Engine response time

[Piepal 1]

Hi all, I am Peter, a new member of this forum. Can I have your opinion on thrust/engine response time? I think that PMDG isn't so close to real so this can be a new upgrade idea... 

 

[JRBarrett 3,035]

What do you think is incorrect about engine response times?  Too fast? Too slow? In flight? On the ground? 

Most importantly, (and I know that Kyle from PMDG would ask this first), what real-world data are you using to determine that the engine response in the sim isn't correct? 

[Piepal 1]

Hi, thank you for your reply. I think pmdg 737-800 (sp. d) ngx is very fast in gaining/loosing rpm (v1 values) after moving throttles. I read many post regarding throttle of 737-800. If during approach I have 5-6 kts over I can pull back to idle my throttle loosing fast engine and knots. At my speed I set throttle at 50-60 of n1 (circa obviously) and engine mantain suddenly that speed. 

On ground if I sett full thrust in 1 second to verify t/o config horn my aircraft gain suddenly 3/4 kts so I have to perform this challenge with max brakes. 

[scandinavian13 5,192]

Hi Peter,

Welcome to the forum. Full names - first and last - are required in your posts.

As Jim mentioned, when it comes to things like this, we don't go off of simple notional "it doesn't feel right," and instead absolutely require hard data to go by. Even our Tech Team (essentially a beta team of people who have verified, logged hours on the aircraft we've simulated) provides some form of explanation of what is right by using very specific details, or (usually) a video showing their actions and the results.

It's usually best to provide this sort of evidence before accusing a developer of getting something incorrect, and asking that it's an idea of something that needs to be fixed.

[Piepal 1]

Hi scandinavian, thank you, no I won't accuse anyone at all, I know pmdg are really the best developers. In fact my post have the question in mind. I don't have raw data at all neither videos. I will try the real 738 simulator on april so I need that my tech skill are correct and supported by real behaviour of aircraft. If pmdg says that thrust are correct I believe and I say thank you one more time. I'm so sorry but my tread were misunderstood. And for clarify I say again that is my impression of fast response but I don't know real a/c.

In many post real pilot say that from idle to 40 n1 engines need near 8 seconds and same if you pull back in idle for approach and set again for 50 n1 so it is'nt a good thing to do during an approach for example. Again this is what I think, no accuse. This is only data I have. Maybe my settings are wrong... So please help me to understand. 

If I offended anyone I absolutely apologize and please feel free to tell me so I delete this tread (or if you want you please do yourself) 

Pietro

[Guest]

Hi Pietro,

I wouldn't worry too much, the PMDG NGX really is an incredibly accurate product and maximum kudos to the guys who make this program. 

FNTP Level II and FTD Training devices (these are fixed base sims which cost in the region of £1.2 million) don't have identical engine response times to the real aircraft, sometimes don't match the real thing and the spool up time is similar to what you get with NGX but for training purposes like MCC training and the initial part of a type rating which doesn't require motion it's as real as it gets! 

Unless you've got +£10 million for a Level D sim, a massive hanger, an empty room for a bank of computers and a team of engineers you'll never fully match the real thing and even these simulators have their own quirks! (CTRL, ALT, DEL doesn't work....  )

I'm a complete novice when it comes to computer programming, I leave that to the clever guys who make these brilliant programs! Whatever they use to program the NGX is darn impressive and it's why I use it "unofficially" to practice real life situations before teaching it in the real thing. 

Best of luck!

 

 

[Chock 17,769]

Hi Peter, welcome to Avsim.

To explain a bit about engine thrust and spool times....

It can take a long time for a gas turbine engine to spool up, in fact, that is one of the reasons that you will rarely see a pilot shove the throttles all the way forward from idle if taking off (except in bad movies). Instead, in real life they will push them up a bit, make sure they are both accelerating the same, then push them the rest of the way. They don't want one engine to get a surge or something and the other one to spool normally, because if that happened, they'd get aysmmetrical thrust and would not be going fast enough to be able to counteract that with the rudder.

Another problem with gas turbine engines, is that, as with all engines, their efficiency is affected by the ambient temperature (this is what all that derated thrust take off stuff is about which you see in the FMC).

