AP Altitude Hold

[Areeda]

fighting a cranky autopilot is often times worse than not using it at all.

Damn straight!

 

[Greg Bockelman]

Hey Greg,

Do you have one in the 195? Does using it allow you to monitor your oil level more closely?

LMAO.

 

[Ted]

All the planes I've flown in with altitude hold do a fine job of maintaining altitude. Variations depend on how the system is designed, and what condition it's in. The real benefits are in the more turbulent flight conditions where trying to juggle keeping the plane straight and level with all the other tasks going on adds significantly to workload.

As to use of one, a lot of my flying is the sort where an autopilot would be nice. I'm pretty worn out at the end of a lot of these days. Even though for me instrument flight isn't any more strenuous than VMC flight at this point (sometimes I'm more tired after a long VFR XC, probably due to the extra direct sun exposure), when you're flying IFR and get "have no routing advise ready to copy", then have to copy it, find it on the charts, input it into whatever your navigation is, and do all that other stuff, it would be nice to have an autopilot, regardless of whether you're in IMC or VMC. The real thing comes down to the extra-high workload situations. Bad weather, approach, missed approach, contingency planning...

As to why you'd want to use an A/P during other types of flying:

- Training flights on how to use the autopilot (this should be a part of your IFR training at some point, and a good idea to figure out how to use it before flying into IMC in an unfamiliar plane)
- Proficiency flights on how to use the autopilot (similar - making sure you can still automatically hit the proper buttons)
- Fun flights where you want to take pictures or just enjoy the scenery

It does wear you out. I've had long days of hand flying the Aztec the whole time, including in bad weather, and the autopilot does allow you to feel a lot more refreshed when you get there, mentally if not physically. Even if I've got another pilot with me, there aren't a number of people who I can just let fly my plane without me monitoring what they're doing. That monitoring takes brain power. When you've got 8-12 hours to fly in a day, that's a lot of brain use that wears you out.

 

[wabower]

I guess we should save the brain size discussion for another thread?

After the JFK, Jr. accident, the prevailing opinion seemed to be that he could have safely accomplished his flight (or returned home to regroup) if he had been competent in the use of the autopilot. Various avmags published articles re. A/P use by GA pilots. They pointed out that pilots of bigger airplanes are trained (required) to demonstrate proficiency with A/P use, and the training organizations and examiners (including the Fed) expect to see significant usage by pilots during routine tasks.

They also mentioned an underlying philosophical difference within the GA ranks, in that smaller airplane pilots (and their CFI's) seem more inclined to the notion that they should hand-fly rather than use the A/P. The primary reason cited was because "you never know when George will decide to stop working" and they therefore place less emphasis on A/P use and pilot proficiency.

They further pointed out that the primary concern re whether a pilot is capable of hand-flying the airplane should never be in doubt, but that the fear of autopilot failure is a vestige of the old days, and that modern A/P's are more dependable. I use the autopilot in my small plane the same way I used them in the big planes, other than the obvious functional limitations (won't track glideslope).

When you're PIC, it's your call.

All the planes I've flown in with altitude hold do a fine job of maintaining altitude. Variations depend on how the system is designed, and what condition it's in. The real benefits are in the more turbulent flight conditions where trying to juggle keeping the plane straight and level with all the other tasks going on adds significantly to workload.

As to use of one, a lot of my flying is the sort where an autopilot would be nice. I'm pretty worn out at the end of a lot of these days. Even though for me instrument flight isn't any more strenuous than VMC flight at this point (sometimes I'm more tired after a long VFR XC, probably due to the extra direct sun exposure), when you're flying IFR and get "have no routing advise ready to copy", then have to copy it, find it on the charts, input it into whatever your navigation is, and do all that other stuff, it would be nice to have an autopilot, regardless of whether you're in IMC or VMC. The real thing comes down to the extra-high workload situations. Bad weather, approach, missed approach, contingency planning...

As to why you'd want to use an A/P during other types of flying:

- Training flights on how to use the autopilot (this should be a part of your IFR training at some point, and a good idea to figure out how to use it before flying into IMC in an unfamiliar plane)
- Proficiency flights on how to use the autopilot (similar - making sure you can still automatically hit the proper buttons)
- Fun flights where you want to take pictures or just enjoy the scenery

It does wear you out. I've had long days of hand flying the Aztec the whole time, including in bad weather, and the autopilot does allow you to feel a lot more refreshed when you get there, mentally if not physically. Even if I've got another pilot with me, there aren't a number of people who I can just let fly my plane without me monitoring what they're doing. That monitoring takes brain power. When you've got 8-12 hours to fly in a day, that's a lot of brain use that wears you out.

