ASN Wikibase Occurrence # 75957
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| Date: | 28-JUL-2010 |
| Time: | 13:42 |
| Type: |  Eurocopter AS 350B3 Ecureuil |
| Owner/operator: | Air Methods Corp (AMC) |
| Registration: | N509AM |
| MSN: | 4698 |
| Fatalities: | Fatalities: 3 / Occupants: 3 |
| Aircraft damage: | Substantial |
| Category: | Accident |
| Location: | Tucson, AZ -
 United States of America
|
| Phase: | En route |
| Nature: | Ferry/positioning |
| Departure airport: | Marana Regional Airport, AZ (AVW/KAVQ) |
| Douglas Municipal Airport, AZ (DGL/KDGL) | |
| Investigating agency: | NTSB |
| Confidence Rating: |  Accident investigation report completed and information captured |
The single-engine air ambulance helicopter was operating near its maximum gross weight and was on a repositioning flight back to its home base. About 6 minutes into the flight, cruising at 800 feet above ground level (agl), the helicopter experienced a complete loss of engine power. Witnesses observed the helicopter, which had been flying steadily in a southeast direction, suddenly descend rapidly into a densely populated residential area. Descent rates calculated from the last 10 seconds of radar data were consistent with an autorotation. The witnesses reported that, as the helicopter neared the ground, its descent became increasingly vertical. Examination of the accident site revealed that the helicopter was in a level attitude with little forward speed when it impacted a 5-foot-high concrete wall, which penetrated the fuselage and ruptured the fuel tank. A postimpact fire consumed the cabin and main fuselage of the helicopter.
An open roadway intersection was located about 300 feet beyond the accident site, in line with the helicopter’s flight path. It is likely that the pilot was attempting to make an autorotative approach to the open area; however, he was unable to reach it because he had to maneuver the helicopter over a row of 40-foot-tall power lines that crossed the helicopter’s flight path near the accident site. This maneuver depleted the rotor rpm, which, as reported by the witnesses, caused the helicopter’s descent to become near vertical before it impacted the concrete wall, which was across the street from the power lines.
The pilot had no training flights during the 317 days since his most recent 14 Code of Federal Regulations Part 135 check flight. The lack of recent autorotation training/practice, although not required, may have negatively impacted the pilot’s ability to maintain proficiency in engine failure emergency procedures and autorotations. However, because the engine failed suddenly at low altitude over a congested area, more recent training may not have changed the outcome.
External examination of the engine at the accident site revealed that the fuel inlet union that connected to the fuel injection manifold and provided fuel from the hyrdomechanical unit to the combustion section had become detached from the boss on the compressor case. The two attachment bolts and associated nuts were not present on the union flange nor were they located within the helicopter wreckage debris. Separation of the fuel inlet union from the fuel injection manifold interrupted the supply of fuel to the engine and resulted in a loss of engine power. Postaccident engine runs performed with an exemplar engine showed that, with loose attachment bolts and nuts, the union initially remained installed and fuel would not immediately leak. As the engine continued to operate, the loose nuts would progressively unscrew themselves from the bolts. With the bolts removed, the union would ultimately eject from the boss, and the engine would lose power due to fuel starvation.
The helicopter's engine had undergone maintenance over several days preceding the accident. The maintenance was related to fuel coking of the fuel injection manifold. The operator's mechanics removed the engine from the helicopter and separated the modules. Another engine with the identical problem was also undergoing the same maintenance procedure at the time. A repair station technician was contracted to complete the maintenance on both engines. The operator's mechanics and the repair station technician disassembled the accident engine and set it aside. They then performed the required maintenance on the other engine, before returning to complete the work on the accident engine. While working on the accident engine, the repair station technician disassembled module 3, replaced the fuel injection manifold, and then reassembled the engine. This work required that the fuel inlet union be removed and reinstalled. It is likely that the technician did not tighten the bolts and nuts securing the union with a torque wrench and only finger tightened them. The eng
Probable Cause: The repair station technician did not properly install the fuel inlet union during reassembly of the engine; the operator’s maintenance personnel did not adequately inspect the technician's work; and the pilot who performed the post maintenance check flight did not follow the helicopter manufacturer's procedures. Also causal were the lack of requirements by the Federal Aviation Administration, the operator, and the repair station for an independent inspection of the work performed by the technician. A contributing factor was the inadequate oversight of the repair station by the Federal Aviation Administration, which resulted in the repair station performing recurring maintenance at the operator’s facilities without authorization.
Sources:
http://registry.faa.gov/aircraftinquiry/NNum_Results.aspx?omni=Home-N-Number&nNumberTxt=509AM
https://flightsafety.org/asw-article/installation-error/
Accident investigation:
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Location
Revision history:
| Date/time | Contributor | Updates |
|---|---|---|
| 28-Jul-2010 23:13 | slowkid | Added |
| 29-Jul-2010 04:28 | RobertMB | Updated [Aircraft type, Registration, Cn, Operator, Location, Nature, Destination airport, Source, Narrative] |
| 30-Jul-2014 20:26 | Dr. John Smith | Updated [Time, Departure airport, Source, Embed code, Narrative] |
| 30-Jul-2014 20:36 | Dr. John Smith | Updated [Operator, Source, Narrative] |
| 29-Mar-2015 12:43 | Aerossurance | Updated [Source, Narrative] |
| 31-Oct-2015 14:35 | Aerossurance | Updated [Source] |
| 21-Dec-2016 19:25 | ASN Update Bot | Updated [Time, Damage, Category, Investigating agency] |
| 26-Nov-2017 17:59 | ASN Update Bot | Updated [Operator, Other fatalities, Departure airport, Destination airport, Source, Embed code, Narrative] |
| 26-Oct-2022 17:44 | Aerossurance | Updated [Operator, Location, Departure airport, Destination airport, Source, Embed code, Narrative] |
| 26-Oct-2022 17:45 | Aerossurance | Updated [Operator, Source] |
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