# NTSB FTW00LA242: Mitsubishi MU-2B-35, N770MA, Corsicana, TX, August 24, 2000 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: August 24, 2000, 02:15 local (07:15 UTC) - Place: Corsicana, TX - Coordinates: 32.079231, -96.46051 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=32.0792&lon=-96.4605&mi=10 - Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 2 uninjured - Status: Completed, report July 17, 2001 - Report: Approved by Office Director - NTSB case key (mKey): 50091 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=FTW00LA242 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/50091/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=50091 - Page: https://openairdocs.org/ntsb/FTW00LA242 ## Probable cause the in-flight separation of the propeller blade, which resulted from intergranular corrosion and fatigue cracking. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Propeller failure/malfunction, cruise - normal - propeller system/accessories,blade - corroded (cause) - propeller system/accessories,blade - fatigue (cause) - propeller system/accessories,blade - separation (cause) ## Analysis The twin-engine airplane was in cruise flight at 11,000 feet msl, when the pilot heard a loud bang and felt the airplane vibrate. The pilot shut down the right engine, feathered the propeller, and landed the airplane at an alternate airport. Examination of the right propeller revealed that one of its propeller blades separated approximately 8.5 inches from the hub. This blade was manufactured from 7076 aluminum alloy, which is referred to as a 'hard alloy.' Examination of the propeller blade's fracture surface revealed that it failed as a result of fatigue cracking, which originated from an area of intergranular corrosion. The propeller was overhauled 222 hours prior to the accident. According to the overhaul facility, they used a process of Alodining the blades for corrosion control, which is an approved method. According to the propeller manufacturer, the preferred method of corrosion control is anodizing due to its corrosion detection properties; however, it is a more expensive and time consuming corrosion control process. As a result of this accident, the propeller manufacturer issued a service bulletin indicating that 'hard alloy' blades should be removed from service due to their susceptibility to intergranular corrosion. ## Factual narrative On August 24, 2000, at 0215 central daylight time, a Mitsubishi MU-2B-35 twin-engine airplane, N770MA, was substantially damaged when one of the propeller blades from the right propeller separated while in cruise flight near Corsicana, Texas. The commercial pilot-in-command and his co-pilot were not injured. The airplane was registered to and operated by International Business Aircraft, Inc., of Tulsa, Oklahoma. Dark night visual meteorological conditions prevailed and an instrument flight rules flight plan was filed for the 14 Code of Federal Regulations Part 135 on-demand air taxi flight. The cargo flight originated from Dallas, Texas, and was destined for Houston, Texas. According to the pilot, the airplane was in cruise flight at 11,000 feet msl, when he heard a loud bang and felt the airplane vibrate. The pilot shut down the right engine, feathered the propeller, and then received vectors from air traffic control to the Corsicana Airport. Upon landing, the pilot noticed that one of the propeller blades had separated from the right engine's propeller. According to the operator's chief mechanic, the right engine firewall was buckled, and the right wing was twisted. The right engine cowling and nacelle were damaged. The airplane was transported to the operator's facility and the Hartzell HC-B3TN-5E propeller was disassembled on August 30, 2000, under the supervision of an FAA inspector. The fractured propeller blade was shipped to the NTSB Materials Laboratory in Washington, D.C., for further examination. According to the NTSB Materials Laboratory report, the propeller blade (serial number C24680) was fractured approximately 8.5 inches from the butt end of the blade, and was covered with a solid layer of gray paint and black topcoat on the flat side. In addition, part of the de-ice boot remained firmly bonded in place. The blade was manufactured with 7076 aluminum alloy. The blade's fracture surface displayed characteristics of fatigue cracking. The fracture marking indicated that the fatigue had initiated on the camber surface about 2.25 inches from the