Learjet 35A IAS mode

[Deltaair1212 53]

Can anyone inform me of how to properly use IAS mode? I took off out of my departure airport and once I cleaned up the aircraft, I trimmed it out to where I wanted it. Afterwards I hit ENG, then NAV, then ALT (hold) but once I hit SPD my Lear started to pitch up and down uncontrollably. I was still under 10,000ft so I was just under 250kts. How does this button work exactly? To my understanding it is suppose to dictate your VS by where your throttles are. I had them pulled back to maybe 50% when I pressed the button but then my aircraft got all squirrelly on me...

[ark 1,288]

41 minutes ago, Deltaair1212 said:

Can anyone inform me of how to properly use IAS mode? I took off out of my departure airport and once I cleaned up the aircraft, I trimmed it out to where I wanted it. Afterwards I hit ENG, then NAV, then ALT (hold) but once I hit SPD my Lear started to pitch up and down uncontrollably. I was still under 10,000ft so I was just under 250kts. How does this button work exactly? To my understanding it is suppose to dictate your VS by where your throttles are. I had them pulled back to maybe 50% when I pressed the button but then my aircraft got all squirrelly on me...

The purpose of SPD is to climb or descend at a desired speed. If you are climbing or descending, I don't understand why you would turn on ALT HLD before reaching your target altitude.  The typical way to use SPD is to trim the a/c for the desired climb or descent speed, and then turn on SPD (I'm assuming the AP is on). If you want the AP to climb or descend to a particular altitude, you should put the target altitude in the Altitude Alerter, trim the a/c for the desired climb or descent speed, turn on ALT SEL, and then SPD.  The a/c should level off at about the desired altitude, SPD and ALT SEL should turn off and ALT HLD should come on.  Note it is important to have the a/c in trim (climbing or descending) before engaging SPD.

Al

Edited January 5, 2019 by ark

[QuaxTheSnoopie 16]

You talk of ALT SEL ?

I experienced the same behaviour with SPD.  What worked for me (actually pre-4 version):
Speed about ~180-200 KIAS, aircraft kept trimmed out by stick, climb a minute with stable speed and stable climb rate, then select SPD and don't touch the throttle for a further minute.

Pushed SPD too early, I got this "self oscillating" roller-coaster (speed decreases -> nose down -> speed increases -> nose up).

Btw I "engage" the AP on the ground with only ALT SEL selected, after being airborne, I hit HDG, so the plane climbs in a straight way: I think, SPD (in the initial phase) don't like changes in any direction (up, down, left, right).

When the plane climbed with SPD one or two minutes in a stable way, I change the heading to intercept the next leg, approaching it, I hit NAV.

The lateral correction tends to overshoot and oscillate in the same way, this can be damped by the "half angle banking".

I don't know how the new 4.1 version will behave (currently downloading ;-), but the former version 3 is very sensitive, especially on takeoff and approach and will be flown with all your mind and heart 😁

[ark 1,288]

18 hours ago, QuaxTheSnoopie said:

Pushed SPD too early, I got this "self oscillating" roller-coaster (speed decreases -> nose down -> speed increases -> nose up).

I think it is helpful to understand the basics of how the AP speed (IAS or MACH) mode works. Since there is no autothrottle function in the Lear35, the only way the AP can control climb or descent speed is by using the elevator to control pitch and thus the vertical speed (VS). When the IAS button is engaged, the AP notes the current VS which it now uses as the reference VS for holding the target IAS. If the IAS should increase above the target IAS, the AP increases the VS by adding an appropriate increment to the reference VS. This now becomes the new reference VS. This causes pitch to increase and IAS to drop since the thrust (throttle position) has not changed. Likewise, if the IAS drops below the target IAS, the AP subtracts an appropriate increment from the reference VS. This now becomes the new reference VS. This causes the pitch to decrease and the IAS to increase.  So it is very important that the initial reference VS be 'valid'  -- that is, the a/c should be in trim at the target IAS with the throttle set when the AP IAS mode is engaged. If this is not the case, the reference VS is not valid, and the AP continually 'hunts' to find a VS that results in the target IAS.

