ECON SPD versus SPD TRANS and V/S mode

[Alex]

A very good day Hardy,

Also we need to fix these 2 additional items:

1 ) If an airplane will fly a very short leg for whatever reason let's say only at 9,000 FT or FL90 it will not accelerate at ECON SPD it will stay at SPD TRANS in a clean configuration (minimum clean speed) based on aircraft gross weight.
At 10,000FT/FL100 and above the aircraft will accelerate at ECON SPD
To replicate this try a short flight from VMMC to VHHH, usually this is a ferry flight and you will get 6,000FT as CRZ ALT.
Not to create confusion, if our min clean speed based on aircraft weight is let's say 237 KTS or 240 KTS the airplane will accelerate in our case at 250 KTS, or if our min clean speed is 255 KTS based on aircraft weight the airplane will maintain 255 KTS even the ECON SPD might be higher (let's say 280) for CRZ.
For heavies 250 IAS below 10,000 FT applies unless minimum clean speed is higher.

When the airplane reaches CRZ ALT of 6,000FT will not accelerate to ECON SPD, will stay at SPD TRANS as I mentioned before at minimum clean speed for that weight until TD where will slow down at 240 IAS.

2) We need to fix the airplane descending below MCP ALT in V/S after the altitude is reached and mode annunciation is in ALT. At this point the airplane will level off and will not descend below MCP ALT even we select a negative V/S value (unless we preselect another lower altitude in ALT MCP window), system logic will prevent the aircraft from descending in these conditions.

Regards
Alex

[Hardy Heinlin]

Hi Alex,

FMC stuff is critical stuff. Not trivial. Perhaps I can try this next year.

For now, for a ferry flight below SPD TRANS, you can simply enter 250 SEL SPD instead of ECON before takeoff. So, when CRZ becomes active, it will then switch to SEL SPD instead of ECON. I did this in the St. Maarten situ, for example.

Re 2) ... I don't understand what you mean. What does PSX do, and what does the real aircraft do? I see PSX levels off correctly from V/S to ALT; it will not fly through the MCP ALT. Once the ALT is captured and engaged, you can re-engage V/S and fly away from the ALT. Are you saying V/S cannot be re-engaged when the aircraft is at the MCP ALT? Did you try this on the real equipment and it didn't work?


Regards,

|-|ardy

[Alex]

Hi Hardy,

This is correct:
1) I see PSX levels off correctly from V/S to ALT
2) Once the ALT is captured and engaged, you can re-engage V/S and fly away from the ALT
Refrence to statement 2 the aircraft will fly away to a higher altitude but not below.

 What is not correct is that after ALT is captured in V/S you cannot go below MCP ALT.
So as an example I am at 7,000 Ft and preselect 5,000 in MCP ALT window and select -700 FT ROD,  the airplane will descend and maintain 5,000 FT.
If I want to further descend let's say to 3,000 Ft I cannot select -700 FT ROD to start descending, first we need to preselect a new MCP ALT of 3,000 and then I can select a negative V/S.
At the present moment PSX will not do this, it will fly direct into the ground.

Regarding the SPD transition issue it is disappointing not to have it fixed.

Regards,
Alex

[Britjet]

Alex, if I understand you correctly, you are saying that you should only be able to select V/S if the ALTSEL has first been selected to a different altitude. This is not so...
Whether up or down, The V/S will operate until it hits an ALTSEL that the aircraft is passing, so PSX is correct.
The real aircraft will indeed fly into the ground if V/S is selected down and there is no ALTSEL in the way...

Brit.

[Jeroen Hoppenbrouwers]

I believe the reason why it is like Britjet says is that you can descend and level off at MCP alt, set to minimum descent altitude, until you are ready to go down, dial in 700 fpm and descend while winding the MCP up to the missed approach altitude. Your next immediate altitude target is the runway so no MCP required, it will stop by itself  :-)  while the missed approach altitude definitely is relevant.

This kind of stuff is philosophy, not logic or technology. Different airframes have different philosophies, from how to fly it down to how to ground avionics.


