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Clarify For Me Please,  
User currently offlineMirrodie From United States of America, joined Apr 2000, 7443 posts, RR: 62
Posted (13 years 4 months 2 weeks 8 hours ago) and read 1269 times:
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Well, after reading a pretty good book about flying basics and watching a JUST PLANES video, I have a few more questions.

As I understand, when you start descent ~20 mins from touchdown, you reduce thrust right? Now here's where I lose it...at some point, they say you reduce all the way to idle thrust. Meaning you are, in all practical terms, "coasting" like in a car? But does that mean that with idle thrust, the fan still turn as do the turbines, but no fuel is going into it??

So then why is it that as I watch those jets landing on the video, right before touchdown, I still "see thrust" coming from the engines right before the thrust reverser is set. By that, I mean I see the distortion on the air suggestive of thrust. Take a look just aft the engine in this picture:

photoid:67106

Please educate me!


Forum moderator 2001-2010; He's a pedantic, pontificating, pretentious bastard, a belligerent old fart, a worthless st
17 replies: All unread, jump to last
 
User currently offlineMirrodie From United States of America, joined Apr 2000, 7443 posts, RR: 62
Reply 1, posted (13 years 4 months 2 weeks 8 hours ago) and read 1156 times:
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Click for large version
Click here for full size photo!

Photo © Allan Rossmore



hope it worked that time



Forum moderator 2001-2010; He's a pedantic, pontificating, pretentious bastard, a belligerent old fart, a worthless st
User currently offlineJT-8D From United States of America, joined Dec 2000, 423 posts, RR: 3
Reply 2, posted (13 years 4 months 2 weeks 7 hours ago) and read 1148 times:

The engines do not shut down at any time during the flight. They do however go to a power setting known as Flight Idle. On some planes, this can be different power settings depending if the landing gear and/or flaps are down. On others it is simply a question of if the plane is in the air or on the ground. By the way, idle thrust is set long before 20 mins before landing, depending on the plane, the weight, and the length of the flight, it could be an hour or more. Usually it is set at what is known at tod, or top of decent. The auto pilot, auto throttles, Flight mgt computer, or performance mgt computer will decide tod based on inputs from the crew before the flight leaves the ground. Hope that helps..

User currently offlineWindshear From Denmark, joined Mar 2000, 2330 posts, RR: 11
Reply 3, posted (13 years 4 months 2 weeks 7 hours ago) and read 1138 times:

Idle thrust is simply, roughly speaking, the lowest amount of thrust, how low back you can move the throttles.
The other thing is cut off, where you remove fuel flow to the engines, and therefor shutting them off, if you shut off the engines then you would stall before you could reach the airport, you also need thrust for taxiing to the gate right?!!

another word for idle thrust, is to retard them, meaning minimum thrust about 25% or more N1 RPM.

The modern airports STAR (Standart insTrument ARrival) are complicated and filled with alt. and spd. ristrictions, even holdings are common, and for that you need, yes, thrust.
The fianl approach is not at an angel at which you could fly with out engine power either.
So shutting down the engines on descent wouldn't be smart Smile/happy/getting dizzy

Hope I could shead some light on it all...



"If you believe breaking is possible, believe in fixing also"-Rebbe Nachman
User currently offlineFDXmech From United States of America, joined Mar 2000, 3251 posts, RR: 34
Reply 4, posted (13 years 4 months 2 weeks 7 hours ago) and read 1128 times:

I hate to use a car/airplane analogy but picture driving down a long hill. Normally you take your foot off the gas pedal and the car coasts down the hill essentially maintaining its speed. The car engine isn't turned off, its just not contributing to (in any meaningful measure) the forward momentum of the car because its just.......idling.


You're only as good as your last departure.
User currently offlinePanman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 5, posted (13 years 4 months 2 weeks 7 hours ago) and read 1129 times:

At idle the engines are using the minimal amount of fuel - not no fuel. Thrust is still being produced but it's not enough to counteract the effects of weight and drag that are bringing the aircraft down.

That distortion of the air is also heat that you are seeing.