Al

[ark 1,288]

19 hours ago, QuaxTheSnoopie said:

When the plane climbed with SPD one or two minutes in a stable way, I change the heading to intercept the next leg, approaching it, I hit NAV.

The lateral correction tends to overshoot and oscillate in the same way, this can be damped by the "half angle banking".

 

The two key factors when intercepting a course with the AP are IAS and the intercept angle.  The combination of high speed and large intercept angle will certainly result in an overshoot which is not surprising since the max bank angle the AP can command is 25 or 30 degrees for most APs.  The higher the IAS, the smaller the intercept angle needs to be. Intercept angles in the 30 degree ballpark typically work best. Intercept angles greater than 45 degrees will almost always result in an initial overshoot.

Al

Edited January 20, 2019 by ark

[capceo 117]

Just to add a tip to this thread, in the real airplane we never use IAS to descend. Its always IAS going up and VS coming down. Asking the autopilot to climb at a specific VS can be dangerous.

[ark 1,288]

2 hours ago, capceo said:

Just to add a tip to this thread, in the real airplane we never use IAS to descend. Its always IAS going up and VS coming down. Asking the autopilot to climb at a specific VS can be dangerous.

 I understand that using VS to climb is dangerous because it can lead to  a stall, but though using IAS below 10,000ft when descending might be a good way to avoid busting the 250kt rule.

Al

Edited January 20, 2019 by ark

[QuaxTheSnoopie 16]

Quote

that is, the a/c should be in trim at the target IAS

Regarding the initial post: The Lear needs some seconds to recognize the trim, but during takeoff, the target IAS varies (my experience). This leads to the mentioned oscillation.

In addition: if ENG, then NAV, then SPD is pressed in a very short time, the lear isn't in a very stable manner (NAV may need lateral correction that leads to changes in pitch/speed).

That's why I first fly with HDG in runway direction to concentrate on pitch and throttle. Then, after a stable time of trim over 20-30 seconds minimum, I press SPD, keeping manually pitch for some additional seconds. Only after this relaxation time, the Lear AP takes over stable IAS control.

If I do not wait but only press the buttons, the vertical oscillation happens.

Quote

using IAS below 10,000ft when descending

In my opinion, the Lear should be flown manually as far as possible - as you said: the AP controls only pitch but not throttle. As i have to control throttle anyway, I use VS on descent, keeping an eye on my airspeed.

What I like in the Flysimware Lear: it's an aircraft that should be flown by pilot, not by automation 😉.

[richjb2 195]

On 1/19/2019 at 11:15 PM, ark said:

I think it is helpful to understand the basics of how the AP speed (IAS or MACH) mode works. Since there is no autothrottle function in the Lear35, the only way the AP can control climb or descent speed is by using the elevator to control pitch and thus the vertical speed (VS). When the IAS button is engaged, the AP notes the current VS which it now uses as the reference VS for holding the target IAS. If the IAS should increase above the target IAS, the AP increases the VS by adding an appropriate increment to the reference VS. This now becomes the new reference VS. This causes pitch to increase and IAS to drop since the thrust (throttle position) has not changed. Likewise, if the IAS drops below the target IAS, the AP subtracts an appropriate increment from the reference VS. This now becomes the new reference VS. This causes the pitch to decrease and the IAS to increase.  So it is very important that the initial reference VS be 'valid'  -- that is, the a/c should be in trim at the target IAS with the throttle set when the AP IAS mode is engaged. If this is not the case, the reference VS is not valid, and the AP continually 'hunts' to find a VS that results in the target IAS.

Al

Al,

That's not how the FC530 or FC200 IAS or MACH modes works. the FCS does not reference vertical speed in IAS mode nor does it try to hold a target vertical speed.

In IAS or MACH, the FCS targets pitch change within a defined g load to maintain the IAS or Mach at the time the button is pressed.  The amount of pitch change up or down required to maintain the IAS or MACH is dependent on the amount of thrust applied or available.  As thrust or environmental conditions change, small pitch changes will occur up or down to maintain the IAS or MACH.  Because the Mach meter in the Lear 35 is not digital (i.e., not tied to an electronic air data computer) it is less susceptible to high altitude temperature changes that cause proposing on more modern airplanes with ADCs, like the Lear 45.  

