Initiating VNAV descent: question

[Hardy Heinlin]

The DES ECON speed system modification is now available in PSX update 10.178:

https://aerowinx.com/board/index.php/topic,4191.0.html


|-|ardy

[Mariano]

Thank you, Hardy.

I was just playing with this and noticed that the Legacy FMC commands a descent Mach number higher than the NG FMC (in the situation I was using, Legacy commanded a descent Mach number of .842, versus .810 for NG (Cost Index of 100)). Descent CAS is the same for both versions.

Perhaps this is intentional and accurate.

Thanks again and best regards,

Mariano

[Hardy Heinlin]

You know, the legacy FMC starts the DES later and requires a shorter idle DES route. Maybe this allowed another step climb which caused different TOD conditions than in the NG FMC.

The new mod affects both legacy and NG equally. But don't forget that the TODs have different positions in the different FMC types.

Also, if you load old situ files, you need to wait some seconds to get the new computations.

|-|ardy

[Mariano]

Thank you, Hardy.

Best regards,

Mariano

[Hardy Heinlin]

I just checked. If you have any step size entry and you switch from legacy FMC to NG FMC, the step size will be reset to 0. This is because the NG has a different step size memory (with RVSM). So you will lose all step climbs and the Mach at TOD will refer to a much lower flight level.

If you want to compare the FMCs, you need to enter the same step size for either FMC type.


|-|

[Mariano]

There it is!

I was just switching versions without further research (I did have several step climbs in the original situation).

Thank you.

Mariano

[Will]

Thanks, Hardy. Your support for your work is beyond excellent.

[simonijs]

Hi Hardy,

Thank you for update 178.

With regard to the (new) VNAV descent schedule, I think it is OK for the Mach-number part, but not for the KIAS part of this schedule. So far I have seen KIAS numbers that are just a few knots above, equal to or lower than the numbers one will see on the PFD just before T/D. While starting the descent, almost instantly the magenta speed indication above the speed tape changes to KIAS.

The descent process would be the reverse of the climb, where the aircraft is climbing with a constant KIAS (KCAS) and increasing Mach-numbers until the altitude where the transition takes place from KIAS to MACH (with numbers shown on the VNAV CLB page). From then on, the MACH-number is more or less fixed while KIAS starts to decrease (as you explained to Will in another post). These decreasing IAS numbers are also shown on the VNAV CLB page, adjacent to the magenta Mach-number. So, in reverse during a descent with a (more or less) fixed Mach-number, KIAS will start to increase until the altitude where the transition takes place from Mach to KIAS.

Just tested a scenario at FL370 in ISA and M0,84 (TAS 482 and IAS 274), in which the descent schedule was set by PSX to .84/266. Instantly the transition to KIAS occurred with the aircraft decelerating. More normal descent speeds after the transition to KIAS would be 280, 290 or 300 KIAS. With .84/290 on the VNAV DES page, the transition would occur at approximately 34557 ft; for .84/300 at 33046 ft. The transition from Mach to IAS with a descent schedule such as .84/266 for my FL370 example would normally occur at an altitude above the current flight level (= 38297 ft).

Yesterday I did a flight at FL410 (just before T/D: -50, M0.831, TAS 484, IAS 247). If I recall well, VNAV DES was set to .836/253 kts (transition would be occuring at 40174 ft). Such descent speeds seem to be very low all the way down to 10.000 ft, especially for ATC. While preparing the descent, I am now changing the numbers on the VNAV DES page to higher IAS values (or using speed intervention with a fixed Mach-number until I get to the desired higher IAS). For this last flight, a situation file is available.

Kind regards,
Simon

[Hardy Heinlin]

Hi Simon,

the new ECON DES KIAS values agree exactly with the data in certain 747 manuals. If the change-over from Mach to KIAS now occurs earlier in DES, it may be due to the ECON CRZ Mach in PSX being a bit higher than in the real FMC. (Which I don't want to modify; that would be a year-long can-of-worms work.)

What is your source of information for the absolute ECON DES KIAS values? Or are you referring to their relation (to Mach) only?


Regards,

|-|ardy

From then on, the MACH-number is more or less fixed while KIAS starts to decrease (as you explained to Will in another post). These decreasing IAS numbers are also shown on the VNAV CLB page, adjacent to the magenta Mach-number. So, in reverse during a descent with a (more or less) fixed Mach-number, KIAS will start to increase until the altitude where the transition takes place from Mach to KIAS.

The command KIAS on the DES page should gradually change while the aircraft is at high altitudes in Mach mode? Is this a theory or a fact?

I mean, the change-over reference is never an altitude but the crossing of a fixed speed: In CLB it happens when the KIAS crosses the fixed Mach. In DES it happens when the Mach crosses the fixed KIAS. If the KIAS is not fixed during the entire in ECON DES, then vice versa the Mach shouldn't be fixed during the entire ECON CLB. But it is fixed. Similarly, the ECON DES KIAS should be fixed. It's been fixed in all PS versions since 1995.