PanMan


User currently offlineMirrodie From United States of America, joined Apr 2000, 7443 posts, RR: 62
Reply 6, posted (13 years 4 months 1 week 6 days 6 hours ago) and read 1079 times:
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I appreciate all your help. FDXmech, I kind of understand your most.
From what you have all said, it's as if the "IDLE THRUST" is a state of stability where the the engines are not off but not accelerating either and Its all a function of drag, slats, flaps, etc, right?? If that's a decent explanation, then I got it. Thanks.
 Big thumbs up



Forum moderator 2001-2010; He's a pedantic, pontificating, pretentious bastard, a belligerent old fart, a worthless st
User currently offlineMonocleman From United States of America, joined Apr 2001, 137 posts, RR: 0
Reply 7, posted (13 years 4 months 1 week 4 days 23 hours ago) and read 1059 times:

Uh, did anyone else realize that the pic given is a TAKEOFF?! The pic caption even says it. Maybe you should look next time, Mirrodie....
-Will


User currently offlineGyro From , joined Dec 1969, posts, RR:
Reply 8, posted (13 years 4 months 1 week 4 days 22 hours ago) and read 1044 times:

To further add to the plane/car explanation... Your car in idle still has up to 1,000 RPM's!!! This is enough to give power to your radio, air conditioning, lights etc...But it's not enough to move forward (unless you play with the clutch of course...).

User currently offlineWindshear From Denmark, joined Mar 2000, 2330 posts, RR: 11
Reply 9, posted (13 years 4 months 1 week 4 days 16 hours ago) and read 1034 times:

I too noticed that it was a plane at take off, but please give him a brake, he's obviously a beginner, in the field and what's the problem in that?!!

I think it's great that he asks instead of just keep wondering...

Wouldn't you think?



"If you believe breaking is possible, believe in fixing also"-Rebbe Nachman
User currently offlineMonocleman From United States of America, joined Apr 2001, 137 posts, RR: 0
Reply 10, posted (13 years 4 months 1 week 4 days 1 hour ago) and read 1013 times:

You're right, Wind... Sorry. His question was perfectly viable.
-Will


User currently offlineSpeedbird002 From Canada, joined May 2006, 0 posts, RR: 0
Reply 11, posted (13 years 4 months 1 week 3 days 13 hours ago) and read 1005 times:

Could someone please confirm or correct what I am saying, but I thought that modern jets would switch to idle power when they began their descent and then apply some power on final approach. I'm not quite sure if its during final approach, but I was pretty sure that they reapply power at some point before touch down.
As a passenger I have also noticed the engines spooling up as the aircraft approached touch down.
I assumed that as the plane reaches a landing flap configuration, the extra drag had to be counteracted by thrust. I assume this also helps with in case of go-around as less spooling up time is needed.
Am I correct in my assumption?


User currently offlineMirrodie From United States of America, joined Apr 2000, 7443 posts, RR: 62
Reply 12, posted (13 years 4 months 1 week 3 days 10 hours ago) and read 997 times:
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Will, Boaz, check my profile  Smile/happy/getting dizzy Will, even you are newer than me. But hey, it's all good. You're from my old hometown! I lived b/w JFK and LGA. I bet you love it where you live!
I just used that picture to illustrate the air distortion I was talking about and that picture shows it well
(even though it's at takeoff!).
if you are ever up for spottin, email me.
have a good weekend and thanks for the "heads up" on the photo.  Wink/being sarcastic





Forum moderator 2001-2010; He's a pedantic, pontificating, pretentious bastard, a belligerent old fart, a worthless st
User currently offlineJT-8D From United States of America, joined Dec 2000, 423 posts, RR: 3
Reply 13, posted (13 years 4 months 1 week 3 days 9 hours ago) and read 1004 times:

Speedbird, you are correct that idle thrust is set long before landing. On most planes, There is Take off thrust, Go-around thrust, mct(max continuous thrust), cruise, Flight idle, and idle. Like I mentioned before, some planes will compensate for the extra drag automatically, by changing the idle speed as flaps and gear are lowered. If the auto throttle is used, power will be adjusted to maintain a certain airspeed, as set by the crew. This may have to be adjusted to meet ATC commands to stay clear of other A/C. As far as spool up time is concerned, it is an important factor. Idle speeds on our older 737s where raised several years ago, just to add a safety factor. Im not sure how technical you want to get, but if I havnt helped yet(see my previous post) please ask..JT

User currently offlineSpeedbird002 From Canada, joined May 2006, 0 posts, RR: 0
Reply 14, posted (13 years 4 months 1 week 3 days 7 hours ago) and read 992 times:

Yes, thank you JT-8D. Your first post is clear enough, I just wanted to make sure my assumptions were correct!

