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# tailwaind approach question

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Hi Captains,

I've a question that I can't solve in my mind...I've tried to use the "search" button but I couldn't find anything about it anyway I do my apologizes if I've lost same topics regards it...

Here it's the question:

1) let's assume you're performing a normal -3° slope ILS approach flaps 30 in a headwind condition at normal Vref+5 airspeed in such a case you'll obtain with your NGX a normal pitch-N1% value that matches with Boeing 737 manual (i.e. what stated into "unreliable airspeed" speed/pitch figure at 1500 ft gear down 3° glideslope (also if that table is calculated for a Vref+10 condition to be exact)) let's assume ZFW 55 tons+3 tons of fuel i.e. 54% N1 and 2.0 pitch..

2) now repeat that approach but this time with a slight tailwind condtion (i.e. runway 360, wind 180/5) not important if you're using ASN or FSX weather engine...normal condition Vref+5 (in B738 FCTM infact is stated: "do not apply wind correction for tail winds . Set command speed at VREF+5 knots (autothrottle engaged or disengaged)" page 50.)

But when this happens with NGX I got N1=45% !

Now, of course that having a slight higher groundspeed (i.e. 145 kts instead of 140) I would have also a slight higher descent rate (i.e. in a 3 degrees glideslope 766 ft/min instead that 740) and that climb or descent rate is given by a formula like (Vertical speed)=(T-D)/W*(true airspeed) where T=thrust developped by the engines, D=airplane total drag, W=aircraft weight but a so slight difference in vertical speed couldn't IMHO justify a so big difference in engine thrust rating (45%N1 instead of 54%), also because if airspeed is the same also lift and drag coefficients and so total drag (at the same weight of course) will be the same... that's a quite big different thrust developped indeed! Only in case of passing throught the updraft front of a whindshear front I could have a so lower thrust setting...could that be a small bug of NGX ? (In fact in that fixed base B738 sim I've never seen a so big difference in N1% when wind change direction), or am I missing something in my ideas?

Best Regards

Ciao

Andrea Buono

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For a higher vertical speed, with the same weight and drag, you need less thrust. Based on your figures total thrust would be about 1040 N less with the tailwind (520 N per engine), not enough for 9% N1 difference. It's probably an FSX issue rather than the NGX. Try the same comparison in the default 737-800.

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Be carefull, there is not a linear relationship between %N1 and thrust.  If you had a EPR measurement I'm sure things would make more sense.

Dan Downs KCRP

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Are you not landing on the wrong runway with a TW

Vernon Howells

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Not if noise abatement or a host of other reasons say otherwise.

Mike Dryden

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No, I'm legal in observing the 10 kts tailwind limit anyway my question is different.

Ciao

Andrea

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Thats why i gave the wink! Take my hat off to you for going into such detail i'll see what i can learn from this!!!

Vernon Howells

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Hi,

Thank you guys!

Kevin you're right regarding too much a 9% N1 difference, but I don't think is a FSX problem 'cause as you know NGX has got is own costumized CFM56 engine model (infact is very realistic)..and another fsx based software and also that Fixed base trainer (running on FSX) haven't this big difference...

Hi Dan thanks: I know that there's no proportional relationship in any jet engine between n1 and thrust for that 9% of N1 speed difference is a too big thrust difference, regarding EPR of course but also "corrected N1%" is good to define "corrected thrust" .

Hi Vernon you great! Effectively executing a landing by applying the "golden rule" of headwind runway would solve my problem immediately!  B)

Ciao

Andrea Buono

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Kevin you're right regarding too much a 9% N1 difference, but I don't think is a FSX problem 'cause as you know NGX has got is own costumized CFM56 engine model (infact is very realistic)..and another fsx based software and also that Fixed base trainer (running on FSX) haven't this big difference...

The NGX does have accurate CFM56 indications but I'm fairly sure it uses the core of the FSX engine model.

I've just tried this myself, but I get a different result to you. At 58 tonnes Vref30 was 139 knots. However to match your speed I selected 140 knots in the MCP. Stabilised N1 was about 44.1% with the tailwind and 44.6% without. However during the zero wind approach, at around 1500 ft, when the AP did its autoland checks, N1 had increased to around 51%, but came back to 44-45% soon afterwards. Not sure why. Anyway it all looks reasonable.

