where an airplane in perfect trim landed by itself
I don't think many modern IFR tourers will do that. I don't think any of them are roll stable. Every one I know will go into a spiral dive.
it's not the autopilot that does it, it's the turn anticipation function of the GPS, which can be performed by any autopilot with roll steering.
Yes, AIUI, the GPS transmits, over ARINC429, roll steering commands which are in the form of "fly roll angle X". Initially I expect the command will be for something like Rate 1 (which is about 20-22 degrees for most light IFR planes in cruise) and as the track approaches the track of the leg being intercepted, the GPS reduces the value of X, so you get a convergence.
There must be the usual control loop stability (overshoot/undershoot versus dynamic performance) issues, but most avionics use a very low proportional gain, a large integral gain, and little if any derivative gain, so unless something is badly shagged, it tends to be stable.
Envelope protection...
The "Straight & Level" button which you can use anytime you lose control of the airplane in clouds is another nice feature, and it works perfectly, at least it did from all legal maneuvers I flew.
I see these two things more as items that fix issues that catch out the unwary user of these advanced autopilots. Envelope protection prevents the AP induced stall, and the LVL button is for the "WTF does it do now" moments. Personally I use the AP disconnect for that, but maybe if the LVL button were on the yoke it would be more appropriate for these situations.
I put a GFC700 Cirrus into a spiral descent I would expect any student pilot to recover from easily (60 degree AOB), and could not let the LVL button finish the job because it was too slow and we would have exceeded Vne, and come a bit too close to the ground despite starting the manoeuvre at 2,500ft and pressing the button at 2,000ft...
The latest GFC700/G1000 versions with what they call "ESP" intervenes before a high bank is reached by applying aileron EVEN IF THE AP IS OFF, and if they did that in pitch you would have a stick pusher to keep you out of the stall...
Cobalt, i have not tried that one yet, but will and then report. But of course non of this can/should be trusted 100%... but i do think that the DFC90 is a great package with many helpful features for not too much money. I actually like the IAS mode the best, and the flight director is very precise too
Another benefit of the LVL button is for use by a non pilot passenger in the event of pilot incapacitation whilst the pilot was hand flying.
Other than testing it when I first got the A/P, I have never used the LVL button myself but it did work very well for me and I'm glad I have it.
Resurrecting this old thread with a slightly different focus.
I usually fly a P210 but during the last few days I have been cross-country (XC) flying a Cessna 182 (w/Petersen conversion , an interesting bird…) and a beautiful PA28-181, both without an autopilot.
Our P210 has a 40+YO but completely functional 2-axis autopilot (AP) which, coupled to Aspen PRO MAX PFD and GTN750 offers a lot of very useful functionality. For single-pilot (SP) IFR I must admit I am a bit spoilt since holds, level changes, procedure turns, missed approaches, or even orbits to “troubleshoot” a minor problem or anxious pax before entering a certain phase of flight that requires full attention are easy peasy.
Yes, one has the yearly IR review with the examiner which is mostly hand-flown.
Yes I did have one case where the a/p failed on me at the start of a 700+NM IFR flight back home 20% in IMC “sentencing” the PIC+PF (SP anyway) to 4 hrs hand flying including an ILS in IMC at the end. But even then FULL attention was not required since the P210 is very stable and the flight controls are quite firm: they stay where you put them so you can look back at the pax or down to grab a bite or a kneeboard and then look forward 30 secs later and all stays put. I was very tired at the end of that flight but it was managed reasonably safely. I however consider a functioning AP a dispatch requirement for flying out of base for any XC of more than 1 hr.
Some call that P210 behaviour truck-like and I don’t blame them, but it does make my SP life easier even w/o AP. If flying with fwd CG, the 210 requires a lot of trim change with speed or climb-cruise mode change which requires some effort, whereas the AP will gladly do it easy for you: you just set the power, adjust rudder trim, close the cowl flaps, and forget the otherwise very attention-getting progressive trimming for one full minute or more as you level-off for cruise. If you do not devote such attention you will end up chasing the airplane for speed and altitude in a never ending oscillation which when mastered while hand-flying leaves one with a satisfied grin at the end of every level change. OK once or twice, but why bother all the time when the AP will do it for u?
There have been a couple 2-hr legs with the 182 and PA28: one IFR and one VFR. It had been some time since I had last flow either type (there are at least 100hrs of the latter in my book) but I was surprised to see both aircraft have relatively light controls. Probably not as light as a Bonanza or even a TB (speaking only yoke-driven aircraft , sticks are a different feeling) but light nonetheless for XC flying for someone mostly used nowadays to a 210, especially the case for the PA28. I even found a friend’s King Air much lighter and very Bonanza-like on controls.
On this occasion I was SP and I found it a lot of work to fly with precision for a long time. 1hr is OK, but after 2 hrs with the 182, and that was 99% VMC, I was much more tired than after 4 hrs with my 210 and then I would be covering thrice as much ground!.
No doubt because I do not practice handflying enough…
But even after 2 hrs VFR SP (in the PA28) , having to care for my pax as well (our kids) , I found it bothersome that every time I looked back for 10 seconds when I looked forwards again the aircraft was 5-10 degrees off on roll and maybe as many in heading…again tiring after two hours…adjusting rudder trim precisely to match changing fuel imbalance helped but did not solve it entirely.
I even reached the conclusion it would not be safe to do it like that for more than two hours…I feel I would need to bring along either self-caring pax (I find even adults need some attention, or maybe I spoil my pax too much?) or another pilot…
How much of that is because of my lack of handflying practice and how much because of the different type?
How are you guys dealing with long XC’s SP w/o autopilot? Anyone doing it IFR routinely?
Not AP related but interesting comments about stability. I am quite frustrated that I find it hard to hold altitude in my new-to-me TB20. I’m not talking huge variations, but if I don’t pay attention I can easily find myself 100 feet off altitude. Whereas with my beloved TR182, I could hold altitude to within 20 feet in reasonably still air without any effort at all. I’m sure it’s just a question of practice. A 182 mya be “light” compared to a 210, but compared to just about anything else it is pretty heavy in pitch – and correspondingly stable.
Antonio wrote:
How are you guys dealing with long XC’s SP w/o autopilot? Anyone doing it IFR routinely?
2.3hrs, landing + food, then 4.3hrs. IFR in either real or simulated IMC.
It’s OK, just very tiring.
Edit: My bad! -0.5hr each for taxi/takeoff/landing.
arj1 wrote:
2.3hrs, landing + food, then 4.3hrs. IFR in either real or simulated IMC.
It’s OK, just very tiring.
Edit: My bad! -0.5hr each for taxi/takeoff/landing.
Which type are you flying?
johnh wrote:
A 182 mya be “light”
You are right: a 182 is “light” in roll, but not so much in pitch, whereas PA28 is lighter in both axis.
How are you guys dealing with long XC’s SP w/o autopilot?
No idea… there really are some crazy daredevils out there.
