I have done three flights with my 737NGX, and on two of them I have had problem with authrottle. When I engage autothrottle and speed button below it doesn't follow speed I have set on MCP panel, but instead it just keeps accelerating to around 320 knots no matter what I do. It seems to be related to FMC somehow, on CLB / CRZ page on FMC it shows target speed as 320 knots and I can't change that. Target speed seems to be growing higher while aircraft accelerates, and when it finally reaches 320 knots it stops growing. Then target speed will start to decrease to around 290 knots and throttle power goes lower again until target speed starts to grow again.

 

Also I get Buffet Alert and Unable crz alt on FMC, I don't know why. I tried speed invertion switch or whatever it is called but it made no difference.

 

So how can I stop autopilot from taking speeds from FMC so I can myself manually control them from autopilot panel? Before I got NGX I used Ifly 737 and had no problems with it.

 

Regards

 

Joona Lehto

Speed mode is completely independent from FMC program (it will use only weights info for minimum speeds)

So, to test what is going wrong:

Take off without programming FMC and use speed button.

Remember to use autopilot in combination with AT, for example use HDG SEL, V/S, and SPD.

Also, make sure SPD button is illuminated and MCP SPD is shown on FMA.

Pay attention to FMA, it is more important than the lights on the MCP, the fma is the upper part of the PFD display, it contains all the autopilot modes that are active.

Remember also that pushing VNAV will extinguish SPD and speed will be controlled by FMC.

If the problem is still present without programming FMC, then you need to re-install.

Two suggestions:

 

1. Change your COST INDEX to something around 30 or simply change your climb speed on the FMC CLB page. You can do that.

 

2. Don't touch the SPEED button. Don't. Leave it alone. Achtung! Verboten!

 

Arm the A/T on takeoff and use V/S, LVL CHG, or VNAV, but don't touch that SPEED button.

Ok thanks for your help, first I tried to go without touching FMC at all, no problem.

 

Then I tried to do things like I had done before, but I didn't touch SPEED button although it engaged itself later. Also I set cost index 30.

 

On FMA it says that following autopilot modes are engaged: MCP SPD, LNAV, V/S

 

On IAS/MACH selector I have set 240 knots as speed, however aircraft accelerated around 270 knots below 10 000 ft. I went to CLB page on FMC and set TGT speed 250 knots, however it has no effect. Autothrottle seems to accelerate to SPD REST speed in CLB page no matter what I do, and that speed seems to be growing all the time.

 

Also I keep getting UNABLE CRZ ALT and BUFFET ALERT on FMC, and can't set V speeds, but I don't know if I have forgotten to set something in weight page right, I must admit that I haven't read manual completely as I had never problems with Ifly... But I can't see why I am still unable to control speed all by myself instead of it coming from FMC.

 

 

 

Joona Lehto

Sounds like a controller issue, maybe. Maybe your hardware is controlling when it should be the sim. Try changing the autothrottle override settings in the FMC in the PMDG menus.

On IAS/MACH selector I have set 240 knots as speed, however aircraft accelerated around 270 knots below 10 000 ft. I went to CLB page on FMC and set TGT speed 250 knots, however it has no effect. Autothrottle seems to accelerate to SPD REST speed in CLB page no matter what I do, and that speed seems to be growing all the time.

 

Also I keep getting UNABLE CRZ ALT and BUFFET ALERT on FMC, and can't set V speeds, but I don't know if I have forgotten to set something in weight page right, I must admit that I haven't read manual completely as I had never problems with Ifly... But I can't see why I am still unable to control speed all by myself instead of it coming from FMC.

 

The high speed under 10,000 in addition to the UNABLE CRZ ALT makes me think you've set the plane up to look for weights in KG, but you're giving it weights in LBS. If that's what's happening, the computers essentially think you're about twice the weight you really are, so it will command higher speeds and assume you can't make it up to higher altitudes.

 

The manual is definitely a useful reference, but I'd suggest the tutorials. They're short, have lots of illustrations, and you learn as you go, instead of practicing after reading. Since you're coming over from the iFly, you have some of the basics. You just need to convert those basics to the slightly higher demands of the more realistic NGX.

Oh I think you are totally right now, on sim actions page where it allows me to choose weights they are in KG's, but somehow I assumed that they would be in lbs on FMC page where it asks for those weights... I will try to set them in KG now and see what happens.

 

 

Joona Lehto

Hi Joona

 

The first thing that came to my mind is your "A/T manual override" option. What do you have it set to?

 

You can adjust this in the FMC by selecting:

 

<Menu->PMDG Setup->Options->Simulation>

 

It's on page two. I have mine set to "IN HOLD/ARM MODE ONLY" and have never had an issue.

Ok it seems that switching FMC units from KG to LBS fixed it, thank you all for your help :)

 

 

Many of my past favorite addons had FMC units available only lbs, and even those that support both units still have lbs by default so I didn't really notice this difference. NGX truly is amazing bird to fly, will look more closely to tutorials when I have time so I can take get full experience from this plane.

 

 

Joona Lehto

Ok it seems that switching FMC units from KG to LBS fixed it, thank you all for your help :)

 

Glad that helped. To be honest, the only way I figured that out is by making that same mistake.