I wonder whether the N1 you saw on your zero wind approach was truly stabilised. However as your Vref is different to mine something else might be set up differently.

Incidentally the default 738 was similar, less than 1% difference in N1 with and without wind.

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How important is monitoring your N1% would like to know more about it

Vernon Howells

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How important is monitoring your N1% would like to know more about it

Unusual throttle position is a warning sign for windshear. The 737 has good configuration warning software, but configuration problems could be caught by unusual N1.

Matt Cee

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Hi Kevin Thank you!

According to me anyway at 58 tons 140 kias 3° slope at 1500 ft a value of 44-45%N1 is not normal..in fact iaccording with B738 "unreliable airpspeed" (final approach 1500ft 3° glideslope flaps 30 gear down -configuration table at page 541 FCOM Volume 1°) pitch/power setting (of course there is for Vref+10 not for Vref+5) it should be pitch: about 2 degrees nose up (infact it matches ) but N1% is given: 52% at 50 tons and 56% at 60 tons so you should have been at 54-55%...on the other hand infact when on that Fixed base B738 sim i've always seen values in that range for flaps 30 (and about 5% more with flaps 40°) but I've another suspect: maybe some approach in FSX is not with a 3° glideslope but slight steeper (with using add-ons sceneries) so I want to test with another runways (the strange thing is that it seems to me that sometimes on the same runways N1 is normal some others is low...)...I'll call you back...(the fixed base trainer where I go is runs on FSX but uses a different very realistic scenario named "SIRX)

Best Regards

Andrea B.

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Hi Gentlemen,

definitively a probable steeper approach due to add-on scenery here...I've tested another approach (LIMF rwy 36 ILS app):

1st) Test: no wind ISA (no weather conditions): Weight during approach= about 59,2 tons flaps 30, Vref=141; approach speed= Vref+5=146 kts autoland (because with manual landing you've to have also manual throttle so it easier to have more "transient settings" not giving a stabilized engine setting) value measured at 1500 ft MSL (500 ft AGL in that case: I choosed 1500 ft to have a comparable value with B738 Boeing manual): about 55% N1 FF 1.16-1.14= Ok matching perfectly into Boeing Manual range of pitch/power settings for that weight!

2nd test: same situation as above but this time with a slight tailwind (180/5): in this case at 1500 ft I could read about 52,5% N1% (FF 1.13-1.12) ok with Boeing pitch/power setting range!

so it's  a glislope angle problem..some approaches I was flying are a little steeper than the real ones...

Ciao

Andrea

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Hi Kevin Thank you!

According to me anyway at 58 tons 140 kias 3° slope at 1500 ft a value of 44-45%N1 is not normal..in fact iaccording with B738 "unreliable airpspeed" (final approach 1500ft 3° glideslope flaps 30 gear down -configuration table at page 541 FCOM Volume 1°) pitch/power setting (of course there is for Vref+10 not for Vref+5) it should be pitch: about 2 degrees nose up (infact it matches ) but N1% is given: 52% at 50 tons and 56% at 60 tons so you should have been at 54-55%...on the other hand infact when on that Fixed base B738 sim i've always seen values in that range for flaps 30 (and about 5% more with flaps 40°) but I've another suspect: maybe some approach in FSX is not with a 3° glideslope but slight steeper (with using add-ons sceneries) so I want to test with another runways (the strange thing is that it seems to me that sometimes on the same runways N1 is normal some others is low...)...I'll call you back...(the fixed base trainer where I go is runs on FSX but uses a different very realistic scenario named "SIRX)

Best Regards

Andrea B.

Andrea,

The unreliable airspeed table in the QRH is for Vref+10. You flew the tailwind case at 140 knots which is too slow to correctly compare with this table at 58 tonnes. I just tried a flaps 30 approach at 60 tonnes GW (which is one of the weights in the QRH table). The FMC calculated a Vref of 142 knots for Flap 30. So I set 152 knots on the MCP.

For zero wind, N1 was 57.6% (the QRH table says 58%).

For 5 knots tailwind N1 was 57.2%. Slightly less, as is to be expected.

I don't see a problem. The NGX checks out close to the table and the tailwind effect is reasonable.

Kevin

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Hi Kevin

You right: Your test confirms that I was Flying a steeper approach when I had read those "wrong numbers".

Thank you.

Best Regards and Happy Landings.

Andrea

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