To pitch trim or not to pitch trim...

[MikeOH]

1 hour ago, Matthew P said:

Gotta think resale

Yeah, losing half of every dollar I spend when I sell.

[Matthew P]

49 minutes ago, MikeOH said:

Yeah, losing half of every dollar I spend when I sell.

Well, you gotta enjoy it while you have it and that goes without saying in aviation..never gonna get rich.. nobody thinks like that when they get married, you automatically have 50% or more waiting to walk out the door..

 

[kortopates]

Most Cessna 172’s in a club i instruct in have the Garmin GFC-500 along with GTN-650 and G5’s and a EDM-900. It’s kind of what new pilots are expecting these days. But most don’t have pitch trim so one manually changes the trim when the G5 says too. But these are trainers and time builders, not serious IFR traveling birds.

All modern Mooney AP installations had pitch trim in their BK or Century AP by the factory (and a few STEC). So I’d think lack of pitch trim a down grade.

But a 67 E only got PC when it left the factory. So a minimal GFC-500 installation is still a significant step up - at least from original.
So i don’t see a big concern either way and understand not wanting to spend $$$ with a low hull value to start. Makes perfect sense and it’s still very marketable.

That said comments about preferring to use manual trim miss the point. I too prefer to use more precise manual trim wheel; especially in cruise. but when I want to hand the plane over to the AP i want to be able to use the AP in climb or descent too - not just cruise without having to deal with trim. As much as i like my manual trim i much prefer to use electric trim during a go around so i can use that hand for others things like throttle, flaps and gear while the trim is moving.

Arguments about the autopilot are going to kill you after you make a silly mistake twisting in the wrong value are just as absurd since a GFC-500 provides a huge safety benefit in the form of ESP and the Blue level button. If more of these instrument pilots lacking proficiency getting into trouble in IMC had a Garmin AP and could push the Blue level button we wouldn’t have had two fatal IMC LOC-I accidents recently here in SOCAL.

But these decisions really are about personal choices. But not all the arguments here make sense IMO to someone that actually uses one.


Sent from my iPhone using Tapatalk

[dkkim73]

10 hours ago, TaildraggerPilot said:

Pitch Trim isn’t a third axis, unless I missed something in the past 30 years….

I think it's an axis in a control-theoretic sense. Ie. a degree of freedom with feedback etc. Obviously not orthogonal to the pitch axis, but not trivially related, either. 

Geometrically, you're of course right, it's about the same spatial axis. 

 

[dkkim73]

9 hours ago, M20F said:

Mistakes are what kill people.  Instead of twisting 500FPM you accidentally twist 5000 and look at your iPad and it potentially becomes a bad day. 

Mistakes don't kill people. Guns don't kill people. iPads kill people. Those, and people with mustaches.... I'm making a list.

[dkkim73]

On 7/21/2025 at 8:19 PM, M20F said:

I am a huge fan of 2 axis automation in GA but personally I don’t think we are there yet on 3 axis.   History has also proven the third axis to be what kill you. 

Your post is pretty thought-provoking for me. I don't think I agree about it (PTRM) being bad to include in an autopilot, on balance. I can see the other side of it. E.g. You point out it is has a few potentially sinister failure modes. OTOH so does flying a coupled approach without an autothrottle. 

I suppose I'm generally pretty aware of the trim state, and sometimes use the AP to trim for me. E.g. doing engine runs (working out power settings) or slow flight... I hit ALT or adjust power and I am aware it's trimming for me. 

But sometimes it can do things you aren't specifically thinking of that moment, like on a level-off. That would still happen with only an elevator (PTCH) axis. And if you get way behind it takes longer to undo. 

Intersting thread. It actually had never occurred to me that a GFC500 or GFC700 would be installed without a trim servo. 

 

 

[M20F]

9 hours ago, kortopates said:

Arguments about the autopilot are going to kill you after you make a silly mistake twisting in the wrong value are just as absurd

The crew of AA965 would disagree.  Automation will do what you ask it to.  The more automation you have the more potential to have it do something unintended.  

[Echo]

13 hours ago, Matthew P said:

Well, you gotta enjoy it while you have it and that goes without saying in aviation..never gonna get rich.. nobody thinks like that when they get married, you automatically have 50% or more waiting to walk out the door..

 

Lol.  What a terrible analogy.  Marriage is a lifelong partnership.  Choose wisely.  Buying a plane NOT equipped to your liking and expecting a positive return on your investment after upgrading is a CERTAINTY of financial failure.  Purchase wisely.

[Jackk]

Always thought modern day marriage was a legal solution to a romantic problem

[TaildraggerPilot]

12 hours ago, M20F said:

The crew of AA965 would disagree.  Automation will do what you ask it to.  The more automation you have the more potential to have it do something unintended.  

I’m not seeing the relevance here. That crew was battling non-standard beacon ID’s. 

[kortopates]

The crew of AA965 would disagree.  Automation will do what you ask it to.  The more automation you have the more potential to have it do something unintended.  

That's was fascinating accident case study. Cool that would you bring this one up as its an excellent discussion on its own. Its one of the most studied and one of the text book examples of the swiss cheese analogy of accidents building on several mistakes. The pilots made so many mistakes all in their haste to get the plane on the ground after taking off a few hours late so that FAA pilot rest regulations would not delay their departure on the next leg. And sadly Cali approach radar was down from an insurgents attack and both pilots lost complete situational awareness because they became too focused changing from a circling approach to a straight in approach when they were already too high and too fast to get down to do it; but then they had become to there whereabouts after miss communications with Cali approach and the Captain did a direct to the FAF bypassing fixes that would have separated them from the terrain. What they were trying to do was actually ludicrous if they had some time to think it through but instead they pressed on trying to go direct to FAF and then make a 180 back to start the arrival not realizing they had already passed the starting fix for the arrival. Even the crew acknowledges on the CVR that they really screwed up getting lost which came about from not following airline procedures to have both pilots verify before making the flight plan changes they did. Absolutely there confusion was aided by pilot controller miss-communication and trouble finding the FAF waypoint.
The accident report doesn't at all blame it directly on automation out right as you suggest but I'll just copy paste from the report:

3.2 Probable Cause

Aeronautica Civil determines that the probable causes of this accident were:

1. The flightcrew's failure to adequately plan and execute the approach to runway 19 at SKCL and their inadequate use of automation.

