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GeirG

N1, approach and EEC

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Good day, all! After work yesterday (I'm working on an airport) I got the chance to sit on jump seat a few legs and home.While flying we talked a lot about the difference between the -800W compared to lighter 737s.The -800W is known to be an amazing glider, that's for sure, but after a demonstration of a "steep" approach (5000ft QFE/12nm) I have some questions for the PMDG 738. As you see on the picture, the landing gear is out and flaps is set to 5. Still the N1 is 32.7%. If I try to do this in the sim, when extending the gear, N1 increases to about 40%.I didn't get a picture of it, but in this real 738, N1 stayed at 32% even when flaps were set to 15+... I tired to set up my simulator just like on the picture bellow, at 4800ft 12nm from the runway, but it seems almost impossible to me with NGX, when N1 idle is 40%+. This is btw the second time in a short while I've seen the "proof" of approach idle at 32% N1 in a B738, a few weeks back the ATC director forgot us. tongue.png Any suggestions or comments? If it's a bug, would it be possible to get this fixed? NB:- I'm flying NGX on a Win7 64bit computer.- A/T is irrelevant to this problem.- Tried with and without ice protection.- Tried all versions of NGX,- Happens at least bellow 6000ft (tested) Anyway, this is the only issue I have with NGX. A master piece of an addon. im%20Not%20Worthy.gif


Best regards,

Geir Gronlien

www.hypoxiafilm.com

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I don't think the PMDG ought to be that precise, but if it is...*******!...it would just confirm once again how awesome this addon is...the best in FS history I'd dare to say. The other day I was flying from EHAM to EGGW and just before entering the runway I got a doors message master caution.- I was like: "what the heck!?" Doors were closed and pressurization was normal. I thought it was a bug. This actually happens sometimes in the real plane due to obstructed or dirty sensors and FCOM recomends to continue normally if pressurization is normal so I did and it was an great flight and the master caution disapeared after 1500Ft. Thanks PMDG for such an awesome product! Now if Geir PG achives a side by side resonance between NGX operation in the matters of N1 based on aircraft physical state and the real aircraft provided it is just a bug and the assumptions are correct, I'll just ###### my pants. Can't wait for the 777! I wish you guys made an Airbus too! Cheers,Fernando A very happy PMDG custumer

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The TMD is showing that the plane is still in cruise !!!
Nice one! Do you think thats the reason for the low N1?

John Rubens
PMDG_ngx_T7_sig.jpg

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Furthermore, regarding this whole normal/approach idle discussion, IIRC the idles are set by reference to N2, not N1. Nevertheless, the numbers are off on the NGX, at least when comparing to the documentation. Reported that ages ago, but I already suspected back then that it could be a FSX limitation after all - e. g. I could imagine the add-on would probably have to set way too high thrust settings to hit the correct numbers, resulting in unacceptable performance (in the sim). Other than that, what I also don't know is how accurate the real thing is. For example, you can try to set a very low temperature on ground, like -25C. This is not exactly an unrealistic number for winter in moderate latitudes (like 50-60°). In any case N2 should not drop below 58% so as to to ensure correct IDG operation. Setting such a very low temperature will force numbers less than that on the NGX. As I said, I dunno about the real thing, but apparently this ain't according to the docs, that's all I'm saying. Also, flight idle (not approach idle!) shall be around 72% N2 at all times - try setting that manually on the NGX (because it's mostly lower), and you'll probably have a hard time to get down in time. Think we'd once again need an actual NG driver to comment on this, or do some observations on their upcoming flights. sig.gif

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The TMD is showing that the plane is still in cruise !!!
Nice one! Do you think thats the reason for the low N1?
Yes ! The plane is in level change and descending with the autothrottles in the ARM position.The APP light is still on so the glideslope has not been captured yet.The plane seems to be turning into an approach in the photo......about 10 miles from the runway.Such a pity we can't see the PFD !
I've been wrong before, but I don't think thrust limit and EEC idle thrust are related. Interesting discussion, though. Check out the N2. It's at the right speed for Approach Idle, but the flaps aren't and the TAI isn't on. Maybe the T/L weren't in idle. Dunno.The N1 looks right for their position. I was J/Sing into KSEA right after this discussion first came up, so I was watching. I saw 32% on our base turn. (Normally, I'm not paying that close attention!)

