# NTSB WPR14LA253: Mcdonnell Douglas Helicopter 500N, N522GS, Santa Monica, CA, April 18, 2014 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: April 18, 2014, 13:40 local (20:40 UTC) - Place: Santa Monica, CA - Coordinates: 34.016109, -118.451385 - Nearby events: https://openairdocs.org/map.md?lat=34.0161&lon=-118.4514&mi=10 - Phase of flight: [Landing](https://openairdocs.org/ntsb-phases/550) (OpenAirDocs' grouping) - Defining event: [Landing: Landing gear collapse](https://openairdocs.org/ntsb-events/094) - Highest injury: No injuries - Injuries: 4 uninjured - Status: Completed, report September 14, 2016 - Report: Approved by Office Director - NTSB office: Western Pacific Region - Parties: Federal Aviation Administration - ADs cited in the narratives: [AD 2007-12-23](https://openairdocs.org/ad/2007-12-23) (OpenAirDocs) - NTSB case key (mKey): 89491 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=WPR14LA253 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/89491/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=89491 - Page: https://openairdocs.org/ntsb/WPR14LA253 ## Probable cause The fracture of the right aft landing gear strut due to undetected fatigue cracking in an inboard rivet hole. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Landing gear system: main gear strut/axle/truck](https://openairdocs.org/ntsb-findings/01023213): Aircraft › Aircraft systems · Fatigue/wear/corrosion (probable cause); cases with it: [All aircraft, 86 cases](https://openairdocs.org/ntsb-findings/01023213/cases.md) - [Landing gear system: main gear strut/axle/truck](https://openairdocs.org/ntsb-findings/01023213): Aircraft › Aircraft systems · Failure (probable cause); cases with it: [All aircraft, 86 cases](https://openairdocs.org/ntsb-findings/01023213/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Landing: Landing gear collapse](https://openairdocs.org/ntsb-events/094) (defining event); cases with it: [All aircraft, 1,787 cases](https://openairdocs.org/ntsb-events/094/sequence.md) ## Analysis The private pilot/owner of the helicopter was conducting a local personal flight. The pilot reported that, upon landing, he slowly sat the helicopter down on a landing pad and then felt it "drastically" slip to the right. He then lifted the helicopter off the pad, at which time a passenger verified that the right aft landing gear strut was broken. A passenger reported that, upon touchdown, he heard a "loud pop." The Federal Aviation Administration (FAA) had previously issued an airworthiness directive (AD) applicable to the accident helicopter make and model to detect cracks that could result in the failure of the strut and subsequent loss of helicopter control during landing. The AD required modifications to the landing gear strut rivet holes and fairings and subsequent initial dye penetrant inspections followed by continuing 10x magnified visual inspections at intervals not to exceed every 100 hours time in service or during each annual inspection. According to the FAA, the AD was complied with at an airframe total time of 2,248.3 hours. At the time of the accident, the total airframe time was 2,274 hours. Examination of the right aft landing gear strut revealed that it had fractured due to undetected fatigue cracking in an inboard rivet hole. The fatigue cracks were very small, and given their size, they likely would not have been detectible by the existing required visual inspections. The cracks initiated after the rivet hole diameter was increased; therefore, they initiated after the initial penetrant inspection. Although it is possible that the fatigue cracks were the result of a degradation in material properties or, more likely, due to greater loads from a previous hard landing or normal landing on primarily the right skid, the investigation was not able to determine the origin of the fatigue cracks based on the available information. ## Factual narrative HISTORY OF FLIGHT On April 18, 2014, about 1340 Pacific daylight time (PDT), a McDonnell Douglas Helicopter (MDHI) 500N, N522GS, right rear landing gear strut failed during landing, and while attempting to set the helicopter on the ground, the front strut also collapsed at Santa Monica Airport (SMO), Santa Monica, California. The owner/pilot was operating the helicopter under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The private pilot and three passengers were not injured; the helicopter sustained substantial damage to the front landing gear structure. The local personal flight departed Camarillo, California, about 1315, with a planned destination of Santa Monica. