Servo transparency

January 8, 2010 by Tim McAdams

Pilots who learn to fly in smaller helicopters probably hear very little about servo transparency, yet this phenomenon has caused or played a role in several accidents. When giving flight reviews I have found some helicopter pilots who totally misunderstand why and how it happens. However, the concept is not too difficult to understand.

Because of the higher control forces in larger helicopters, hydraulically boosted servo actuators are used to assist the flight controls. The maximum force that these servo actuators can produce is constant and is a function of hydraulic pressure and servo characteristics. Engineers design the hydraulic system to adequately handle all aerodynamic forces required during approved maneuvers. Even so, with certain aggressive maneuvering it is possible for the aerodynamic forces in the rotor system to exceed the maximum force produced by the servo actuators. At this point, the force required to move the flight controls becomes relatively high and could give an unaware pilot the impression that the controls are jammed. To prevent servo transparency, pilots should avoid abrupt and aggressive maneuvering with combinations of high airspeed, high collective pitch, high gross weight, and high-density altitude.

The good news is that this phenomenon occurs smoothly, and can be managed properly if the pilot anticipates it during an abrupt or high-G load maneuver. On clockwise-turning main rotor systems the right servo receives the highest load, so servo transparency produces an un-commanded right and aft cyclic movement accompanied by down collective. The pilot should follow (not fight) the control movement and allow the collective pitch to decrease while monitoring rotor rpm, especially at very low collective pitch settings. The objective is to reduce the overall load on the main rotor system. It normally takes about two seconds for the load to ease and hydraulic assistance to be restored. However, be aware that if the pilot is fighting the controls when this happens, the force being applied to the controls could result in an abrupt undesired opposite control movement.

Many of these accidents have happened while aggressively flying the helicopter at low altitudes, leaving very little time to recover. Most important for avoiding this kind of accident is to follow the aircraft limitations published in the helicopter’s flight manual.


  • Kris Sundberg

    You are correct. I never heard of this phenom until I was reading one of the accident reports (i.e., Grand Canyon) you posted last week. Very interesting reading. Thanks! Kris Sundberg, Mercer Island, WA

  • Ehud Gavron

    I just got my PPL 5 months ago and in the 14 months I’ve been studying and flying nobody ever once mentioned this! As I fly in an R44 this is definitely applicable. Thanks!

    Ehud Gavron
    Tucson AZ

  • Kevin

    The following UK Air Accident Investigation Board report is a good example:

    Colin MacRae – the pilot – was a big name in rally driving. The report speculates/states that during high G low-level flying through a tree-lined valley, servo transparency or Jack stall probably occurred resulting in an uncommanded roll to the right with the result of the aircraft hitting the trees at very high speed. Death was instantaneous for him, another adult and two small children. It was a tragedy and the report made for a sobering read. I’m a student pilot at the moment and it’s stories like this that give me a lot of respect for the aircraft we’re flying in. They are not toys. Anyone flying a helicopter with hydraulically assisted controls needs to know about this phenomenon and I think its study should at least be part of the type rating if not the PPL.

  • whytech

    Helicopters with hydrauliclly boosted controls are not equally prone to servo transparency. This is related to the design margins incorporated in the servo system. A review of accident reports seems to indicate that this is much more prevalent in the Eurocopter AS 350 series than in the R44 and Bell light helicopters.

  • Brendan Fitzpatrick

    Just shows how much more things we have to deal with as helicopter pilots (I fly fixed and rotary) Thanks for the information TIm – Will you be at Heli-Expo in Houston next month?

  • Tim McAdams
  • Richard Woodbury

    I have never even heard of this in over 30 years and 15,000 hours of helicopter experience. It may be something that the engineering design staff knows about….but not working pilots. This article is a waste of space…..just learn the manual and do not fly in a stupid fashion.

  • Ty Burlingham

    Very good information. We operate R-44’s as well as Jet Rangers at our flight school in oregon; and now that is posted on our bulletin board!

    Thanks for the info,

    Ty Burlingham
    Director of Flight Training
    Sunset Helicopters, Inc.

  • Edward L. Renoux

    As an aerial applicator in Bell 47s I am very aware of it. First time it happened was during transition training and I was pulling up like normal, started to bank left then changed my mind and decided to go right. Of course it pretty well locked up the cyclic and my instructor laughed at me for changing my mind in mid turnaround. It left a lasting impression on me and it happens every now and then when I am in a new area or otherwise occupied. It always recovers before the ground intersects (so far). I suppose those who never really get near the edge of the flight envelope need never be aware of it (unless something uncommon happens at which time it may be fatal).

  • William M.

    I believe this is a video of the phenomenon you are discussing;

  • Kevin

    @Richard Woodbury

    That’s an interesting attitude. Can I sum it up as – you have 15,000 hours in helicopters, and therefore know everything worth knowing about helicopters? Pretty arrogant…and wrong.

    My view is that having knowledge and an understanding the helicopter you fly might be a good thing and the more you have the better. Yes, you shouldn’t fly in a stupid fashion and yes the manual should be studied. However, the manual doesn’t cater for all possible situations and there may come a day where you, for whatever reason, have to make demands on the helicopter that the hydraulics may not be able to cope with, possibly resulting in “servo transparency”, knowing what to do in that situation might save your life.

  • Avi Weiss

    Meant to comment earlier but lost track.

    Yes, Have seen this issue a couple of times in B206, and as you point out, is not something one typically hears about during training. It is EXACTLY this type of “advanced operational lore” that I’d love to see more of in “Hover Power”. It is these kind of “corner case” details that tend to get overlooked and forgotten, right up to the point where we need them. Review via “discussion sources” such as this blog are invaluable and could certainly come in handy…

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  • http:[email protected] hassan zandieh

    dear sirs. Az i understand the servo transparency is a natural phenomena that can occure for any flyable helicopter .my question isthat: is there any differece between the two types of blades(fully articulated & semi rigid) in recovering the helicopter encounterd with the mentioned phenomena with the respect to the assumption that pilot on board has acomprehensive undrestaniding of the phenomena?

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