B738 FMC vnav descent speed?

my_controls

my_controls

Well-Known Member
Let's say you're at FL300 cruising. You're cleared to FL200, so you dial it up and start the descent in VNAV. 200 appears in the scratchpad now and if you enter it on the cruise page I've noticed that the FMC commands a slower speed all of a sudden.

I was told that the new speed is the cruise speed you would have at the target altitude, in this case FL200. Is this correct and if so, why is this feature there? I would have expected it to maintain the path (I'm assuming it's a VNAV PTH descent) and leave the speed setting be.
 
my_controls said:
Let's say you're at FL300 cruising. You're cleared to FL200, so you dial it up and start the descent in VNAV. 200 appears in the scratchpad now and if you enter it on the cruise page I've noticed that the FMC commands a slower speed all of a sudden.

I was told that the new speed is the cruise speed you would have at the target altitude, in this case FL200. Is this correct and if so, why is this feature there? I would have expected it to maintain the path (I'm assuming it's a VNAV PTH descent) and leave the speed setting be.
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Lots of assumptions here but I'll try to help. If you are in VNAV at FL300 and you are cleared to FL200 you have several choices. If as you have said, FL200 appears in the scratch pad and you insert it as the new cruise altitude (not recommended), you are now in a cruise descent which will descend you at 1000 fpm until FL200. The commanded speed will be a product of your cost index and whether you have selected LRC, Econ and a few other things considered by the FMC - this could be the cause of your slow airspeed command. You can of course manually insert whatever descent speed you would like into the FMC.

When doing a cruise descent, you do not have a path. You are in cruise mode and not in a descent mode. The airplane will not look at any path until you are level at FL200 and you have established a descent profile. You would still have a path if you would of cleared the scratchpad of FL200 and just selected Descend now. This option would keep you in the descent mode. The airplane would descent at the descent speed programmed into the FMC descent page at 1000 fpm until the path is intercepted and then the throttles would come to idle and the path would be maintained. The airplane will try to maintain the airspeed you have programmed; however, if you have a tailwind, the airspeed will increase to maintain the path. Path has priority!!!

Not sure if this answers your questions but it seems to me you are a little confused by features of a cruise descent and a path descent. If you want to PM me, I will be happy to discuss further or whatever? Are you actually flying the 737-800 or is this a simulation question?
 
Hm, I don't think that's what it does. If I leave 300 for 200 earlier than the computed T/D point then it will do a cruise descent but only until it recaptures the computed path. If it does intercept the path prior to FL200 then it will leave the cruise descent and continue in VNAV PTH down to FL200. This is why you have to watch out sometimes you get clearance to descend early and you reach the path say, 2-3000 feet before the cleared level and the plane noses over to maintain the path. At best uncomfortable at worst you bust the level I suppose although it seems to me it captures the altitudes quite well, even at high ROD/ROC.

I'm aware of the wind, but this phenomenon is instant. Basically here's what happens. I'm at, say, FL300. I get cleared FL200 so I select it and push VNAV intervention initiating a cruise descent at this point in time. I then go down to the CDU and enter FL200 in the cruise page. Immediately when I execute this entry I can see the speed bug move to a lower speed. This to me is the mystery.

Maybe I'm not making myself very clear :-)

It's the actual aircraft I'm talking about, forgot to mention that. Software 10.7 in the FMC I think it was.
 
my_controls said:
Hm, I don't think that's what it does. If I leave 300 for 200 earlier than the computed T/D point then it will do a cruise descent but only until it recaptures the computed path. If it does intercept the path prior to FL200 then it will leave the cruise descent and continue in VNAV PTH down to FL200. This is why you have to watch out sometimes you get clearance to descend early and you reach the path say, 2-3000 feet before the cleared level and the plane noses over to maintain the path. At best uncomfortable at worst you bust the level I suppose although it seems to me it captures the altitudes quite well, even at high ROD/ROC.

I'm aware of the wind, but this phenomenon is instant. Basically here's what happens. I'm at, say, FL300. I get cleared FL200 so I select it and push VNAV intervention initiating a cruise descent at this point in time. I then go down to the CDU and enter FL200 in the cruise page. Immediately when I execute this entry I can see the speed bug move to a lower speed. This to me is the mystery.

Maybe I'm not making myself very clear :-)

It's the actual aircraft I'm talking about, forgot to mention that. Software 10.7 in the FMC I think it was.
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If you put FL200 in the MCP window and it "pops up" in the FMC scratch pad as 200 and you then insert it into the FMC L1 position, you have told the FMC you want to descent to FL200 at 1000 FPM (a cruise descent). There is no path above FL200 now. Any path will be computed from your new cruise altitude of FL200 and not FL300 (your earlier cruise altitude). In VNAV the aircraft will descent to FL200 and then look at your VNAV path once you reach FL200. That is the danger of using cruise descents close in because any down line crossing restriction will not be honored or looked at in VNAV before you reach your new selected cruising altitude. Unless your software is significantly different than ours at CAL, it seems to me that you are not understanding the difference between VNAV cruise descents and VNAV path descents. Talk with a check airman or refer to Capt Bill Bulfer's FMC study manual which is available online and at many aviation outlets. Bill is considered the industry expert on the Smith and Honeywell FMCs.