If you're down low an push the throttles up fast, the airplane will pitch up fast, up to its g limit rate programmed in the flight control computer (certification thing, can't be changed by the pilot). Same thing goes if you pull the throttles rapidly back to idle, the nose drops rapidly.  That leads to passenger discomfort.  Smooth movement of the thrust levels is necessary. 

i don't ever recall using VS when I flew the Lear 35 since there is no way to set a specific VS value. Used IAS and MACH every once an awhile.  Usually, I used the control wheel barrel without pressing the trim button to change  the pitch of the airplane. 

Rich Boll

ATP, LR-JET, LR-45

FlightSafety instructor/TCE LR-JET & LR-45 1996-2000 

[ark 1,288]

6 hours ago, richjb2 said:

That's not how the FC530 or FC200 IAS or MACH modes works. the FCS does not reference vertical speed in IAS mode nor does it try to hold a target vertical speed.

Hello Rich,

Thanks for the info on how IAS works in the real world. What I was describing is exactly how the Lear's IAS mode code works in the sim. Note in IAS mode the sim uses VS as a 'measure' of the pitch change needed to hold the desired IAS. So ultimately it is pitch that is adjusted to hold the IAS target.

6 hours ago, richjb2 said:

i don't ever recall using VS when I flew the Lear 35 since there is no way to set a specific VS value.

I don't understand this. If the VS gauge indicates about 2000FPM when you activate the AP VS mode, doesn't the AP try to maintain the indicated VS? I thought that was the reason why using VS to climb is dangerous -- if necessary, the AP will pitch the a/c up and sacrifice IAS to maintain the climb VS, which could lead to a stall.  I realize there is some error in the gauge, but I would think what the gauge reads is specific enough in most cases, such as when descending to an assigned altitude.

Thx,

Al

Edited November 2, 2019 by ark

[richjb2 195]

Hi Al, 

Yes, if you establish a specific Vertical speed and press the VS button, the FD will command a pitch attitude to satisfy that vertical speed. My point is that there is no way to set a specific vertical speed, or IAS/MACH for that matter, using the flight guidance panel like in the Lear 45  moving the pitch trim switch without pressing the arming button doesn’t change the selected VS or IAS/MACH, rather it refers back to pitch hold mode.  When I flew the airplane, most folks didn’t bother with these other modes   Pitch mode could get you into trouble, but not as fast as VS mode.  

As you correctly noted In a climb, if that vertical speed is excessive, the FD and autopilot, if engaged, will pitch up to meet that vertical speed at the expense of airspeed.  That was a causal factor in the loss of control/high altitude stall accident of the CRJ at Jefferson City.  

[richjb2 195]

7 hours ago, ark said:

Thanks for the info on how IAS works in the real world. What I was describing is exactly how the Lear's IAS mode code works in the sim. Note in IAS mode the sim uses VS as a 'measure' of the pitch change needed to hold the desired IAS. So ultimately it is pitch that is adjusted to hold the IAS target.

Thanks for the clarification. Coding must be fun for these types of autopilots! 

Any working being done to duplicate the autopilot pitch mode using the control wheel barrel switch? 

I just started flying this Lear 35 simulator as means to get familiar with the Garmin FMS  I speed Collins and Universal quite well, but Garmin’s new to me.  I’m enjoying the add-on!

Thanks!

Rich 

[capceo 117]

1 hour ago, richjb2 said:

Thanks for the clarification. Coding must be fun for these types of autopilots! 

Any working being done to duplicate the autopilot pitch mode using the control wheel barrel switch? 

I just started flying this Lear 35 simulator as means to get familiar with the Garmin FMS  I speed Collins and Universal quite well, but Garmin’s new to me.  I’m enjoying the add-on!

Thanks!

Rich 

 

I fly two FC530 equipped 35A's, and an FC550 equipped 55. We almost exclusively use pitch mode for climbs and descents, with the occasional engagement of VS mode in a descent. IAS/MACH is useless from a passenger comfort standpoint. It constantly hunts around for the target speed, and never really stabilizes. As you know, its a reactionary system. We spend most climbs and descents constantly bumping the trim barrel to keep it where we want it.