If you have a car or a motorcycle, you will find that it will probably be a tiny bit faster on acceleration and top speed on a colder day, and it is often the same with aeroplanes and especially ones with gas turbine engines. Basically the problem is this; when the air is warmer, it is less dense, so there is less air available to mix with the fuel to help it work efficiently on a hotter day. This can be compounded by an airport's height above sea level too, for example, Mexico City International Airport is well over 7,000 feet above sea level, so on a hot day when the air is thinner (and it is already thin owing to the increased altitude at somewhere like Mexico City) that can severely affect how a jet engine works, not only that, there will be less lift available to the wings because of that thin air, to the extent that some aircraft might not even be able to take off from somewhere like there on a very hot day (helicopters with a gas turbine engine, such as the Bell 206 JetRanger, would probably have a hard time there on a very hot day). This is why Mexico City has very long runways - both are nearly 13,000 feet long, which is around 3,000 feet longer than most other big airports which aren't up at 7,000 above sea level!

At most airports it's not a really big deal, but it will still have an affect on how the engine works unless you are at an airport which is right at sea level and the QNH is exactly 1013Mb, in which case they will behave exactly 'by the book'. So even at somewhere like Manchester Airport in the UK, which isn't exactly noted for having blisteringly hot days lol, you are still about 250 feet above sea level when on the runway, so even that has some effect on the engine performance.

Now you'd think that ramming the throttles all the way to the stops would be a good idea if taking off from somewhere like Mexico City, but what would actually happen if you did that, is that the engine will be getting too much fuel and not enough air, so it wouldn't develop thrust in the most efficient way possible. So although it might not seem to make sense on the face of it, it is actually more efficient for an engine in those circumstances to accelerate a bit more slowly, to ensure enough air is coming into it to mix properly, otherwise it could be swamped with fuel. The reverse can happen too with jet engines in some weather conditions, where there might be too much air coming in for the fuel. This is why putting all the correct temperature and airport data into the FMC matters with realistically simulated aeroplanes such as PMDG's 737 NG, and it is also why you might see the engines spool up slowly if the FMC is managing thrust, because it will be taking the ambient temperature into account as well as the speed of the oncoming air as you go down the runway, etc, etc.

Anyway, if you want exact numbers, I guess you can check it, so here they are: On a average day (where the pressure is at about 1013Mb or 29.92Hg on the ground), a CFM 56 engine (depending a bit on which exact model of engine it is) will take about four seconds to go from flight idle (which is 29 percent of N1), up to 67 percent of N1, then one more second to go to 86 percent of N1, then just over another second to go to 100 percent. this will depend somewhat on your airspeed too, since high airspeeds ram a lot more air into the engine, so the engine has to deal with that too.

So it will take that engine about six seconds to go to full throttle from flight idle, on an average day, but possibly more or less time depending on the weather and the settings you put into the FMC for things like the departure airport, runway length, weight etc, etc, which is why you won't get V-Speeds displaying on a jet airliner's PFD unless you put all that stuff in, because the FMC needs to know all of that stuff to be able to calculate those speeds.

Airlines also consider other things too where engine thrust is concerned, since they want to save money (which is what all that Cost Index stuff in the FMC is about). So, a longer take off run at a reduced thrust may put less wear on the engine, and even things like how hot the engine gets and how much is it is cooled down by the fuel going into it before it gets ignited can have a (tiny) effect on how much wear there is on an engine. But of course if you use less thrust and do a slightly longer take off run, you might save a bit of wear on the engines, but then put more wear on the tires because they are rolling along the ground more. But airlines all want to save money on the cost of running their jets, so they have to think about that stuff, they even tell pilots to put the wheels down later on final approach these days so that there is less thrust required to maintain speed when still a few miles from touchdown.

[achutchison 26]

I've noticed that the higher n1 target you set, the harder the engine tries to get there. That's not the case in the jet aircraft I'm familiar with but I don't know if this is the case for the 737

[Guest]

15 hours ago, achutchison said:

I've noticed that the higher n1 target you set, the harder the engine tries to get there. That's not the case in the jet aircraft I'm familiar with but I don't know if this is the case for the 737

One of the biggest factors in spool up time is your density altitude. The higher you fly the more thrust limited you become. When you're cruising above your FMC OPT and near your FMC MAX Flight Level you are typically quite close to your N1 Limit thrust setting (this is where Turbofan jet engines are at their most efficient). There is very little residual thrust to accelerate the aircraft.

During the 737 Type Rating we demonstrate slow engine acceleration at high altitude. We close the thrust levers and wait for the thrust to reach flight idle. As soon as we reach flight idle we set the N1 limit for CRZ thrust and start timing. 

