VNAV Early Descent feature at low airspeeds

[Hardy Heinlin]

Good evening,

as we all know, when within 50 nm from the T/D, we can start an early descent by pushing the MCP ALT knob or the DES NOW key.

The THR mode reduces the thrust, and HOLD engages when circa -1250 fpm are reached.

1250 is a nice path intercept rate when the airspeed is high.

But it's too steep at airspeeds below 200 kt with flaps out, on a short missed approach cruise, for example. -500 fpm would be a better path intercept rate then.

Has anyone ever tried an early descent (the DES NOW feature) from a missed approach cruise on the real 744 or in the big sim?

As we all know, this path intercept method with THR | | VNAV SPD is also used when a route modification puts the aircraft below the new idle descent path: In that case, the A/T aims at a descent rate that is half as high as the original descent rate. Then it goes to HOLD. Perhaps this value "1250" that all manuals mention is not a fixed value but a dynamic, calculated value that is just circa 1250 in most cases, but not necessarily always. When the FMC takes 50% of the original descent rate, that original descent rate is -- maybe -- not a sensed value but a computed value. If this is true, "DES NOW" will not aim for -1250 fpm at low airspeeds when actually something like -500 is required.


Cheers,

|-|ardy

[Will]

I don't know the answer, but it's questions like that that make me love this forum.

[Britjet]

I'm afraid I don't know, Hardy. Such an event would be quite rare. Most missed approaches in the big sim would be vectored using basic modes before perhaps engaging a VNAV profile again.
My gut feeling is that the 747 system won't have any refinements like this. 1250fpm would merely cause an early altitude capture which wouldn't do any harm..
Peter.

[Hardy Heinlin]

I just noticed that too. Also the following speed reductions for flap extension at idle thrust will reduce the descent rate anyway.


|-|ardy

[emerydc8]

I know this is contradictory to what is said in other parts of the same manual, but check out this from the Bulfer book under the CLIMB chapter, ALTITUDE INTERVENTION, p.10.3 (Jan. 99).

[Hardy Heinlin]

Yes, that's what I mean. That 50% feature is only for "resumption" of the descent, but the interesting words in this text are "... for the descent path". The FMC knows the planned descent path because it's computed by the FMC.

In most cases, at high altitudes, the initial descent rate is typically circa 2500 fpm. The FMC knows that. The half of it is 1250. So it could be that those famous 1250 fpm are not a constant in the FMC but a variable, always freshly calculated by the FMC when a descent path interception from below is to be started.

"Missed approach cruise" is not a good example. Start an idle descent at, say, FL150 at 280 KIAS, and your descent rate may be 1500 fpm. In that case I don't think the FMC will aim at 1250 fpm. Rather the half of 1500. That is, 750.


|-|

[emerydc8]

I wonder if this is like FLCH. All the books used to say that FLCH wants to do everything in two minutes, so if you had 2000' to descend it would do it at 1000 FPM. If you only had 1000' to lose it would descend at 500 FPM. What I have seen in the real airplanes on both the 744 and 767 is that FLCH will descend but it doesn't necessarily seem to be keyed into getting it done in 2 minutes. In fact, some of the newer FCOMs just say that FLCH will descend or climb at an appropriate rate. I wonder if the software got more refined over the years.

[ADDENDUM]: I stopped using FLCH if we are really light and they keep us say 2000' below our cruise altitude and then clear us up 2000 more feet once level. FLCH seems to climb at such a high rate that it's almost as bad as VNAV using THR REF. There is a possibility that we could set off a TA or RA for an aircraft above our assigned cruise altitude due to the rate of climb.

When given this scenario, I have been using V/S to climb the last few thousand feet at 500FPM. The looks I get when I do this are interesting. It's almost like some of these guys think we are going to fall out of the sky upon selecting V/S.

[G-CIVA]

When given this scenario, I have been using V/S to climb the last few thousand feet at 500FPM. The looks I get when I do this are interesting. It's almost like some of these guys think we are going to fall out of the sky upon selecting V/S.

You know why Jon .... you did exactly what that late gentleman from American Airlines told people to start doing 20 years ago ...

You took things down a gear or two, selected a simpler & more appropriate level of automation to control the velocity of your airplane & this enabled you to remain ahead of it whilst keeping a wary eye on the possible decay in the speed trend so that you could use the V/S wheel to keep the 'needle' nicely between the two sets of red bricks .... not exactly rocket science but very simple & more sedate.

Tut Tut Tsk Tsk!

[United744]

I thought it was based on angles, and the intercept angle to the flight path? Ergo, the descent rate will vary based on ground speed.

[Jeroen Hoppenbrouwers]

Interesting possibility.

