738 PMDG realistic Ap

[Hoverlow 0]

Hi guys.

 

This is my first post and I'm sorry for my bad English. My name is Eduardo and I'm from Brazil.

 

I'm trying to get my best out of a PMDG experience and I wanna try to fly the PMDG with the AP realistic mode on.

 

I have some questions thoug.

 

I know to get realistic AP on I must be over 400 feet. Also, I must release the joystick first and have the trim set accordingly the FMC.

 

Is there something else? Must the plane leveled (VS and Lateral Position on 0)?

 

And If I miss something and can’t the AP on, what should I do in order to get it back??

 

Thank you.

[Vagabondo 23]

You got it about right. The aircraft must be in trim, with no force being applied to the joystick (you don't have to let go of it) and with zero aileron trim. You don't have to be in level flight, you can climb and turn. The most reliable methods is with wing more or less level, centered on the flight directors and correctly trimmed so that you don't need to push or pull the joystick.

[hopskip 253]

Autopilot will only come on if:

On the overhead

Both left and right IRS are Alligned. (This can only be done on the ground and takes 10 mins - an emergency "ATT" allignment mode is possible airborne)

Electrical busses are connected to an operating power source (ie a running engine 1, engine 2 or APU, suggest using engines 1 and 2 both when in the sky, with APU off.)

Hydraulic pumps on (at least one on each side, preferably both on each side).

The best way to check this is look at the overhead and check any lights that are not window heat or pressurization related are switched in their "not lit" mode. Look for Amber and Blue lights on the overhead as a guide. If there is Amber or Blue, you might want to check that out (Pressurization & air conditioning will not effect autopilot but may have some blue or amber lights associated. Get the amber out of there otherwise you will get a "Takeoff config warning" passing 12,000ft... and then the passenger oxygen masks will deploy).

 

On the MCP, check:

Flight Director switched on (PF at least, both is good).

A flight Director mode selected (HDG, LNAV, ALT HOLD, FLCH, SPD, N1/TOGA etc).

Aircraft in trim and no force on joystick (ie not changing pitch and the stick neutral).

 

The aircraft CAN BE climbing, decending or turning (constant angle of bank with stick centered is a must). It is quite possible to engage the autopilot with the nose up at 15 degrees pitch, rolled right 30 degree bank and climbing at 3000ft/min, as long as this attitude bears some resemblance to what the flight director is commanding, and the yoke/stick is totally neutral/centred when the CMD (A or B ) button is pressed.

[kevinh 409]

Let's not confuse or over complicate things. The main requirement is that no input is applied to the controls in pitch or roll. Basically the aircraft is flying hands free.

 

You do not need an FD mode selected. You can select a mode after the AP is engaged.

 

There can be some aileron trim input, it does not need to be zero.

 

The aircraft doesn't have to be exactly in trim, but it should be reasonably stabilised.

 

Blue lights on the overhead won't stop the AP engaging.

[SierraHotel 982]

The best and easiest way is to say " Arm command A" to your FS2Crew co-pilot...;-)

[jbassue 181]

Come on guys, the guy barely understands his own English.

[Vagabondo 23]

There can be some aileron trim input, it does not need to be zero.

Just for info, if you want to keep it real, it is a limitation that AP must not be used with aileron trim. However, you are correct in that it does not stop the AP being engaged, just that you shouldn't

[ethanhawx 3]

I love PMDG because its the real deal in simulation... I remember doing this one flight and after T/O i did everything in order its supposed to be and tried to engage the autopilot but it woudnt engage.... the a/c was in a stable climb requiring very little joystick input, I was within 5 dgrees for sucessful lnav capture and deployment but still the autopilot woudnt engage...

 

Then it struck me .. the service based failures was active and hence the FCC had failed so autopilot A woudnt engage but B did....

[kevinh 409]

Just for info, if you want to keep it real, it is a limitation that AP must not be used with aileron trim. However, you are correct in that it does not stop the AP being engaged, just that you shouldn't

Actually it's the other way around. You must not use aileron trim with the AP engaged. Engaging the AP with some aileron trim applied is not a problem. Just don't touch the trim after that.

