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Precision Simulator and so on

... getting ready for takeoff (assumed temperature ...)

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Member
Registered: Oct 2009
Posts: 32
Location: LOWL
Hi there!

Im always trying to improve my (basic) knowledge of flying the queen .... today i was trying to resolve the mysteries of assumed temperature methode.
Looking through my rather new FCOM I found some examples for take off data ...
Take Off at EHAM rwy 24, OAT 6C on wet runway; wind 240/10, QNH 1003 hPa and A/I on:
Estimated TOW 325.2 to
=> Assumed Temperature 55C
V1 WET 136kt
VR 156kt
V2 165kt

setting up a simular situation in PS1 ...
EHAM same runway, winds, fuel ... etc. trying to enter the assumed temperature in the FMS ... but the FMS doesnt accept 55C (not in TO,TO1,TO2) ... :|
(looking up the data at psmanual p. 250: runway limit brake release weight for 6 and 3400m is 396.9 and climb limit 430.8 disregarding adjustments for headwind and a/i ...)

ok, maybe somethings wrong in the way I read the data from the FCOM ... rechecking with Topcat and to be on the safe side with another source (virgin...)
results: topcat uses 52C D-TO1 (122,154,164 vspeeds) and virgin uses 43C D-TO2 (140,156,165 vspeeds) ... but none of this temperatures are accepted by PS1 ...

Now, after more or less 2 hrs, Im quite puzzled and sure that I "flirt" with some basic error respectively lack of knowledge. So maybe somebody give my brain a little push in the right direction ... :oops:

thx in advance ...
« Last edit by skin on Sun, 21 Feb 2010 09:03:40 +0000. »
Moderator
Registered: May 2009
Posts: 3269
Hi Skin,

PS1.3 has a simplified Assumed Temp model, it doesn't work in all ranges.


Regards,

|-|ardy
Member
Registered: May 2009
Posts: 301
Skin,

A while back, a made a chart that helps you figure out what to enter in PS1's CDU for assumed temperature. To make it work, though, you need the real-world reduced N1 (or you can get it from TOPCAT too; it doesn't necessarily have to be real-world.) If you are interested, let me know and I'll send it to you (I also made a P&W chart.)

Mariano
Member
Registered: Oct 2009
Posts: 38
Mariano, could you send that to me as well please?

probott at gmail dot com

Cheers! :)
Member
Registered: Oct 2009
Posts: 32
Location: LOWL
mariano,
would be great ... BIG thx!

peter66 at gmx dot at
« Last edit by skin on Sun, 21 Feb 2010 09:02:52 +0000. »
Member
Registered: May 2009
Posts: 301
Sorry about the delay guys. I will be home tomorrow and send it to you soon afterwards.

Mariano
Member
Registered: May 2009
Posts: 2
Location: Melbourne, Australia
Mariano, me too please.
Much Appreciated.
:lol:
jkitt at optusnet dot com dot au
Member
Registered: Oct 2009
Posts: 38
Maybe we should just create a mailing list and anything that one person requests, it gets sent to all of us since we all want it :P
Moderator
Registered: May 2009
Posts: 3269
Good evening,

I just started to prepare the assumed temp function for PSX.

As noted in the docs, PS1.3 used for the RR and PW engines a simplified EPR model based on the (accurate) N1 performance model of the GE engine (which has no EPR).

Regarding the assumed temp function, I think there were two problems in PS1.3:

- In real life (and PS1.3), the max N1 limits for the GE is not rising and falling with OAT: instead, the N1 limit is low when OAT is low or high, i.e. the N1 limit is high only when the OAT is in a mid range (all depending on pressure altitude). Now, when the assumed temp calculation won't spit out a lower thust limit, the PS1.3 FMC will say INVALID ENTRY. This happens when the value is "behind the mid range hill".

- In real life, the max EPR limits for RR and PW rises and falls with OAT: Of course, this is not compatible to the GE's N1 "hill shape" distribution. Edit: They, too, have a "hill" in the mid range.


That's my initial theory at the moment (without having digged into the original PS1.3 code yet).


