Help!!two things are happening that I have no idea why, I set the FMC as instructed, enter weights, V1 thru to VR, Full Route with a cruise altitude. When I get airborn I can arm the throttle but when i select LNAV LVL change, or VNAV the plane goes nose down and tries to go supersonic!! What am I doing wrong?I also see in the FMC from time to time Unable Cruz Alt, I have searched the documentation and Gogle away but nothing comes back for this. Oh and also I get a flashing A next to the selected speed which i believe is low speed unachievable??? Again not sure what I am doing wrong. please help a frustrated man!

Colin,On the first issue - post a screenshot of what the main panel looks like when you're seeing this please.UNABLE CRZ ALT can mean several things - it can mean that the cruise altitude entered is too high for the airplane at its current weight - it won't be able to actually reach it. It can also mean that the route is too short to reach the altitude you've entered - T/C and T/D would be the same point at a lower altitude...

Pulled back on yoke to try and get some height and stop the plane going back in time! The distance between the airports should be fine it is KRFD and KSDF, so has a good distance between TOC and TOD with a FL of either 29 or 30.

Edited February 1, 201214 yr by devilsdale

Post a pic of your PFD and it'll be easier to diagnose your autopilot problems.

Yeah when I said main panel I meant the displays and the MCP mainly.

Help!!two things are happening that I have no idea why, I set the FMC as instructed, enter weights, V1 thru to VR, Full Route with a cruise altitude. When I get airborn I can arm the throttle but when i select LNAV LVL change, or VNAV the plane goes nose down and tries to go supersonic!! What am I doing wrong?I also see in the FMC from time to time Unable Cruz Alt, I have searched the documentation and Gogle away but nothing comes back for this. Oh and also I get a flashing A next to the selected speed which i believe is low speed unachievable??? Again not sure what I am doing wrong. please help a frustrated man!

First, as Ryan mentioned, UNABLE CRZ ALT means you're doing something wrong: short route, or too heavy.Second, while no mode of the autopilot is more correct for cruise flight, you should really be letting the FMC take care of the speed using VNAV. The only thing I see on the MCP that's actually enabled is LNAV. Furthermore, at that altitude, you shouldn't be using IAS, you should be using Mach.To be honest, it looks like you could use a run through the tutorial. I don't mean to be offensive, but there are some fundamental knowledge issues that seem to be present here.

Thanks for the response guys, I was letting VNav do the flying but had disable it becasue I don't like being a lawn dart, and was flicking between the two to see what the heck was going wrong, I don't doubt I need training but I am confused, I have been using the Wilco 737 for years and only recently went to PMDG 737, I realise these are different animals but in respect of VNAV and LNAV they are quite similar. I had entered the FMC as instructed both in the manuals and via numerous videos on YouTube and everything appears fine until the plane gets airborne.And the point about letting the aircraft do the speed is the problem I have, if I let the aircraft do the speed and select an altitude via the mcp, plus FL300 is set in the FMC even though N1 might say 92.5 it goes to 92.5 but does not climb. It might even level off and try and go back in time.The Mach has no bearing on climb or descent, it is just a measurement of the speed of sound, whether the MCP is in Mach or IAS should not effect the cruise / climb rate.

Did you hit the SPEED button while you were doing all of this?

Thanks for the response guys, I was letting VNav do the flying but had disable it becasue I don't like being a lawn dart, and was flicking between the two to see what the heck was going wrong, I don't doubt I need training but I am confused, I have been using the Wilco 737 for years and only recently went to PMDG 737, I realise these are different animals but in respect of VNAV and LNAV they are quite similar. I had entered the FMC as instructed both in the manuals and via numerous videos on YouTube and everything appears fine until the plane gets airborne.And the point about letting the aircraft do the speed is the problem I have, if I let the aircraft do the speed and select an altitude via the mcp, plus FL300 is set in the FMC even though N1 might say 92.5 it goes to 92.5 but does not climb. It might even level off and try and go back in time.The Mach has no bearing on climb or descent, it is just a measurement of the speed of sound, whether the MCP is in Mach or IAS should not effect the cruise / climb rate.

...and this is why I said there's a knowledge issue.Aboute IAS/Mach: Higher up, there is a larger error in the indicated airspeed, so your speed should be measured as a proportion of the speed of sound. There is a more accurate readout of that particular metric, so at altitude, the only thing you really see used is Mach. Since the error is so large with IAS, speeds that were appropriate at lower altitudes are no longer appropriate (which is why you have the 'A' before the speed), and the Mach curve better fits those speed limitations. If you actually break it down, higher up, climbing at a constant Mach versus climbing at a constant IAS does cause a disparity between climb speeds.There are other things I could mention, but instead of trying to address them without the necessary information, here's what we need:-A picture of your FMS (WEIGHT) INIT page-A picture of your MCP (Mode Control Panel, where the AP buttons are); and-A picture of your FMA (Flight Mode Annunciator).All of that will help us determine what the aircraft thinks you're telling it to do. From what you've typed in your posts, it seems that you just haven't set it up properly.If you have Realistic AP Engagement set to on, and your aircraft is not trimmed properly, or you're trying to arm VNAV too early, you could definitely get the behavior you describe. Also, make sure you hit CMD (I know that sounds stupid, but there are several posts on this forum where people forget to actually tell the AP to take over and just tell it what modes to use, by selecting the modes without hitting CMD afterwards).

You mis-understood IAS and TAS. If you try and maintain an IAS (e.g. 250 kts IAS is quite possible at 35000 ft!), the TAS increases as you climb. You can fly a constant IAS if you understand what you are doing, but it is not optimal.As you climb TAS increases for a given IAS (actually it is CAS - the ADC accounts for error due to compressibility). At sea level on a standard day, 250 kts IAS is 250 kts TAS, but at altitude (e.g. 35000 ft) 250 kts IAS is actually 450 kts TAS! On a standard day the temperature at 35000 ft is -56 degrees C, and so Mach at that altitude and speed is M 0.782. If we go up to 40000 ft, the temperature is still -56 degrees C, IAS is still 250 kts but TAS is now 496 kts and Mach is now M0.87. We would be exceeding MMO the aircraft if we tried to fly at our constant 250 kts IAS. The aircraft would also run out of excess thrust and could no longer climb.If however we climbed at e.g. M0.75, as we climb the IAS would reduce, and whilst TAS would still increase, it would not result in an overspeed, and at high altitude we would still have excess thrust to climb to this altitude to begin with.This is a short, oversimplified explanation of why you climb using Mach at altitude. You need to look at compressibility effects and altitude, and how IAS (CAS), TAS, Mach, and stall speed are affected by them.Best regards,Robin.

Fly the tutorial. Always follow the checklists. I mean always, 100% of the time.

Sorry all I am being a nutter!Ran through the tutorials and everything worked fine, loaded FSPassengers and all went ** up, i suspect it is something to do with the weights and N1 settings going tonto. Great plane really enjoyed the flight from KSDF to KSRD via Vatsim I just have to work the weights in FSPX.Thanks all for the help.A quick question on the mach and speed indicator, when in VNAV and LNAV is it usual not to have any speed / mach number displayed on the MCP or have I turned something off by mistake AGAIN?Thanks once again.

loaded FSPassengers and all went ** up,

I hope you're not letting FSP load your plane for you, right?

Yes, this is a big no no - FSP tries to mess with things internally - turn OFF the loading and whatever else's it's automatically doing to the airplane.

A quick question on the mach and speed indicator, when in VNAV and LNAV is it usual not to have any speed / mach number displayed on the MCP or have I turned something off by mistake AGAIN?

No, as soon as VNAV is active SPD window should go blank.