DFC90 and the "Servo Limit" Message

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   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:

**************************************************************

GENERAL PERFORMANCE – CIRRUS AIRCRAFT 

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. 

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AviJake Members Profile Send Private Message Find Members Posts Add to Buddy List Admin Group Joined: 26 Mar 2009 Location: Lincoln MA Status: Offline Points: 2815	Post Options Post Reply Quote AviJake Report Post Thanks(0) Quote Reply Topic: DFC90 and the "Servo Limit" Message 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: ************************************************************ GENERAL PERFORMANCE – CIRRUS AIRCRAFT 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.
	Steve Jacobson sjacobson@avidyne.com

Robercom Members Profile Send Private Message Find Members Posts Add to Buddy List Newbie Joined: 24 Jan 2015 Location: Alabama Status: Offline Points: 2	Post Options Post Reply Quote Robercom Report Post Thanks(0) Quote Reply Posted: 06 Mar 2015 at 6:13am
	AviJake, I was hoping you could shed some light on an MSR Fail message I have been getting. The plane is a 2008 Cirrus SR20 with DFC90 and Avidyne. I purchased in November of 2014. Recently I have been getting an MSR Fail message that will eventually go off but I lose all aural alerts for the remainder of the flight. I am new to the forum and am not sure what release I have or even how to find that information. Any assistance you can give would be appreciated.

AviJake Members Profile Send Private Message Find Members Posts Add to Buddy List Admin Group Joined: 26 Mar 2009 Location: Lincoln MA Status: Offline Points: 2815	Post Options Post Reply Quote AviJake Report Post Thanks(0) Quote Reply Posted: 06 Mar 2015 at 9:12am
	Robercom, Congratulations on the purchase. Here's some background on the MSR in case you hadn't read it before: "MSR" stands for "Maintenance & Safety Recorder". It is physically a SD card that is housed inside the DFC90 unit and serves two functions: 1. Data logger storage unit recording internal autopilot-relevant data. 2. Hosts the aural alerts including the disconnect tone, that the autopilot can issue. As you've experienced, when you have a "MSR Fail" you lose all aural alerts including the disconnect tone. The autopilot will still be fully functional but the supplemental aural alerting will be missing so extra vigilance to the autopilot annunciator status messages across the top strip of the PFD is warranted. In 2012, we switched vendors for that SD card for reliability improvement reasons. Units with the "new" SD cards do not experience this type of "MSR Fail" issue so it's worth sending your DFC90 in when you can for service (replace the SD card, and load current software, if not already in the unit). As for determining what software version of the autopilot you have, you can determine that from the PFD Maintenance Mode pages (description of page is on page 6-4 of the DFC90 Pilot Guide). It's really a combination of the PFD software version and the autopilot software version that results in described functionality - see page 6-2 of the DFC90 Pilot Guide for that software version combination set of notes.
	Steve Jacobson sjacobson@avidyne.com

Robercom Members Profile Send Private Message Find Members Posts Add to Buddy List Newbie Joined: 24 Jan 2015 Location: Alabama Status: Offline Points: 2	Post Options Post Reply Quote Robercom Report Post Thanks(0) Quote Reply Posted: 06 Mar 2015 at 9:26am
	Thanks for the response Jake. What would be the cost of the updated software and SD update? Am I allowed to remove the unit and send it in or would I have to have a Cirrus service center do that?

AviJake Members Profile Send Private Message Find Members Posts Add to Buddy List Admin Group Joined: 26 Mar 2009 Location: Lincoln MA Status: Offline Points: 2815	Post Options Post Reply Quote AviJake Report Post Thanks(0) Quote Reply Posted: 06 Mar 2015 at 9:34am
	Software upgrade does not cost anything but I'm afraid you'd have to check with the Avidyne Tech Support crew (888-723-7592) to determine if there is a cost for the SD card update and the logistics of pulling that off. Try to have the serial number of your DFC90 when you call. If that's not feasible, hopefully they can determine that information just from your tail number.
	Steve Jacobson sjacobson@avidyne.com