This is the internet so the only thing we know about you is what you tell us. You have told us that you either dont read or dont respect the forum rules (all posts require your full name). You have told us that you are not familiar with, or do not respect, standard operating procedures (If you always fly with clear skies, then you never need AI). You have told us that you had read the small print (but only after it was pointed out to you).

 

So, lets assume you do know that LVL-CHNG works by using pitch to maintain IAS at target speed. So what will happen if your IAS is above target? As yours is according to your screen shot?

 

No words! :rolleyes:

What's your N2% while all this is going on?

I see 500 fpm through 4000 at 220 knots clean in LVL CHG with TAI on. I don't think this is out of the realm of being realistic - you have no drag at all on the airplane and the engines have to come up to account for the TAI - 37% or so is correct for the N1 according to the Boeing data we have. This is always going to vary based on the actual atmospheric conditions though. (here it's nearly 38% for instance) This is *not* approach idle - I'm 100% sure of that - there are very specific places approach idle activates in the code and this is not one of them.

 

Also if you go from no TAI on to TAI on, the engine thrust comes up and causes the speed to get above the LVL CHG target slightly - this causes the pitch to come up to compensate for it until it gets back on target - at that point it resumes descending at the normal rate.

 

Hi Ryan, the idles are 3, ground, flight and approach.

Approach idle is triggered by either gears, flaps, or antiice.

Just in memory it is working fine (I cannot confirm the v/s rates) on the ngx

Yeah, with the NG there are three idles - I'm sure Ryan doesn't need being told. Approach idle is selected if, while in flight, the flaps are configured for landing or the anti-ice is on for either engine. Whether or not the NGX is coded in such a way approach idle is selected with any engine anti-ice on - it is supposed to according to the manuals.

Yeah, with the NG there are three idles - I'm sure Ryan doesn't need being told. Approach idle is selected if, while in flight, the flaps are configured for landing or the anti-ice is on for either engine. Whether or not the NGX is coded in such a way approach idle is selected with any engine anti-ice on - it is supposed to according to the manuals.

He knows, but probably he forgot that, too much work on the triple seven can do that

He knows, but probably he forgot that, too much work on the triple seven can do that

 

Haha, the T7 works in pretty much the same way. It has the same three idles, just the criteria for approach idle are slightly different. It'll still go into approach idle if the engine anti-ice is in use, also if the flaps are set to 25 or above, if the opposite engine bleed air valve is closed and if one hydraulic air-driven pump is inoperative and the flaps aren't in the up position - they'll all individually result in the EEC selecting approach idle.

Hi Ryan, the idles are 3, ground, flight and approach.

Approach idle is triggered by either gears, flaps, or antiice.

Just in memory it is working fine (I cannot confirm the v/s rates) on the ngx

 

Really? Well I learn something new every day.

 

I thought there is only one idle. I thought though the idle speed though is different depending on what the pressure and temperature sensors pick up due to the fact that pressure and temperature affect the mass air flow into the compressor. Then as a result the computer will schedule more or less fuel in relation to mass airflow to reduce the risk of compressor stall/surge and flameout blah blah etc...

 

Honestly, I'm not being sarcastic but I thought the above description was how a turbojet works and didn't realise that aircraft configuration had much to do with it.

Really? Well I learn something new every day.

 

I thought there is only one idle. I thought though the idle speed though is different depending on what the pressure and temperature sensors pick up due to the fact that pressure and temperature affect the mass air flow into the compressor. Then as a result the computer will schedule more or less fuel in relation to mass airflow to reduce the risk of compressor stall/surge and flameout blah blah etc...

 

Honestly, I'm not being sarcastic but I thought the above description was how a turbojet works and didn't realise that aircraft configuration had much to do with it.

 

The reason there's two different idles when in flight (both flight and approach idle) is because in the event of a go around you obviously want to accelerate very quickly. If you notice, in the NGX, it pretty much takes as long to go from ground idle (of ~20% N1) to 40% N1 as it does to go from 40% N1 to ~95% N1. So if there was only one idle, say ground idle, then initiating go arounds would take twice as long. The reason for the difference between ground idle and normal flight idle I believe can be for various reasons depending on the engine. Some aircraft are ridiculously slow at spooling up at lower N1s, so on some engines the difference between ground idle and flight idle might be just a few % on the N1, but the difference in spool up time can be 50%. Some consideration might be made for the fact you need the engines to provide sufficient bleed air for pressurisation when at higher altitudes, so in a descent, if the engines were to go all the way down to ground idle there may not be enough bleed air.

Some aircraft I know have low speed ground idle and flight idle...some switchable by buttons and that now-a-days, flight idle is called approach idle, the speed which is set by the FADEC corresponding to atmospheric conditions.

 

The low speed ground was just so save power and reduce thrust on taxi/on the ground and flight idle was for flight, and there are specifics put in place that will not allow you to be able to select ground idle while there is no weight on wheels ie aircraft in flight. This is why I said one idle because I thought we were talking about flight only.

 

As far as the bleed air for pressurisation is concerned, aircraft pressurisation control is achieved by regulating the amount of air leaving the cabin. Bleed air is regulated from the engines to a much lower value for the air-con system since 60 psi of air blowing in the passenger's face might be a bit too much So I believe that low engine speeds will not affect the pressurisation system as many aircraft can be pressurised to maximum differential by use of just the apu alone.

 

I think thee is some regulation which also says that on approach, engines should be able to go from approach speed/idle not sure what it's called in 6 seconds, which the controlled by the fadec and also slightly overfuelling the engine to be able react to the need for the acceleration.

 

I've just spouted a load of stuff from the top of my head....now I've forgotten what the subject was!

 

Time for a cuppa me thinks!

 

Edit:-

Head's back on track now. I think at some time, I will have to look in to this idle stuff, as the many different idles is news to me or is it we're just calling the same thing different names?

The 737 classic only uses 2 idles.

However the NGs are a bit different.

The ground idle is also not costant, will change from the starting to a "warmer" condition expecially at low temperatures.

What it is important to know is that IDLE control is a N2 parameter, so the engine computer EEC will give an N2 target, N1 is a derivate value.

So, We must speak of apprach idle of about 70% of N2(memory, I must check values if needed, but they were already discussed in the past) N2, not 36% of N1.

But, as the major part of the thrust is given by the FAN, the N1 number is the most discussed and viewed. It is a common mistake (but sometimes it is better when we talk) to use N1 values.

Not getting off topic, but I just flew a miseed approach and holding pattern for a while , and I noticed that the FMC commanded a speed of 223 (not an economical speed rather low thrust to burn the least fuel , since youre not going anywhere , you're holding) . Anyway at this speed at 3000 ft altitude, the thrust settled at about 42% N1. Since the thrust is going to be around 39-41% with TAI on (so as to keep bleed pressures up) that doesn't leave you that much headroom to descend, you need to have a higher forward speed to have a higher angle of descent. So observing this I would suspect that PMDG's thrust model is correct and these things are slippery at low altitudes after all.

Eric W.