Servo transparency
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 [5]
Background
Helicopter flight controls are connected to the
Phenomenon
The servomechanisms employed on helicopters are typically
Onset and recovery
During forward flight, the pitch angle – and therefore the
Therefore, on clockwise-turning rotors, the right servo will reach its maximum design load first, and if the manoeuvring persists, the lift on the right side of the rotor disk will become insufficient, creating a rolling moment to the right.
The helicopter's natural response to servo transparency is to a degree self-correcting, in that the pitch-up normally results in a reduction of
When encountering servo transparency conditions, pilots are advised to immediately reduce the severity of the manoeuvre by following the controls movements, which allows the collective pitch to decrease naturally, thus lowering the rotor loading. The tendency to roll towards the retreating blades should be countered smoothly, to avoid abrupt roll inputs in the opposite direction once servo effectiveness is restored.[2]
Prevalence
In general, factors that increase the risk of encountering servo transparency include:[7]
- High airspeed
- High collective pitch input
- High gross weight
- High g-loads
- High density altitude
- Low servo hydraulic pressure
Servo transparency has been cited as a possible or likely contributing factor in several accidents occurred to helicopters of the
In a widely circulated 2003 service letter on the subject,
Nevertheless, in 2023, the European civil aviation regulator
Notable accidents involving servo transparency
- 15 September 2007British rally driver AS350 helicopter he was flying. Servo transparency was cited as a possible factor in the accident.[4]:
- 31 August 2019An H125 helicopter crashed in Alta, Norway, after entering servo transparency conditions. All six people on board were killed.[11] :
References
- ^ a b c "Servo Transparency". SKYbrary. Retrieved 28 November 2023.
- ^ Eurocopter. 4 December 2003. Archived(PDF) from the original on 22 May 2023. Retrieved 2 December 2023.
- ^ Lacagnina, Mark (June 2007). "EMS Control Loss" (PDF). AeroSafetyWorld, Flight Safety Foundation: 35–36.
- ^ a b 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.
- ^ "Safety Information Notice No. 3287-S-67" (PDF). Airbus Helicopters. Archived (PDF) from the original on 21 January 2024. Retrieved 21 January 2024.
- ^ Accident Investigation Board Norway. Archived(PDF) from the original on 6 October 2022. Retrieved 21 January 2024.
- ^ 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.
- ^ "Servo Transparency Cited". Canadian Aviator Magazine. 29 March 2018. Archived from the original on 9 June 2023. Retrieved 3 February 2024.
- ^ Aviation Investigation Report A07W0138 (Technical report). Transportation Safety Board of Canada. 10 June 2008. Archived from the original on 4 February 2024. Retrieved 4 February 2024.
- ^ from the original on 21 September 2023. Retrieved 3 February 2024.
- ^ Perry, Dominic (22 March 2022). "Norwegian H125 crash probe calls for EASA action on helicopter fuel system safety". FlightGlobal. Retrieved 8 December 2023.
External links
- Unveiling the Secrets of Servo Transparency in Helicopters – Instructional video by training provider Eurosafety International