Descent options?

[GodAtum 69]

I'm just trying to get my head clear on how to descend when the ATC directs you. From what I've read there are 3 options:

1. Press Alt INTV and set desired altitude
   
2. Press LVL Change and set desired altitude
   
3. Press VS and select desired rate of descent and altitude
   

What are the advantages and disadvantages of each option?

[richardnobes14 13]

I'm no expert here but I guess it depends on what sort of descent instruction was given:

 

If the instruction was "Descend when ready to FLxxx" Then I would use the standard VNAV method, adjusting the ALT knob and pressing ALT INTV when given a new altitude. If however I was required to maintain a flight level for an extended period of time then I would begin to lose the VNAV PTH and further descending with VNAV would be in VNAV SPD rather than PTH, therefore I would swap over to LVL CHG set a the required speed and use speedbrakes if required to assist with recapturing the PTH should conditions and ATC instructions permit.

 

If the instruction was "Expedite your descent or descend quickly" I would immediately use LVL CHG with a fairly high speed set in the MCP to put the aircraft into a "mini dive" sometimes putting out the speed brakes for a higher rate of decent- Again only if ATC and traffic conditions permit.

 

Ahh V/S, well there really are no advantages to this because most importantly potentially lethal if not monitored e.g. It will fly you into the ground if you aren't watching your Altitude. V/S also sports no SPD or FLAP protection leaving you vulnerable to over-speeding your flaps or even stalling. I would only ever use V/S if a very specific rate of descent was required and only when closely monitoring the speed situation. For example dial in a 4000fpm descent rate and before you know it you will gaining a truck load of speed and will suddenly come to the realization that you won't be able to slow down in time for the approach and that you cant put the flaps or gear out because you are going to fast! V/S can be helpful in a CDA approach as it can assist in capturing the glide slope in the early stages of an approach but BE CAREFUL!

[Vagabondo 23]

Mosty wot he said above

 

ALT INT refers to a VNAV descent. VNAV is generally the planned or intended descent profile. You would use ALT INT if instructed to "DESCEND NOW" and before the T/D. You might also use ALT INT when level off to satisfy a VNAV at or below constraint takes you below the nominal 3º descent profile. In that case VNAV would prefer to level off to the next T/D point. However, ATC might want you to continue the descent, so just press ALT INT when past the constraint.

 

LVL CHG and VS are generally tactical modes.

 

If vectored, it is generally best to use LVL CHG in preference to NVAV. However, you may be way below the profile in which case VS and something like -1000 FPM works a treat, this is to keep the descent going and re-capture the profile. I even use -500 FPM when decelerating and configuring to capture the G/S or G/P from below.

 

If the instruction was "Expedite your descent or descend quickly" I would immediately use LVL CHG with a fairly high speed set in the MCP to put the aircraft into a "mini dive" sometimes putting out the speed brakes for a higher rate of decent- Again only if ATC and traffic conditions permit.

Indeed. This is also useful if high on the profile too. This can be accomplished with VNAV SPD too, but I prefer LVL CHG as it is a more direct mode of automation. VNAV SPD can be entered easily by using the SPD INT button, but the you have automatically entered VNAV SPD due to being too fast on VNAV PTH, then it will revert back to VNAV PATH when back on profile and you end up with thrust and speedbrake!

 

Ahh V/S, well there really are no advantages to this because most importantly potentially lethal if not monitored e.g. It will fly you into the ground if you aren't watching your Altitude. V/S also sports no SPD or FLAP protection leaving you vulnerable to over-speeding your flaps or even stalling.

VS isn't lethal. It will fly to the MCP altitude and enter ALT ACQ and ALT HOLD to level off. If too slow or too fast, with normal AT/AP conditions, VS will revert to LVL CHG. It isn't lethal, it is a lower mode of automation and requires a certain level of situational awareness, is all.

[GodAtum 69]

Thanks for the detailed explanation. the issue I sometimes have is ATC tell me to descend before TOD on the FMC. I press ALT INT but nothing happens so i have to use LVL CHG.

[Lek767 37]

Press Alt INTV and set desired altitude

 

Note that you have to dial your desired altitude into the MCP altitude window before pressing the ALT INTV button. Not much of anything will happen if you do it the other way around...

[GodAtum 69]

Note that you have to dial your desired altitude into the MCP altitude window before pressing the ALT INTV button. Not much of anything will happen if you do it the other way around...