Are all these high-level automatics programmed based on aerodynamics, or on geometrics? Who knows?

Is the thing following physical laws and attempting to be smooth in terms of airspeed, angle of attack, vertical speed?

Or is the thing following straight lines drawn with a ruler in mathematical space, using whatever controls it has to remain on the line like a train on rails?


Hoppie

[Hardy Heinlin]

Re FLCH (off-topic): You can stop the throttle movement manually, e.g. when reaching 500 fpm, before the A/T goes to the thrust limit. If you stop it, the A/T goes to HOLD. For more questions on this subject:
http://aerowinx.com/board/index.php?topic=3527.msg39156#msg39156


Re groundspeed: Yes, the sine function is not linear. But we're talking about path angles in the range of up to 5° only. In that sector the sine is pretty linear (remember the 3° thumb rule; it works very well). This is even more precise than the A/T can be when setting an initial approximate target descent rate with the throttles. You may expect a tolerance of about +/-200 fpm anyway.


VNAV descent path prediction calculation has always a geographic target. Speed limit points, waypoints, runways, altitude constraints etc. are all hard geographic objects. Every initial idle descent path (which is always weather and performance dependent) has to end at such a geographic object. So, geographic geometry has always the last word, no matter what the aerodynamics say. However, the shape of the idle path is not linear. It's shaped by forecasted groundspeed variations, OAT, anti-ice altitude, TL ... -- Anyway, regarding early descent interception, that interception path is simply based on HOLD | | VNAV SPD. Take about 50% and wait for the interception. No rocket science required.

The primary, ruling parameters for the idle descent path profile are the sinkrate at idle thrust and the first geographic target point. These two parameters are given by the performance database and the nav database. Everything else is adjusted for these two parameters. The FMC cannot vary the idle sinkrate and cannot vary the first geographic target point. The FMC can only move the location of the T/D. That's all. The idle sinkrate cannot be changed by the system. The idle sinkrate is determined by the planned command speed, the current gross weight, and atmospheric variables; those parameters cannot be adjusted by the system.

(The FMC can vary the idle sinkrate once the descent has started, by shifting the target airspeed away from the command airspeed. But before the descent starts, there has to be a plan. One plan with one starting point using one sinkrate at one command airspeed.)


|-|ardy

[emerydc8]

Re FLCH (off-topic): You can stop the throttle movement manually, e.g. when reaching 500 fpm, before the A/T goes to the thrust limit. If you stop it, the A/T goes to HOLD. For more questions on this subject:
http://aerowinx.com/board/index.php?topic=3527.msg39156#msg39156

True, but you still get the initial surge in power and high climb rate with FLCH by the time you pull the thrust back to adjust for 500 FPM. It's easier just to roll in 500 on the V/S. You might be able to fight the throttle servos to keep the power from initially increasing so much in FLCH, but I'm not even sure whether holding them in one spot while the servos try to increase thrust will result in a HOLD FMA. I know if they are moved a certain amount once THR is set they will go to HOLD, but not sure what the effect of just physically holding the throttles will have. In any case, it's just not worth the effort.

[Will]

With VNAV descent, why is it desirable to do a shallow descent followed by a steep descent in the first place?

[emerydc8]

Very often, ATC will start you down early because they need you out of that altitude. Rather than do an idle descent immediately, it is better to descend at 1250 FPM until capturing the path, then it will resume an idle descent.

[Hardy Heinlin]

In other words, it's not desired by the FMC or by the crew; it's ATC that sometimes instructs a descent before you reach the T/D. If you then do this early descent at idle, you will reach the first altitude constraint earlier, and so you will have to fly level at that constraint for a while, which is not economic.

[Jeroen Hoppenbrouwers]

Basically you're telling ATC that you hate them and complying just enough to not get scolded   :-P

[Will]

Okay, I get it. But the optimal vertical path would be a parabola, right? You want to balance time spent at fuel-efficient altitudes with achieving your constraints on time. With the "1250-fpm-then-idle" method, you comply with ATC but you get to stay higher longer.


But even better would be to start at 500 fpm, and aim to slowly reduce thrust such that idle power is reached at the moment the idle descent path is reached.

[emerydc8]

I can verify that the 767 Pegasus doesn't use 1250 FPM for a resumption of descent while under the path -- it will take roughly half the angle to re-intercept the descent path.

On a descent this afternoon, we were given an early descent that put us a few thousand feet below the original path once we leveled off. When cleared even lower, I rolled the lower altitude into the MCP and pressed VNAV (the initial descent was done in FLCH). I would have thought that being below the path it would go to 1250 FPM (THR/HOLD|VNAV SPD) until it captured the path from below. Instead it descended at 500 FPM until the path was captured.