Hoppie

[torrence]

I was just reading my Bulfer FMC guide again, and noted that  right up front they suggest regarding V/S mode as "VERY SPECIAL" to be use only for specific situations.  One of which is as Jeroen describes, a non-precision approach.  Which, from other sources, I gather is effectively never flown in a 747 in real operations but is still on the check ride list of required items.  Other cases, fairly limited, include slow changes in FL etc. when contact with the ground isn't an issue.

Cheers,
Torrence

[Hardy Heinlin]

Hi Alex,

re 1) I could try this: When the FMC changes from CLB to CRZ while the aircraft is below SPD TRANS, the FMC ignores the pilot's ECON mode selection and replaces it by SEL SPD mode, and sets its value to Vref+100 or 250 whichever is higher.

re 2) How should the MCP behave then? ALT is captured, aircraft is at MCP ALT. You push the V/S button. The window unblanks and shows 0000 fpm. When you now wind it nose down, what will happen? Will the window promptly blank again, and ALT will re-engage?


Cheers,

|-|ardy

[Alex]

Hi Hardy,
On #1 that sounds pretty good.
On #2 again, it's correct but please allow me 2 days before I get out of here, (DEL) and I can verify in the airplane. Or maybe there is somebody-else in 744 or 748 and check it out before I do it.

Regards,
Alex

[Magoo]

Hi Alex,
Stuck in DEL hey? I'm guessing you and I work for the same outfit...
I've always tought  that V/S would fly away from an MCP Altitude but looking forward to find out.

[Alex]

Hi Hardy,

During CRZ at CRZ ALT I pushed V/S and window did open with 0000, FMA changed to V/S in green, on vertical speed selector I dialed DN it changed briefly to a -200 or -300 value and immediately closed VERT SPD window and went in ALT on FMA and on MCP ALT HOLD light got illuminated indicating altitude hold mode active. The airplane did not descend. Pushed again V/S, same indications as previously mentioned and now I dialed UP and the airplane starts climbing.
On the next leg we changed equipment to a 748 and both (744 & 748 ) behave in the same way.
I hope it helps.

Kind regards,
Alex

PS, there is a software upgrade to the both FMC's in the near future with new enhancements, when I don't know yet.

[Britjet]

Hi Hardy,

Ref your (2)...

Selecting V/S up or down with the ALTSEL at the same altitude as the aircraft sometimes works for real and sometimes doesn't! The reason is that if the aircraft is *slightly* different to the ALTSEL ( maybe only 10ft or so) then the system will interpret this as an altitude capture from the new selected V/S  - which can be a damn nuisance if you are trying to start a final descent in a procedure!

The better way to do it is to press ALT HOLD, then wind the ALTSEL up or down, then select V/S..

Regards,

Brit.

[John H Watson]

Selecting V/S up or down with the ALTSEL at the same altitude as the aircraft sometimes works for real and sometimes doesn't! The reason is that if the aircraft is *slightly* different to the ALTSEL ( maybe only 10ft or so)

I recall that in some modes there is an inertial element (IRUs) in cruise, perhaps to prevent continual hunting for minor baro changes? (although I may be thinking of later model 767's). There are times when the actual altitude disagrees with both captain's and f/o's altitude indications.

Rgds
JHW

[Jeroen D]

I recall that in some modes there is an inertial element (IRUs) in cruise, perhaps to prevent continual hunting for minor baro changes? (although I may be thinking of later model 767's). There are times when the actual altitude disagrees with both captain's and f/o's altitude indications.

During cruise barometers would be set to standard. That is a fixed value. I can understand there might be a difference between the left and right standard barometer setting, but why would you get this hunting?

Jeroen

[John H Watson]

I probably used the wrong expression. By baro changes, I meant absolute altitude compared to baro altitude. With a fixed baro setting you are following a fixed pressure altitude. If the air is not laminar over a small distance, then a purely pressure guided A/P system will follow the irregularity. If some of this is smoothed by the IRUs, I assume you will get a smoother flight.

Perhaps someone can find some answers in this PPRuNe thread (although it tends to digress and there is some denial of inertial smoothing).

http://www.pprune.org/tech-log/398441-inertial-altitude-boeing.html

Rgds
JHW

[Hardy Heinlin]

Hi Alex, hi Britjet,

I'll try to model this effect you observed. Thanks!