Matt Cee

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Thank you.I do not think this is related to neither of the automatic flight systems, or flight modes. I think this is related to the electronic engine control, that increases the idle. It might be a limitation between N1 and N2 in FSX, but hopefully something that would be possible to fix.:)


Best regards,

Geir Gronlien

www.hypoxiafilm.com

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During flight, the EEC controls engine idle to satisfy idle speed requirements.The EEC goes to the approach idle mode when the airplane is in flight and one of these conditions occur:Cowl thermal anti-ice switch is in the on position for engine 1 or engine 2Below 15,500 feet and the left or right main gear is down and lockedBelow 15,500 feet and the left or right flaps are equal to or more than 15.No glideslope or approach is needed, only flap settings and landing gears.


Regards

Andrea Daviero

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Why in the real one this thing is not happened (gears are down and altitude seems to be less than 15500fts) I don't know.


Regards

Andrea Daviero

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Andrea, you're a mechanic/engineer, aren't you? You might have the more accurate details in the maintenance manuals. However, the FCOM says the flaps or anti-ice are the controlling factors, not altitude unless those signals to the EEC are lost.


Matt Cee

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Excactly! I cold not find the exact requrements for the eec to switch to approach mode.At this picture you therefore see an aircraft with approach idle. I've also asked quite many pilots here in Scandinavia now, and none of them recognizes the very high N1 I experience with NGX. Two times the last few moths they have even demonstrated this, and this last time I grabbed my camera to take a picture. It's hard enough trying to brake an -800 with 32 on N1, and sometimes you might need the landing gear to get some extra drag. This "secret weapon" is almost no use now, if N1 increases to over 40%.


Best regards,

Geir Gronlien

www.hypoxiafilm.com

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Well, the exact conditions have been outlined above. However, as I said EEC sets idle by N2, not N1. Flight idle shall be maintained at 72% N2, while approach idle could be 72 up to 79%. As far as a very quick test shows (ISA), you won't find these numbers on the NGX. Test altitude of 10000', speed somewhere around 250. Idle will drop N2 to about 65%. Dropping the gear, hence going into approach idle mode, will yield just shy of 68% on the N2 shaft. Setting N2 to 72% manually (=actual flight idle) yields already 50+% on the N1 rotation. And just for the heck of it, setting it to the highest approach idle of 79% N2 (whereas I don't know what exactly causes that value to go up to its maximum) will yield a whoppin 67+% N1. Totally not idle. In fact, in my quick test scenario at 10000' and about 210 initial speed it will even slightly accelerate with that setting. Now imagine that "idle" on approach and you ain't havin fun gettin down there. My hypothesis still stands: I'd assume it to be a sim limitation. It's probably not practical to tune the flight model in a way so the numbers were spot on in each and every case. Maybe it would just turn out that you get way too much thrust if you set the N2 values so high for the idle modes. The numbers might be correct, but the thrust output might not. Probably tweaking the flight model by decreasing the thrust (hence higher engine indications, but relatively less thrust to what we have now) would probably have vast consequences on the rest of the add-on systems, so it might not be a viable way. sig.gif

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Manuals tells about the 72% of N2 for approach idle. 15500ft of altitude, gears and flaps, plus antiice as confirmed by badderjet.Maybe that other factors like altitude and so on will adjust those things.N1 is a slave of N2, and EEC will normally change N2 value to get the target N1 (as N1 is the most important thing for thrust for a CFM56 engine), For this same reason you can see tat in the picture the engine number one is a bit less efficient from the right one, maybe older tan the other, with the same N1 it have 2 different N2 and 2 different fuel flow.It appears a bit strange that on idle control the EEC takes in mind N2 values, but, as the N1 must be equal with thrust levers in the same position, probably the EEC rises the N2 for one of the engines to match the other one N1.In the picture N2 is around 74% so the value is in the range for approach idle, probably as described before, the value is adjusted to let the 2 engine to run at the same thrust.


Regards

Andrea Daviero

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N1 is a slave of N2, and EEC will normally change N2 value to get the target N1 (as N1 is the most important thing for thrust for a CFM56 engine),
Andrea, maybe I'm misunderstanding you or the system, but I thought that N1 is the master for thrust control, N2 is the master for idle. If you want 90% N1 for takeoff, the EEC isn't looking at N2, it's looking at N1 and you'll get whatever N2 it takes to get 90% N1. On approach with the T/Ls at idle, you'll get idle N2 (72-79%) and whatever N1 that happens to be.

