Started flying on VATSIM a few months ago and last night I was doing a standard approach to Heathrow via the LAMB3R arrival and the controller was asking me to be at a certain altitude by passing a waypoint, I know this is very normal but I could not for the life of me find out how to instruct the FMC to do this. A/P was engaged with VNAV and LNAV, MCP altitude was set to the altitude I wanted it to go to. And I tried to enter the altitude into the FMC and select the waypoint in the LEGS page, (That seemed like the logical way to go about this). The numbers on there changed, but the plane didnt do anything.

 

Am I going the wrong way about this or..?

I ended up having to manually set the decent rate via the MCP since I couldn't figure it out, would appreciate some help

That's actually a good way, you lower the level of automation in order to reduce workload.

 

Anyway, VNAV restrictions logic is very simple:

Imagine you are leveled in VNAV mode; when you enter restriction, two things can happen. If you have to descend now, you will get "RESET MCP ALT" message at scratchpad - you need to reset mcp altitude and pres ALT INTV button; if aircraft is unable to descend to restricted altitude, you will get "DES PATH UNACHIEVABLE" MESSAGE. If you do not have to start your descend  right now, you will get T/D symbol at ND. At that moment you can choose to wait for calculated T/D or to reset mcp alt and press ALT INTV to force descend. 

Hmm..allright.

So if I press the altitude intervine switch after setting the altitude for the waypoint im approaching in the FMC it will start decent?

On the legs page, enter /[put your assigned altitude/FL here] (e. g. /120) and up-select it to the right of the corresponding waypoint. Since you're in VNAV, that should about do it.

 

So long,

Oli

 

Edit: right, you will have to change the MCP altitude, too, of course.

 

Oliver Branaschky

On the legs page, enter /[put your assigned altitude/FL here] (e. g. /120) and up-select it to the right of the corresponding waypoint. Since you're in VNAV, that should about do it.

 

So long,

Oli

 

 

Oliver Branaschky

 

That is what i explained I was doing. Quoting: "And I tried to enter the altitude into the FMC and select the waypoint in the LEGS page, (That seemed like the logical way to go about this).".

 

It set the alttiude for the waypoint but when i pressed execute it just continued flying leveled

Hmm..allright.

So if I press the altitude intervine switch after setting the altitude for the waypoint im approaching in the FMC it will start decent?

Remember to reset MCP altitude first. You see, in VNAV mode, altitude is double protected - with profile restrictions, and with mcp altitude, so aircraft will stop descending if hits any of those two. 

Remember to reset MCP altitude first. You see, in VNAV mode, altitude is double protected - with profile restrictions, and with mcp altitude, so aircraft will stop descending if hits any of those two. 

 

I did that too as I stated in my initial post "MCP altitude was set to the altitude I wanted it to go to." 

You were leveled or you were already descending?

 

How many miles was that restriction fix distant and how many feet you are asked to descend?

You were leveled or you already was already descending?

 

How many miles was that restriction fix distant and how many feet you are asked to descend?

 

I was at FL280 (cruise altitude) I was asked to descent to FL220 by the next waypoint which was 12nm away. i had set MCP altitude to fl220 and selected the next waypoint and set its altitude to 220. Nothing happened

That's too short distance for normal descend in my opinion, but it can be achieved. When you got the restriction you should enter the restriction first, than "reset mcp alt" message pop up, you reset it to FL220 and press ALT INTV, and then you get "request drag" message, you open speedbrakes, and it's uncertain whether you reach it or not.

 

You see, you were 12nm, away, but when you finish all of this you are 2 miles closer. That's why lower level of automation is better, you need 2 seconds to reset mcp alt and press level change. I would increase speed knob to limit in order to maximize descend rate when situation is too tight.

That's too short distance for normal descend in my opinion, but it can be achieved. When you got the restriction you should enter the restriction first, than "reset mcp alt" message pop up, you reset it to FL220 and press ALT INTV, and then you get "request drag" message, you open speedbrakes, and it's uncertain whether you reach it or not.

 

You see, you were 12nm, away, but when you finish all of this you are 2 miles closer. That's why lower level of automation is better, you need 2 seconds to reset mcp alt and press level change. I would increase speed knob to limit in order to maximize descend rate when situation is too tight.

 

Hmm.. allright, thanks for your help!

 

I was at FL280 (cruise altitude) I was asked to descent to FL220 by the next waypoint which was 12nm away. i had set MCP altitude to fl220 and selected the next waypoint and set its altitude to 220. Nothing happened

Hi Carl?

OK, I would do it this way, maybe not correct, but works for me, depending on what my altitude is and time to get to the target altitude.

1. On the Legs page, find the waypoint and on its right hand side, /220 (be at the waypoint at FL220)

2.I would then have to Activate this command LSKR6 and maybe have to push the Execute button to make this waypoint active on the Vnav descent.

3.You will also need to set the Altitude to at least 22000, By now, if descending via Vnav, I would have put in some lower height restriction, or if non, the transition/ ILS entry altitude.

4.So If you are cruising at 28000, now with the altitude set at say 10,000, then to tip the a/c down, push the Alt/Intv button. You might also have to select the v/s button to say 800f/m to ensure the a/c energetically points down, as you do not have much time. Make sure though you then reselect the LNAV and VNAV buttons when you start descending so as to make the fmc do the calculations to reach the waypoint at 22000.

 

Your way should have also worked. I would have selected 22000 altitude, used alt intv, and again might have had to use v/s to push the a/c down. You could have used LVL CHG as well...so many choices...

 

Hope this helps. Maybe some one with more skill on the 737 is better to answer you.

 

Regards

 

 

 

As mentioned above, keep the level of automation as low as possible for the situation. There is a video somewhere here in the forum of an American Airlines seminar that addresses levels of aircraft automation for the situation. One thing mentioned is that pilots should not go "heads down" and start banging data into the FMC for situations that can be initally addressed by a simplier task. In your situation, get the aircraft going down via a simple MCP input and address the crossing restriction after that.

As mentioned above, keep the level of automation as low as possible for the situation. There is a video somewhere here in the forum of an American Airlines seminar that addresses levels of aircraft automation for the situation. One thing mentioned is that pilots should not go "heads down" and start banging data into the FMC for situations that can be initally addressed by a simplier task. In your situation, get the aircraft going down via a simple MCP input and address the crossing restriction after that.

 

Exactly...you are in a 'tactical' situation & VNAV is oft referred to as a 'strategic tool'...use a lower level of automation - also referred to as a 'tactical mode'

 

The MCP offers a couple:

 

LVL CH - perhaps 'winding in' airspeed to get below the profile if you are high or combine LVL CH with additional drag from the speed brake ('dive' - normally at idle thrust)

 

 V/S can be useful if a much more shallower angle is required ('drive' - normally with some power on)

 

Once the 'GREEN Banana' is where you want it in relation to the magenta line/FIX then go heads down & punch some data to back up the 'tactical' automation.

 

Rule #1 - Fly the aeroplane!

 

The key to much of this is checking all the necessary information is in the CDU prior to the TOD...after that any 'tactical' changes required by ATC are best dealt with by using 'tactical' modes of automation on the MCP i.e. LVL CH, V/S & HDG SEL. 

It would also help if you noted your FMA. Were you in ALT HOLD, VNAV ALT, or VNAV PATH?