User currently offlineMirrodie From United States of America, joined Apr 2000, 7443 posts, RR: 62
Reply 15, posted (13 years 4 months 1 week 3 days 7 hours ago) and read 989 times:
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Thank you all for explaining that Ilde thrust isn't just some random number but is a function of many variables.

You guys (ALL OF YOU) are all real awesome. Thanks for the info.

have a good weekend



Forum moderator 2001-2010; He's a pedantic, pontificating, pretentious bastard, a belligerent old fart, a worthless st
User currently offlineMusang From United Kingdom, joined Apr 2001, 864 posts, RR: 7
Reply 16, posted (13 years 4 months 1 week 3 days 6 hours ago) and read 1002 times:

Mirrodie - thought you might like some actual examples. I drive Avro RJ 100s. As in most jets, the most fuel efficient way to descend is to spend as long as poss. at cruise level, then reduce power to flight idle.

We have a basic rule of thumb for calculating where to chop the power - Multiply the number of thousands of feet to lose by 3, then correct the resulting number of nautical miles for :-

(1)weight (lighter aircraft, greater descent rate - I took a while getting my head round that too! Its opposite to what you would expect),

(2) head or tailwind during the decsent, and

(3) whether or not your using the airframe de/anti-icing kit. This saps power so the engines are automatically spooled up a bit to compensate, and it won't descend so rapidly.

Typically one has to slow to 250 knots by 10,000 ft so we add 5 miles for that.

So to run an example - a 38 tonne aircraft with a 60 knot tailwind, icing kit required on for 5000 ft of the descent, cruising at 33,000 ft, to get down to pattern altitude of 3,000 ft.

30,000 ft to lose = a starting figure of 90 miles.

2 tonnes lighter than the reference figure of 40, so subtract 2 miles per tonne = -4 miles (its lighter so requires less distance to descend due to the higher rate)

Take 2/3 of the tailwind component, 40, add a mile per 10 kts = +4 miles.

The deceleration = +5 miles.

1 mile per each thousand feet the icing kit is on = +5 miles.

So 90-4+4+5+5= 100 miles out, about 20 minutes, which is a remarkably consistent figure for all types of jet transport.

As we get closer to the field ATC restrictions start messing with our planning, but the descent progress is monitored all the way down and if we're too high, add some airbrake, too low, add some thrust. In either case the planning wasn't quite right, but there are so many variables affecting the descent, its more a black art than a science.

Sure, in some types the vertical nav mode of the Flight Management Computer works out the TOD, top of descent point, but they work on a 3 degree slope, and don't use idle thrust. They therefore compensate for all the abovementioned variables by changing thrust automatically, and the bottom of descent point is more predictable. Avro RJs and to my knowledge 146s don't have VNAV in the sense that modern types do.

Ground idle in our RJ 100s is approx 25% N1 (Fan Speed, representative of thrust). Flight idle could be up to about 50% N1 at high levels and this figure also varies for a number of reasons as mentioned in earlier posts. When de/anti-ice is selected the min. speed of the engine's core is increased to ensure it copes with the air bleed demand, but there is no automatic compensation on this type for flaps/gear. Small engines need less time to spool up in case its necessary.

On final approach with flap 33 (max) and gear down, the thrust will be around 60%, i.e. typically from about three miles out or wherever we get into the landing configuration. This is reduced to idle at about 50 feet.

So the objective, and apologies if this sounds familiar, I mentioned it in a reply to someone else's topic recently, is to go to flight idle at cruise level and not have to add any power until final approach. It rarely happens, but is very satisfying if it does. Its called a CDA, continuous descent approach, and is beneficial in terms of noise for residents near airports. The less time spent flying level in the terminal area, i.e. with thrust on, the better.

So there you go. Been to Australia yet?

Regards - Musang.


User currently offlineGE From Singapore, joined Mar 2000, 320 posts, RR: 6
Reply 17, posted (13 years 4 months 1 week 1 day 16 hours ago) and read 959 times:

 Smile/happy/getting dizzy Thanks for the excellent description, Musang!  Smile/happy/getting dizzy


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