Hi,

I am in cruise flight in VNAV PATH at FL390 i put mcp altitude below my current altitude, my next constraint is 9500 in (big white numbers) in the CDU which I also want deleted as well as an early descent....nothing happens. If I try put mcp altitude above FL390 and press alt intv...nothing happens. Is this a bug? Can't seem to make it work. Also I can make it go into ALT HOLD by cutting off its VNAV path descent with the MCP altitude window. If i press alt intv then it seems to agree to a VNAV SPD descent and then PATH again.

 

 

I am in cruise flight in VNAV PATH at FL390 i put mcp altitude below my current altitude, my next constraint is 9500 in (big white numbers) in the CDU which I also want deleted as well as an early descent....nothing happens.

 

ALT INTV will delete the next restriction, but will not force the descent. Forcing a descent requires hitting DES NOW> on the VNAV DES page.

 

 

 

If I try put mcp altitude above FL390 and press alt intv...nothing happens. Is this a bug?

 

No. To climb in VNAV, you'll need to change the cruise altitude to an altitude higher than the current cruise altitude in the FMC.

 

 

 

Also I can make it go into ALT HOLD by cutting off its VNAV path descent with the MCP altitude window. If i press alt intv then it seems to agree to a VNAV SPD descent and then PATH again.

 

This is how it's supposed to work.

 

My bet is that you'd learn a lot from flying the tutorials. You'll learn a bunch about the autopilot modes, to include VNAV.

Scandinavian13 Hi,

I did try this in the tutorial for 20nm to TOD. I tried to get rid of all the constraints for testing purposes, but it did not yield the desired result, using alt intv, as well as no early descent. Isn't alt intv supposed to make an early descent at 1000fpm AND cancel the next constraint with each click of it button, or is having to go into the cdu the only way for the pmdg 737 sim?

Reference:-

"Altitude intervention switch: 
- If mcp altitude is set higher than current FMC altitude, FMC cruise altitude will be changed to selected MCP altitude and a cruise climb will be initiated.
- If mcp altitude is set below the current FMC cruise altitude, an early descent will be initiated (same as selecting DES NOW on descent page). Aircraft will fly a cruise descent (1000FPM) until mcp altitude is reached or VNAV path is captured."

Scandinavian13 Hi,

 

I did try this in the tutorial for 20nm to TOD. I tried to get rid of all the constraints for testing purposes, but it did not yield the desired result, using alt intv, as well as no early descent. Isn't alt intv supposed to make an early descent at 1000fpm AND cancel the next constraint with each click of it button, or is having to go into the cdu the only way for the pmdg 737 sim?

 

Reference:-

 

"Altitude intervention switch: 

- If mcp altitude is set higher than current FMC altitude, FMC cruise altitude will be changed to selected MCP altitude and a cruise climb will be initiated.

- If mcp altitude is set below the current FMC cruise altitude, an early descent will be initiated (same as selecting DES NOW on descent page). Aircraft will fly a cruise descent (1000FPM) until mcp altitude is reached or VNAV path is captured."

 

Not at my sim to verify, but from my version of the FCOM, FCOMv2 11.42.22 states that early descents are initiated with the DES NOW> prompt.

 

Perhaps you're thinking of the 777? I know it'll do exactly what you're assuming the NGX should be doing.

 

 

ALT INTV will delete the next restriction, but will not force the descent. Forcing a descent requires hitting DES NOW> on the VNAV DES page.

 

In the airplane you can initiate an early descent with the ALT INTV button.  Setting a lower altitude in the MCP and pressing the ALT INTV button does the same thing as selecting DES NOW on the CDU, that's how I do it all the time.  I will have to look it up but I think you need to be within a certain distance of the TOD for it to work.

FCOM V2 11.31.36 under early descent, "An early descent can also be started by pushing the altitude intervention switch."  I didn't see anything about being within a certain distance of the TOD, I may have been thinking of the 777 on that.  You do need to be within the allowable cross-track error for LNAV to initiate a path descent, perhaps the OP wasn't within the lateral limits.

Except that, I think, pressing ALT INV will delete the next altitude constraint if there is one and using DES NOW from the CDU will not.

 

Dave

Except that, I think, pressing ALT INV will delete the next altitude constraint if there is one and using DES NOW from the CDU will not.