2. Failure of the flightcrew to discontinue the approach into Cali, despite numerous cues alerting them of the inadvisability of continuing the approach.

3. The lack of situational awareness of the flightcrew regarding vertical navigation, proximity to terrain, and the relative location of critical radio aids.

4. Failure of the flightcrew to revert to basic radio navigation at the time when the FMS-assisted navigation became confusing and demanded an excessive workload in a critical phase of the flight.

3.3 Contributing Factors

Contributing to the cause of the accident were:

1. The flightcrew's ongoing efforts to expedite their approach and landing in order to avoid potential delays.

2. The flightcrew's execution of the GPWS escape maneuver while the speedbrakes remained deployed.

3. FMS logic that dropped all intermediate fixes from the display(s) in the event of execution of a direct routing.

4. FMS-generated navigational information that used a different naming convention from that published in navigational charts.

 

But with that tangent out of the way for a minute, lets focus specifically on autopilots and specifically the modern digital GFC-500.

Your argument seems to be that although your in your favor of lateral nav automation with an autopilot you don't believe the automation/autopilot is safe enough for vertical nav - although you called in 3 axis but I interpreted it as vertical nav/up & down.

I though consider this autopilot to be large advance in safety for any pilot that allows themselves to get into trouble in IMC. Its not going going to make up for airmanship mistakes like twisting in 010 and when they were told 100 degrees. But using your examples on the vertical nav is non-sensical to anyone that has flown with a GFC-500. You can't command it to climb at 5000 FPM and the autopilot will not prevent you from miss using it; but using your example of too steep of a climb rate the AP will simply lower the nose when it approaches its Min Speed (69 kts for J&K); similarly in a descent  if your about to exceed the AP max speed it will lift the nose to slow down. And then rather than let a pilot become a loss of control statistic, even without using a lateral mode and/or vertical mode the autopilot ESP (Electronic Stability and Protection) will work to prevent excess rates of bank beyond 45 degree and pitch beyond +20 and -15. And a big but simple feature for a pilot loosing control in IMC is to push the Blue Level Button to level the wings and regain control. This could have saved multiple lives here very recently from LOC-I fatal accidents. You can still program your flight plan to fly into a mountain but newer digital AP technology can save a pilot from becoming a LOC-I statistic and IMO represents a huge safety improvement in protecting a pilot and their pax.

[Igor_U]

13 hours ago, kortopates said:

That's was fascinating accident case study. Cool that would you bring this one up as its an excellent discussion on its own. Its one of the most studied and one of the text book examples of the swiss cheese analogy of accidents building on several mistakes. The pilots made so many mistakes all in their haste to get the plane on the ground after taking off a few hours late so that FAA pilot rest regulations would not delay their departure on the next leg. And sadly Cali approach radar was down from an insurgents attack and both pilots lost complete situational awareness because they became too focused changing from a circling approach to a straight in approach when they were already too high and too fast to get down to do it; but then they had become to there whereabouts after miss communications with Cali approach and the Captain did a direct to the FAF bypassing fixes that would have separated them from the terrain. What they were trying to do was actually ludicrous if they had some time to think it through but instead they pressed on trying to go direct to FAF and then make a 180 back to start the arrival not realizing they had already passed the starting fix for the arrival. Even the crew acknowledges on the CVR that they really screwed up getting lost which came about from not following airline procedures to have both pilots verify before making the flight plan changes they did. Absolutely there confusion was aided by pilot controller miss-communication and trouble finding the FAF waypoint.
The accident report doesn't at all blame it directly on automation out right as you suggest but I'll just copy paste from the report:

 

Good article on the AA965 accident:

 

https://admiralcloudberg.medium.com/children-of-the-magenta-the-crash-of-american-airlines-flight-965-b16f57c34cfe

 

 

[redbaron1982]

If you have manual trim with the AP enable, how do you know the airplane is trimmed? I mean, you don't "feel" the force the AP is doing to keep level flight. So how's the process to trim it out? Just curious.

If I'm flying with the AP engaged I'd like to have a significant attitude change if for whatever reason the AP decides to disengage, so automatic trim seems like an additional safety feature: in a AP disconnect event the ship should continue (more or less) straight and level flight. Am I wrong here?

[dkkim73]

13 minutes ago, redbaron1982 said:

If you have manual trim with the AP enable, how do you know the airplane is trimmed? I mean, you don't "feel" the force the AP is doing to keep level flight. So how's the process to trim it out? Just curious.

If I'm flying with the AP engaged I'd like to have a significant attitude change if for whatever reason the AP decides to disengage, so automatic trim seems like an additional safety feature: in a AP disconnect event the ship should continue (more or less) straight and level flight. Am I wrong here?

No, you're not wrong. The argument is that if you get runaway trim, or the trim state is very different than you think, it will fight you when the AP clicks itself off (or you click it off). 

IIUC from Junkman's description above, the PFD/AP display will tell you to trim with a little arrow. In effect, you become the pitch trim axis servo in that design. IIUC The argument is that that makes you more aware.