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Hypothetical IMC autopilot scenario


Skunkworks85

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I am fortunate enough to have about 10 hours of actual IMC(not PIC but hand flying). But I have often thought that IF I were to ever find myself in IMC in my CTSW. I most likely would engage the autopilot and have it fly the 180 turn back to known VMC.

That got me thinking the other night, and this whole question is based on the premise of; when the truetrak reaches the min airspeed threshold, it automatically lowers the nose.

Since almost all CTs do not have heated pitot tubes, if a pilot is using the autopilot in IMC (presumably to get out of trouble) and the pitot tube starts to ice over, thereby indicating a lower airspeed, will the autopilot point the plane at the ground?

I think the Dynon has a back up, that if the pitot system fails, it will revert to gps speed, with a big warning. Will the autopilot use that number?

Lemme know what you know.

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In theory if a pitot system ices over, the pressure in the line would be held at the last point of being just open, and airspeed would be held at whatever correlates to that pressure, then as time progresses, any leak allowing drop of pressure in the pitot system would allow the pressure to slowly decay, and airspeed would slowly drop over time following that level of decay.  I can't speak to what the autopilot might do - your thoughts appear on the right track, the system will try to correct for what it is programed to do.  If I found myself in icing conditions and suspect the pitot might freeze over, I'd be hand flying it immediately.

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The problem is that detecting if the pitot system has failed is pretty hard to begin with - best you can do is to use some heuristics ( like for instance drastic changes in airspeed or some kind of GPS correlation ) but unless you have two systems where you can compare outputs , what do you verify against to tell you if it is a valid reading or a corrupted data ...

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12 hours ago, Warmi said:

The problem is that detecting if the pitot system has failed is pretty hard to begin with - best you can do is to use some heuristics ( like for instance drastic changes in airspeed or some kind of GPS correlation ) but unless you have two systems where you can compare outputs , what do you verify against to tell you if it is a valid reading or a corrupted data ...

image.png.40ab8d692f5b25c0f5fcb1c298c29823.png

 

So the question is, Does the trutrak get its airspeed from the dynon? and if so, in the event of pitot airspeed loss, will it substitute the GPS speed for actual in the autopilot?

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In level flight it is going to try and do 2 things. One is maintain altitude, and this is based on static pressure, and second maintain airspeed. The reference in the manual for minimum airspeed is to lower the nose in a climb when there is no longer enough power to maintain the climb rate. I suspect in level flight that altitude will win out. 

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I did have a conversation with trutrak (bendixking now) about this question.

He did indeed say that the autopilot will continue to pitch the aircraft at about 5 degrees down until the min airspeed threshold is higher. (Or until you hit the ground) He also noted, that if the pitot where to become unfrozen, the autopilot would then climb back to original set altitude.

 

This is a good limitation to understand about the autopilot. 

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On 11/19/2019 at 11:11 AM, Tom Baker said:

In level flight it is going to try and do 2 things. One is maintain altitude, and this is based on static pressure, and second maintain airspeed. The reference in the manual for minimum airspeed is to lower the nose in a climb when there is no longer enough power to maintain the climb rate. I suspect in level flight that altitude will win out. 

Will a standard two axis AP try to maintain airspeed?  I don't think it cares what speed you are at, unless it exceeds a minimum/maximum airspeed set in the config.  The AP will maintain altitude and heading to the best of its abilities (assuming altitude hold).  Weather the airplane is at 70kt or 120kt, the AP will adjust pitch as required to keep altitude.

The only question I have is what happens if the ice is bad enough that IAS drops below the airspeed minimum configured.  I have seen my AP flash “MIN AS” at me when I was playing around with it, but I’m not sure if that happens long enough if the AP will disengage.  If so, the loss of AP in IMC would be a Bad Thing.  If not, and it just complains, the AP should keep flying normally by adjusting pitch to maintain altitude.

EDIT:  I see what TruTrak said.  Interesting.  I guess push the nose to maintain altitude is preferable to letting the airplane stall in cases like pilot incapacitation, but not so much In icing.  Moral is: AVOID ICING AND IMC.

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3 minutes ago, FlyingMonkey said:

Will a standard two axis AP try to maintain airspeed?  I don't think it cares what speed you are at, unless it exceeds a minimum/maximum airspeed set in the config.  The AP will maintain altitude and heading to the best of its abilities (assuming altitude hold).  Weather the airplane is at 70kt or 120kt, the AP will adjust pitch as required to keep altitude.

The only question I have is what happens if the ice is bad enough that IAS drops below the airspeed minimum configured.  I have seen my AP flash “MIN AS” at me when I was playing around with it, but I’m not sure if that happens long enough if the AP will disengage.  If so, the loss of AP in IMC would be a Bad Thing.  If not, and it just complains, the AP should keep flying normally by adjusting pitch to maintain altitude.

