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DFC90 and the "Servo Limit" Message

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AviJake View Drop Down
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    Posted: 03 Mar 2012 at 12:49pm
With the deployment of DFC90 Release 2 software, there are some pilots now observing a "SERVO LIMIT" message appear on their PFD and are asking as to why that is now showing up in Rel 2.

Common questions are:

Why didn't I see these messages in the 1st versions of DFC90?
What does this mean?
I never got this when I had as STec 55X installed, why now?
Does this mean I have a bad DFC90?

This message was added in Release 2 of the DFC90 software.  It was not included in the initial release.

This message only applies in the roll axis and only in Cirrus aircraft that have not had the aftermarket roll servo modification done.  As described in a snippet from the DFC90 Pilot Guide pasted below, this message implies the roll trim spring cartridge has been fully compressed in one direction and is no longer pulling current and moving.

This is not a failure of the DFC90 autopilot and it is not "caused" by the DFC90 or a "bad" DFC90.  While the DFC90 does command more frequent roll commands than a 55X, they are much smaller and precise inputs.  By themselves, they are not the type of commands that will wind up a spring cartridge to its fully compressed state.

It is likely that for those aircraft that are now exhibiting the message/behavior, they have been tripping the internal roll trim spring cartridge micro switch for some time (DFC90 Rel 1 and STec55X) but you've had no way to know that until now.

We added the message with the intent of it being an aid to the pilot.  If this message is presenting itself frequently, especially in non-turbulent air conditions, and/or you are certain there are no pilot inputs on the yoke or rudders, Avidyne recommends you have your roll axis affecting aircraft components checked out.  This could be the roll trim cartridge itself, it could be the aileron-rudder interconnect bungee, or it could be the physical roll trim tab being out of proper alignment.

It is not a normal set of circumstances for this alert to be frequently observed.

Despite some speculation to the contrary, aside from the "MSR FAIL" issue (that has been root caused and resolved), the DFC90s just do not fail.   They are not analog devices and are not variable from one unit to the next.   Because they operate far more precisely than the predecessor autopilots they replace, they are more sensitive to out-of-tolerance, or close-to-the-edge inputs (e.g. poorly seated connectors, high startup voltage servos, out of spec bungees, misbent trim tabs, etc) than their predecessors.  In effect, they are serving as a form of aircraft level circuit breaker and are highlighting existing imperfections at the airplane level.

Whenever we hear of a report of a misbehaving DFC90 equipped airplane, if it is not 100% obvious that some other component is causing the issue, we often swap autopilots, not because we think the autopilot is failed, but typically so that we can get access to the data logs inside the autopilot that will help point out the true causal issue.

From the DFC90 Pilot Guide:



The input forces required for roll-axis control surface actuation are demonstrably light and it is not difficult to force the servo (roll trim spring cartridge) to drive to its limit.  By design, reaching the physical limit causes a micro-switch to be tripped which effectively decouples the autopilot commands from the flight  control surfaces. 

Pilot-induced inputs on the system including yoke input, rudder input, p-factor, fuel imbalance, airspeed and general aircraft trim alignment can all contribute to this condition. 

In the event the DFC system believes the roll-axis servo limit has been reached, a yellow “SERVO LIMIT” text message is displayed along the top edge of the PFD pages.
WARNING:   Reaching Servo Limits Can Cause Uncommanded Rolls                                              
If the roll servo limit is reached, the autopilot may stop following lateral commands.  This could appear  to the pilot as an uncommanded roll or a failure to follow the commanded lateral target. 

As a result, it is highly recommended that a pilot avoid making flight control inputs while in coupled autopilot mode operations aside from minor rudder input to maintain coordinated flight.  Moderate rudder or any roll input may result in an inability of the DFC90 autopilot to track the commanded targets. 

It is also important to note that in aircraft equipped with only roll trim motors (no roll servo), the maximum aileron deflection is ½ the total possible aileron throw. 
S. Jacobson
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