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 1 July 2015. It is designed to transport up to 19 passengers.[1]

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,[2] with tactile cues. The system is triple redundant, and is developed in two simulator environments.[3] The 525 is powered by a pair of GE CT7-2F1 turboshaft engines, with a new composite five-blade main rotor system.[4] 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.[5][6]

The Bell 525 is designed to fit the emerging "Super-Medium" size category suited ideally to support offshore oil and gas operations,[7] with 50% of the customers coming from that sector.[3] Helicopters under development in the same class are the Airbus Helicopters H175 and the AgustaWestland AW189.[4] The Bell 525's maiden flight was planned for late 2014.[8] PHI, Inc. was the launch customer for the type,[4] but as of 2016 is no longer the launch customer. [9]

After a six-month delay, the Bell 525 prototype first flew in July 2015. At that time, Bell predicted certification to be completed by the end of 2017.[10][11] The FAA suggested special rules in May 2016, to address the fly-by-wire concept.[12] On July 6, 2016, the prototype crashed during a test flight, killing the two occupants.[13] The aircraft broke up in flight[14] while travelling about 229 mph at an altitude of about 2,000 feet.[15] In January 2018 the National Transportation Safety Board released its findings, saying that the aircraft had suffered from a severe inflight 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 which caused the tailcone to pulsate at 6 cycles/second, 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.[16] The crash delayed certification[15] from 2017 to 2018.[16] In February 2018, Bell predicted certification to be completed by late 2018 or early 2019.[1]

After the accident, Bell amended the control paradigm, improving the filter on side-stick controller inputs to block transmission of stick vibrations to the rotor system. Filtering was also added to the control system to account for gusts and maneuver loads.[17]

The website for the Bell 525 states on its homepage that it is designed for "up to 20 passengers", but the only further reference to the passenger configuration states " . . offers fully flat cabin floor highly configurable with 16 seats in a standard OGP-compliant layout." In February 2018 the company stated it was capable of carrying 19 passengers, more than any existing super-medium-segment at that time.[1]

The 525 is to be certified in Category A Takeoff class, at maximum gross weight. This involves being able to continue a takeoff (or landing) after one of the helicopter's two turbine engines fails at any point.[1]

Specifications (Bell 525)

Data from Bell Helicopter[18][19][20]

General characteristics

  • Crew: one or two
  • Capacity: 16 or 20 passengers
    8,200 pounds (3,700 kg) useful load
  • Gross weight: 20,500[1] lb (9,299 kg)
  • Max takeoff weight: 20,500 lb (9,299 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. ^ a b c d e Bell Pressing Ahead with Fly-by-Wire Certification. Aviation Week & Space Technology, 26 February 2018
  2. ^ "Bell's 525 is Relentless". Sport Aviation: 14. April 2012. 
  3. ^ a b Maher, Guy. "'Flight Testing' the Bell 525 Relentless" Vertical, 10 July 2014. Accessed: 23 July 2014. Archived on 23 July 2014.
  4. ^ a b c Norris, Guy (20 February 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. 
  5. ^ "Bell 525 Brochure" Archived October 5, 2013, at the Wayback Machine.. Bell Helicopter, February 2012. pdf
  6. ^ "‘Relentless’ 525 To Be Largest Bell Helicopter". AINOnline, February 2012.
  7. ^ Oil Prices Create Super Uncertainty. Flight Global
  8. ^ "Bell Helicopter Reveals the Latest in the Bell 525 Program during HELI-EXPO 2014". Bell Helicopter, February 25, 2014.
  9. ^ Drew, James. "VIDEO: Bell relentlessly pursuing 525 despite market troubles". FlightGlobal. Reed Business Information. Retrieved 15 August 2018. 
  10. ^ Perry, Dominic (July 2, 2015). "VIDEO: Successful first flight for new Bell 525 Relentless". Retrieved August 16, 2015. 
  11. ^ Bell Completes First Flight of Model 525. Aviation Week & Space Technology, 1 July 2015
  12. ^ "Federal Register - Special Conditions: Bell Helicopter Textron, Inc. (BHTI), Model 525 Helicopters; Interaction of Systems and Structures". Retrieved 6 August 2016. 
  13. ^ Ramirez, Domingo (July 6, 2016). "Bell helicopter crashes in Ellis County, two reportedly killed". Fort Worth Star-Telegram. Retrieved July 6, 2016. 
  14. ^ "DCA16FA199". NTSB. Retrieved 6 August 2016. 
  15. ^ a b "NTSB releases preliminary report on Bell Helicopter crash". Fort Worth Star-Telegram. Retrieved 6 August 2016. 
  16. ^ a b Grady, Mary (16 January 2018). "NTSB Cites 'Adverse Feedback Loops' In Bell Crash". AVweb. Retrieved 17 January 2018. 
  17. ^ Grady, Mary (23 January 2018). "Bell: Changes Made To 525 After Investigation". AVweb. Retrieved 24 January 2018. 
  18. ^ Specifications
  19. ^ Technical specifications
  20. ^ Bell Confident on Further 525 Performance Boost. Flight Global

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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