocs.org/ntsb-findings/02062025/cases.md?model=056375K), [All aircraft, 30 cases](https://openairdocs.org/ntsb-findings/02062025/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Approach-VFR pattern base: Loss of engine power (partial)](https://openairdocs.org/ntsb-events/342) (defining event); cases with it: [Soto William J CH701, 1 case](https://openairdocs.org/ntsb-events/342/sequence.md?model=056375K), [All aircraft, 1,858 cases](https://openairdocs.org/ntsb-events/342/sequence.md) ## Analysis The pilot stated that he fabricated and installed a cowling on the airframe prior to the flight. During the initial climb, the engine experienced a partial loss of power and the water temperature climbed above the maximum temperature. The pilot made a forced landing at the departure airport and encountered wind during the approach. The airplane was pushed off the runway and landed hard in the grass; the airplane came to rest inverted. The engine manufacturer provided instructions to builders to ensure there is proper venting over the engine. The pilot stated that he did not use any plans to build the cowling and did not perform any tests of the venting. The cowling was open on the bottom but did not have aft venting. As a result, the air pressure behind the radiator likely increased to be equivalent to that of the ram air, resulting in the engine overheating and experiencing detonation, leading to the loss of engine power. ## Factual narrative On November 29, 2019, about 1545 Pacific standard time, an experimental amateur built Zenith CH-701 STOL airplane, N701WJ, was substantially damaged when it was involved in an accident near Byron Airport, Byron, California. The pilot was not injured. The airplane was operated under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. The pilot stated that he had recently fabricated and installed a cowling for the airplane. After completing an uneventful high-speed taxi test run, he decided to depart and remain in the pattern. The pilot configured the airplane for takeoff on runway 23 and applied full power. He turned onto the crosswind leg of the traffic pattern and felt an abnormal vibration accompanied with a slight loss in engine rpm. He continued onto the downwind leg and the airplane was about 500 ft above ground level (agl) when he was midfield. The airplane started to lose altitude and he observed the water temperature gauge indicating 250° F; maximum operating temperature was 220° F. The pilot further stated that the airplane continued to descend and at about 100 ft agl he attempted to land on runway 30. While trying to align with the runway, the wind pushed the airplane onto the grass. The airplane touched down hard and nosed over, coming to rest inverted. The airplane sustained substantial damage to the wing struts. The airplane was equipped with the originally installed AeroMomentum AM13 (serial number AM0314A). the pilot stated that the airframe and engine had accumulated a total time of 115 hours. A representative from AeroMomentum stated that because of the design of the airplane, specifically with slats on the leading edge, it was capable of being configured at an unusually high angle of attack during departure. With such a high angle of attack, the bottom of the airplane would have a high air pressure at the firewall. With no venting aft, the air pressure behind the radiator would likely build to be equivalent to that of the ram air. This would result in overheating and the engine would subsequently experience detonation. The AeroMomentum installation manual stated that: The radiator must be mounted so that there is a minimum of 3 inches of water differential air pressure between the front and back. If you do not have this differential air pressure there will not be adequate airflow for cooling. You can use a water manometer to verify this required pressure and airflow. In general, the inlet should have ram air and the outlet should be in a low-pressure area. The inlet area must be no less than 40% the area of the radiator (more for slower aircraft) and the outlet should be about twice the total inlet area. Keep in mind aerodynamics and how they affect the airflow both in and out of the cowling. Sharp corners at the lower edge of the firewall can have a profoundly reduce the exit airflow. High pressure at the bottom of the aircraft especially during climb can reduce the exit airflow. Obstructions likethe tailpipe can reduce the exit area and airflow. The manual further stated that "high engine temperatures can also cause preignition, detonation and knocking that will reduce power and can cause catastrophic engine failure." The pilot stated that he built the cowling with no plans and did not perform any tests of air pressures over the engine. The cowling was open on the bottom but did not have aft venting. ## Aircraft - Registration: [N701WJ](https://openairdocs.org/aircraft/N701WJ), registered now, S/N 7-10529, built 2019 (OpenAirDocs) - Reported make and model: ZENITH STOL CH701 - Registry model: [Soto William J CH701](https://openairdocs.org/model/soto-william-j-ch701) (registry code 056375K) (OpenAirDocs) - Registry engine: [Aeromoment AM13 series](https://openairdocs.org/engine/aeromoment-am13-series) (registry code 00450, 4 cycle) (OpenAirDocs) - Category: Airplane, amateur-built - Serial number: 7-10529 - Year built: 2019 - Airframe hours: 115 - Engines: AEROMOMENTUM AM13 100 horsepower reciprocating - Damage: Substantial damage - Flight: Flight test, Part 91 - Airworthiness certificate: Special, Experimental (Special) - Departed: C83, Byron, CA - Destination: C83, Byron, CA - Airport: C83, Byron Airport, runway 23, 3,000 × 75 ft, asphalt - Equipment: Electronic flight bag or handheld device, Handheld GPS ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 56; injuries: none; private; airplane ratings: single-engine land; medical: class 3, with waivers/limitations; last flight review October 23, 2018; seat: left; restraint: 3-point; flight hours: 339 total, 87 in this make and model, 297 as pilot in command, 36 in the last 90 days, 6 in the last 30 days, 1 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 140° at 4 knots - Visibility: 10 statute miles - Lowest ceiling: 7,500 ft broken - Temperature: 52 °F, dew point 37 °F - Altimeter: 29.88 inHg - Observed at: KSCK, 19 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 20191129022279A](https://openairdocs.org/incident/20191129022279A), Nov 29, 2019: Soto William J CH701, Byron, CA, Substantial damage. N701WJ accident @ Byron airport (C83) on 11/30/19, 0000Z. Crashed about 700 ft short of runway 23. POB:1; injuries: none; damage: substantial. ## Other investigations on N701WJ (1) 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 WPR26LA324](https://openairdocs.org/ntsb/WPR26LA324), Aug 30, 2026: Zenith CH701 (N701WJ), Byron, CA, substantial damage, investigation ongoing ## Docket (4 files) The files NTSB released with the investigation on June 7, 2022, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=100634. 4.1 MB in all. - Pilot/operator aircraft accident report, NTSB form 6120.1 (item 1, PDF, 11 pages, 3.3 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=9142169&FileExtension=pdf&FileName=Z%20-%20Attachment%2001%20-%206120_Redacted-Rel.pdf - Records of conversation (item 2, PDF, 1 page, 81 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=9143025&FileExtension=pdf&FileName=Z%20-%20Attachment%2004%20-Record%20of%20Conversations-Rel.pdf - Aeromomentum engine manual excerpt (item 3, PDF, 6 pages, 555 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=9143118&FileExtension=pdf&FileName=Z%20-%20Attachment%2002%20-Engine%20Manual%20Excerpts-Rel.pdf - Post accident pictures (provided by pilot) (item 4, PDF, 2 pages, 230 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=9143127&FileExtension=pdf&FileName=Z%20-%20Attachment%2003%20-Post%20Accident%20Pictures-Rel.pdf 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.