[Vagabondo 23]

Actually it's the other way around. You must not use aileron trim with the AP engaged. Engaging the AP with some aileron trim applied is not a problem. Just don't touch the trim after that.

That is a very optimistic interpretation if I may say so I don't have a direct line to Boeing to be sure so you maybe correct, but I would be very very surprised.

 

All I know is that it the AFM and FCOM says the "use of aileron trim with the AP engaged is prohibited." I would define "use" as any aileron trim applied and that therefore the sequence of events is not relevant.

 

The FCOM also describes the negative effects of the aileron trim and AP combination regardless of what happened first. My interpretation is that the AP was clearly not designed to be used with any aileron trim.

 

The FCTM only talks about aileron trim in relation to the alternate rudder trim technique with the AP disengaged. Typical Boeing, in that it isn't completely clear what the context is, but to me it is clear the section is talking about controlability with the AP disengaged (i.e. final approach).

[kevinh 409]

That is a very optimistic interpretation if I may say so I don't have a direct line to Boeing to be sure so you maybe correct, but I would be very very surprised.

 

All I know is that it the AFM and FCOM says the "use of aileron trim with the AP engaged is prohibited." I would define "use" as any aileron trim applied and that therefore the sequence of events is not relevant.

 

The FCOM also describes the negative effects of the aileron trim and AP combination regardless of what happened first. My interpretation is that the AP was clearly not designed to be used with any aileron trim.

 

The FCTM only talks about aileron trim in relation to the alternate rudder trim technique with the AP disengaged. Typical Boeing, in that it isn't completely clear what the context is, but to me it is clear the section is talking about controlability with the AP disengaged (i.e. final approach).

 

The FCOM says this:

 

"If aileron trim is used with the autopilot engaged, the trim is not reflected in the control wheel position. The autopilot overpowers the trim and holds the control wheel where it is required for heading/track control. Any aileron trim applied when the autopilot is engaged can result in an out of trim condition and an abrupt rolling movement when the autopilot is disconnected."

 

This says to me "do not make any aileron trim inputs while the AP is engaged". It does not say "the aileron trim must be zero before engaging the AP". It's no different to stab trim, you simply would not make trim inputs with AP engaged and if you do the AP might trip off. Essentially what would happen if you change the aileron trim offset is that the would AP correct automatically to keep the wheel where it is, but the trim input has added an out of balance force in the controls which is not reflected in wheel position. When you disengage the AP the trim you applied (but which didn't move the wheel) now becomes effective and will create an unexpected wheel offset. It will be interesting to try this in the NGX to see how it reacts.

 

All the aileron trim does is offset wheel position, changing the zero force point so it occurs with some aileron offset. You'd only apply trim if you had some lateral out of balance issue. I don't think Boeing intend you to take out any aileron trim there is before engaging AP. If you did so, and engaged the AP with zero aileron trim the AP would have to apply wheel again to keep the wings level because of the lateral asymmetry. But this is exactly what the trim was doing. Then when you disengage AP you'd have to put the trim back in. So removing the trim offset before engaging AP does not make any sense. Not making trim inputs while AP is engaged makes perfect sense.

 

Also on a practical level, if you take that out the aileron trim you now have to use the wheel to keep wings level and so are applying force. Because of that force the AP won't engage. So you'd have to release the wheel and let the aircraft roll as you engaged AP. Not a good idea.

[Vagabondo 23]

It's no different to stab trim, you simply would not make trim inputs with AP engaged and if you do the AP might trip off.

The operation of aileron trim is fundamentally different to stab trim! Good point about the AP trip off...would the AP trip off or would it eventually go into CWS R? I suspect the latter.

 

Just an edit to say I that I tried this in NGX and got a result I cant seem to replicate. What I expected is that the AP would revert to CWS R, it doesn't. In fact nothing happens. When you disconnect after applying aileron trim you just an undesirable out of trim condition. If setting the trim before AP it seems to work fine. Which concurs with the FCOM system description. I will ask around.

 

You'd only apply trim if you had some lateral out of balance issue.

