hi guys, can suggest  why:

 

1) upon hitting TO/GA i lose the LNAV and VNAV not for few seconds (as per tutorial 1)  but for much longer until I am off the ground and they magically reappear and can engage Autopilot

2) I am getting NO DES AFTER "WAYPOINT" in FMC when in fact I am sure I have no dicontinuity across my route (i checked and rechecked)

and finally

3) PATH NOT ACHIEVABLE (i think this is because i may be going too fast but not sure)

 

thanks

Andrea De Biase

 

 

 

1) upon hitting TO/GA i lose the LNAV and VNAV not for few seconds (as per tutorial 1)  but for much longer until I am off the ground and they magically reappear and can engage Autopilot

 

This is correct. The Tutorial was written in a generic sense. There are limits as described in the Intro Manual and FCOMs.

 

 

 

2) I am getting NO DES AFTER "WAYPOINT" in FMC when in fact I am sure I have no dicontinuity across my route (i checked and rechecked)

 

This is normal. The lack of a discontinuity isn't an issue. The FMC is basically saying that it can't plot a realistic descent after that point, or there is no information available to compute one.

 

 

 

3) PATH NOT ACHIEVABLE (i think this is because i may be going too fast but not sure)

 

Related to the above. This usually happens in cases where people incorrectly "connect the dots" or connect the STAR to an approach when you're not supposed to. The LENDY STAR into JFK is a good example. It drops you off at LGA at FL190, and that's 20nm from JFK. The FMC doesn't know you get extensive vectors on the other side of LGA before getting to the approach.

so i am looking at the LENDY 6 STAR map and it does have a hard FL19 at LENDY and you are saying, correctly, that this is too close for JFK, sure, so I should not connect my route to LENDY but why, then,  is LENDY a valid STAR point for JFK? what am i missing here?

btw thank you

 

Andrea De Biase

why, then,  is LENDY a valid STAR point for JFK?

 

Because airports are not independent. Traffic flow is dependent on all the airports in the area. NY is a busy area, and there are large traffic flows into JFK, LGA, EWR and TEB. The traffic flows from the west into JFK, being the furthest airport to the east of that group, have to be kept above the other flows into and out of the other airports.

 

 

 

what am i missing here?

 

As I mentioned in the earlier post:

"The FMC doesn't know you get extensive vectors on the other side of LGA before getting to the approach." In other words, given the amount of traffic - and highly varied traffic (small props to the A380) - into JFK, they use vectors to tactically sequence.

 

Here's a visual explanation (to include a couple of the things you'd brought up):

 

(Sorry for the minor mic clipping. I tried to boost my mic prior to recording as my mic wasn't the best and required me to speak pretty loudly to be heard clearly.)

so i am looking at the LENDY 6 STAR map and it does have a hard FL19 at LENDY and you are saying, correctly, that this is too close for JFK, sure, so I should not connect my route to LENDY but why, then,  is LENDY a valid STAR point for JFK? what am i missing here?

btw thank you

 

Andrea De Biase

The first time I flew this arrival to JFK I though they forgot about us and left us high. After doing it a lot I found out this is the norm. If they are landing on the 31 or 22 runways they will start vectoring you down south when you are around EWR and around counterclockwise to the landing runway. If they are landing on the 4 or 13 runways they will bring you over LGA and then bring you around clockwise to the landing runway. Either way you will end up using the speedbrakes from a long time on that descent in order to get down in time. 

 The LENDY STAR into JFK is a good example. It drops you off at LGA at FL190, and that's 20nm from JFK. The FMC doesn't know you get extensive vectors on the other side of LGA before getting to the approach.

 

That's something I love about ATC in the US. The guys definitely deserve their pay-checks.

 

In Europe you'll never see route discontinuities or 20nm gaps but rather endless transitions.