The trutrak autopilot will pitch the aircraft down at the min airspeed threshold,(MIN AS) this is designed to prevent a stall while engaged.

 

According to the guy at bendixking, it will continue this until it either, airspeed goes above min threshold, or hits the ground.

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On 11/18/2019 at 6:20 PM, Skunkworks85 said:

I am fortunate enough to have about 10 hours of actual IMC(not PIC but hand flying). But I have often thought that IF I were to ever find myself in IMC in my CTSW. I most likely would engage the autopilot and have it fly the 180 turn back to known VMC.

That got me thinking the other night, and this whole question is based on the premise of; when the truetrak reaches the min airspeed threshold, it automatically lowers the nose.

Since almost all CTs do not have heated pitot tubes, if a pilot is using the autopilot in IMC (presumably to get out of trouble) and the pitot tube starts to ice over, thereby indicating a lower airspeed, will the autopilot point the plane at the ground?

I think the Dynon has a back up, that if the pitot system fails, it will revert to gps speed, with a big warning. Will the autopilot use that number?

Lemme know what you know.

You also have Foreflight or the Garmin app, which in your scenario I will be using. Depending with model of iPad you have and which ADS-B you have, will give you ground speed and GPS altitude at the very least and hopefully attitude. Not directly answering your question but just thinking through it. 

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On 11/20/2019 at 7:36 PM, Skunkworks85 said:

The trutrak autopilot will pitch the aircraft down at the min airspeed threshold,(MIN AS) this is designed to prevent a stall while engaged.

 

According to the guy at bendixking, it will continue this until it either, airspeed goes above min threshold, or hits the ground.

That’s pretty alarming ... my only experience with TT autopilot in this regime was with their flight director ( without actual AP being engaged ) during landing - it was constantly barking at me  “ holding minimum airspeed” - I presume if I had the actual AP engaged , it would pitch the plane down right into the runway ...

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9 hours ago, Warmi said:

That’s pretty alarming ... my only experience with TT autopilot in this regime was with their flight director ( without actual AP being engaged ) during landing - it was constantly barking at me  “ holding minimum airspeed” - I presume if I had the actual AP engaged , it would pitch the plane down right into the runway ...

Only if you let it. The AP servos use a stepper motor. It uses magnetic force to hold the control in the position it wants. The strength of the magnetic force is set in the AP programming. It should be set strong enough that it can hold the control position, yet be overcome by the pilot. With the CT I have even seen when it would jump from one step to the next because the trim was not set right. While not ideal you should be able to land the airplane with the AP engaged. Also remember since it is an electrical magnet if you shut off its electrical power the servo will disengage. 

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On 11/26/2019 at 9:34 AM, Tom Baker said:

 Also remember since it is an electrical magnet if you shut off its electrical power the servo will disengage. 

Right, I think the first response to an AP doing something unexpected or alarming should be to pop the AP breaker.  You can always restart it and diagnose at your convenience (either later in the flight or on the ground).

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On 11/26/2019 at 8:34 AM, Tom Baker said:

Only if you let it. The AP servos use a stepper motor. It uses magnetic force to hold the control in the position it wants. The strength of the magnetic force is set in the AP programming. It should be set strong enough that it can hold the control position, yet be overcome by the pilot. With the CT I have even seen when it would jump from one step to the next because the trim was not set right. While not ideal you should be able to land the airplane with the AP engaged. Also remember since it is an electrical magnet if you shut off its electrical power the servo will disengage. 

Additionally, I did forget to mention the trutrak rep did say there is a (firmware controlled) max pitch down attitude of around 5 degrees for this "min AS" warning,  so it is not like the it will be a dramatic nose down attitude.

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I have had my pitot freeze up twice over the 2200 hrs of CT flying. It happens over nite after having flown through some rain the day before and temp drops that nite. After takeoff in the morning with normal airspeed, (plane was in heated hanger) in the climb the airspeed starts indicating higher than normal air speeds and keeps increasing the higher u go. After it happened the 2 Nd time my mechanic put a little reservoir in the pitot system to collect that little bit of moisture and it evaporates by itself in the heated hanger. 

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36 minutes ago, Scrapman1959 said:

I have had my pitot freeze up twice over the 2200 hrs of CT flying. It happens over nite after having flown through some rain the day before and temp drops that nite. After takeoff in the morning with normal airspeed, (plane was in heated hanger) in the climb the airspeed starts indicating higher than normal air speeds and keeps increasing the higher u go. After it happened the 2 Nd time my mechanic put a little reservoir in the pitot system to collect that little bit of moisture and it evaporates by itself in the heated hanger. 

I wondered if there was some sort of drain for rain/moisture in the pitot.  Surprised there isn't. 

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