In aviation, and in particular in helicopters, servo transparency (also called servo reversibility or jack stall),[1] is a phenomenon affecting the servomechanisms (or servos) that assist a helicopter's flight controls, which, in certain flight conditions, can result in a significant stiffening of the controls handled by the pilot.[2][3] The effect, if not promptly recognised by the pilot, can be hazardous as it can lead to partial or total loss of control, which, if encountered at low altitude, could result in impact with terrain.[4]: 101 

Background

A helicopter's main rotor hub. The vertical rods are at the end of the control chain that starts with the pilot controls.

Helicopter flight controls are connected to the main and tail rotors, and include a cyclic stick, broadly to control forward-aft and left-right movements, a collective lever, broadly to control vertical movements, and anti-torque pedals, to control left and right yaw. The forces applied to such controls to manoeuvre the helicopter are opposed by aerodynamic forces acting on the rotors, and in all but the lightest helicopter types (such as the two-seater Robinson R22), they are too big for a human alone to handle. Therefore, most helicopters are fitted with servo systems that aid the control effort and effectively isolate the controls from the aerodynamic forces acting on the rotors.[2] The concept is similar to that of power steering in automotive technology.

Phenomenon

The servomechanisms employed on helicopters are typically hydraulic actuators. The maximum force that such servos can impart is set, and depends primarily on the design service pressure delivered by the aircraft's hydraulic system. Within the approved flight envelope (that is in ordinary flight conditions), the aerodynamic forces acting on the rotor generally remain within the servos' ability to oppose them, but in certain conditions, for example during aggressive manoeuvring, such aerodynamic forces can exceed the maximum force that the servos can exert.[2] When that happens, the excess force is transmitted unabated down the control chain and to the cyclic and collective levers, giving the pilot the impression that the controls are either moving of their own accord or are jammed. The servos have become 'transparent', in that, relatively to the excess force, it is as if they were absent.[1]

Factors that increase the risk of servo transparency include:[5]

Notable accidents involving servo transparency

  • 15 September 2007 (2007-09-15): British rally driver Colin McRae and three passengers were killed in the crash of the AS350 helicopter he was flying. Servo transparency was cited as a possible factor in the accident.[4]
  • 31 August 2019 (2019-08-31): An H125 helicopter crashed in Alta, Norway, after entering servo transparency conditions. All six people on board were killed.[6]

References

  1. 1 2 "Servo Transparency". SKYbrary. Retrieved 28 November 2023.
  2. 1 2 3 "HYDRAULIC POWER SYSTEM: Servo Transparency" (PDF). Eurocopter. 4 December 2003. Archived (PDF) from the original on 22 May 2023. Retrieved 2 December 2023.
  3. ↑ Lacagnina, Mark (June 2007). "EMS Control Loss" (PDF). AeroSafetyWorld, Flight Safety Foundation: 35–36.
  4. 1 2 Eurocopter AS350B2 Squirrel, G-CBHL, 15 September 2007 (Technical report). Air Accidents Investigation Branch. February 2009. Archived (PDF) from the original on 7 Sep 2023. Retrieved 8 December 2023.
  5. ↑ Aviation Investigation Report A16P0045 (Technical report). Transportation Safety Board of Canada. 28 March 2018. Archived from the original on 16 Sep 2022. Retrieved 10 December 2023.
  6. ↑ Perry, Dominic (22 March 2022). "Norwegian H125 crash probe calls for EASA action on helicopter fuel system safety". FlightGlobal. Retrieved 8 December 2023.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.