Not really. Have a look at the FCTM. Aileron trim's only recommended use is with the alternate rudder trim technique. In other words rudder trim is the primary roll trim method, but if you have serious roll control problems, aileron trim maybe required too.

[kevinh 409]

The operation of aileron trim is fundamentally different to stab trim! Good point about the AP trip off...would the AP trip off or would it eventually go into CWS R? I suspect the latter. So I tried this in NGX to see what it would do and the result was so obvious that we both overlooked it. Regardless if you applied the trim before or after selecting the AP, the result would be the same, the AP would revert to CWS R! Now NGX is just $60 software, but this is in line with the FCOM system descriptions for Autoflight.

 

 

Not really. Have a look at the FCTM. Aileron trim only recommended use is with the alternate rudder trim technique. In other words rudder trim is the primary roll trim method, but if you have serious roll control problems, aileron trim maybe required too. Given the above, in that case, use of the AP is out of the question.

I realise stab trim is different in operation to aileron trim, but it is an example of a case where a trim input produces an effect on AP logic.

 

Look carefully at the FCOM wording again. It's telling you why you should not adjust roll trim while AP is engaged. It is not telling you you can't engage AP with some aileron trim already in place. That aileron trim is the less desirable technique in the FCTM is not the issue. Lateral and directional trim are separate things but normally directional trim is all you need in a jet. Wings usually stay level with an engine out and correct rudder trim applied, you almost never need aileron trim on top of rudder trim.

 

What you are really saying is the pilot should zero aileron trim before engaging AP, but this is not practical, nor is it mentioned in the manuals as a procedure for AP engagement. The problem arises if you adjust aileron trim when the AP is engaged. It puts in a trim bias that is not apparent with AP engaged but becomes effective as soon as you disengage it. There's no mention of it dropping to CWS mode if you do this. If you are flying trimmed hands free with some aileron trim applied and you then engage AP there is no force on the wheel, no out of trim condition and no force build up in the controls circuit.

 

The FCOM descriptions make no mention of the AP only engaging in CWS mode as a result. Surely if this was a design feature this would be mentioned? If only to explain to the crew why they could only get CWS mode. Anyway, CWS mode has the same problem as CMD mode if you adjusted aileron trim while AP was engaged.

 

Only someone with experience of the real aircraft could answer this question for certain, a real world pilot or a maintenance engineer. Quite often a manual will say you shouldn't do something, but there is nothing in the aircraft system which actually prevents you from doing it.

[kevinh 409]

I just tested AP engagement in the NGX with and without roll trim applied, and changing roll trim with AP engaged and it performed exactly as I would have expected. No sign of it dropping to CWS R. I did get CWS P, but that was because I had forgotten to reselect a pitch mode. It even did what the FCOM says after I applied roll trim in AP CMD mode. When AP disengaged the wheel moved to an out of trim position in the direction I had applied roll trim.

[por930 51]

Guys, This bloke's English aint that great, so a full blown discussion on the finer points of Autopilot engagement will not help him too much just yet....

 

Hoverlow..Be good if you signed your name to make conversations easier....

 

Until you get the hang of flying this fantastic girl, I suggest you modify the the simulation settings to make the autopilot switch on easier:

FMC>Setup>Options>Simulation>“Realistic Engagement” set to off.

 

 

Realistic Autopilot Engagement: This option can be used to

simplify the autopilot engagement process if desired. When set to

“Realistic Engagement” the autopilot will require that the airplane

is in a balanced trim condition prior to accepting a pilot command

to activate. Thus, if you are holding control input in place to

maintain the desired flight path, you would need to re-trim the

airplane until control force is no longer required to maintain the

desired flight path. Selecting “OFF” will simplify the engagement

logic for those who are just learning how to fly the simulator, thus

reducing your workload. If you notice the autopilot failing to take

control of the airplane when you press the CMD button, refer back

to this paragraph as a refresher!

Remember, if you have this option set to ON, you should be

certain that the airplane is trimmed, the flight director is centered

and you are not operating the electric trim at the time you press

the autopilot CMD button.

Regards