It typically takes 15-20s for the engine to accelerate and we lose a significant amount of speed and it takes a VERY long time for the speed to recover and in some instances you're forced to descend. That's why we teach partial speedbrake deployment in the event of an overspeed instead of being tempted to disconnect the autothrottle and close the thrust levers! 

The engines at lower altitude are a lot more responsive at higher thrust settings. Initial spool up when setting 40% prior to take off is 5-6 seconds. I'd say 40% to the N1 limit takes less than 2 seconds.

One of the reasons we use flaps for landing (apart from allowing us to fly slower) is that we have to have more thrust on final approach due to the increased drag with F30 and F40. Whilst it might not be good for jolly old Polar Bear and the people living near an airport it means in the event of a go-around we can set GA Thrust a lot quicker!

On 2/28/2017 at 8:32 PM, Chock said:

But airlines all want to save money on the cost of running their jets, so they have to think about that stuff, they even tell pilots to put the wheels down later on final approach these days so that there is less thrust required to maintain speed when still a few miles from touchdown.

That's an interesting practice! Not one we teach for sure! 

[vololiberista 205]

20 minutes ago, flightdeck2sim said:

On 28/02/2017 at 9:32 PM, Chock said:

But airlines all want to save money on the cost of running their jets, so they have to think about that stuff, they even tell pilots to put the wheels down later on final approach these days so that there is less thrust required to maintain speed when still a few miles from touchdown.

That's an interesting practice! Not one we teach for sure! 

In the days when aircraft were aircraft (see avatar top left!) the gear would come down at the start of the glideslope with flaps2 i.e. 20%. Then at about 6DME full flaps. The 10 would be slightly nose up during the whole approach leading to a very slight pull to flare at touch down. Otherwise a long float down the runway would ensue. Many Captains actually flew the 10 onto the deck aka aircraft carrier style to avoid floating.

 

 

[Chock 17,769]

37 minutes ago, flightdeck2sim said:

That's an interesting practice! Not one we teach for sure! 

Usually the low budget carriers.

[Guest]

2 minutes ago, vololiberista said:

In the days when aircraft were aircraft (see avatar top left!) the gear would come down at the start of the glideslope with flaps2 i.e. 20%. Then at about 6DME full flaps. The 10 would be slightly nose up during the whole approach leading to a very slight pull to flare at touch down. Otherwise a long float down the runway would ensue. Many Captains actually flew the 10 onto the deck aka aircraft carrier style to avoid floating.

 

 

Haha what a beast! 

Certainly this century the priority has shifted to efficiency and noise reduction. I can only vouch for Europe but certainly most major airports monitor CDA compliance. Looking to the future, with noise and environmental pollution such an issue and the huge increase of GPS based approaches, we might see airports adopt steeper approaches to further lower noise.

Aviation authorities are under pressure to reduce pollution and the aviation industry is slowly being pushed into the 21st century. (We still have NDB approaches at some of our destinations!). Modern engines like the 737 don't have massive spool up issues unlike that old school VC10! It probably took 15 years for those bad boys to spool up!!

 

[Guest]

2 minutes ago, Chock said:

Usually the low budget carriers.

Interesting! I hope it's one we don't adopt soon, some of our destinations have runways less than 1800m! If we don't land in the touchdown zone it's TO/GA, GA F15 set GA thrust and have another go! 

[achutchison 26]

@flightdeck2sim I wasn't talking about at different altitudes. If i'm on the ground and i set a target n1 of 30% then the engine will take it's sweet time getting there, but if I need thrust and I need it now, ill set a target of 60% and the engine accelerates to 30% n1 much quicker whereupon I retard the target to 30% in order to maintain the thrust I want. As far as I know this shouldn't happen.

[scandinavian13 5,192]

2 minutes ago, achutchison said:

As far as I know this shouldn't happen.

Based on operational experience, I assume?

 

I'm not seeing the logic in the statement, either. If you're looking for 30% and you set it, the controller will increase thrust to that value such that it will not accel past that value. If you're looking for 30% by setting 60% and retarding the original setting, then of course you'll end up at 30% faster, as the controller is aiming a lot higher, and is not worrying about overshooting 30% - it's worried about 60%. I'm not sure of the specific curve, either, but I highly doubt it's very aggressive through 30%. There's little to no operational reason that it should be.

We're dealing with a jet engine on a controller here. Not a piston banger, or a supercharged engine with nothing but cables to the throttle lever.