Just lately, I have been getting a problem with over speed. Where the NGX end up doing 300kts very soon after take off. It seems to be a intermittent problem.

Some take offs are a joy to behold, with a nice climb out on the V2 speed. And then scheduling the flaps.

 

It is probably 'user error' somewhere along the line, but I would like to know what is causing it, because it is very bad if a airliner is doing 300kts over a built up area of London., at 3000f. I was experimenting with the debate thrust modes TO2 and using climb 2. Which produced a 87.7  N1, and a assumed temp of 47c  Using FLAPS5. The take off weight was 59181KG, so quite light.  I got this info from TOPCAT.

 

 

 

Ok here is the take off. SAM2M out of EGKK.  The A/T is armed, LNAV and VNAV not armed.

I start the take off and I can hear that the engines are a little bit sluggish as they aim for the 87 N1. Soon after V1 which was bugged at 125K speed increase almost immediately to 170K by 200f, blasting through the Flaps 1 marker.  Speed then remains at 160KT by 2000f. The auto pilot is defiantly on.

 

 

Then at 5000f I switch on VNAV (I think this is the problem)  the speed trend vector shoots up to 270KT even though I am at 5000f.  I can't remember of the top of my head but I think the speeds on the SID in the FMC where something like 250/3000  250/4000  by MID.   I don't know why the VNAV is ignoring the speeds in the FMC?   The speed shot up to 300K, so I had to disconnect all the automatics and hand fly to get things back to normal.

 

I was messing about with a setting in the PMDG options.  Throttle overrides A/T, could this be it?  I was flying very light, and the climb out speed was barely above the V1 VR speeds. 

 

Any way please let me know what is going on?  As I said it doesn't always do this, some take offs are a dream. 

 

 

 

the speed trend vector shoots up to 270KT even though I am at 5000f. I can't remember of the top of my head but I think the speeds on the SID in the FMC where something like 250/3000 250/4000 by MID. I don't know why the VNAV is ignoring the speeds in the FMC? The speed shot up to 300K, so I had to disconnect all the automatics and hand fly to get things back to normal.

 

 

What is your transition altitude set to in the FMC? That can and will cause you to accelerate to 270kts below 10k. It's happene to me before when I've forgotten to change it.

 

 

 

I was messing about with a setting in the PMDG options. Throttle overrides A/T, could this be it? I was flying very light, and the climb out speed was barely above the V1 VR speeds.

 

If your throttles are overriding the A/T, then, well, yes that can also definitely happen. If you're light and the engines are screaming at 100%+ N1....

 

 

Also, it seems pretty normal to me that you're still at T/O (Not Climb) Thrust at only 200 ft. AGL and accelerating, especially if you're pretty light. If you have altitude restrictions then the A/T should throttle back to meet speed requirements/restrictions in the SID. If it's not, then you're probably overriding the A/T.

Unless you've got a very expensive servo-operated throttle quadrant right beside you, you should generally choose the "IN HOLD/ARM MODE ONLY" option for the "MANUAL A/T OVERRIDE" setting. Using "ALWAYS" will lead to the kind of issues you're describing, and "NEVER" will unnecessarily limit the manual use of your throttle during various phases of the flight.

 

You do have to ascertain that your throttles are not returning jittery values at their max thrust setting, or the NGX will interpret that as a valid input and firewall the throttles during takeoff when the thrust mode switches to THR HOLD. If that's the case, and calibrating your hardware throttle with a sufficiently large dead zone at both ends of its full range doesn't resolve the issue, using the "NEVER" mode might be your only viable course of action.

 

By the way, these kind of issues tend to be resolved more quickly when you post a couple of cockpit screenshots showing the problem as it occurs.

What is your transition altitude set to in the FMC? That can and will cause you to accelerate to 270kts below 10k. It's happene to me before when I've forgotten to change it.

 

The transition is 6000f, in UK airspace, but this should not make any difference. We always fly this trans in the UK and europe has similar low transition levels.

 

 

If your throttles are overriding the A/T, then, well, yes that can also definitely happen. If you're light and the engines are screaming at 100%+ N1....

 

This could be the problem.

 

 

Unless you've got a very expensive servo-operated throttle quadrant right beside you, you should generally choose the "IN HOLD/ARM MODE ONLY" option for the "MANUAL A/T OVERRIDE" setting. Using "ALWAYS" will lead to the kind of issues you're describing, and "NEVER" will unnecessarily limit the manual use of your throttle during various phases of the flight.

 

 

I think I better be switching that option back.