 

[Ted]

I guess we should save the brain size discussion for another thread?

I figured I didn't need to point out the size of my brain or the fact that I am human and as such have limitations - someone with greater brain power and no limitations will surely do so for me.

After the JFK, Jr. accident, the prevailing opinion seemed to be that he could have safely accomplished his flight (or returned home to regroup) if he had been competent in the use of the autopilot. Various avmags published articles re. A/P use by GA pilots. They pointed out that pilots of bigger airplanes are trained (required) to demonstrate proficiency with A/P use, and the training organizations and examiners (including the Fed) expect to see significant usage by pilots during routine tasks.

Smaller airplane pilots (and their CFI's) seem more inclined to the notion that they should hand-fly rather than use the A/P because "you never know when George will decide to stop working" and place less emphasis on A/P's. They further pointed out that the primary concern re whether a pilot is capable of hand-flying the airplane should never be in doubt, but that the fear of autopilot failure is a vestige of the old days, and that modern A/P's are more dependable. I use the autopilot in my small plane the same way I used them in the big planes, other than the obvious functional limitations (won't track glideslope).

When you're PIC, it's your call.

Definitely important to be able to handle not having an autopilot, but like anything else, it's a tool that can help you. Not knowing how to use it is only hurting yourself.

 

[John Collins]

I can trim it to perfect, hit Alt hold, and we begin the excursions (+/- 100') until the altitude is re-captured.

Annoying and pointless.

The C2000 has a bad habit of porpoising in altitude hold. The pitch cable tensions have to be within specification, on the tighter side. I was never happy with the lack of quality in the C2000, so we stopped installing them, way too many failures IMHO.

 

[John Collins]

I use my Stec 60-2 autopilot a good deal of the time, when flying under or over class B airspace, when flying IFR, when VFR but on long legs. I normally turn off the altitude hold when in vertical turbulence, particularly when in Cumulus clouds or mountain waves. Most autopilots with attitude input will do a better job holding altitude in turbulence, but the rate based autopilots have a tougher time handling the turbulence.

 

[Bill]

A question for those here with A/P with altitude hold...

Do you use it?

We have a KFC-200, and it has Alt Hold, and we use it all the time. Works exceedingly well.

 

[Bill]

Nick, it isn't just weather. You really don't realize just how much brain power it takes to keep the plane straight and level and navigate on even three hour flights. Even in VMC. I didn't realize that until I had access to a wing leveler in an Archer. I was amazed at just how much better I felt after a three hour VFR cross country with just a wing leveler.

Absolutely. You arrive at the destination after a 3+ hour flight fresh as a daisy, ready to tackle that gusty crosswind or difficult approach to minimums.

 

[Bill]

They also mentioned an underlying philosophical difference within the GA ranks, in that smaller airplane pilots (and their CFI's) seem more inclined to the notion that they should hand-fly rather than use the A/P. The primary reason cited was because "you never know when George will decide to stop working" and they therefore place less emphasis on A/P use and pilot proficiency.

Trust but verify. Applies to George as well. I use him on long-d flights from maybe 2000agl to about the same at the destination. He mostly does a fine job, but I still watch him like a hawk.

When I do practice approaches, I do many by hand, but also do a good number using George. He not only tracks the LOC, but also couples the GS, so he can fly to mins.

Just another tool in the bag, Nick, and a might fine one at that.

 

[dmccormack]

We have a KFC-200, and it has Alt Hold, and we use it all the time. Works exceedingly well.

This S-TEC is a fine A/P, I just don't use the Altitude hold feature any more -- more work than trimming.

For tracking a course, it's awesome. It doesn't do a coupled approach, but using it to track a heading (it hunts too much to use it for the LOC) helps reduce the Approach workload significantly.

I agree it should be used based on the situation -- and that over-use or dependence on the A/P is a bad thing.

But I also think even VFR pilots should know how the wing leveler function works as a bare minimum. Next they should know how to use the A/P to make a 180 out of IMC.