Matt Cee

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Yes, but N1 for all engines is a slave of N2.The difference in how the engine is controlled is made by the EEC wich changes the engine parameters (N2) to match the required N1, when idle thrust changes from ground to air or from air to approach, or approach to ground, the EEC changes the values taking in mind the N2 required ranges (72-79 for example in approach mode) and programming a target N1 calculated by the ADIRU inputs and the engine parameters but, in this range both N2 values can differ from engine to engine to match the N1 value that EEC calculates, those values of N1, with both levers in idle, must be the same (with equal inputs from the sensors). This by remaining inside the 72-79% (N2) in the approach mode.This means that with a certain mach speed, at a certain P0, a certain altitude, and with N2 in the idle range, the EEC calculates its N1 setting, N2 will move inside this range to reach the N1 setting. This setting will change as the parameters changes, taking in mind always te N2 range.This is as far as I can remember as on Ngs I worked for few times and never inflight.But this is what can be read from the manual, it never tell that N1 setting is disregarded during idle, it only tells about the range of N2 in idle modes, but from what is written, never the EEC in normal mode goes to follow exactly N2 disregarding N1.Take a look at it, so you can better understand if i'm confusing a bit (I'm an avionic Hug.gif )

ENGINE FUEL AND CONTROL - ENGINE CONTROL - THRUST CONTROL - FUNCTIONAL DESCRIPTION Engine Thrust ControlThe EEC uses N1 speed to control engine thrust. The EEC calculates six N1 reference speeds based on this data:
  • Airplane model
  • Engine thrust rating
  • P0 (ambient static pressure)
  • Mach number (air speed divided by the speed of sound at the current ambient conditions).

The N1 reference speeds are at the same thrust lever resolver and thrust lever angles for all engine thrust ratings and airplane models. These are the N1 reference speed names, the thrust lever resolver angles (TRA), thrust lever angles (TLAs), and reverse lever angles (RLA) (note: the angles are in degrees):

  • Maximum reverse thrust (8 TRA, 104 RLA)
  • Idle reverse (24 TRA, 62 RLA)
  • Idle (36 to 38 TRA, 0 to 2.4 TLA)
  • Maximum climb (72 TRA, 44 TLA)
  • Maximum take-Off/go-around (78 TRA, 52 TLA)
  • Maximum certified thrust (82.5 TRA, 58 TLA).

You can see TRA angles in engine BITE input monitoring pages on the CDU. See the training information point pages in this section for more information on input monitoring pages.The EEC calculates the commanded N1 speed based on the position of the thrust levers with respect to the N1 reference speed thrust lever positions. When the thrust lever is between two N1 reference speeds, the EEC does a linear interpolation to find the commanded N1 speed. When the thrust lever is moved forward, the commanded N1 speed is more than the actual N1 speed. The EEC controls the engine servo systems to accelerate the engine to the commanded N1 speed. When the thrust lever is moved back, the commanded N1 speed is less than the actual N1 speed. The EEC again controls the engine servo system to slow the engine to the commanded N1 speed.The EEC adjusts the commanded N1 value for the amount of bleed air the airplane takes from the engine. If bleed air demand increases, N1 speed decreases to compensate for the additional load. This keeps engine hot section in limits for the current engine thrust rating. The EEC gets this airplane bleed configuration data from the DEU to find the airplane bleed load:

  • Right pack on or off
  • Right pack in high or normal flow
  • Left pack on or off
  • Left pack in high or normal flow
  • Isolation valve open or closed
  • Bleed valve for opposite engine open and engine running
  • Wing anti-ice on or off
  • Cowl thermal anti-ice on or off.

The EEC locks the airplane bleed load in the current configuration during takeoff when airspeed is more than 65 knots. The EEC unlocks airplane bleed load if the airplane is 400 feet above takeoff altitude and one of these conditions is true:

  • Airspeed more than 300 knots
  • Altitude more than 4500 feet above takeoff altitude
  • TRA decreases more than 3 percent.

The EEC uses fan trim balance to adjust the engine fan speed for engine variations. After engine assembly, engine tests are done to make sure it meets engine performance requirements. One of these tests is to measure engine thrust and fan speed. To make sure all engines have the same takeoff and early climb thrust for the same indicated N1 speed, the fan trim is used to adjust indicated and commanded N1 speeds. This adjustment is used to decrease the commanded N1 speed and increase the indicated N1 speed. When the EEC changes between N1 fan trim and no N1 fan trim, the transition is slow to make sure there is not a sudden change in thrust. This adjustment is effective when these conditions occur:

  • Altitude is below 15,000 ft (4572 m)
  • Mach number is less than 0.40
  • N1 speed is between 75 and 99.54 percent.

