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How Much Power Used During Descent?  
User currently offlinebristolflyer From United Kingdom, joined May 2004, 2290 posts, RR: 0
Posted (1 year 1 month 4 weeks 1 day 15 hours ago) and read 4273 times:

I was playing on Flight Sim on one of the missions and the instruction was 'reduce thrust to zero' during descent. How much thrust is typically used during descent? If you're doing a simple landing (without complex routing) is it possible that the thrust could be at or near zero for most of the descent (I realize that there's often thrust used for small corrections on final)?

Thanks in advance.


Fortune favours the brave
14 replies: All unread, jump to last
 
User currently offlineN243NW From United States of America, joined Jul 2003, 1630 posts, RR: 20
Reply 1, posted (1 year 1 month 4 weeks 1 day 14 hours ago) and read 4238 times:

Sure - many (if not most) descents are done with idle thrust the majority of the way down. If you're ever flying in back and see the speedbrakes come up - that's normally because idle thrust is still too much for the rate of descent needed. Of course, the engines will still be producing a small amount of thrust at idle, but it's the lowest you can go without shutting them down.

The ideal descent for the lowest fuel burn is usually idle thrust and no speedbrakes all the way from the top of descent until configured on final, at which the thrust returns to maintain speed with the flaps and gear down. As you noted, though, this rarely goes perfectly according to plan, since ATC restrictions and other traffic makes it very difficult.



B-52s don't take off. They scare the ground away.
User currently offlinevikkyvik From United States of America, joined Jul 2003, 9910 posts, RR: 26
Reply 2, posted (1 year 1 month 4 weeks 1 day 11 hours ago) and read 4148 times:
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Quoting N243NW (Reply 1):
that's normally because idle thrust is still too much for the rate of descent needed.

That wording seems a bit confusing. You can always get a higher descent rate by pointing the nose down at a greater angle. Problem is you may overspeed the aircraft, or bust through whatever speed restriction ATC has given. In a lot of cases, descent + deceleration = speedbrakes necessary.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineGoBoeing From United States of America, joined Jun 2000, 2693 posts, RR: 15
Reply 3, posted (1 year 1 month 4 weeks 1 day 11 hours ago) and read 4141 times:

Bristolflyer for the plane I fly for example, we attempt to plan the descent at idle from cruise altitude on down to the bottom of the arrival we are flying. As in, generally there is a fix about 30-40 miles from the airport that we can expect to cross around 10,000 to 15,000 feet. Idle to that point. After that, it's going to depend on the air traffic configuration etc.

In the E-170/175, in calm wind I'll use 3.7-4.0 degrees of descent path to have an idle descent of around 270-300 KIAS. From there we adjust for any headwind or tailwind we may have.


User currently offlinewoodreau From United States of America, joined Sep 2001, 1032 posts, RR: 6
Reply 4, posted (1 year 1 month 4 weeks 1 day 11 hours ago) and read 4140 times:

As you fly and gain experience flying you get to know what your airplane is capable of doing.

For instance I normally plan an idle descent but also flying a certain speed. In absence of any speed restrictions by ATC, I'm' shooting for a 3500-4000 fpm descent at idle thrust which will give me a descent just a few knots shy of 330kts.

If ATC wants me to fly 280kts then I adjust my descent to around 2800-3000fpm to give me the idle descent at 280kts
Or 2000fpm to give me an idle descent at 250kts.

But usually I'll be able to descend from TOD at idle at 3500-4000fpm then decelerate from 330 to 250kts between 11000 and 10000 ft and continue the descent at 2000fpm at 250kts at idle until its time to start throwing out flaps and gear, etc.



Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.
User currently offlinefrancoflier From France, joined Oct 2001, 3741 posts, RR: 11
Reply 5, posted (1 year 1 month 4 weeks 1 day 11 hours ago) and read 4133 times:

Idle descent is the most economical way of descending. The other factor is the speed at which you're descending. Go faster and you'll descend faster, but it will be less economical. The flight computer is usually programmed with an econ descent speed, which the pilot may choose to use (or not) when programming his descent.

