ENG FAILURES

[Planedriver]

Does anybody know which answer is correct?


(1) The thrust limit changes to CON when selecting and executing an E/O page while an E/O condition is detected.

(2) The thrust limit changes to CON when selecting and executing an E/O page, no matter wether an E/O condition is detected or not.


|-|

[Hardy Heinlin]

Thanks for the confirmation, Planedriver :-)

Hi Steve.

FL 330 - W 344T TAT within 1°, SAT within 3° - CI 80 ECON SPD
M = .874(PSX) .844(real)

My performance database refers to -50°C at FL330. My ISA deviations are based on this reference.

With cost index 0 I get max range cruise. With zero wind, 340 tons, no deviations, that should result in Mach .841. If I set -57°C at FL330, it's Mach .859.

LRC is Mach .859 in my database at -50°C at FL330, 340 tons. According to Boeing, LRC data is used at cost index 230 (LRC speed plus cost index dependent wind additives).

The difference here between CI 0 and CI 230 refers to Mach .841 and .859. In PSX, cost index 80 will set an interpolated Mach number between .841 and .859, that is Mach .849. This is very close to your real-world value .843. -- 2 knots more or less at 314 KIAS high speed, this is peanuts :-)

However, ... heck ... in the current PSX version, for some reason, I'm getting an LRC of .874 instead of .859 -- this is obviously an unwanted disagreement between the FMC output and the FMC database. This also affects the CI system. I'll try to fix it ...

Nevertheless, even with that error, CI 80 gives me only Mach .854 and not .874 like in your test. It's only 0.01 Mach difference. Perhaps you used a higher OAT.


Regards,

|-|ardy

[Hardy Heinlin]

Hi.

FL 330 - W 344T TAT within 1°, SAT within 3° - CI 80 ECON SPD
M = .874(PSX) .844(real)

This stuff is now adjusted in version 10.0.3-beta11 (not uploaded yet).

For the engine out, PSX is showing the same MAX ALT for the LRC SPD and the E/O DD SPD, which denies the definition of Max Range (Min drag) and Long Range options offered by the FMC. The level off altitude following the E/O DD SPD must always be higher than the LRC procedure.

Normally, the MAX ALT rises when a higher Mach number is selected.

Normally, E/O speed is lower than LRC speed.

So why should the MAX E/O ALT be higher when E/O speed is selected instead of LRC speed? I would expect that LRC gives a higher MAX ALT because LRC speed is higher than E/O speed.


Regards,

|-|ardy

[Skybird]

Hi Hardy,

If you have a QRH, have a look at the following PERFO tables:
1 ENGINE INOP Driftdown Speed/Level Off Altitude, and
1 ENGINE INOP Long Range Cruise Altitude Capability.

The Driftdown (at E/O SPEED) will give you a maximum level off which is higher than the LR CRUISE level off (at LR SPD), with same initial weight, same ISA deviation.

During a DD PROC, you will burn more fuel and will consequently be lighter when reaching your level off altitude.
During a LRC descent, you will burn less fuel and have to level off at a lower altitude.

The time to reach your one engine out altitude will be longer with the DD Procedure...

I checked the QRH values vs the onboard FMC and they do match...

On the one engine out page, real FMC:
+The OPT FL are the same for E/O and LRC
+The MAX FL on the LRC SPD page is lower than the one on the E/O SPD page (by 1000ft approx. for 345T)

PSX gives the same MAX ALT should you perform a LR or an E/O DD procedure...

Steve

[Skybird]

Nevertheless, even with that error, CI 80 gives me only Mach .854 and not .874 like in your test. It's only 0.01 Mach difference. Perhaps you used a higher OAT.

I don't understand what you mean with the OAT. The SAT is 3° from the one of the real flight. I still have a .872 Mach in ECON with CI 80 (10.0.3 Beta13).

[Hardy Heinlin]

Hi Steve,

I must ask again. What do you mean by "TAT within 1°, SAT within 3°" at FL330?

FL 330 - W 344T TAT within 1°, SAT within 3° - CI 80 ECON SPD
M = .874(PSX) .844(real)

What exactly is your SAT at FL330?

3°C? That's impossible.

3°C within ISA? The performance tables use -50°C at FL330.

I set this on Situation > Weather > Global:

Altitude of tropopause: 33000 ft
Temperature at tropopause: -50°
Wind at tropopause: 000 / 000 / 0 / 0

Aircraft is at FL330
Cost index is 80
GW is 344 tons

With SAT -50, ECON SPD is .852
With SAT -49, ECON SPD is .850
With SAT -48, ECON SPD is .848
With SAT -47, ECON SPD is .846

With SAT -47, and 10 kt tailwind, ECON SPD is .844

I.e. within 3° SAT I can say my ECON SPD matches the real world value.

Version 10.0.3-beta13


Cheers,

|-|ardy

The SAT is 3° from the one of the real flight.

You had 3°C SAT on that real flight at FL330?
Or in your test you used a value that was within 3°C of your real-world SAT? If so, I need to know what your SAT was. It influences the speed very much, of course.