How many end up as NTSB reports when a functioning A/P was available but not used?

 

[MikePapa]

This S-TEC is a fine A/P, I just don't use the Altitude hold feature any more -- more work than trimming.

For tracking a course, it's awesome. It doesn't do a coupled approach, but using it to track a heading (it hunts too much to use it for the LOC) helps reduce the Approach workload significantly.

I have an S-TEC 20 and as long as the air is relatively stable, it tracks the LOC just fine. My a/p doesn't have any sort of altitude hold, but I have a lot of hours behind the KAP 140 and a few behind the GFC700 and both are exceptional in both lateral and vertical.

There's a few caveats, especially with the KAP 140. If you're not really familiar with the a/p, it's easy to set the system to a vertical speed and not arm the altitude capture point. On descent it will literally run you in the ground if you don't pay attention to the altitude alarms as you pass through your selected altitude.

 

[dmccormack]

I have an S-TEC 20 and as long as the air is relatively stable, it tracks the LOC just fine. My a/p doesn't have any sort of altitude hold, but I have a lot of hours behind the KAP 140 and a few behind the GFC700 and both are exceptional in both lateral and vertical.

There's a few caveats, especially with the KAP 140. If you're not really familiar with the a/p, it's easy to set the system to a vertical speed and not arm the altitude capture point. On descent it will literally run you in the ground if you don't pay attention to the altitude alarms as you pass through your selected altitude.

Yikes! I suppose that's a "feature."

 

[Areeda]

OK, Century 2k and now S-TEC 30.

I can trim the airplane to 0 on the VSI and it will stay there for minutes, with only a slight nudge with a finger to maintain within +/- 10 FPM.

I've seen this behavior a few times.

The fix is usually either a new logic board in the A/P, adjusting the tension on servo friction coupling, or occasionally a new pitch servo.

Joe

 

[MikePapa]

Yikes! I suppose that's a "feature."

It's just like any computer. They only do what you tell them to do, not necessarily what you want them to do.

 

[dmccormack]

I've seen this behavior a few times.

The fix is usually either a new logic board in the A/P, adjusting the tension on servo friction coupling, or occasionally a new pitch servo.

Joe

This is a fairly new (>2 years) unit with maybe 200 hours on it (maybe 50 hours of actual use).

I'll ask the IA who installed it -- perhaps the new cables have stretched?

 

[John Collins]

This is a fairly new (>2 years) unit with maybe 200 hours on it (maybe 50 hours of actual use).

I'll ask the IA who installed it -- perhaps the new cables have stretched?

Dan,

The Stec 30 altitude hold feature should be rock solid, so if yours is porpoising, something is definitely wrong. It could be cable tensions, bridle tensions, high start-up voltage for the pitch servo, a bad pressure sensor, water in the static system, etc. A shop that knows your autopilot should be able to determine and fix the problem.

 

[dmccormack]

Dan,

The Stec 30 altitude hold feature should be rock solid, so if yours is porpoising, something is definitely wrong. It could be cable tensions, bridle tensions, high start-up voltage for the pitch servo, a bad pressure sensor, water in the static system, etc. A shop that knows your autopilot should be able to determine and fix the problem.

Yeah -- it definately needs to be checked. Scheduled for a look-see next week when the Garmin 430 is upgraded with WAAS.

 

[gismo]

I've got a question about Century III altitude hold and whether it could be 'upgraded'. If an Aspen can take a gps input and give you GPSS for the autopilot (Century III), is it possible to also eventually have an 'altitude select' upgrade? I know they are working on being able to get attitude information from their own ADHRS. I wonder whether 'atitude select' would even be a possible upgrade.

It's technically feasible and likely pretty easy but given the limited market and significant certification hurdles (cost) I doubt we'll ever see it.

On a related note, I have an Icarus SAM interfaced to my Century III and I know it would be easy for Icarus to add the ability to capture a GPSS course from any angle using the heading bug to set the intercept heading. I have added a request for this feature to their forum. If any of you have this combination (or better yet would consider adding a SAM to your C-III if the feature existed) it would help motivate their management and development folks if you'd sign up for the forum and post a similar message.

http://www.centurionci.com/cgi-bin/ikonboard.cgi

Unfortunately the SAM does not currently interface with the altitude axis of any autopilot and therefore cannot easily incorporate an altitude pre-select function.