Re "baro changes": There is also the influence of the baro selector on the EFIS control. There are two barometric altitude hold modes: -- 1. ALT on MCP ALT bug follows the bug on the PFD tape. -- 2. VS-0000 follows the pressure altitude value that was recorded in the moment when the aircraft leveled off and reached 0000 fpm -- this is independent of the baro selections on the EFIS control, because the actual goal of V/S-0000 is to maintain zero vertical speed; which, in fact, is internally a pressure altitude hold mode since every microscopic disturbance on the vertical speed would, in the long run, result in a non-zero vertical speed if the aircraft were not to return to the original pressure altitude after each, and ever so slightly, altitude deviation. -- Pushing ALT when the MCP ALT bug is not captured, has the same effect as V/S-0000 (following pressure altitude, not EFIS altitude), except that the target altitude is recorded in the moment when the ALT is engaged, i.e. when a vertical speed is present, it will overshoot and return to the target altitude.


Cheers,

|-|ardy

[Jeroen D]

I probably used the wrong expression. By baro changes, I meant absolute altitude compared to baro altitude. With a fixed baro setting you are following a fixed pressure altitude. If the air is not laminar over a small distance, then a purely pressure guided A/P system will follow the irregularity. If some of this is smoothed by the IRUs, I assume you will get a smoother flight.

thanks John,

So I assume the pressure guided A/P system could be the situation Hardy describes as VC-0000?

When you say this could be smoothed by the IRU's, are they seperate from the three IRU's I'm familiar with on the 744?

I always assumed they provide lateral navigation only. But I guess in principle the could also provide vertical guidance?

[Jeroen Hoppenbrouwers]

There are just three IRUs on the 744, but the standby integrated display has independent ones that don't feed outside it as far as I know (totally self-contained unit, brilliant piece of kit, really).

Yes these IRUs do 6 DOF. They provide fast, direct input to the V/S indicator as baro takes quite a bit to stabilize. They also work with the yaw dampers, they provide fast accelleration inputs to the AP/FD system, they stabilize the flight path along the radio waves of the ILS, they modulate the autobrakes, they control antenna positioning, etc.


Hoppie

[Hardy Heinlin]

Hi Hardy,

During CRZ at CRZ ALT I pushed V/S and window did open with 0000, FMA changed to V/S in green, on vertical speed selector I dialed DN it changed briefly to a -200 or -300 value and immediately closed VERT SPD window and went in ALT on FMA and on MCP ALT HOLD light got illuminated indicating altitude hold mode active. The airplane did not descend. Pushed again V/S, same indications as previously mentioned and now I dialed UP and the airplane starts climbing.
On the next leg we changed equipment to a 748 and both (744 & 748 ) behave in the same way.
I hope it helps.

Kind regards,
Alex

PS, there is a software upgrade to the both FMC's in the near future with new enhancements, when I don't know yet.

Hi Hardy,

Ref your (2)...

Selecting V/S up or down with the ALTSEL at the same altitude as the aircraft sometimes works for real and sometimes doesn't! The reason is that if the aircraft is *slightly* different to the ALTSEL ( maybe only 10ft or so) then the system will interpret this as an altitude capture from the new selected V/S - which can be a damn nuisance if you are trying to start a final descent in a procedure!

The better way to do it is to press ALT HOLD, then wind the ALTSEL up or down, then select V/S..

Regards,

Brit.

Good morning,

Alex and Britjet, would you agree with the following modification in Alpha 25?

===================

Action: V/S is engaged and a negative fpm value is being set while the aircraft is within the magenta altitude bug.

• When doing this action while the aircraft is above MCP ALT + 10 and sinking, the ALT mode will re-engage.

• Otherwise, the ALT mode will not re-engage.

===================


Regards,

|-|ardy

[Magoo]

Sorry to stir things up but Ive just tried something on a -8 In cruise at fl 370 I selected V/S without changing the altitude on the MCP. V/S windows opened at 0000 and selecting down -100 the aircraft started to descend... so as far as I can tell v/s will take you down without changing MCP alt.

[Jeroen Hoppenbrouwers]

Magoo, this is not entirely convincing yet as the aircraft may have been at 370001 and climbing at 1 ft/min which would, according to some other reports, allow it to not capture ALT immediately.  We think it is either a subtle interplay between mini-modes or a different design in various APFD releases, but nobody has the definitive answer yet.


Hoppie