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ASN Wikibase Occurrence # 188544
Last updated: 12 April 2021
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Time:c. 11:48 LT
Type:Bell 525 Relentless
Owner/operator:Bell Helicopter
Registration: N525TA
C/n / msn: 62001
Fatalities:Fatalities: 2 / Occupants: 2
Other fatalities:0
Aircraft damage: Written off (damaged beyond repair)
Location:Ellis County, NW of Italy, TX -   United States of America
Phase: En route
Departure airport:Arlington Municipal Airport, TX (GKY)
Destination airport:Arlington Municipal Airport, TX (GKY)
Investigating agency: NTSB
The first Bell 525 Relentless helicopter prototype (Flight Test Vehicle 1 - FTV1) was destroyed when it impacted terrain in Ellis County, Texas near the towns of Bell Branch and Italy. Both pilots were killed. There was a post crash fire.

Bell said in a statement its helicopter was "conducting developmental flight test operations on the Bell 525 at our Xworx facility in Arlington, Texas, that resulted in a helicopter accident." A Bell spokesperson said there was one "chase" helicopter present as well, which is standard in test flight scenarios.

According to an NTSB spokesman, the main rotors "departed their normal plane of rotation", and struck the tail and nose of the aircraft as a result.

Flightradar24 records show the helicopter departing Arlington at 10:39 LT (15:39 UTC). It proceeded to the south were it flew pattern at altitudes between 2000 and 3000 feet. Last data point is at 1975 feet, at a groundpeed of 199 kts at 11:47 hours. At the time, it was tracking south to north with a 20-knot tailwind, equating to an approximate airspeed of 179 knots. Bell’s projected high-speed cruise for the 525 is 162 knots.

FTV1 first flew on 1 July 2015.

Probable Cause : "The NTSB determined that the probable cause of the accident was a severe vibration of the helicopter that led to the crew’s inability to maintain sufficient rotor rotation speed (Nr), leading to excessive main rotor blade flapping, subsequent main rotor blade contact with the tail boom, and the resultant in-flight breakup. Contributing to the severity and sustainment of the vibration, which was not predicted during development, were (1) the collective biomechanical feedback and (2) the attitude and heading reference system response, both of which occurred due to the lack of protections in the flight control laws against the sustainment and growth of adverse feedback loops when the 6-hertz (Hz) airframe vibration initiated. Contributing to the crew’s inability to maintain sufficient Nr in the severe vibration environment were (1) the lack of an automated safeguard in the modified one-engine-inoperative (OEI) software used during flight testing to exit at a critical Nr threshold and (2) the lack of distinct and unambiguous cues for low Nr.

Sources: <- NTSB Report


Safety recommendations:

Safety recommendation A-18-1 issued 9 January 2018 by NTSB to Flight Test Safety Committee
Safety recommendation A-18-2 issued 9 January 2018 by NTSB to Bell Helicopter Textron


Revision history:

06-Jul-2016 18:28 harro Added
06-Jul-2016 18:31 harro Updated [Time, Departure airport, Source, Embed code, Narrative]
06-Jul-2016 19:06 harro Updated [Embed code]
06-Jul-2016 22:01 Aerossurance Updated [Time, Destination airport, Source, Embed code, Narrative]
06-Jul-2016 23:51 Geno Updated [Operator, Location, Source]
07-Jul-2016 15:07 bovine Updated [Source]
08-Jul-2016 08:14 Aerossurance Updated [Operator, Narrative]
09-Jul-2016 14:42 Aerossurance Updated [Source]
20-Jul-2016 19:20 Aerossurance Updated [Source, Narrative]
05-Aug-2016 10:27 Aerossurance Updated [Source]
20-Sep-2016 20:11 Nic8895 Updated [Source, Narrative]
20-Jan-2018 10:17 Captain Adam Updated [Source, Narrative]

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