Bell 525 Relentless

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Bell 525 Relentless
525 GKY (31495047476).jpg
Prototype of the Bell 525 in flight
Role Medium-lift utility helicopter
National origin United States
Manufacturer Bell Helicopter
First flight 1 July 2015
Status Under development
Produced 2015–present

The Bell 525 Relentless is an American medium-lift helicopter, under development by Bell Helicopter. The Bell 525 was unveiled at the 2012 Heli-Expo in Dallas, Texas in February 2012. The helicopter first flew on July 1, 2015.

Design and development

The Bell 525 is being developed to meet a requirement for a medium-lift helicopter. It will be constructed primarily from composites and metal and is to be the first commercial helicopter to incorporate fly-by-wire flight controls,[1] with tactile cues. The system is triple redundant, and is developed in two simulator environments.[2] The 525 will be powered by a pair of GE CT7-2F1 turboshaft engines, with a new composite five-blade main rotor system.[3] The cost of the 525 has not yet been determined, but it is expected to be cost competitive on missions between 50 and 400 nmi, performed by helicopters such as the AgustaWestland AW139 and Sikorsky S-92.[4][5]

The Bell 525 is designed to fit the emerging "Super-Medium" size category suited ideally to support offshore oil and gas operations,[6] with 50% of the customers coming from that sector.[2] Helicopters under development in the same class are the Airbus Helicopters H175 and the AgustaWestland AW189.[3] The Bell 525's maiden flight was planned for late 2014.[7] PHI, Inc. is the launch customer for the type.[3]

After a six-month delay, the Bell 525 made its first flight on July 1, 2015. Bell expects certification in 2017.[8][9] The FAA suggested special rules in May 2016.[10] On July 6, 2016, a Bell 525 crashed during a test flight, killing both occupants.[11] The aircraft broke up in flight[12] while travelling about 229 mph at an altitude of about 2,000 feet. In January 2018 the National Transportation Safety Board released its findings, saying that the aircraft had suffered from inflight severe vibrations, which resulted in a loss of rotor RPM, subsequent rotor flapping and rotor impact with the tailboom, causing the inflight break-up. Contributing causes were collective biomechanical feedback, plus 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 airframe vibration initiated." Further causes included the lack of software safeguards designed in and the lack of a low rotor RPM indicator. The investigation was hampered by Bell not having employed any video or audio recording during the test flying process.[13] The crash delayed certification[14] from 2017 to 2018.[13]

As a result of the accident, Bell implemented design changes to the 525, including better filtering to the side stick controller to prevent the transmission of stick vibrations to the rotor system. Filtering was also added to the control system to account for gusts and maneuver loads.[15]

Specifications (Bell 525)

Data from Bell Helicopter[16][17][18]

General characteristics

  • Crew: one or two
  • Capacity: 16 or 20 passengers
    8,200 pounds (3,700 kg) useful load
  • Gross weight: 20,000 lb (9,072 kg)
  • Fuel capacity: 2,400 liters (530 imp gal; 630 U.S. gal)
  • Powerplant: 2 × General Electric CT7-2F1 turboshaft, 1,800 shp (1,300 kW) each
  • Main rotor diameter: 54 ft 6 in (16.61 m)


  • Maximum speed: 190 mph; 306 km/h (165 kn)
  • Cruise speed: 178 mph; 287 km/h (155 kn)
  • Range: 575 mi; 926 km (500 nmi) with a 1,530 pounds (690 kg) payload
  • Service ceiling: 20,000 ft (6,100 m) with 12,000 ft (3,700 m) hover in ground effect, 6,000 ft (1,800 m) HOGE


See also

Aircraft of comparable role, configuration and era


  1. ^ "Bell's 525 is Relentless". Sport Aviation: 14. April 2012. 
  2. ^ a b Maher, Guy. "'Flight Testing' the Bell 525 Relentless" Vertical, 10 July 2014. Accessed: 23 July 2014. Archived on 23 July 2014.
  3. ^ a b c Norris, Guy (February 20, 2012). "Bigger Bell (web title: Bell Rings In Changes With Super-Medium 525 Relentless)". Aviation Week & Space Technology. New York: McGraw-Hill. 174 (7): 36–37. 
  4. ^ "Bell 525 Brochure" Archived October 5, 2013, at the Wayback Machine.. Bell Helicopter, February 2012. pdf
  5. ^ "‘Relentless’ 525 To Be Largest Bell Helicopter". AINOnline, February 2012.
  6. ^
  7. ^ "Bell Helicopter Reveals the Latest in the Bell 525 Program during HELI-EXPO 2014". Bell Helicopter, February 25, 2014.
  8. ^ Perry, Dominic (July 2, 2015). "VIDEO: Successful first flight for new Bell 525 Relentless". Retrieved August 16, 2015. 
  9. ^ Osborne, Tony (July 1, 2015). "Bell Helicopter Completes First Flight Of Model 525". Aviation Week & Space Technology. McGraw-Hill. (Registration required (help)). 
  10. ^ "Federal Register - Special Conditions: Bell Helicopter Textron, Inc. (BHTI), Model 525 Helicopters; Interaction of Systems and Structures". Retrieved 6 August 2016. 
  11. ^ Ramirez, Domingo (July 6, 2016). "Bell helicopter crashes in Ellis County, two reportedly killed". Fort Worth Star-Telegram. Retrieved July 6, 2016. 
  12. ^ "DCA16FA199". NTSB. Retrieved 6 August 2016. 
  13. ^ a b Grady, Mary (16 January 2018). "NTSB Cites 'Adverse Feedback Loops' In Bell Crash". AVweb. Retrieved 17 January 2018. 
  14. ^ "NTSB releases preliminary report on Bell Helicopter crash". Fort Worth Star-Telegram. Retrieved 6 August 2016. 
  15. ^ Grady, Mary (23 January 2018). "Bell: Changes Made To 525 After Investigation". AVweb. Retrieved 24 January 2018. 
  16. ^ Specifications
  17. ^ Technical specifications
  18. ^

External links

  • Bell 525 Web site
  • ANALYSIS: Bell 525 Relentless cutaway and technical description
  • Cutaway drawing of 525
  • Mark Huber (January 17, 2018). "NTSB: Severe Vibration Triggered Bell 525 Breakup". AINonline. 
  • Stephen Trimble (17 Jan 2018). "NTSB report faults control system gaps in fatal 525 crash". Flightglobal. 
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