"TAKEOFF SPEEDS DELETED" message

[Dirk Schepmann]

Hi Hardy,

Yesterday, I made an interesting observation in PSX (version 10.73).

I was already lined up and ready for takeoff when I noticed that I forgot to enter the assumed temperature in the THRUST LIM page. When I entered the temperature, the V-Speeds were deleted as usual, but the related FMC message "TAKEOFF SPEEDS DELETED" didn't appear.

Today I tried to reproduce the problem - and it was the same on various SITU files (i.e. Sydney-Los Angeles, Tel Aviv-Hong Kong and one of my SITU files). It's not only limited to the entry of the assumed temperature, also the change to a different fixed thrust rating causes the speeds to be deleted without the FMC message.

But here comes the interesting thing: it only happens with the first modification. If you re-confirm the V-speeds, subsequent changes on the thrust limit page generate the message as usual.

Can you reproduce this?

Best regards,
Dirk

[simonijs]

Something I observed regarding TAKEOFF SPEEDS (for some months now, so with previous versions of PSX as well), is that you cannot insert lower values than the PSX calculated values for V1, VR, V2. If I want to lower a V1 of 154 kts to - say - 153 kts, it will return to V1MIN and 154 kts. This also applies to values for VR and V2.

Minimum V-speeds are documented in the AOM/FCOM and are approximately 125 kts for both VMCG and VRMIN at MSL, and 146 kts for V2 (PAX/Combi at ISA). These values decrease with higher Pressure Altitudes to 115/115/134 (8000 ft PA and 10 degrees C). Freighter values are slightly different: at MSL and ISA they are 133 (VMCG) and 138 (VRMIN).

I would expect a very low V1 (at a very low weight) to be increased to V1MIN, but I would not expect a V1 - well above VMCG - to indicate V1MIN if I try to lower the PSX calculated value by 1 or 2 knots.

Kind regards,
Simon

[Martin Baker]

And a question sort of related to this: using BACARS, when the final loadsheet comes while taxiing and says the entered data is within limits, but the ZFW is slightly different to the one in the FMC, what is the correct procedure?

It seems that any change in the entered ZFW, no matter how small, will result in deleted take-off speeds. Does one leave the ZFW untouched as the change is so slight that it doesn't affect performance?

Thanks,

Martin

[skelsey]

Hi Martin,

As far as I know, the normal procedure provided the loadsheet is in compliance is to leave everything as-is.

After takeoff, at a safe height (i.e. >FL100) and convenient moment, you can then enter the updated ZFW and CG in to the FMC to provide more accurate predictions.

[Hardy Heinlin]

Hi Dirk,

I'm not at work at the moment, but I think this is intentional because at a certain point after preflight -- e.g. during takeoff -- you want to be sure the V speeds won't be deleted anymore due to an unusual OAT change etc.


|-|ardy

[Martin Baker]

Hi Martin,

As far as I know, the normal procedure provided the loadsheet is in compliance is to leave everything as-is.

After takeoff, at a safe height (i.e. >FL100) and convenient moment, you can then enter the updated ZFW and CG in to the FMC to provide more accurate predictions.

Ah, I see - that makes sense. Very helpful, thanks! Martin

[Britjet]

Just to clarify this point in updating the 'ZFW and the CG".
The ZFW update is from the final loadsheet - you just type into the FMC what you have been sent. Simple, and quite important - however...
The "CG" referred to is actually the cruise CG and of course is entered on the CRZ page. This figure has to be calculated from the Cruise CG graph in the QRH, using the LIZFW (Laden Index Zero Fuel Weight) that is given in the loadsheet, so taking into account the fuel that you have on board when you do the calculation. It is usually best to do this when the centre tank fuel has been used (assuming that you have some) because the centre tank fuel affects the CG greatly, and by the time you've done the calculation it will be out of date!
It's not really a big deal if you don't update the Cruise CG anyway - at most it makes only about 400ft difference to the MAX ALT figure.
HTH.

Peter.

[Hardy Heinlin]

Re "TAKEOFF SPEEDS DELETED" message:

PSX only triggers this message when a V speed is computed and displayed in big font. The first condition ("speed computed") is only true when it has been computed. The V speed computation is executed whenever something related has changed. When loading a situ file, nothing related has changed. The message trigger won't be armed just by loading a situ file. Something must change after situ file loading. And only then the message will be triggered.

I guess I can improve this in one of the next updates.

I can also add an option to inhibit V speed computations, so that small font values will never appear. I've been avoiding this option because not all users have access to external V speed tables and programs. But I think I can bypass this problem by providing a V speed suggestion display on one of the Instructor screens.


Regards,

|-|ardy

[Hardy Heinlin]

How should the option "Show FMC computed V speeds" be arranged?


Three checkboxes?

[x] Show FMC computed V1
[x] Show FMC computed VR
[x] Show FMC computed V2


Or just one checkbox?