There are eight N1 fan trim levels. These fan trim levels are N1 trim 0 through 7 with 0 as no trim and 7 the maximum trim of 2.36 percent. The N1 trim level for the engine is stored in the engine identification plug. The N1 trim level shows on the EEC CDU IDENT/CONFIG page of EEC BITE. See the TRAINING INFORMATION POINT pages in this section for more information on EEC BITE. EEC Mode DescriptionThe EEC gets ambient total pressure (PT) from the ADIRUs or calculates it from ambient total air temperature (TAT) and ambient static pressure (P0). The EEC gets P0 from the ADIRUs or from the P0 transducers in the EEC. The EEC gets TAT from the ADIRUs or from the T12 sensor on the engine.These are the three EEC operation modes:

  • Normal mode
  • Soft alternate mode
  • Hard alternate mode.

The EEC is in normal mode when all these conditions occur:

  • PT is valid
  • EEC SWITCH on the P5 aft overhead panel is in the ON position.

PT is valid when these conditions occur:

  • PT signal from both ADIRUs are in limits
  • PT signals agree
  • Pitot probe heat for at least one PT probe is on
  • Pitot probe heat off, airplane is on the ground and TRA is less than 53 degrees.

In the normal mode, the EEC calculates Mach number with the two PT values from the ADIRUs and P0. The Mach number is one of the parameters used to calculate the N1 reference speeds. If outside air pressure, temperature, and Mach number change, the N1 reference speeds also change. This makes sure engine thrust is satisfactory for airplane performance.If PT is not valid or the EEC switch is put in the OFF position, the EEC goes to one of the alternate modes. The EEC energizes the ALTN light on the P5 aft overhead panel when one of these occur:

  • EEC is in soft alternate mode for 15 seconds
  • EEC is in hard alternate mode
  • EEC switch is selected to OFF (this puts the EEC in hard alternate mode).

The EEC goes to the soft alternate mode when PT is not valid. If PT becomes valid within 15 seconds, the EEC goes back to the normal mode and the ALTN light does not come on. After the EEC is in the soft alternate mode for 15 seconds, the ALTN light comes on. The EEC goes back to the normal mode and the ALTN light goes off if these conditions occur:

  • PT becomes valid
  • EEC is in soft alternate mode
  • Engine thrust change when the EEC mode changes back to normal mode is small or thrust levers are near to idle (TRA less than 51.6 degrees).

In the soft alternate mode, the EEC uses this data to estimate Mach number:

  • Total air temperature (TAT)
  • Standard day temperature (from P0)
  • Last valid difference between standard day temperature and static temperature (TO).

The soft alternate mode makes sure the engine thrust does not have large changes when PT data is not valid. Engine thrust can be less than normal or engine exceedances can occur if outside air conditions change while the EEC is in soft alternate mode. This occurs because the EEC estimates Mach number using TAT, standard day temperature, and the last valid value of delta air temperature from standard day. Delta temperature from standard day is usually calculated from standard day temperature and static air temperature. While in normal mode, static air temperature is calculated from TAT and Mach number. Because Mach number is not available in the soft alternate mode, the EEC uses the last valid value for delta air temperature from standard day. This estimate is accurate only if the outside air stays the same.The EEC goes to the hard alternate or reversionary mode when these conditions occur:

  • EEC is in the soft alternate mode for more than 15 seconds (ALTN light on) and the thrust lever is less than 19 degrees above the idle stop
  • EEC switch is in the OFF position.

NOTE: If one EEC is in the normal mode and the other EEC is in the soft alternate mode, it can cause thrust lever stagger. When this condition occurs, the pilots must put both engine EECs in the hard alternate mode. When both EECs are in the hard alternate mode, it prevents thrust lever stagger. At low thrust levels, there is a small thrust difference between soft alternate and hard alternate modes. At higher thrust levels there can be an uncommanded large thrust change when the EEC changes from the soft alternate mode to the hard alternate mode. Large uncommanded thrust changes are not acceptable. The EEC will not change automatically from soft alternate mode to hard alternate mode if there is a large thrust change.In the hard alternate mode, the EEC uses static pressure (P0) to get an assumed Mach number. To make sure the airplane will have enough thrust for satisfactory airplane performance in all conditions, the EEC assumes the outside air temperature with the highest thrust requirement. In this mode, large maximum thrust rating exceedances are possible during hot day conditions. This can cause EGT exceedances during hot day conditions.The EEC returns to the normal mode from the hard alternate mode and the ALTN light goes off if one of these occur:

  • EEC switch goes from the OFF to ON position and PT is valid
  • The engine is shutdown and restarted, PT is valid and the EEC switch is in ON position.


Regards

Andrea Daviero

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