[Skybird]

TAT -24
SAT -55

I just mean that between the real flight and the PSX, the TAT was close by 1°, and the SAT by 3°.

All I can tell, Hardy, is that with a cost index of 80, I never ever saw the aircraft cruising at a Mach number higher than .855 as far as I remember. The usual Mach is around .845, and this is valid for flights going East and West...

[Hardy Heinlin]

In PSX you need to allow a tiny little tolerance only in the ratio of cost index versus airspeed.

TAT is absolutely correct in PSX because -- thank goodness -- it's based on a law of nature, not on a law of the office. It's a function of Mach and SAT etc.

If I fly your real-world Mach number and set the same SAT and GW, I get the same TAT, IAS and TAS as on your real-world flight.

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

Cost index, on the other hand, is a function of the FMC model, not of the physical model.

For cost index 80, for example, PSX calculates the ECON SPD like this:

Say, GW is 340 tons, FL330, SAT -50°C, zero wind:

Maximum range cruise Mach is: .845 (PSX), .841 (real world)
So for cost index 0 PSX sets .845

Long range cruise Mach is: .859 (PSX), .859 (real world)
So for cost index 230 PSX sets .859

For cost index 80 PSX uses the ratio R:

R = 80 / 230 = 0.348

And applies R to the airspeed difference:

( .859 - .845 ) * R = .005

Result:

.845 + .005 = .850 ECON SPD for CI 80

If the cost index was 230, the result would be equal to LRC speed. If the cost index was 0, the result would be equal to maximum range cruise speed.

The difference on the third digit after the Mach decimal is so small that it lies within the width of the speed bug on the PFD, and even less than a millimeter on the standby airspeed indicator, which is even smaller than the instrument error of the standby airspeed itself :-)

I could improve it even more if had another year free, but why should I? I guess this tolerance now (version 10.0.3-beta14) is not greater than the performance differences between real-world 744 aircraft within an airline fleet.

For even more accuracy the sim development costs rise exponentially because the complexity of the algorithms rise exponentially: Pay 200$ for a model tolerance of 10%, 400$ for 5%, 800$ for 2%, 2000$ for 1%, 5000$ for 0.5%, 10000$ for 0.2%, 20000$ for 0.1% ... and so on ... that's about the principle (theoretically) :-)


Cheers,

|-|ardy


This is the source and reason why I set CI 230 equal to LRC (in zero wind):
http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/article_05_3.html

[emerydc8]

Google "autothrottle wake-up"...

Thanks. From Google I'm now learning that this is feature is installed on the 747-8 and some 747-400, and that they have been considering installing it on all models as recommended by the FAA in 2011 or 2013 (Asiana accident).

So it looks like it was airline specific in the past? Or it is still optional today?


Cheers,

|-|ardy

Okay, I give.  I'm not seeing anything in Google that says the autothrottle wakeup was applied to any B744. Hardy, do you remember the article you saw?

[Hardy Heinlin]

I don't remember, sorry ...

[emerydc8]

Thanks, Hardy. I'm not doubting it -- I'd just like to see something in a manual or bulletin that explains the autothrottle wakeup system of the 744 and whether it is installed on all aircraft or it was just an option. I'll keep searching.

[Britjet]

I don't think BA does have wake-up as described, despite what I said in an earlier post. There is no mention of it in FCOM or FCTM whereas it is specifically mentioned in the 777 case. Maybe I was thinking of 777. One tends to think of all Boeings being alike, but we know that isn't the case.
I will do a little test in big sim soon if I get chance.
Hardy is on a private island somewhere at the moment anyway..:-)

Peter.

[emerydc8]

I hope Hardy enjoys his vacation. No doubt it is well-deserved.

Thanks for looking into this, Peter. The FMCs on both aircraft are similar enough, and the wakeup feature is seen so infrequently, that it is understandable that you would think of both FMCs as being alike. I think the only difference on the CDU is the ALT button. Maybe we can have more information on the wakeup feature by the time Hardy returns.

Jon D.

[Hardy Heinlin]

This protection applies to the B744.

The low speed protection acts at [Min MAN Speed - 8kts]

It engages the SPD mode of the A/THR in the following conditions:
-A/THR in ARM and fully serviceable (Active or not)
-Above 100ft Gnd for Approach / Above 400ft Gnd after Takeoff
-The AFDS vertical mode FLCH SPD, VNAV SPD, TOGA are not the active modes

Hi Skybird,

are you absolutely sure this applies to the 744 in your airline? Sorry if I'm asking again :-) But we all, including myself, sometimes misunderstand our own memories ... :-)

A new feature???    Nooo... I saw it somewhere in the books.
There is also a high speed protection...

Same question for the "high speed protection". You mean the 744 engages the SPD mode when not elevator-on-speed, and A/T armed, and approaching overspeed?



Regards,

|-|ardy


The wake-up feature is now discussed in this thread; let's continue the discussion there:
http://aerowinx.com/board/index.php?topic=3493.0