VNAV Disconnect During Flight - legs have no speed/alt

[bhorv67 14]

I have to say I've had very little issues with the NGX. I've had the plane going on 18 months now, maybe more. I'm using P3dv4.

Today will be second time I've been in flight where I go to enter the arrival portion of the route with the star & transition, and as soon as I press 'exec' on the FMS, the legs have their alt & speed erased (about half of the legs in the remainder of the flight). Vnav of course disconnects when this happens. 

My procedures have been the same since I learned the AC and trained from real NG pilots. I don't believe there is anything I'm doing that's causing the problem. 

In fact on this flight, at takeoff, once established on the departure and I engage the AP, both Lnva & Vnav are active, but the plane is not following the route nor the programmed v/s. I had to disconnect the AP completely, re-establish on route and vnav path, then engage AP, then it was ok. Very strange. 

Could this be some oddity from the last update?

[downscc 1,327]

13 hours ago, bhorv67 said:

Could this be some oddity from the last update?

Nope.  Please be more specific.  What is your route? What arrival and approach are you adding?  Where are you on the route when you add the arrival and approach?

[Spin737 102]

What does the FMA say?

[spesimen 135]

it's strange, seen a lot of people post about this one lately. wonder why it's so popular a source of confusion recently?

check your speeds on the vnav descent page. it is very likely that your CI is on the high side, and is calculating an econ descent speed that is impossible to maintain while also hitting the speed and altitude restrictions in the STAR. this creates an ambiguity in the vertical profile so it removes the calculations. many STARS have a restriction of 280 knots for example, but at a CI of 100 and with a light load it will try to command a descent up to 320 or 330 knots. when combined with some tight altitude restrictions you can get into a situation where you are basically saying you want to decelerate by 50 knots and drop a few thousand feet at the same time and it's impossible.

 the simplest way to fix it is to force your vnav descent speed to ~280 knots to match the restriction, or to reduce your CI so the calculated ECON descent is closer to that value.

now please understand it could be something else.. without knowing your CI, weights, and info about the plan and star you are using, it is difficult to verify. but two other people asked this exact question just in the last week haha!

good luck!

cheers,-andy crosby