Regards,

|-|ardy
« Last edit by Hardy Heinlin on Sat, 27 Mar 2010 00:12:25 +0000. »
Moderator
Registered: May 2009
Posts: 3269
Hardy Heinlin wrote
Now, when the assumed temp calculation won't spit out a lower thust limit, the PS1.3 FMC will say INVALID ENTRY.

Not INVALID ENTRY. The temperature entry will be replaced by dashes.

Here's an example of the above problem:
Real world MAX takeoff N1 at sea level:
OAT = 10 --- N1 = 101.9%
OAT = 20 --- N1 = 103.7%
OAT = 30 --- N1 = 105.4%
OAT = 40 --- N1 = 104.9%
OAT = 50 --- N1 = 103.4%

When the true OAT is 10 and the pilot enters 30, the PS1.3 FMC will not accept the pilot entry because the calculated N1 is higher instead of being derated.

Only when an OAT above 30 is entered, the calculated N1 will decrease.

I wonder what the real FMC will do if the entry does not lead to a derate. This effect seems to occur only with engines that use N1 as the primary thrust parameter.


|-|


Edit: I just noticed that real-world "EPR"-engines have "rise-and-fall" characteristics also. It's not specific to "N1"-engines.
« Last edit by Hardy Heinlin on Fri, 26 Mar 2010 23:31:42 +0000. »
Moderator
Registered: May 2009
Posts: 3269
Next theory:

In the real world, any OAT entry between 0-99C or 32-210F are valid (well, that's a fact). I guess, when the entry does not achieve a derated thrust limit, the entered OAT remains entered and dashes will not occur -- only the "D-" prefix will be removed.

This leads to another question: An entered assumed OAT that is higher than the real OAT, may not necessarily derate the thrust limit; it may in some cases produce a higher thrust limit. Isn't that an infringement of the operating limits?

As long as the "fake" OAT decreases the thrust limit, no problem, that's the very purpose. But if it increases the limit, it may lead to a damage.

Something must be wrong in my theory.

|-|
Member
Registered: Jun 2009
Posts: 297
Location: Northwest Pacific
Hardy Heinlin wrote
This leads to another question: An entered assumed OAT that is higher than the real OAT, may not necessarily derate the thrust limit; it may in some cases produce a higher thrust limit. Isn't that an infringement of the operating limits?

As long as the "fake" OAT decreases the thrust limit, no problem, that's the very purpose. But if it increases the limit, it may lead to a damage.

Something must be wrong in my theory.

|-|

a. Simplification of turbine engine aircraft theory: Thrust available depends on power setting (direct function), pressure altitude, Mach, and ambient temperature. (inverse function)

b. Varying takeoff assumed temperature can be viewed in two ways: a) changing thrust, and b) changing airflow temperature. Either way, the function is according to theory continuous. One possibility to a function discontinuity in these trends can be a certain engine manufacturer imposed limitation, due to design or maintainability considerations.

c. Utilizing TOPCAT for reviewing temperature effects on EPR (PW4062) and N1 (CFM 80C2B1F) data, it is consistent with the theory above up to 74 deg C (program limit).
_______________
Regards, Zinger
Member
Registered: May 2009
Posts: 309
Location: Winchester, UK
Can I just ask that someone tells me why, in a higher OAT, a de-rate is appropriate?

I just cannot get my head around why, when air density drops and therefore lift decreases, I would want to decrease the take-off thrust.

Help!
Regards, Richard
_______________
Cheers, Richard
Member
Registered: May 2009
Posts: 730
Location: Sydney, Australia
The way it was explained that made sense to me was;

At a higher temperature, you will use more runway to get get airborne at the same thrust setting than at a colder temperature. At higher temps, pressure is higher but density is lower - those molecules are moving around faster so they arn't 'sitting around' as much. This makes sense if thinking of an extreme - steam can be at a high pressure, but the density of that steam would still be quite low compared to an equivelant volume of room temperature water.

Entering an assumed temp is kind of telling the FMC to calculate a thrust level that will result in using the same amount of runway to get airborne as I would have if it were hotter.

ie

* at 10C I use 1000m of runway to get airborne at max TO thrust.
* at 30C I use 1500m of runway to get airborne at max TO thrust.