In your example, the airplane slows down as it is unable to compute exactly where and when it will reach FL200 and then be able to make any crossing restrictions down line from there. It slows down as a defense mechanism to give it some cushion to work with once it gets to FL200. Think of it this way: You must define a VNAV path with a beginning altitude and an ending altitude. The beginning is the cruise altitude and the end is the crossing restriction you program. If you are above your cruise altitude (in this case above FL200) you have no path until you reach FL200. Hopefully that helps and I am sorry if it still doesn't make sense to you. Good luck!!
 
Hm interesting, I just don't see our aircraft doing that though. If I use cruise descent to descend prior to my T/D then as I get closer to the original path, the path that was computed back when I was level, the plane will pitch down and reduce power to idle and capture the path.

As stated in the manual regarding early descent:

"A descent in VNAV started before the top of descent point is an early descent. If a path descent is planned, VNAV commands a 1000 fpm descent until the idle descent path is intercepted."

The way I interpret your post you're saying that the plane will cruise descent down to the altitude in the MCP window at 1000 fpm, and that it won't have any path information until it reaches this altitude. Is this a correct interpretation?

I did however find another segment in the manual about a special program called energy compensation which may be what I was seeing before.

"The FMC uses a special program called energy compensation at certain times during an ACT PATH DES. This program goes into effect when the MCP has been temporarily set to an altitude above the planned descent path. The airspeed cursor will slowly move toward a slower airspeed while the "TARGET" speed on the FMC remains constant. The airspeed reduction improves the capability of recapturing the descent path. When the airplane is cleared to resume the descent, the airspeed will slowly build up to the FMC target speed as the airplane recaptures the planned descent path."

I have seen that happen before, usually when you pass the T/D point without descending. The plane starts slowing down towards the min clean speed. I don't think this is what's happening though but I could be wrong.

On a side note I still don't fully understand the difference betweet a FMC SPD descent and normal lvl change either.
 
Have you flown the airplane yet? This is some very basic stuff that will be clear after your first flight or two.
 
my_controls said:
"A descent in VNAV started before the top of descent point is an early descent. If a path descent is planned, VNAV commands a 1000 fpm descent until the idle descent path is intercepted."

The way I interpret your post you're saying that the plane will cruise descent down to the altitude in the MCP window at 1000 fpm, and that it won't have any path information until it reaches this altitude. Is this a correct interpretation?

On a side note I still don't fully understand the difference betweet a FMC SPD descent and normal lvl change either.
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The way to accomplish what you are talking about is to select the "descend now" function in the FMC which will descend the plane at 1000 FPM until the VNAV path is reached at which time the aircraft will descend to stay on the path. Using this feature you have not changed the cruise altitude to FL200 that you did in your earlier example. If you change the cruise altitude to a lower altitude you will have no accurate path information until you reach your "NEW" cruising altitude.

Think of level change as all or nothing. It will climb at climb power at the speed commanded in the MCP (mode control panel) and it will descend at idle power at the commanded MCP speed. This level of automation does not employ the FMC in any fashion. FMC speed descent will employ the VNAV function as you may know. In a normal path descent, priority is given to the path and the speed will go to whatever is necessary to maintain the path. If you have a big tailwind, the airplane may have to descent at 320 knots even though you have assigned a 300 knot descent speed in the FMC. In a speed descent mode, you are telling the FMC that speed should take priority over the path. So in the above example, you will maintain the 300 knots but may very well go high on the path. Speed brakes may become necessary to make it all work. As you use various levels of automation and learn how it all works you will develop your individual ways to make it all happen. There are many ways to make the airplane do things and it is quite entertaining to watch various pilots display their own special techniques. As long as you get what needs to get done, there is no right answer as to how to do it.
 
I see, so basically the way to go about it if I do descend using VNAV is to let it level off at the new altitude and THEN enter that altitude in the cruise page (assuming I want too, I like to do this early out on the descent to get a new T/D point for myself). I just didn't realise that you took information away from the FMC by inputting a new cruise level while you're descending like that.

Yes I've flown the plane, I'm not so sure I'd call this stuff "basic" though. I've flown with plenty of captains who seem to have a pretty limited understanding of VNAV ;-)
 
my_controls said:
I see, so basically the way to go about it if I do descend using VNAV is to let it level off at the new altitude and THEN enter that altitude in the cruise page (assuming I want too, I like to do this early out on the descent to get a new T/D point for myself). I just didn't realise that you took information away from the FMC by inputting a new cruise level while you're descending like that.

Yes I've flown the plane, I'm not so sure I'd call this stuff "basic" though. I've flown with plenty of captains who seem to have a pretty limited understanding of VNAV ;-)
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I have flown this equipment for 10 years. I am still surprised from time to time as I discover things I don't know. You're right, some of the concepts of VNAV are complex. That's why so many pilots utilize more basic modes of automation since it is easier to understand. Good Luck!
 
You know how to tell how long someone has been flying NGs?

New guys say, "Why is it doing that?"
Seasoned guys say, "I've never seen it do that before."
Old heads say, "It does that sometimes."
 
I've had to ride an NG jumpseat to JFK a few times.

I've certainly heard "I've never seen it do that before" and "It does that sometimes" on a variety of occasions.
 
Velocipede said:
You know how to tell how long someone has been flying NGs?

New guys say, "Why is it doing that?"
Seasoned guys say, "I've never seen it do that before."
Old heads say, "It does that sometimes."
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I have said all three at various times in my career. I have even used:

"Screw it, just use Level Change or Vertical Speed.":laff:
 
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