I have on occasion considered developing "enhancements" to the C-III such as alt pre-select, Nav intercept, or even a complete replacement for the analog control electronics, console, and couplers but I have no idea how well this would sell.

 

[dmccormack]

I have on occasion considered developing "enhancements" to the C-III such as alt pre-select, Nav intercept, or even a complete replacement for the analog control electronics, console, and couplers but I have no idea how well this would sell.

Do this enhancements really add much? After all, if the A/P maintains heading or tracks a Nav input, all you've got left is to manage power and airspeed...

It seems anything beyond tracking is nice, but comes at a momumental cost for minor upgrade in overall capability....?

 

[gismo]

There's a few caveats, especially with the KAP 140. If you're not really familiar with the a/p, it's easy to set the system to a vertical speed and not arm the altitude capture point. On descent it will literally run you in the ground if you don't pay attention to the altitude alarms as you pass through your selected altitude.

One caveat with any altitude hold and worse yet a vertical speed mode is that you can end up in a stall if you're not careful. With alt hold it would normally require a reduction of power but leveling off from a descent can present the same opportunity. An example would be using the alt hold to level off at the MDA on a non-precision approach. This typically involves a fairly steep descent in a dirty configuration at a low power setting and if you fail to restore sufficient power the airspeed can decay to a dangerously low value close to the ground. A stall at that point especially in the clag could be disastrous. Similarly, if a vertical speed climb is selected the climb airspeed will decay as the increasing altitude reduces power possibly leading to a stall.

 

[gismo]

Do this enhancements really add much? After all, if the A/P maintains heading or tracks a Nav input, all you've got left is to manage power and airspeed...

It seems anything beyond tracking is nice, but comes at a momumental cost for minor upgrade in overall capability....?

One thing that could be quite valuable (and would only be feasible with a complete controller replacement) is some redundancy and/or internal self checking. And one of the simplest things I'd really like to add is an audible (preferably verbal) indication of autopilot mode changes (especially disengagement).

 

[dmccormack]

One thing that could be quite valuable (and would only be feasible with a complete controller replacement) is some redundancy and/or internal self checking. And one of the simplest things I'd really like to add is an audible (preferably verbal) indication of autopilot mode changes (especially disengagement).

The S-TEC beeps and flashes a mode LED when mode changes or A/P is dis-engaged.

 

[Areeda]

One caveat with any altitude hold and worse yet a vertical speed mode is that you can end up in a stall if you're not careful.

I like to practice autopilot induced stalls like these on BFRs. It makes a lot of pilots very nervous to watch it, which is good. It's just like any other stall demonstration except you press the disconnect button which is also good.

Joe

 

[dmccormack]

I like to practice autopilot induced stalls like these on BFRs. It makes a lot of pilots very nervous to watch it, which is good. It's just like any other stall demonstration except you press the disconnect button which is also good.

Joe

Excellent tip! I will certainly add that to BFRs and to any training in an A/P equipped a/c.

 

[wsuffa]

Dan,

The Stec 30 altitude hold feature should be rock solid, so if yours is porpoising, something is definitely wrong. It could be cable tensions, bridle tensions, high start-up voltage for the pitch servo, a bad pressure sensor, water in the static system, etc. A shop that knows your autopilot should be able to determine and fix the problem.

Agree.

On my 60-2 it started porpoising lightly and got progressively worse. Within about 50 hours it was so bad I took it into the shop - they said the servo was "toasted". With an overhaul, it's been rock solid.

Dan, it should do a lot better than that. It's worth a visit to the shop.

 

[dmccormack]

Agree.

On my 60-2 it started porpoising lightly and got progressively worse. Within about 50 hours it was so bad I took it into the shop - they said the servo was "toasted". With an overhaul, it's been rock solid.

Dan, it should do a lot better than that. It's worth a visit to the shop.

Wilco -- scheduled for this Monday.

 

[John Collins]

Do this enhancements really add much? After all, if the A/P maintains heading or tracks a Nav input, all you've got left is to manage power and airspeed...

It seems anything beyond tracking is nice, but comes at a momumental cost for minor upgrade in overall capability....?