[x] Show FMC computed takeoff V speeds


Regards,

|-|ardy

[G-CIVA]

How should the option "Show FMC computed V speeds" be arranged?

Since we can already inhibit the V1 SPD via the CustomCode options my vote is for 1 check box.

Cheers

[simonijs]

Moin moin,

So...: if all votes were for one checkbox, which is kept empty (i.e. no FMC computed V speeds will be shown), does the FMC then still provide protection in case the aircraft takes off at very light weights with (own) calculated V-speeds being below the minimum ones from the AOM/FCOM? Say I diverted to EHRD, and now want to ferry back to EHAM at 205,0T. V-speeds from the tables are like 101/114/139 (Flaps 10), but the Minimum speeds should be 125/126/149. Will I get "a message"?

Maybe such a checkbox is not really necessary. If you don't have access to tables/programs, you leave the figures as they are calculated by the FMC. If you do have access to tables/programs, you should be able to adjust to slightly lower values as long as you do not enter figures that are below the minimum values of VMCG, VRMIN, V2 for VRMIN. That is where the protection of the FMC should pop up (I think).

Regards,
Simon

[G-CIVA]

Maybe such a checkbox is not really necessary.

Why?  Not opting for any kind of checkbox option penalises those who want to have the empty boxes option enabled & who might have access to tables/programmes.

Some aircraft still have the empty box option installed as a customer option.

If you don't have access to tables/programmes then surely you are not bothered by the checkbox option anyway & you leave it ticked.

The whole purpose of using tables/programmes is to work out the numbers in a safe manner - thus you should not get near the minimum values - the tables contain such data.

[Martin Baker]

Just to clarify this point in updating the 'ZFW and the CG".
The ZFW update is from the final loadsheet - you just type into the FMC what you have been sent. Simple, and quite important - however...
The "CG" referred to is actually the cruise CG and of course is entered on the CRZ page. This figure has to be calculated from the Cruise CG graph in the QRH, using the LIZFW (Laden Index Zero Fuel Weight) that is given in the loadsheet, so taking into account the fuel that you have on board when you do the calculation. It is usually best to do this when the centre tank fuel has been used (assuming that you have some) because the centre tank fuel affects the CG greatly, and by the time you've done the calculation it will be out of date!
It's not really a big deal if you don't update the Cruise CG anyway - at most it makes only about 400ft difference to the MAX ALT figure.
HTH.

Peter.

I see, so if the ZFW is compliant but slightly different, you still update the FMC and consequently have to re-enter the V speeds before take off?

[Hardy Heinlin]

Simon, I will implement a special table for V speed minima in the FMC. Currently the minima are equal to the computed speeds.

The minimum feature for a V speed is only enabled if the FMC is enabled to suggest a computed V speed.

Steve, the new option will be on the Instructor > Model > FMS page and will replace the old "V1 disable" bit in the Preferences.

I think BA and Qantas inhibit the V1 computation only. Is it still the case or are all three V speeds manually entered?


|-|ardy

[andrej]

Hardy,

Will this table be part of the PSX, i.e. not visible for example in the Instructor's station but be a internal part of the PSX?
Being amateur, I don't have access to these tables and relly mostly on PSX as well as TOPCAT calculations.

Regarding your question, I would also opt the:
[x] Show FMC computed takeoff V speed

But this made me think a question. Do some airlines have an option to calculate Vr but not V1 or V2 (or some other way? And if so, what are the benefits of such feature?

Cheers,

[Britjet]

I think BA and Qantas inhibit the V1 computation only. Is it still the case or are all three V speeds manually entered?


|-|ardy

BA just has a manual entry for the V1.
Peter

[Hardy Heinlin]

V minima values are not equal to V1, V2, VR values.

V1, V2, VR will appear like this in the checkbox text lines.

Example:

[x] TAKEOFF REF shows FMC computed V1 (currently 123)
[x] TAKEOFF REF shows FMC computed V2 (currently 134)
[x] TAKEOFF REF shows FMC computed VR (currently 145)


Example:

[ ] TAKEOFF REF shows FMC computed V1 (currently 123)
[ ] TAKEOFF REF shows FMC computed V2 (currently 134)
[ ] TAKEOFF REF shows FMC computed VR (currently 145)


|-|ardy

[Britjet]

Martin,

If the loadsheet comes back as "compliant" you don't update anything before take-off. That's the point of the "compliant" message - it means no before-take-off updates are necessary.
At appropriate time after take-off you then enter the updated values.
Peter.

[skelsey]

Hardy,

Another possible solution could be that if the "no V speeds calculation" option is selected, a press of the LSK next to each input line (with no scratchpad input) could place the internally calculated value in to the scratchpad which would obviously then be inserted with a further press?

This would allow those who do not have access to the tables etc to have no calculated V speeds displayed if that is what their chosen airline option is, but still be able to use PSX's internal calculation without reference to the IOS?

(I suspect I've not explained that very well!)

[Hardy Heinlin]

Good idea, but that might look like a feature of the real FMC and confuse the students ... :-)