Worth mentioning is that now, the overwhelming majority of LR35's do, in fact, have ADCs, which are connected to the mach meters. This was an added benefit of the RVSM mandate. Both of our 35's and our 55 have dual ADCs. FSI's LR35 and LR55 simulators at TUS were updated quite a while ago with dual ADCs as well.

Al, your interpretation of how VS mode works is correct. The correct procedure for its use is to manually establish the desired vertical speed while in pitch hold, allow it to stabilize there for several seconds, and then engage VS. The autopilot will now adjust the pitch attitude of the aircraft to maintain that target VS.

[richjb2 195]

11 hours ago, capceo said:

 

I fly two FC530 equipped 35A's, and an FC550 equipped 55. We almost exclusively use pitch mode for climbs and descents, with the occasional engagement of VS mode in a descent. IAS/MACH is useless from a passenger comfort standpoint. It constantly hunts around for the target speed, and never really stabilizes. As you know, its a reactionary system. We spend most climbs and descents constantly bumping the trim barrel to keep it where we want it.

Worth mentioning is that now, the overwhelming majority of LR35's do, in fact, have ADCs, which are connected to the mach meters. This was an added benefit of the RVSM mandate. Both of our 35's and our 55 have dual ADCs. FSI's LR35 and LR55 simulators at TUS were updated quite a while ago with dual ADCs as well.

Al, your interpretation of how VS mode works is correct. The correct procedure for its use is to manually establish the desired vertical speed while in pitch hold, allow it to stabilize there for several seconds, and then engage VS. The autopilot will now adjust the pitch attitude of the aircraft to maintain that target VS.

Hi Joe,

I was under the impression that the ADCs added to make the Lear 35 and 55s RVSM affected only the altimeter.  I didn't believe that the airspeed indicator changed, that it was still just a mechanical instrument only with a Mach compensating bellows.  That modification came after i taught the airplane and after we sold our last Lear 35, so I never really got involved with it.  That's the version we see in the Flightsimware LR35, correct?  If you lost power to the ADC, would you lose airspeed/Mach indications? 

If that's correct and ADC temperature-corrected data is going to the Mach/airspeed indicator, then that would explain the proposing.  I used IAS/Mach a couple of times with old airspeed indicator with just mechanical bellows and it worked pretty good. it was just a pain.  Also, we hand flew the airplane to altitude (before RVSM) in most cases. 

Another question.  I'm using the Flight 1 GTN750 in the Flightsimware LR35 and the Extreme Prototypes LR25.  For VNAV, when in GTN750 LNAV is displayed on the HSI, the glideslope needle provide VNAV path deviation on the Extreme Prototypes LR25, but not the Flightsimware LR35. Is than optional setting or something unique between the two simulator versions?

Thanks!

Rich Boll

[ark 1,288]

46 minutes ago, richjb2 said:

Another question.  I'm using the Flight 1 GTN750 in the Flightsimware LR35 and the Extreme Prototypes LR25.  For VNAV, when in GTN750 LNAV is displayed on the HSI, the glideslope needle provide VNAV path deviation on the Extreme Prototypes LR25, but not the Flightsimware LR35. Is than optional setting or something unique between the two simulator versions?

Hi Rich,

I think I can answer your VNAV question.  In the F1GTN.ini file, which is in the main a/c folder,  under [GTN750.1] you can have an entry that controls whether or not the glide slope needle provides VNAV info.  If you have VNAV GS Needle=0, that turns off the GS needle VNAV function, and VNAV GS Needle=1 (and also maybe no VNAV GS Needle entry at all) turns the GS VNAV needle function on.

I personally don't like using the GS needle for VNAV info, especially in the terminal area. For me, the GS needle is just that, a GS needIe!   🙂   What I do instead is set one of the 4 User Fields in the corners of the Map display to VSR (Vertical Speed Required) which tells me how I'm doing wrt VS.  I don't think the VNAV GS needle function is available iwith the real Garmin GTN750, I think it is just something Flight1 decided to do (but I could be wrong about that). Maybe someone else here can clarify this.

Al

Edited November 3, 2019 by ark