Autopilots and trim

[MattCW]

I'm not sure if this is the correct forum, if not please move it. I've been following the thread about Jenny Blalock's crash and I've learned a lot about autopilots in small planes. I don't currently fly a plane with one (that I can use) but I definitely have my assumptions about them, many of which are proving to be wrong. I think the biggest assumption I had made previously was that the autopilot flies the plane pretty much like the pilot does, at least as far as trim is concerned. In other words, when the A/P is disengaged, the plane would pretty much be trimmed off. But from that topic, it seems like that isn't always the case, some autopilots just fly the elevator, so when you disengage, whatever force the A/P was exerting, is suddenly gone, and the trim "takes over" which might send you on a ride. This seems like an inherently bad design to me. Why is this allowed to persist? Something else hinted in the topic is pilots should set the trim to relieve the force on the A/P similar to how we set it to relieve force on ourselves. But how would someone do that? Do Autopilots have some kind of indicator on how much force they're exerting? I remember specifically seeing something about trim limit lights, but it seems like trim set near but not over a limit could be catastrophic too. I don't foresee myself flying a plane with a two+ axis autopilot in the near future, but I'm still eager to learn about the different ways these things work. Thank you!

 

[flyingron]

In Ms. Blalock's case, the trim really didn't much enter into it. She did not have autopilot controlled trim, despite some loudmouth youtube "expert" analysis. Her autopilot deals with an out of trim condition by prompting the pilot to spin the trim wheel.

In my plane (and I suspect hers IF she had autotrim) the autopilot does trim off the control forces automatically. When you cancel the autopilot it holds whatever mode it was in at the time.

As I mentioned over in the thread on that crash, while it is possible the trim could run away, it's unlikely given safeguard to protect from that. Further, the REQUIRED (but possibly omitted) preflight checks run through all the various ways to get control AWAY from the autopilot should it fail.

 

[Hang 4]

In an autopilot without trim, there are annunciator lights that indicate the need for trim. There is a feedback loop that identifies how much force the elevator is exerting to hold altitude and the direction of that force. If it's excessive, a light or message will indicate the need for trim and what direction (up or down). Just trim so the annunciator stops and all is good. The issue with the pilot that started this discussion was an issue of basic airmanship. Trim is a concept that needs to be understood while hand flying and become automatic, like how rudder pedals work. You go through a period of thinking about which pedal to push until you no longer think about it. Conscious vs unconscious competence. Trim is the same way. After a time, you just do it, vs thinking about it. When you switch to an AP, you would ideally have the aircraft trimmed before engaging the AP. An exception to that would be an AP that can do climbs and descents vs just hold altitude. There you might need to re-trim for different flight regimes.

IMHO there is too much discussion of the AP in the Blalock crash vs a lack of fundamental airmanship. The AP might have exacerbated the issue, but there seemed to be a lack of pretty basic understanding how to fly the plane.

 

[Country Flier]

In Ms. Blalock's case, the trim really didn't much enter into it. She did not have autopilot controlled trim, despite some loudmouth youtube "expert" analysis. Her autopilot deals with an out of trim condition by prompting the pilot to spin the trim wheel.

In my plane (and I suspect hers IF she had autotrim) the autopilot does trim off the control forces automatically. When you cancel the autopilot it holds whatever mode it was in at the time.

As I mentioned over in the thread on that crash, while it is possible the trim could run away, it's unlikely given safeguard to protect from that. Further, the REQUIRED (but possibly omitted) preflight checks run through all the various ways to get control AWAY from the autopilot should it fail.

Actually the trim did enter into it, just not the way you are thinking it would. Although I’ve never flown behind a Century 2000 AP, my understanding is in Ms Blalock’s plane it has an elevator servo, but no trim servo. So the AP can be adjusted for pitch (most likely referencing the AH), while completely out of trim. An AP annunciation light would show that it’s out of trim. Some of her previous videos showed she had a complete lack of understanding on this.

As far as the OPs question as to why it’s made this way, it’s a combination of simplicity vs cost, and the fact that IF you are aware of how it works, then it’s limitations are not dangerous.

 

[Jim K]

You also have to remember these things were designed 50÷ years ago. Think about electronics from that era. Having a device that could hold altitude and heading was pretty damn magical..... and expensive. Another servo to operate the trim might seem like too much extra money when it's so easy to do. I just put a garmin autopilot in my plane, and I decided to not spend the extra money to get a 4th servo to control the rudder.

My old century III did have the optional trim servo, but it also had an indicator to tell you which way to move the trim in case you only had two. Wouldn't be that hard to do, IF you understand that you have to.

