Cyberstudio;

I have made some mods to the xml autopilot gauge that might solve your problem with pitch control; if you have a spare email address that you don't mind posting I can send it to you.

Matt

Matt2218 wrote on Dec 14^th, 2009 at 12:25am:


Please, will someone explain to me what the aircraft weight (at approach time) can possibly have to do with the inability of the AP to properly capture and fly an ILS? Even the old Sperry AP in this aircraft should be capable of intercepting and flying an ILS down to DH or lower irrespective of aircraft weight!

This old airplane does not have a FMS, fuel-burn calculators or any other sophisticated automation. Just an old Sperry autopilot and steam gauges. There is no way that this autopilot even "knows" what the aircraft weight is at any given point in a flight. How, then, could aircraft weight at any point in any flight have any effect whatsoever on autopilot operation?

Imagine taking off in a full-gross-weight 727-200, losing an engine, deciding to immediately land back at the departure airport and finding out that your AP doesn't work except at certain weights... the concept is absurd.

This AP has only 2 axes: pitch and roll. These are aerodynamic parameters, measured by spinning gyros, not weight parameters. Yes, I know that pitch angle to maintain any particular vertical flight path (level, climb, descent) at any given speed is dependent on weight, but the variations are small, fractions of degrees of pitch to 2-3 degrees or so, and even more telling, THAT IS COMPLETELY BESIDE THE POINT!

In approach mode (Auto/GS), this AP should be able to "see" the ILS needles come alive and anticipate (based on intercept angles, simple basic math) how soon BEFORE the actual intercept point it needs to begin turning or pitching to capture the correct path... mine never anticipates... it waits until the intercept point has come and gone, then makes gross overcorrections back toward the LOC and/or GS.

I submitted a ticket on this, and the response was "we do not experience this problem." This is not helpful. I am doing everything "by the book" as far as AP setup and aircraft configuration are concerned. I have read all 3 manuals cover-to-cover several times, and I am following the procedures there TO THE LETTER. Based on postings here and elsewhere, I am not the only one experiencing autopilot "wonkiness" on ILS's.

Since CS apparently won't or can't help, I am hoping that someone here on the forum can help me to understand what I am doing wrong, or at least what needs to be done to make the AP capture and fly an ILS. However, I am BAFFLED by the references to aircraft landing weight in connection with autopilot function.

Please, someone help me understand what aircraft weight has to do with AP function. This (or any other approach-capable) AP should capture and fly the needles much better than I can AT ANY AIRCRAFT WEIGHT INCLUDING 5000 LBS OVER GROSS TAKEOFF WEIGHT!

Thanks in advance to anyone who can help me to understand the bizarre concept of aircraft weight preventing correct AP function.

On its way.  Let me know how you make out.

Cyberstudio - I emailed you...pls let me know if you didn't get it.

Richard Portier wrote on Dec 19^th, 2009 at 2:07pm:


In their response to my ticket on this issue, CS said that I should check aircraft speed, configuration and WEIGHT. They cited these 3 factors in the context of any one of them causing the AP not to be able to capture an ILS.

I can see how SPEED, if too high, would make the AP lag behind the ILS needle indications- simply because they come and go so quickly if you are flying toward the LOC and/or GS at an excessive speed... the AP just can't make adjustments that fast. I get that. I can even understand how flaps configuration could affect the AP because you can't fly the airplane at correct approach speeds without flaps & slats. However, this is only a peripheral connection; the REAL culprit would still be SPEED, because the old Sperry AP neither "knows" nor cares how much flap you have out. If you could fly the airplane on approach at 125KIAS without flaps, the AP should be FINE with that. So "configuration" would be only an indirect factor preventing proper AP function- only because proper config enables slow-enough airspeed.

I find it difficult to believe that CS or any other sim-aircraft developer of their caliber would program in a clearly-irrelevant "weight" parameter in the AUTOPILOT operation of an airplane that has ZERO automation such as FMS. Without FMS or similar functionality, the AP simply has no way of "knowing" aircraft weight at any given point in time- so why would excessive WEIGHT play any part in causing the AP to fail to capture and fly an ILS?

I'm just baffled by CS's suggestion that I should check approach WEIGHT as a possible cause for the AP failure to capture LOC and GS.
Likewise, I'm baffled by those who said here (above somewhere) that the AP works for them on ILS's with an empty airplane but not when loaded. It's just nonsensical to me.

You might as well say that the AP in this airplane only works well if the windshield wipers are turned on. That's how absurd and irrelevant "weight" seems to me as a possible cause of AP failure.

That's why I asked for help in understanding what weight can possibly have to do with AP operation on approaches in this steam-gauge, no-FMS airplane.

I think it would be perfectly safe to re-run the executables...or you could uninstall/reinstall.

I wonder if it could be an FSUIPC version issue?  Grabbing at straws here...maybe check which version of FSUIPC you have installed (I think it's in the 'modules' folder in FSX) and see if it is the most recent one.

FWIW my C-130 is a bit lazy grabbing the localizer...sometimes I have to do the off/on routine with track mode.

HoggyDog wrote on Dec 20^th, 2009 at 7:35pm:


For lack of a better term, this is what I call the "pitch runaway" syndrome of this AP, i.e. AP fails to hold pitch properly. I have submitted a ticket on this and it has been "forwarded to the development team for consideration." So the good news is, CS is able to duplicate the problem and is aware of it.