 

Dave

 

The first press will begin the descent, you would need to press the button a second time to delete the next altitude constraint.  Pressing the ALT INTV button a single time does the exact same thing as selecting DES NOW on the CDU.  One of the things they reinforce in training is that it matters how many times you press the button.

In the airplane you can initiate an early descent with the ALT INTV button.  Setting a lower altitude in the MCP and pressing the ALT INTV button does the same thing as selecting DES NOW on the CDU, that's how I do it all the time.  I will have to look it up but I think you need to be within a certain distance of the TOD for it to work.

FCOM V2 11.31.36 under early descent, "An early descent can also be started by pushing the altitude intervention switch."  I didn't see anything about being within a certain distance of the TOD, I may have been thinking of the 777 on that.  You do need to be within the allowable cross-track error for LNAV to initiate a path descent, perhaps the OP wasn't within the lateral limits.

 

Cool - I actually don't have my FCOM with me. I just caught that excerpt from Google. Thanks for looking. I'm still not sure our sim is behaving improperly. I'll have to verify when I get home.

 

 

Cool - I actually don't have my FCOM with me. I just caught that excerpt from Google. Thanks for looking. I'm still not sure our sim is behaving improperly. I'll have to verify when I get home.

 

Not all the airplanes have the ALT INTV button, the reference you found would be correct for an airplane without the ALT INTV option.

Hey Phil. What should happen in your case is the descent should begin to your MCP set altitude as soon as you press ALT INTV. If you do this before the TOD circle on the ND the descent will be 1000 fpm. It doesn't matter how far you are away from your TOD circle. Cleveland centre is notorious for descending you WAY back before your TOD. As mentioned pressing the ALT INTV twice in a row should eliminate that 9500 hard altitude.

 

As for increasing you MCP altitude above your cruise altitude what will happen is the FMC will reset a new cruise altitude according to your MCP. So if you set FL410, 410 will now appear in you CRZ page and the aircraft will now climb to that altitude in VNAV SPD mode. Hope this helps

 

Jack Colwill

 

 

As for increasing you MCP altitude above your cruise altitude what will happen is the FMC will reset a new cruise altitude according to your MCP. So if you set FL410, 410 will now appear in you CRZ page and the aircraft will now climb to that altitude in VNAV SPD mode.

 

I will add to Jack's information by mentioning that you can only use the ALT INTV button to modify the FMC cruise altitude to a higher one, never a lower one.  If you want to modify the FMC cruise altitude to a lower one you can only do that by changing it in the CDU.

Scandinavian13 Hi,

 

I did try this in the tutorial for 20nm to TOD. I tried to get rid of all the constraints for testing purposes, but it did not yield the desired result, using alt intv, as well as no early descent. Isn't alt intv supposed to make an early descent at 1000fpm AND cancel the next constraint with each click of it button, or is having to go into the cdu the only way for the pmdg 737 sim?

 

Reference:-

 

"Altitude intervention switch: 

- If mcp altitude is set higher than current FMC altitude, FMC cruise altitude will be changed to selected MCP altitude and a cruise climb will be initiated.

- If mcp altitude is set below the current FMC cruise altitude, an early descent will be initiated (same as selecting DES NOW on descent page). Aircraft will fly a cruise descent (1000FPM) until mcp altitude is reached or VNAV path is captured."

 

 

You were too close to the TOD, I reckon. Works as advertised earlier in the cruise and with the latest release (the one with the wx radar :wub:).

Hi Guys,

Cheers for the replies guys. FYI at that time I was on LNAV and VNAV path and I have it working correct now, not sure how but never mind now as it seems all good. 

A few more small questions - say now I press alt intv from cruising in vnav path to a higher level from fl390 to fl410...it is climbing, updates my cruise altitude...all is well...but now atc says...''amended clearance fl400'' how do I update this as alt intv doesn't want to go backwards.

I also find that if you are climbing and press alt intervention for a lower alt and vice versa (descending and set a higher mcp alt) it doesn't want to obey that command. This must be correct though?

 

 

A few more small questions - say now I press alt intv from cruising in vnav path to a higher level from fl390 to fl410...it is climbing, updates my cruise altitude...all is well...but now atc says...''amended clearance fl400'' how do I update this as alt intv doesn't want to go backwards.

 

Just change the cruise altitude to FL400 in the CDU.