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seagull · August 2nd, 2007, 05:39

However, disconnecting it also means you lose many of the protections that it can add. If the routing is pretty close to what the actual will probably be, the FMS on the later aircraft (4th generation onward, for sure) can do a pretty decent job of keeping you from descending too early, or too late.

The two reasons not to use it would be:

1. You have no idea what the actual "miles to run" are, and/or, ATC needs to you at an altitude immediately;

2. More commonly, the crew does not understand the FMS well enough to program it such that it will provide useful information, possibly procedures that lead to not having a useful routing in the system before arriving in the critical phase of flight (I am NOT advocating mucking with it below 10k!), or, too often, not understanding what the system is doing well enough to know that it was going to do a fine job if you just left it alone! All are really training issues.

August 2nd, 2007, 05:39	#5
seagull Old Skool Join Date: Sep 2001 Posts: 2,045	Re: use of VNAV....... However, disconnecting it also means you lose many of the protections that it can add. If the routing is pretty close to what the actual will probably be, the FMS on the later aircraft (4th generation onward, for sure) can do a pretty decent job of keeping you from descending too early, or too late. The two reasons not to use it would be: 1. You have no idea what the actual "miles to run" are, and/or, ATC needs to you at an altitude immediately; 2. More commonly, the crew does not understand the FMS well enough to program it such that it will provide useful information, possibly procedures that lead to not having a useful routing in the system before arriving in the critical phase of flight (I am NOT advocating mucking with it below 10k!), or, too often, not understanding what the system is doing well enough to know that it was going to do a fine job if you just left it alone! All are really training issues.