I feel like we're getting to a point where new pilots don't have any frame of reference for the planes they are flying. 50"s mechanical designs and 70's electronics are very far removed from a world of iPads and Teslas. Flight instructors need to emphasize this. Unfortunately, many of them train in new G1000 skyhawks and don't have any experience with a wonky old analog autopilots.

Another feature my new autopilot has is a loud disconnect chime that plays through the audio panel. I didn't really appreciate that feature until reading about this crash. My old century iii, you could hear the solenoids unlatch if you were listening for it, but more than once I was looking at my attitude indicator wondering why I was starting to roll before I realized the a/p had decided to take a break for a while. You have to watch any autopilot, but those old ones especially. They will try to do something weird once in a while even if they work great 99% of the time. If you don't understand how they work they will try to kill you.

 

[Chip Sylverne]

Even the new a/p's, with airspeed climbs, envelope protection and all manner of safety gizmos, will kill you if you don't pay it the attention it's due.

 

[TrueCourse]

The design is, as mentioned, a less expensive version of an AP. Not a bad design, if the operator understands how it works and what its limitations are. One of the planes I fly now has only bank and pitch and you must manually trim to keep the AP happy. A little more work, but not a big deal. It doesn’t even have an UP and DN attitude feature.

In her case, that prompting lights (UP or DN) was blinking a lot when it was turbulent. She complained about it at least a few times. She knew what the lights meant as proven by her statements a few times. BUT, she wasn’t paying attention to them as much as she should. You can see her occasionally manually trimming in response. They would flash and then extinguish even without her trimming. That occurred a lot. The AP seemed overly sensitive to me in this regard.

Here is the explanation straight from the operating manual of the Century 2000:

“When the auto pilot displays a flashing, TRIM UP or TRIM DN on the annunciator, the pilot should manually move the trim control of his aircraft in the direction indicated on the auto pilot. When the auto pilot determines that the trim condition is satisfied, the trim lamp on the annunciator will extinguish and the pilot should stop his trim action. There are two degrees of trim prompting. For a small trim error the prompt will flash approximately once each second. A large trim error will cause the prompt to flash approximately three times per second. A large trim error not corrected for a period of approximately two minutes will sound an alert for five seconds. The alert ruler repeat every two minutes until the large error is corrected.”

I don’t think this warning is a good design. A lot can happen in two minutes with a large trim error. And, maybe the alert sound is not even loud enough to hear. In one of her videos she didn’t trim for over two minutes because the lights were flashing three times per second. Given an attentive pilot, this all works. When some understanding of all of this is lacking combined with lack of attention or ignoring it, it can be a disaster waiting to happen. The ATT feature allowing pitch control by pushing a button was also something that was an invitation to get her in trouble.

The more modern versions will do a better job warning when out of trim. So, if you get to fly one, don’t fret. Just learn and apply. ALWAYS know the various methods to turn off or kill the AP and practice reaching for the less used methods periodically.

 

[TrueCourse]

Here is something else worth emphasizing @MattCW with these UP and DN buttons which control the elevator and change the attitude of the airplane. Let's say you are straight and level with the airplane manually trimmed. You select ON and the AP goes into HDG and ATT mode. It is simply holding an ATTitude. You can leave it in ATT, or select ALT which is altitude hold mode. Or if in ALT switch back to ATT mode by touching the UP or DN button. Regardless, when you start using the UP or DN button and it goes into pitch control mode it must be soon followed by manual trim if you leave the engine power alone and don't keep the IAS where it was at before the pitch change. Otherwise as the plane goes slower in a climb or faster in a descent the plane becomes out of trim. The AP would be holding the pitch but only monitoring pitch control forces. The TRIM UP and TRIM DN lights would start flashing. Without an autopilot this would be like pulling or pushing on the yoke, changing the attitude of the airplane, keeping it in that attitude forcefully. Then letting it slow down or speed up without retriming, and staying in that untrimmed state for awhile. This was done in several of the videos.

With newer two-axis autopilots you can deactivate ALT hold mode while keeping HDG or NAV tracking active, change the pitch manually, change power, then once you reach desired altitude, pitch, set power, and trim as speed stabilizes. When not in ALT hold mode you are constantly aware of the trim needs. When in doubt of pitch control forces while in cruise, disengage ALT hold mode, check trim state, engage ALT mode again. I don't care for the UP or DN capability without AP electric trim capability. With this Century 2000, climbing in ATT mode, then when reaching desired altitude hitting the ALT button is not good practice in my mind. Because when it pitches to hold that altitude you start manually trimming based simply on the trim prompt lights. You don't "feel" it in the yoke anymore.

Long explanation for a simple concept she didn't appear to fully understand.