My question is, during a VNAV descent (or climb) when should you change the altitude window setting for the next waypoint so that you don't enter ALT HOLD or require the use of ALT INTV?  For example, suppose I am descending toward waypoint X where the altitude requirement is 9000ft, and so 9000 is set in the Altitude Window. If the altitude requirement for the next waypoint  is 5000ft, should I reset the Altitude Window to 5000 just before arriving at waypoint X, or just after, to avoid entering an ALT HOLD condition or the need to push ALT INTV?

I am asking this because a number of times after resetting the Altitude Window for the next waypoint I’ve been surprised to find myself in Altitude Hold and I’m not quite sure why. And if in ALT HOLD, inorder to begin the descent to the next waypoint altitude as set in the altitude window, is it enough to just turn off ALT HOLD, or do I need to also push ALT INTV?

 

I am probably making this harder than it is. It just seems I should be able to reset the altitude window at the appropriate times so a descent or climb "continues smoothly" without requiring dealing with ALT HOLD or ALT INTV.

 

Thx,

Al

 

 

during a VNAV descent (or climb) when should you change the altitude window setting for the next waypoint so that you don't enter ALT HOLD or require the use of ALT INTV?

 

When you've been cleared lower.  If the controller hasn't cleared you lower, then, as annoying at it may be that the plane captures the altitude and stops the descent, at least you won't bust a regulation.

 

 

 

And if in ALT HOLD, inorder to begin the descent to the next waypoint altitude as set in the altitude window, is it enough to just turn off ALT HOLD, or do I need to also push ALT INTV?

 

Press ALT INTV.  There's no need to do anything to ALT HOLD.

 

 

 

I am probably making this harder than it is. It just seems I should be able to reset the altitude window at the appropriate times so a descent or climb "continues smoothly" without requiring dealing with ALT HOLD or ALT INTV.

 

You are...

Set the altitude you're cleared to.  In the case of multiple altitudes in a profile STAR like that, the controller will usually say "descend via the [sTAR Name], [additional information, as appropriate]."

 

In those cases, there's some FAA guidance that contradicts what I'm about to tell you (but it's more complicated than it needs to be for someone using Flight Sim), but basically: set it to the lowest altitude listed on the profile STAR and just monitor your performance to make sure you're not too high/low.

 

If you forget and level off: ALT INTV and back on your merry way.

And remember you can set altitude restrictions for VNAV in the LEGS page on the CDU. Only VNAV will take those into consideration. Even with those, the ALT window will have priority, so VNAV will never descend lower or climb higher than what you selected in the ALT window.

 

In your case, if you want to cross that waypoint at 9000ft then the next at 5000ft, and ATC cleared you to 5000ft, just set 5000ft in the ALT window, and VNAV will cross the waypoints at the altitudes you set in the LEGS page (9000 and 5000ft), if the descent profile is not unreasonable, of course. It will then level off at 5000ft.

You can indeed create FMC altitude restrictions to do this using VNAV but I would argue that it's far easier to use the ALT SEL window as and when cleared by ATC. A vertical constraint is tied to a waypoint and that could delay your descent unnecessarily leaving you too high too close to the airport.

Thanks all for the replies.

 

What I'm trying to do is understand the operation of the MCP wrt altitude window settings, ALT HOLD, and ALT INTV divorced from other considerations like ATC, etc. So based on the replies, if I understand correctly (probably not likely):  

 

1. When descending to a waypoint altitude as set in the altitude window and on the legs page, if the a/c reaches that altitude before the altitude window setting has not been changed, then the a/c will enter ALT HOLD, and after that whenever you decide to continue the descent you will need to reset the altitude window and push ALT INTV.

 

2. If before reaching the first waypoint and the waypoint altitude, you change the altitude window to a lower value for the next waypoint, the descent will continue to the new lower altitude and you don't have to also push ALT INTV ( ie,  the MCP will ignore the original altitude value entered on the legs page for the first waypoint).

 

Thx,

Al

 

 

2. If before reaching the first waypoint and the waypoint altitude, you change the altitude window to a lower value for the next waypoint, the descent will continue to the new lower altitude and you don't have to also push ALT INTV ( ie, the MCP will ignore the original altitude value entered on the legs page for the first waypoint).

 

You're in VNAV.

 

The FMC will follow the stepdowns and limits up to the point where the MCP ALT is.

 

If you have a STAR that has several stepdowns, say 9000, 7000, and 5000. You could

1. Set each altitude in the window.
2. Set the "bottom" in the window.

Option 1. is a pain in the &@($*. You don't wait to level to change the MCP ALT, however. You make sure that the crossing altitude is assured and then  you change the MCP. eg. If you're coming up on a fix that you're supposed to cross at 9000ft and everything shows that you will cross it at 9000ft, then you can set the next restriction in for the next fix, 7000ft. Then approaching the 7000ft fix, if it looks good you set the 5000ft. Change the MCP before you get to the fix.

 

2. is much easier.

 

For Boeing's take on all of this check out the FCTM section  "MCP Altitude Setting Techniques Using VNAV."

You're in VNAV.

 

The FMC will follow the stepdowns and limits up to the point where the MCP ALT is.

 

If you have a STAR that has several stepdowns, say 9000, 7000, and 5000. You could

1. Set each altitude in the window.
2. Set the "bottom" in the window.

Option 1. is a pain in the &@($*. You don't wait to level to change the MCP ALT, however. You make sure that the crossing altitude is assured and then  you change the MCP. eg. If you're coming up on a fix that you're supposed to cross at 9000ft and everything shows that you will cross it at 9000ft, then you can set the next restriction in for the next fix, 7000ft. Then approaching the 7000ft fix, if it looks good you set the 5000ft. Change the MCP before you get to the fix.

 

2. is much easier.

 

For Boeing's take on all of this check out the FCTM section  "MCP Altitude Setting Techniques Using VNAV."

Thanks much. I think I understand now. Your recommendation to "set the bottom in the window" was Kyle's suggestion as well.

 

I'll take a look at the FCTM MCP Altitude Setting Techniques Using VNAV section. I probably should have searched for and read that before posting in the first place.

 

Al