 

Sorry typo. i set desired altitude and press Alt INTV

[Gazzareth 808]

Thanks for the detailed explanation. the issue I sometimes have is ATC tell me to descend before TOD on the FMC. I press ALT INT but nothing happens so i have to use LVL CHG.

 

When that happens I change the set altitude in the MCP, go into the DES page in the CDU, hit "DES NOW" and execute it - that will put you into a 1000fpm descent until it intersects with the VNAV Path and then it will follow that....

 

G

[GodAtum 69]

When that happens I change the set altitude in the MCP, go into the DES page in the CDU, hit "DES NOW" and execute it - that will put you into a 1000fpm descent until it intersects with the VNAV Path and then it will follow that....

 

G

 

Ah I'll give that a try next time thanks

[Spin737 102]

When that happens I change the set altitude in the MCP, go into the DES page in the CDU, hit "DES NOW" and execute it - that will put you into a 1000fpm descent until it intersects with the VNAV Path and then it will follow that....

 

G

That's the same as ALT INT, if you have it installed.

[richardnobes14 13]

VS isn't lethal. It will fly to the MCP altitude and enter ALT ACQ and ALT HOLD to level off.

 

Set a higher or the same altitude than you currently are flying at in the MCP window and then click V/S, you will notice you will be descending or climbing (depending on what V/S you set) as I'm sure you are aware of regardless of what the altitude in the MCP window is. Here's a made up scenario: You level off at 1,000ft after descending in V/S. You then forget to change the altitude in the MCP window but you set the V/S window to -800fpm. You will descend and the plane can do nothing to stop it and as you are flying a normal approach V/S you wont get any GPWS warnings provided that by 1000ft all gear and flap would be configured. If you take your eye off the ball you will could end up flying straight into the ground in particular circumstances, again which i'm sure you are very aware of. I was just raising awareness of that particular situation.

 

But then properly monitored V/S has many a benefit, it's only when the simmer get distracted things go wrong. Especially when I have set +1000 instead of -1000fpm

[Vagabondo 23]

Set a higher or the same altitude than you currently are flying at in the MCP window and then click V/S, you will notice you will be descending or climbing (depending on what V/S you set) as I'm sure you are aware of regardless of what the altitude in the MCP window is

This is very true, but unless you are doing a non-precision CDA approach, this is not the way to use VS. Use of VS should be limited to the applicable FCOM1 Supplementary Procedure in section 4. If you operate the aircraft outside of the required procedures (not just V/S), then it certainly has the potential to be lethal.

You level off at 1,000ft after descending in V/S. You then forget to change the altitude in the MCP window but you set the V/S window to -800fpm

I have to admit I haven't tried this in NGX, but VS should be inhibited after leveling off in VS with the MCP altitude unchanged, but this can happen with ALT ACQ but only if you change the MCP altitude.

 

However, your point is valid, because this feature of VS, where setting a higher altitude in the MCP and then commanding an FPM descent is required for non-precision approaches using VS. Reference the FCOM1 Supplementary Procedure, the procedure was even a Normal Procedure until recently, but has not been relegated to a Supplementary.

If you take your eye off the ball

We don't do that

[GodAtum 69]

Are there any good tutorials on VOR landings? I keep heading off the runway or too short or too high! I use the AP until about 500ft then disengage and fly manually.

[Spin737 102]

Are there any good tutorials on VOR landings? I keep heading off the runway or too short or too high! I use the AP until about 500ft then disengage and fly manually.

That's probably a topic in itself.

[G-CIVA 62]

Are there any good tutorials on VOR landings?

 

I know that referring people to the manuals is not considered as fashionable here these days. They do contain the correct info that answers just about every Q raised within these hallowed walls & they also deliver it in a reasonably clear format.

 

TBH most of what is posted out there on YT is not pretty to watch & is factually incorrect often containing many an FS'ism.

 

You could go right to the documentation PMDG provided with the product...specifically...

 

FCTM 5.64 (pp 240)

 

QRH 2.7 (pp415)

 

Whilst not containing any pretty pictures (just easy to follow line diagrams & plenty of text) they contain the same info as would be used IRL (in addition to any company specific SOPs) & they are a good starting point...as is every single page of documentation PMDG provided with the product.

 

Hope that gets you started.

[GodAtum 69]

Thanks i have read the manuals and done the 2 PMDG tutorials. Maybe I just need practice?