TacomaSailor wrote on Dec 20^th, 2009 at 12:16am:


This sounds like a combination of three well-known and previously-discussed issues:

1. Triple flameout due to improper crossfeed setup (see JETPILOT's informative response to this nasty inflight surprise elsewhere in the forums). It is conceivable that you could still show a "normal" N1 speed due to windmilling of the flamed-out engines as you plummet to the ground with no power; and

2. The VSI is inaccurate. Matt2218 has addressed this in another thread in this forum; and

3. The aerodynamic modeling of this airplane is WAY off, such that power (throttle) has TOO MUCH impact on speed, and pitch has TOO LITTLE impact on aircraft speed. Again, our hero (and RL 727 pilot) JETPILOT has discussed this long-standing issue elsewhere (possibly here, possibly on AVSIM). The exact problem he discussed was that it is impossible to descend the airplane with throttles correctly set at idle no matter how large the rate of descent is- you have to carry some power in the descent to maintain airspeed even on a "clean" (no flaps, spoilers or gear out) airplane- which is just silly. The 727 is a fast, "slick" airplane and should maintain airspeed during a descent at idle throttle with descent rates of anything over 1000 fpm.

TacomaSailor wrote on Dec 20^th, 2009 at 12:16am:


This is what my AP does as it tries to capture a GS in Auto/GS mode AND when engaged during the first segment climb after takeoff- it's the AP "pitch runaway" issue, Part Deux. The bottom line here is that the AP pitch control simply doesn't work, period. Hopefully, CS will fix this sooner rather than later.

TacomaSailor wrote on Dec 20^th, 2009 at 12:16am:


Haven't experienced this one (yet). If you're sure your course needle was set correctly (it can be confusing, depending on whether you intend to fly TO or FROM the VOR) then this is just another example of autopilot "wonkiness" in the CS 727 (FSX).

TacomaSailor wrote on Dec 20^th, 2009 at 12:16am:


I wish I could say the same. My AP has ***NEVER*** captured a LOC or GS without first delaying the roll or pitch toward the correct path until after it has passed and then overcontrolling the airplane after the correct path has come and gone, resulting in endless zigzagging and porpoising... and a crash if I don't disconnect it and try to salvage the approach by hand... and it has ***NEVER*** held pitch on initial engagement after takeoff.

TacomaSailor wrote on Dec 20^th, 2009 at 12:16am:


Um, no. This is the longest and most consistent thread on the entire 727 Forum, even after you disregard some of the posts wanting to know where the alt-preselect or autothrottle-enable switches are (the airplane doesn't have those.) It is CLEAR from the many AP issues discussed here over the past 10 months that CS needs to completely re-work the AP "logic" and functionality in this airplane. Hopefully, they will do so soon. Please, CS, move this to the top of your list and FIX IT!

TacomaSailor wrote on Dec 21^st, 2009 at 3:24am:


Back in the Dark Ages (around the time that early 727's were rolling off the Boeing assembly line) I remember a lively "hangar talk" session about the new Piper Arrows being given T-Tails for no reason other than "visual style" and/or "ramp appeal." Somehow, Piper's Marketing people had prevailed over the aerodynamicists and engineers and convinced Management that the airplane would sell better if it looked more like a jet.

The conclusion of the assembled sages that day was that Piper had shot itself in the foot, and that the adverse aerodynamic characteristics of T-Tails (elevator blanking) at rotation, in the flare and (as you have discovered with the 727) in "slow flight" when level would basically "kill" the Arrow as a marketable airplane. As evidence, they discussed the elevator blanking so painfully obvious in Piper's attempt to unseat the Cessna 150/152 as the most widely-used primary trainer: the Tomahawk, which also had a T-Tail purely for misguided aesthetic reasons and whose entire empennage would shake visibly and audibly while practicing stalls. I turned around to look at it once while it was shaking and flapping around at the onset of a stall, and I immediately released back pressure, resumed normal flight, returned SLOWLY AND GENTLY to the airport and refused to ever fly a Tomahawk again. It's the only time in my long and mostly-routine flying career that I was AFRAID of what was happening and genuinely fearful that I would not make it back alive.

Turns out the sages were right about the T-Tail Arrow- it wasn't long before poor sales moved Piper to have the same epiphany- they put the horizontal stab back where it came from, i.e. where the engineers said it should be. Also, on the used airplane market, the T-Tails are generally the cheapest and least-desirable of all the Arrows, other factors being equal.

Of course, the T-Tails on 727's, DC-9's (etc. etc.) were put there due to engine placement, not aesthetics, but the result is that elevator blanking at high AoA is a very real phenomenon in pretty much ANY T-Tail airplane, large or small. That's why the "normal configuration" for horizontal stabs has them mounted on or very close to the fuselage.

The only part of your post that I'm having any difficulty understanding is where you appear to say that you are reading Mach .71 and 140 KIAS simultaneously at 12,000 ft MSL. Just "don't seem right" to me.  

Also, I was under the impression that ALL autopilots would "kick off" at the onset of the stick shaker. I have no personal RL experience with the 727's Sperry, but I thought that this was a certification requirement. Many large airplanes even have "stick pushers" that take affirmative action to cure an incipient stall- I wonder if the 727 had a stick pusher as well as a stick shaker?

Here is a link to my youtube video tutorial on how to use the 727 autopilot for an ILS landing. http://www.youtube.com/watch?v=fy3utBqi7-4

I hope this is helpful!