If I tell the FMC to assume 30C, I get;

at 10C (30C assumed), I use 1500m of runway to get airborne at less than max TO thrust.

Now this might not be technically correct, but it helped me get things right way 'round in my head...
_______________
« Last edit by John Golin on Sat, 27 Mar 2010 22:45:05 +0000. »
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Registered: May 2009
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Location: KTMB
Assuming you don't want to de-rate, but you want to know the maximum load you can take up on a given runway, it is easier to understand why OAT limits the performance of jet engines.

The hotter the ambient temperature, the hotter the intake air, the hotter the exhaust air.
All jet engines have maximum exhaust gas temperatures. When it is hot out there, you should push the engine less, to not exceed the max EGT.

Now, of course, high OAT means you need more thrust -- and this only makes your problem worse.

When you want to reduce the takeoff thrust to save your engine (assuming you have the light load and sufficient runway), you can fool the FMC into a higher OAT and it will try to be kind on the engines. The beauty of assumed temperature is that, should you want more thrust for whatever reason, you can just slam the thrust levers forward, as there is a fake limit imposed on the engines. The real maximum thrust is much higher than what the FMC thinks.


Jeroen
Member
Registered: May 2009
Posts: 236
Location: LLBG
Hi,

I want to remind everyone the Engine Limitations tutorial Ief Cooreman made for the PS1 community.
I don't know if Hardy can find all the answers for his questions there (read it more than 5 years ago), but it is a very interesting tutorial.
_______________
Avi Adin
LLBG
Member
Registered: Jun 2009
Posts: 297
Location: Northwest Pacific
Avi,
thanks for the link. While the tutorial is a good introduction to turbine engines, it is pretty detailed for the average user. It does however give a good explanation to the GE/ CFM N1 limitation measurement of rise then fall. The P&W EPR shows constant then fall (above ~33 deg C).

Hardy,
what you were seeing appears to be actual behaviour. The data I have is based on TOPCAT, no idea how reliable though.
_______________
Regards, Zinger
« Last edit by Zinger on Sat, 27 Mar 2010 20:25:00 +0000. »
Member
Registered: Jun 2009
Posts: 297
Location: Northwest Pacific
Just a small point to remember, about airframe performance. When OAT goes up, the air density is reduced, which affects the takeoff in two opposing ways: a) the drag is lower. b) The aircraft has to be accelerated to higher ground/ true speed.
Jeroen Hoppenbrouwers wrote
Now, of course, high OAT means you need more thrust -- and this only makes your problem worse.
_______________
Regards, Zinger
Member
Registered: Jun 2009
Posts: 297
Location: Northwest Pacific
One more point about derated thrust and takeoff. I met it when delayed into late evening departure from Tokyo Haneda, surrounded by nearby dense residential areas and strict noise abatement requirements. Studying appropriate configurations for these conditions, I used a combination of 20 degree flaps and substantial thrust reduction, which greatly reduced VMCA and therefore VR, V2 and total ground roll.
_______________
Regards, Zinger
Moderator
Registered: May 2009
Posts: 3269
I'm concluding from old and new thoughts:

Max take-off thrust is a function of:
- Pressure altitude
- OAT
- Packs on/off
- Anti-ice on/off

Theoretically, instead of OAT one could fool the FMC also with an assumedly higher pressure altitude. But the thrust limit variety is greater when OAT is the variable.

On CF6 engines, the peak of the max allowable thrust lies between 10 to 35 Centigrades. Below or above these temps the thrust limit is lower.

A similar peak can be seen when varying the pressure altitude:
- 9000 feet at OAT 15C
- 7000 feet at OAT 25C
- 6000 feet at OAT 50C
Above or below these pressure altitudes, the thrust limit is lower (when keeping the OAT).

PW engines have similar thust limit peaks at mid temps and mid pressure altitudes. However, below these peaks the thrust limit is not lower, but constant (flat rated). I have not yet checked RR engines, but I guess they are flat rated as well. I think this has something to do with the EPR principle and also with simplification of flight planning math (within safety limits).