In the scheme of things, the cost is not monumental, $2500 to $3500. In my opinion, Nav tracking is next to worthless, all the S turning over the VOR, trying to determine a suitable wind correction angle, etc. It is better when tracking a GPS, but even that can be insensitive and one needs to maintain the reference course on the DG or HSI.

What the GPSS would add to your 430 and Stec 30 combo are the following:

1) Ability to intercept a course.
2) Superior course tracking.
3) Nearly instant wind correction without S turning.
4) Ability to fly GPS T style approaches with those 90 degree turns that require turn anticipation.
5) Ability to fly a multi-leg flight plan, including approaches, SIDS and STARS both RNAV and standard and the RNAV T and Q routes.
6) Ability to fly curved paths such as DME arc's, hold entry (430W), holding pattern circuit (430W), and procedure turns (430W).
7) Ability to turn directly to a waypoint when rerouted in flight.
8) Ability to easily track an offset route or a route to a pilot selected User waypoint when required to circumvent weather
9) Ability to follow a localizer backcourse (430W only up to the FAF) or track outbound on the localizer for an ILS approach that is loaded from the 430/430W database.

You can accomplish most of this without GPSS, it is just more work.

 

[dmccormack]

In the scheme of things, the cost is not monumental, $2500 to $3500. In my opinion, Nav tracking is next to worthless, all the S turning over the VOR, trying to determine a suitable wind correction angle, etc. It is better when tracking a GPS, but even that can be insensitive and one needs to maintain the reference course on the DG or HSI.

What the GPSS would add to your 430 and Stec 30 combo are the following:

1) Ability to intercept a course.
2) Superior course tracking.
3) Nearly instant wind correction without S turning.
4) Ability to fly GPS T style approaches with those 90 degree turns that require turn anticipation.
5) Ability to fly a multi-leg flight plan, including approaches, SIDS and STARS both RNAV and standard and the RNAV T and Q routes.
6) Ability to fly curved paths such as DME arc's, hold entry (430W), holding pattern circuit (430W), and procedure turns (430W).
7) Ability to turn directly to a waypoint when rerouted in flight.
8) Ability to easily track an offset route or a route to a pilot selected User waypoint when required to circumvent weather
9) Ability to follow a localizer backcourse (430W only up to the FAF) or track outbound on the localizer for an ILS approach that is loaded from the 430/430W database.

You can accomplish most of this without GPSS, it is just more work.

Interesting -- though except for DME arc and LOC BC (we just don't have many round here) I've done all of those twisting the heading bug.

Thanks for explaining the increased utility, though -- it makes sense -- especially if the upgrade is that little.

 

[John Collins]

Interesting -- though except for DME arc and LOC BC (we just don't have many round here) I've done all of those twisting the heading bug.

Thanks for explaining the increased utility, though -- it makes sense -- especially if the upgrade is that little.

Interesting that you use the heading bug to accomplish the same tasks, in effect the GPSS adapter generates its own heading bug simulation to the autopilot as well. You have to have the Stec 30 in heading mode to follow the GPSS guidance!

 

[TMetzinger]

I like to practice autopilot induced stalls like these on BFRs. It makes a lot of pilots very nervous to watch it, which is good. It's just like any other stall demonstration except you press the disconnect button which is also good.

Joe

Really good point! Trim Runaways are another abnormal procedure to practice.

 

[RotorAndWing]

I like to practice autopilot induced stalls like these on BFRs. It makes a lot of pilots very nervous to watch it, which is good. It's just like any other stall demonstration except you press the disconnect button which is also good.

Joe

One problem that this could create is if the autopilot pitch servo uses a shear pin. If the student doesn't disengage the AP and forces the yoke down the pin shears rendering the AP inop.

 

[RotorAndWing]

Really good point! Trim Runaways are another abnormal procedure to practice.

How do you simulate a runaway trim in an airplane? Do the specific airplane AFM's have an abnormal procedure for trim runaway?

Given that FAR 23.677 states "(d) It must be demonstrated that the airplane is safely controllable and that the pilot can perform all maneuvers and operations necessary to effect a safe landing following any probable powered trim system runaway that reasonably might be expected in service, allowing for appropriate time delay after pilot recognition of the trim system runaway. The demonstration must be conducted at critical airplane weights and center of gravity positions" I don't see the value of "practicing" runaway trim, especially since the disconnect switch is right there on the yoke by the left hand. Flip the switch, event over.

