Caution Underspeed
Printed From: Avidyne
Category: Avidyne General
Forum Name: DFC90/DFC100 Digital Autopilot
Forum Description: Topics on the Avidyne DFC Digital Autopilots
URL: https://avidynelive.com/forum_posts.asp?TID=1210
Printed Date: 26 Mar 2026 at 11:49am
Software Version: Web Wiz Forums 12.07 - https://www.webwizforums.com
Topic: Caution Underspeed
Posted By: jimmyz80
Subject: Caution Underspeed
Date Posted: 28 Jan 2017 at 1:38pm
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Ever since i had my DFC90 installed, I've been getting super annoying "Caution, Underspeed" warnings on landing rollout. So you're rolling down the runway listening for exit instructions from the tower, and the DFC is bitching and complaining that I'm going too slow. :) I assume this doesn't happen to everyone, or it would be a much bigger deal. Any ideas as to why this would be happening? ------------- 2006 Cirrus SR22 - IFD540 IFD440 DFC90 AXP322 MLB100 |
Replies:
Posted By: Melohn
Date Posted: 28 Jan 2017 at 9:14pm
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Assuming you have a Cirrus, you are hearing this because your flap switch is not connected to the DFC90, so it assumes that you are flying with flaps up. You have two choices; have the flap switch connected to the DFC90, or always use the autopilot disconnect when operating at speeds below ~80 knots; you won't hear the caution when the autopilot is in AP READY state. |
Posted By: jimmyz80
Date Posted: 29 Jan 2017 at 8:30am
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The flap switch is properly connected, since I get the appropriate flap overspeed warnings. ------------- 2006 Cirrus SR22 - IFD540 IFD440 DFC90 AXP322 MLB100 |
Posted By: jimmyz80
Date Posted: 25 Sep 2018 at 3:30pm
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Bumping this back to the top since I had forgotten about it, but the warnings still continue. The flap switch is definitely connected to the DFC90 since I get flap overspeed warnings as appropriate. But when landing, even with the AP and FD fully disengaged, I still get underspeed warnings on roll-out. It's super duper annoying since it's inevitably talking over the top of a tower controller trying to give me runway exit instructions. Any ideas why it would be doing this? ------------- 2006 Cirrus SR22 - IFD540 IFD440 DFC90 AXP322 MLB100 |
Posted By: Ibraham
Date Posted: 27 Sep 2018 at 9:33am
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Do you retract the flaps on rollout? try to keep the flaps extended until you exit the runway to see if you still get the alert. On some configurations, full flaps condition suppresses the "underspeed" alert. You could look at the DFC90 Pilot Guide Section 6 "Limitations and Performance" table for your specific configuration.
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Posted By: jimmyz80
Date Posted: 27 Sep 2018 at 9:43am
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I tried that test as well on recent flights, and I get the warning regardless of whether I leave in 100% flaps or retract them while rolling. I see some notes about the warnings being suppressed when "near idle" but that's not quantified in any way. Many times I still have a little taxi power re-applied on the ground to clear the runway quickly at large airports. I have no way to know if that still qualifies as "near idle". ------------- 2006 Cirrus SR22 - IFD540 IFD440 DFC90 AXP322 MLB100 |
Posted By: Ibraham
Date Posted: 27 Sep 2018 at 10:24am
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See this SIL from Avidyne (page 6), it addresses Underspeed Alerting in DFC90 and SIU-Equipped Cirrus Aircraft https://www.avidyne.com/files/downloads/606-00006-002.pdf SR22 S/N:1663 and on have DAU (Cirrus SR22 S/N 435-1662 have SIU)
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Posted By: Melohn
Date Posted: 27 Sep 2018 at 4:23pm
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I read the SIL, but it didn’t seem clear to me what happens in an SIU aircraft with R2 software and latest PFD software that does NOT have the flap modification. It would seem as though the logic should say, no flap input, no Manifold Pressure input, no alerting, but it wasn’t clear to me that’s the case. I happen to have that configuration, but am still running R1, which does not provide the warnings unless you leave the AP in FD mode. I’m quite happy with this, but fear someday I’ll need to send in my DFC90 and it will come back with new software. |
Posted By: jimmyz80
Date Posted: 08 Oct 2018 at 8:05am
My aircraft is DAU equipped, so this SIL doesn't apply. ------------- 2006 Cirrus SR22 - IFD540 IFD440 DFC90 AXP322 MLB100 |
Ibraham wrote:Posted By: dopolinsky
Date Posted: 22 Mar 2019 at 3:38pm
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Jimmyz, I have been working on this problem in my DAU equipped 2007 SR 20 for a year and half. After checking flap wiring and all avionics configurations my attention is now turned to the Manifold Pressure. According to Avidyne tech support Caution Underspeed aural alert that we are hearing during landing should be inhibited by a combination of the Flaps 100 and a manifold pressure below 13.5" I have been looking at my MP and they are about 14-15 when I close the throttle. Our family of engines should have the MP at about 12" when idling with a closed throttle. This can obviously be caused by a bad sensor or an intake leak. I haven't found my intake leak yet but I just started looking. You can sometimes detect it by closing the throttle to 20" or less inches at 5000 feet or so. If the leak is near one cylinder it will run leaner and have a lower EGT than other cylinders. The number 13.5" is not in the pilot manual for DFC 90 and is not well publicized. Owners wit this issue naturally look to avionics shops to resolve it and Avionics techs gravitate toward the electronics and away form the engine manifolds which makes this additionally chaleging to work on.
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Posted By: jimmyz80
Date Posted: 22 Mar 2019 at 5:53pm
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Thanks! I'll have to keep an eye on my manifold pressure when I get the warning. ------------- 2006 Cirrus SR22 - IFD540 IFD440 DFC90 AXP322 MLB100 |
Posted By: Aerochip
Date Posted: 05 Dec 2019 at 7:24am
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Having the same issue on a SR22 with DFC90 that I fly. Caution Underspeed aural alerts on the runway with the AP and FD disconnected. It’s distracting when you can’t hear turnoff instructions. MP is definitely below 10” and it only happens after touchdown.
Were you able to resolve this issue and what was the solution? |
Posted By: dopolinsky
Date Posted: 20 Dec 2019 at 3:39am
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I have not resolved my issue due to problems with fuel pump adjustments at low idle and continued inability to get MP low. The older SIU (not DIU) Cirrus has a different software fix for the PFD software that inhibits "Caution Underspeed" when flaps are 100%. You need that PFD software patch if you have an SIU Cirrus. SIU does not send MP to PFD and AP.
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Posted By: Roger
Date Posted: 18 Jan 2020 at 4:40pm
| Try turning off flight director and see if that makes it stop saying that. |
Posted By: Aerochip
Date Posted: 06 Feb 2020 at 8:31am
| Figured out my issue on this airplane - I was dumping flaps during the landing roll-out, which was triggering the Caution Underspeed alert. If I wait until turning off and then raise the flaps, I don't get the warning. |
Posted By: Roger
Date Posted: 07 Feb 2020 at 5:42am
| I will try that. My flaps are tied into the airspeed system so maybe that will work! |