leading edge. Examination of the fatigue initiation area revealed a small darker appearing region at the fatigue origin. The darker origin area displayed a mixture of intergranular separation and fatigue features. At least two secondary fatigue origins were found emanating directly from intergranular features within the darker region. The 7076 alluminum alloy blades were referred to as "hard alloy" blades. Hartzell Propeller used 2025 aluminum alloy for most other blades, which were referred to as "standard alloy." Hartzell's overhaul and maintenance manual (133C; 05/20/96) stated that "hard alloy blades can be susceptible to intergranular corrosion." According to the propeller maintenance records, the propeller was overhauled on November 11, 1999, by an FAA approved repair station at an aircraft total time of 8,779 hours. At the time of the accident, the propeller had accumulated 222 hours since its last overhaul. According to Hartzell Propellers, their overhaul procedures call for the removal of 0.010 inches of material from the entire blade surface of hard alloy blades at each overhaul. In addition, the Hartzell Propeller overhaul procedures require that the propeller blades be re-anodized or protected with a chemical conversion coating, commonly referred to as Alodine. Alodining is a more cost effective process than anodizing. The anodizing corrosion control process; however, is known to help expose deep-rooted corrosion. According to the propeller overhaul facility, they removed the required amount of material and used the Alodine process on the propeller blade that failed. Upon reviewing the NTSB Materials Laboratory Report, the propeller manufacturer stated that that the failure of the blade resulted from intergranular corrosion and that the corrosion appears to have existed at the time of the propeller overhaul. According to the propeller manufacturer, a Service Bulletin (SB 250) was issued after the accident, indicating that the "hard alloy" blades should be removed from service due to their susceptibility to intergranular corrosion. In addition, the propeller manufacturer revised their overhaul manuals to require the anodizing corrosion control process only, until the "hard alloy" blades are removed from service. ## Aircraft - Registration: [N770MA](https://openairdocs.org/deregistered/N770MA-2007-02-16), registration cancelled 2007-02-16, S/N 625 (OpenAirDocs) - Reported make and model: Mitsubishi MU-2B-35 - Registry model: [Mitsubishi MU-2B-35](https://openairdocs.org/model/mitsubishi-mu-2b-35) (registry code 5780411) (OpenAirDocs) - Registry engine: [AiResearch TPE331-5&6SER](https://openairdocs.org/engine/airesearch-tpe331-5-6ser) (registry code 01515, turbo-prop) (OpenAirDocs) - Category: Airplane - Serial number: 625 - Airframe hours: 9001 - Engines: Garrett TPE-331-6-252 665 horsepower turbo prop - Damage: Substantial damage - Flight: Part 135 - Registered owner: International Business Acft - Airworthiness certificate: Standard, Normal - Last inspection: AAIP, August 14, 2000, 24 hours before - Departed: DAL, DALLAS, TX - Destination: HOU, HOUSTON, TX - Flight plan: IFR - Airport: CRS, C. David Campbell Field, runway 14, 4,999 × 75 ft, asphalt ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 38; commercial, flight instructor; airplane ratings: multi-engine land, single-engine land; instrument ratings: airplane; medical: class 1, valid medical--no waivers/lim.; flight hours: 5,511 total, 3,386 in this make and model, 5,211 as pilot in command, 138 in the last 90 days, 72 in the last 30 days, 5 in the last 24 hours ## Weather - Conditions: Visual (VMC), Night/dark - Wind: 130° at 5 knots - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 84 °F, dew point 61 °F - Altimeter: 29.00 inHg ## 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 20000824015709A](https://openairdocs.org/incident/20000824015709A), Aug 24, 2000: Mtsbsi MU2B35, Corsicana, TX, No damage. (-23) On August 24, 2000, at 02:15 CDT, mitsubishi MU-2B-35 airplane N770MA, was on a flight from dallas, TX. To houston, TX., when one of the propeller blades… ## Other investigations on N770MA (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 CHI81FET04](https://openairdocs.org/ntsb/CHI81FET04), Jan 21, 1981: Mitsubishi MU-2B (N770MA), Wabash, IN, substantial damage ## Docket (20 files) The files NTSB released with the investigation on June 11, 2001, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=50091. - Supplement E - Second Pilot Information (item 1, .PDF, 142 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40475131&FileExtension=.PDF&FileName=Supplement%20E%20-%20Second%20Pilot%20Information-Master.PDF - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 2, .PDF, 534 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40475133&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Master.PDF - NTSB Materials Laboratory Report (item 3, .PDF, 111 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40475134&FileExtension=.PDF&FileName=NTSB%20Materials%20Laboratory%20Report-Master.PDF - NTSB Materials Laboratory Report, Figure 1 (item 4, .PDF, 28 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40141546&FileExtension=.PDF&FileName=NTSB%20Materials%20Laboratory%20Report,%20Figure%201-Master.PDF - NTSB Materials Laboratory Report, Figure 2 (item 5, .PDF, 39 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40141547&FileExtension=.PDF&FileName=NTSB%20Materials%20Laboratory%20Report,%20Figure%202-Master.PDF - NTSB Materials Laboratory Report, Figure 3 (item 6, .PDF, 45 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40141548&FileExtension=.PDF&FileName=NTSB%20Materials%20Laboratory%20Report,%20Figure%203-Master.PDF - NTSB Materials Laboratory Report, Figure 4 (item 7, .PDF, 42 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40141549&FileExtension=.PDF&FileName=NTSB%20Materials%20Laboratory%20Report,%20Figure%204-Master.PDF - NTSB Materials Laboratory Report, Figure 5 (item 8, .PDF, 51 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40141550&FileExtension=.PDF&FileName=NTSB%20Materials%20Laboratory%20Report,%20Figure%205-Master.PDF - NTSB Materials Laboratory Report, Figure 6 (item 9, .PDF, 49 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40141551&FileExtension=.PDF&FileName=NTSB%20Materials%20Laboratory%20Report,%20Figure%206-Master.PDF - NTSB Materials Laboratory Report, Figures 7, 8 & 9 (item 10, .PDF, 39 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40141552&FileExtension=.PDF&FileName=NTSB%20Materials%20Laboratory%20Report,%20Figures%207,%208%20%26%209-Master.PDF - NTSB Materials Laboratory Report, Figure 10 (item 11, .PDF, 31 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40141553&FileExtension=.PDF&FileName=NTSB%20Materials%20Laboratory%20Report,%20Figure%2010-Master.PDF - Propeller Logbook Records (item 12, .PDF, 695 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40475135&FileExtension=.PDF&FileName=Propeller%20Logbook%20Records-Master.PDF - Propeller Manufacturer's Report (item 13, .PDF, 82 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40475136&FileExtension=.PDF&FileName=Propeller%20Manufacturer%27s%20Report-Master.PDF - Overhaul Manual Excerpts (item 14, .PDF, 2.5 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40141556&FileExtension=.PDF&FileName=Overhaul%20Manual%20Excerpts-Master.PDF - Hartzell Service Bulletin SB 250 (item 15, .PDF, 261 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40141557&FileExtension=.PDF&FileName=Hartzell%20Service%20Bulletin%20SB%20250-Master.PDF - NTSB Materials Laboratory Division Receipt of Returned Parts (item 16, .PDF, 158 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40475137&FileExtension=.PDF&FileName=NTSB%20Materials%20Laboratory%20Division%20Receipt%20of%20Returned%20Parts-Master.PDF - Statement of Party Representatives to NTSB Investigation (item 17, .PDF, 120 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40475140&FileExtension=.PDF&FileName=Statement%20of%20Party%20Representatives%20to%20NTSB%20Investigation-Master.PDF - Photo 01: Propeller after removal from engine (No neg. Hartzell) (item 18, .PDF): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40141560&FileExtension=.PDF&FileName=Photo%2001%3A%20Propeller%20after%20removal%20from%20engine%20(No%20neg.%20Hartzell)-Master.PDF - Photo 02: Fracture surface of failed propeller blade (No neg. Harzell) (item 19, .PDF, 62 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40141561&FileExtension=.PDF&FileName=Photo%2002%3A%20Fracture%20surface%20of%20failed%20propeller%20blade%20(No%20neg.%20Harzell)-Master.PDF - Photo 03: Close up view of propeller's fatigue origin (No neg. Hartzell) (item 20, .PDF, 71 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40141562&FileExtension=.PDF&FileName=Photo%2003%3A%20Close%20up%20view%20of%20propeller%27s%20fatigue%20origin%20(No%20neg.%20Hartzell)-Master.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.