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported that on landing he slowly sat the helicopter down on a landing pad and felt the helicopter "drastically" slip to the right. He then lifted the helicopter off the pad at which time a passenger on board verified the right strut was broken. A passenger reported that on touchdown he heard a "loud pop." AIRCRAFT AND AIRWORTHINESS DIRECTIVE The 1993 McDonnell Douglas 500N helicopter was subject to Airworthiness Directive (AD) 2007-12-23, requiring modifications to the strut and fairings and continuing inspections. The AD references MD Helicopter Service Bulletin SB500N-022 that describes the modifications. The increased size of the rivet hole and the polished surface around the hole suggest that the modifications had been performed. The AD required an initial dye penetrant inspection followed by continuing 10x magnified visual inspections at intervals not to exceed every 100 hours time in service or during each annual inspection. According to the Federal Aviation Administration (FAA), AD 2007-12-23 was complied with at a airframe total time of 2,248.3; at the time of the accident the a total airframe time of 2,274 was reported. The FAA notes that there were no dates on the record for the AD compliance and it was assumed that the AD was complied with during the aircraft 100/annual inspection, which was performed in December 2013, according to maintenance records. TESTS AND RESEARCH Magnified visual examinations of the fracture surfaces by a National Transportation Safety Board materials specialist uncovered faceted features and arrest marks consistent with fatigue cracking on both sides of the inboard holes. The remaining fracture area was matte gray and highly textured consistent with overstress separation. The fatigue appeared to initiate at multiple origins on opposite sides of the hole bore. The fatigue cracks propagated circumferentially short distances into the strut wall with slightly greater penetrations adjacent to the outer diameter of the strut. Visually, the fatigue measured about 0.043 inch (aft) and 0.054 inch (forward) at the outer surface of the strut About 1.5 inches above the fracture, the outer diameter of the strut measured approximately 2.368 to 2.374inches with an inner diameter measuring 1.998 to 1.999 inches. The inner and outer diameters were not concentric consistent with the engineering drawing. The wall thickness measured about 0.167 inches thick at the inboard location above the fracture. Engineering representatives of the manufacturer stated that the material was specified as aluminum alloy AA1 7075 in the T73 temper condition per AMS-A-227712. X-ray fluorescent spectroscopy 3 confirmed the composition as consistent with aluminum alloy 7075. For further information see the NTSB Material Lab Factual Report in the docket for this accident. ## Aircraft - Registration: N522GS (not matched to a registry record) - Reported make and model: MCDONNELL DOUGLAS HELICOPTER 500N - Category: Helicopter - Serial number: LN056 - Year built: 1993 - Airframe hours: 2274 - Engines: ROLLS ROYCE 250C20RZ 450 hp turbo shaft - Damage: Substantial damage - Flight: Personal, Part 91 - Operator: V & A Aviation LLC - Registered owner: V & A Aviation LLC - Airworthiness certificate: Standard, Normal - Last inspection: 100 hour, December 16, 2013 - Departed: CMA, CAMARILLO, CA - Destination: SMO, Santa Monica, CA - Airport: SMO, Santa Monica Muni - Equipment: None ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 64; injuries: none; private; airplane ratings: single-engine land; medical: class 3, with waivers/limitations; last flight review May 8, 2012; seat: left; restraint: 4-point; flight hours: 2,110 total, 368.5 in this make and model, 32 in the last 90 days, 22 in the last 30 days, 0 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 240° at 8 knots - Visibility: 10 statute miles - Temperature: 68 °F, dew point 54 °F - Altimeter: 29.92 inHg - Observed at: KSMO ## 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 20140418004629A](https://openairdocs.org/incident/20140418004629A), Apr 18, 2014: Mcdonnell Douglas Helicopter 500N, Santa Monica, CA, Substantial damage. On April 18, 2014 at approx. 1330 PDT, N522GS, a mcdonnel Douglas 500N helicopter, landed at Santa Monica Airport, Santa Monica, California. The landing was… ## Docket (7 files) The files NTSB released with the investigation on June 29, 2016, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=89491. 4.7 MB in all. - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 1, .PDF, 1.5 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40441797&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Master.PDF - Materials Laboratory Factual Report 15-019 (item 2, .PDF, 5 photos, 2.2 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40441798&FileExtension=.PDF&FileName=Materials%20Laboratory%20Factual%20Report%2015-019-Master.PDF - MD Helicopters Service Bulletin (item 3, .PDF, 100 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40441799&FileExtension=.PDF&FileName=MD%20Helicopters%20Service%20Bulletin-Master.PDF - Airworthiness Directive 2007-12-23 (item 4, .PDF, 33 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40441800&FileExtension=.PDF&FileName=Airworthiness%20Directive%202007-12-23-Master.PDF - Pilot's Statement (item 5, .PDF, 68 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40441801&FileExtension=.PDF&FileName=Pilot%27s%20Statement-Master.PDF - Passenger Statement (item 6, .PDF, 68 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40441802&FileExtension=.PDF&FileName=Passenger%20Statement-Master.PDF - Accident Photos (item 7, .PDF, 3 photos, 827 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40441803&FileExtension=.PDF&FileName=Accident%20Photos-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.