[simonijs]

Moin,

Neither the FCOM, nor the AOM (KLM) provide absolute values. There is only one reference to "Typical descent rates" in the FCTM, for 20000 ft and below. For the clean 747-400 that would be 0,83M/340 kts, with a corresponding V/S of 2800 fpm. However, with this speed schedule the transition from Mach to KIAS would already occur at some 26600 ft.

Your picture refers to minimum speeds - it can't be lower than that. But they can be quite a bit higher, there are no maximum speeds listed. Especially in the USA, several STARs require to reduce to 280 kts at certain WPTs (BDEGA 4 into KSFO; ANJLL 4 into KLAX). Well...: I think they want you to reduce there. Amsterdam Area Control often instructs arriving traffic into EHAM, for instance to "maintain 300 or greater to WPT RIVER" descending through FL220.

From "before update 178", I recall that KIAS was normally set to 272 kts on VNAV DES, for whatever Mach-number. I manually adjusted that to 290 or so. CI on my PSX-flights is usually around CI-12 or 20.

Kind regards,
Simon

[simonijs]

Sorry, I didn't see the second part of your post.

The command KIAS on the DES page should gradually change while the aircraft is at high altitudes in Mach mode? Is this a theory or a fact?

No, that is a fixed number. I was referring to the VNAV CLB page, where after transition from KIAS to Mach, you can see the white KIAS value decreasing as the aircraft continues climbing (with a fixed Mach-number in magenta). In reverse, you would see the KIAS value increase during descents but only on the PFD. On the VNAV DES page that is a fixed number.

I know that transition from KIAS to Mach (or vice versa) occurs when passing a fixed KIAS. But you can calculate the altitude at which this happens. And these calculated altitudes match excactly the change-over in PSX.

Kind regards, again,
Simon

[Hardy Heinlin]

Your picture refers to minimum speeds?

Are you sure? The text is the end part of a longer text which discusses miscellaneous aspects of the ECON DES, the last part being the minimum speed. -- The table, however, is titled "ECON DES Speed", not "Minimum ECON DES Speed".

|-|ardy

[simonijs]

In my post #30, the text I wrote appeared in the quote box... Sorry for that!

[Hardy Heinlin]

I guess I'll ignore that original table and add 25 knots to each table value (in the next PSX update). That's the only thing I can do if I keep the Mach system as is.

[Hardy Heinlin]

ECON DES KIAS is now higher again in PSX version 10.179:

https://aerowinx.com/board/index.php/topic,4191.0.html


|-|ardy

[dhob]

For an ECON descent, the minimum KIAS is 250 knots. All weights, CI, and KIAS in the table posted earlier by Hardy are accurate. At CI 0 and weights 650K or less, ECON Des KIAS is 250 knots. At CI 60 and weights 510K or less, the ECON Des KIAS is 250 knots. For CI 300 to 9999, the maximum ECON Des KIAS is 349 knots.
The ECON Des KIAS is fixed. ECON Des maintains the VNAV Des Mach until the crossover altitude and then transitions to the KIAS listed on the Des page.
Operationally, CI 60-80 is usually ideal. However, this CI's FMC ECON Des KIAS is impractical in most ATC environments. In this case, the ECON Des KIAS is manually changed to 300 to 315 knots by entering the last CRZ Mach/300 at 3L on the VNAV Des page.

[simonijs]

For an ECON descent, the minimum KIAS is 250 knots. All weights, CI, and KIAS in the table posted earlier by Hardy are accurate.

Well..: now I feel really sorry to have brought this up. And I must apologize to you, Hardy. I will refrain from "smoking" for the next year or so.

Kind regards,
Simon

[Hardy Heinlin]

Simon, no problem :-)

Dhob, thanks. So when you use CI 70 on the real ship, the change-over to ECON DES KIAS may sometimes occur immediately after TOD as it may already be lower than the current KIAS?


|-|ardy

[dhob]

Yes, at low cost index values. That's why the ECON DES KIAS is usually changed to a higher value (300-320 knots) and entered at 3L.

For example, at a weight of 610,000 lbs., the optimum altitude is FL370. At CI 70, the ECON CRZ Mach is 0.841, and the ECON DES speed profile is approximately 0.841/267, assuming no wind.

At FL370 and M841, the CAS is 274, which indicates that the airplane is already below the crossover altitude for M841/267 (38,215').

In this case, the airplane will immediately initiate deceleration at the TOD to 267 KIAS. The NGFMC creates two TOD points for these cases. The first TOD initiates the deceleration point, the second when the airplane reaches 267 KIAS, for example, and starts to descend.

[Hardy Heinlin]

That second TOD point for deceleration is also modelled in PSX (when the NG FMC is selected).

Well, regarding the cost index stuff, maybe I'll redo my last undo ... :-)