23.677 Trim systems.

(a) Proper precautions must be taken to prevent inadvertent, improper, or abrupt trim tab operation. There must be means near the trim control to indicate to the pilot the direction of trim control movement relative to airplane motion. In addition, there must be means to indicate to the pilot the position of the trim device with respect to both the range of adjustment and, in the case of lateral and directional trim, the neutral position. This means must be visible to the pilot and must be located and designed to prevent confusion. The pitch trim indicator must be clearly marked with a position or range within which it has been demonstrated that take-off is safe for all center of gravity positions and each flap position approved for takeoff.
(b) Trimming devices must be designed so that, when any one connecting or transmitting element in the primary flight control system fails, adequate control for safe flight and landing is available with—
(1) For single-engine airplanes, the longitudinal trimming devices; or
(2) For multiengine airplanes, the longitudinal and directional trimming devices.
(c) Tab controls must be irreversible unless the tab is properly balanced and has no unsafe flutter characteristics. Irreversible tab systems must have adequate rigidity and reliability in the portion of the system from the tab to the attachment of the irreversible unit to the airplane structure.
(d) It must be demonstrated that the airplane is safely controllable and that the pilot can perform all maneuvers and operations necessary to effect a safe landing following any probable powered trim system runaway that reasonably might be expected in service, allowing for appropriate time delay after pilot recognition of the trim system runaway. The demonstration must be conducted at critical airplane weights and center of gravity positions.

 

[dmccormack]

How do you simulate a runaway trim in an airplane? Do the specific airplane AFM's have an abnormal procedure for trim runaway?

Yep -- "DISENGAGE AUTOPILOT"



When all else fails, pull the breaker.

 

[TMetzinger]

Nearly all do. The AFM supplement for the KAP140 with electric trim specifically addresses problems with the autopilots autotrim and the manual electric trim.

Like simulating a depressurization - you "fake" the emergency by saying something like "Hey, why's the trim wheel running forward?" and go through the steps - fly the airplane, disconnect the trim, retrim manually, and pull the AP circuit breaker. Similar steps apply to the GFC700, which is more applicable because that sucker trims RAPIDLY.

In the sims, of course, you can actually cause a trim runaway.

 

[RotorAndWing]

In all my years of flying I have never had a runaway trim situation, especially in a GA airplane. I've trained for it in large airplanes and airplanes with pure electric trim.

Since there is a "Disengage" switch by your left hand it's a non event anyway. Simply switch off and continue.

 

[dmccormack]

Nearly all do. The AFM supplement for the KAP140 with electric trim specifically addresses problems with the autopilots autotrim and the manual electric trim.

Like simulating a depressurization - you "fake" the emergency by saying something like "Hey, why's the trim wheel running forward?" and go through the steps - fly the airplane, disconnect the trim, retrim manually, and pull the AP circuit breaker. Similar steps apply to the GFC700, which is more applicable because that sucker trims RAPIDLY.

In the sims, of course, you can actually cause a trim runaway.

I work through the steps to disconnect and then run through it with eyes closed.

I have owners with A/P show me the disconnect procedures. Haven't had one yet that ended with "When all else fails, pull the breaker."

 

[Areeda]

One problem that this could create is if the autopilot pitch servo uses a shear pin. If the student doesn't disengage the AP and forces the yoke down the pin shears rendering the AP inop.

Hmm, what kind of autopilots are these?

I don't know I'm asking because in the pre-takeoff checklist I have make sure you can override the autopilot and I don't want to shear the shear pin.

Joe

 

[RotorAndWing]

Hmm, what kind of autopilots are these?

I don't know I'm asking because in the pre-takeoff checklist I have make sure you can override the autopilot and I don't want to shear the shear pin.

Joe

Cessna ARC autopilots use the shear pins. Other brands use clutches. Just to be sure I would check what autopilot you have and what method they use.

 

[gismo]

Really good point! Trim Runaways are another abnormal procedure to practice.

How do you practice that? On my autopilot (C-III) operating the electric trim trips the autopilot offline and unless you disable the auto and electric trim you cannot move the trim manually.

One thought if there's an autotrim capability would be to surreptitiously pull back on the yoke while the autopilot rolls in down trim to compensate but then you'd have to release the pressure at just the right time and rate to provide the same effect as a real runaway.