The thrust limit is an important factor for engine life. A small change in thrust limit may cause a big change in engine life.

The air must not be too hot -- and not too dense. Too dense means: too cold. So there's an optimum mix somwhere in the middle.

There's probably a chemical limit (melting metal) as well as a mechanical limit (more thrust by more density).

The FMC Thrust Limit page is not a flight dispatch program. Flight dispatch is done at the flight dispatch office. The officers (and Captains) are responsible for the correct thrust limit calculation based upon various runway parameters, weather, weight and aircraft configurations. When the result is far within the safety limits, the takeoff thrust will be reduced. Well, one method of thrust reduction is the assumed temperature method.

The dispatcher will calculate the assumed temp. This will be entered into the FMC. The FMC itself only knows the actual OAT and actual pressure altitude (given by the sensors and Air Data Computers). The FMC does not know anything about the runway and wind etc., not even the anti-ice and packs status for the take-off (that may be detected later inflight). The FMC is not a flight dispatch program, it just computes the thrust limit for the given temp and pressure.

The entry is valid when it's above the actual OAT, no matter if it achieves a lower thust limit or not. If it's valid, the "D-" prefix will be displayed on the CDU and EICAS. With CF6 engines with the assumed OAT on the cold side of the limit peak, yet hotter than the actual OAT, the calculated thrust limit may be higher, hence less economic. I think the flight planing program should check that in such cases the assumed temp method is not used.

Just thoughts, not necessarily true.


Cheers,

|-|ardy
« Last edit by Hardy Heinlin on Mon, 29 Mar 2010 03:39:03 +0000. »
Member
Registered: Jun 2009
Posts: 297
Location: Northwest Pacific
Hardy,
just to make sure it's clear, it isn't that the thrust is lower below 15 (or up to 35 degrees C in other cases, depending on manufacturer spec) but rather that the parameter limiting available thrust is reduced to maintain nominal FRT.
_______________
Regards, Zinger
Moderator
Registered: May 2009
Posts: 3269
Zinger wrote
it isn't that the thrust is lower below 15

The thrust limit is lower ...

When I mention those peaks, I refer to the max allowable (full rating) takeoff thrust, i.e. the takeoff thrust limit. This limit rises and falls with rising OAT (CF6 engines).

This is not the thrust generated when the levers are at the firewall or anything like that, in case my comments sound like that. That's not what I meant.

Do we mean the same thing?

What is FRT?


|-|ardy
Moderator
Registered: May 2009
Posts: 3269
It would be also nice to know what the FMC would do with the "fake" thrust limit when the actual OAT were rising above the assumed temp in the time between preflight check and takeoff. Would it self-delete the entered assumed temp?


|-|
Member
Registered: May 2009
Posts: 216
Location: between EDDF and EDDN
Hardy Heinlin wrote
....when the actual OAT were rising above the assumed temp in the time between preflight check and takeoff...


Hi Hardy,

I just tried to check what happens with the EDDF1.ini (EDDF - SAEZ)
There an OAT of 14 C is indicated with 70 C as selected temperature.

Playing around shows the following conditions at TO-1:

14 -> N1=102,7
15 - 54 -> no entry possible.
55 -> N1=102,5
60 -> N1=101,6
and so on...

Checking this roughly with several ini files, shows that obviously an assumed temperature less than 40 over the OAT is not accepted.

I just think, if the OAT within this half hour or give them one hour for preflight preparations raises about 41 C to trigger this condition this planet has serious problems.

Do you want to model global warming, vulcano eruption or sun explosion? ;)

I'm not sure if my thoughts are right, but I think this condition will not exist. Nevertheless I will ask my friend. Perhaps in the real 747 there is no 40 difference.

Peter
Moderator
Registered: May 2009
Posts: 3269
Hi Peter,

if you rise the pressure altitude, the max thrust limit peak will drift to colder temps. E.g. at 2000 feet (or 959 hPa) and 30C the N1 limit will be at 108.6%, while at 35C it will already drop to 107.8%.


Cheers,

|-|ardy

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