Setting up the descent in the FMC requires entering the crew's best guess as far as the runway in use, the STAR to be flown with its speed restrictions, modified according to local knowledge. Most ATCs will clear you for a STAR but then start vectoring you around to final, which, depending on traffic, might mean more or less track miles than planned. Given all that data, the FMC computes a top of descent point that theoretically allows for an idle descent all the way to the glideslope intercept, if possible.

That is rarely the case, however, and flight spoilers often end up having to be used to adjust to actual descent clearance.

There is increasing focus from ATC and airlines on trying to achieve idle descents all the way to the runway, as a way to increase fuel economy for airlines and to decrease the noise footprint for airports. In reality, it is quite difficult to apply, especially in high traffic airports. LHR, for instance, despite its best effort and stellar ATC management, simply has too much traffic to permit efficient descent profiles. You invariably end up in a hold somewhere, wasting fuel.



Looks like I picked the wrong week to quit posting...
User currently offlineN243NW From United States of America, joined Jul 2003, 1630 posts, RR: 20
Reply 6, posted (1 year 1 month 4 weeks 1 day 10 hours ago) and read 4116 times:

Quoting vikkyvik (Reply 2):
That wording seems a bit confusing.

Sure - I should have added "for a given airspeed."



B-52s don't take off. They scare the ground away.
User currently offlinewilco737 From Greenland, joined Jun 2004, 8991 posts, RR: 76
Reply 7, posted (1 year 1 month 4 weeks 1 day 6 hours ago) and read 3991 times:
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The Boeing 747-8 is doing a so called "Off-idle" descent. The powersetting during VNAV profile descent is almost at idle, a couple of % above it, so that some changes in the calculated profile can be adjusted with reducing the throttle to idle.

Boeing says that this even reduces the total drag of the airplane as the engines aren't running at idle, but a little out of idle and hence don't produce and drag.

An Idle descent cannot be flown very often. Most airports are very busy, so that you have to descent earlier, faster, slower etc or you don't know exactly how many more miles to fly and you need to adjust your descent profile.

In LAX the RIIVR2 arrival is an excellent example how it can be done even at big airports. You are cleared the RIIVR2 arrival and automatically receive the clearance for the ILS24R. So you follow the descent path of the arrival (which is an almost idle descent) and then you put in flaps and gear and are established on the approach.

wilco737
  



It it's not Boeing, I am not going.
User currently offlineDashTrash From United States of America, joined Aug 2006, 1522 posts, RR: 2
Reply 8, posted (1 year 1 month 4 weeks 19 hours ago) and read 3671 times:

Too many variables, as already has been posted.

Some aircraft cannot do an idle power descent. I used to fly turbojet powered bizjets that couldn't. At top of descent I'd gradually retard the thrust levers until I felt it in my ears, then pitch for whatever speed I wanted to descend at. A few minutes, later, ease a little more power out. Rinse and repeat until 10000 where you could pull them to idle relatively comfortable to continue your descent. Anti ice can also have some effect as you may need more air than the HP bleeds provide.


User currently offlineAA737-823 From United States of America, joined Mar 2000, 5767 posts, RR: 11
Reply 9, posted (1 year 1 month 3 weeks 6 days 18 hours ago) and read 3268 times:

Quoting DashTrash (Reply 8):
Anti ice can also have some effect as you may need more air than the HP bleeds provide.

That's an alarming reality.

Thankfully, the turbofan airliners I work on don't exhibit such weakness.
The 737NG for example, can start descent from 41000 feet at idle, without losing cabin pressure... or TAI, as far as I know.


User currently offlinePihero From France, joined Jan 2005, 4402 posts, RR: 76
Reply 10, posted (1 year 1 month 3 weeks 6 days 17 hours ago) and read 3245 times:

Quoting AA737-823 (Reply 9):
The 737NG for example, can start descent from 41000 feet at idle, without losing cabin pressure... or TAI, as far as I know.

Yes, but AFAIK, the selection of engine A/I triggers an *idle * setting that is in fact higher than *Flight Idle*. It may be transparent to you but the EEC as a matter of fact sets up a thrust that could deal with the bleed requirements of your configuration.
That happens with all airplanes I know that are equipped with an EECU / FADEC ...



Contrail designer
User currently offlineGoBoeing From United States of America, joined Jun 2000, 2693 posts, RR: 15
Reply 11, posted (1 year 1 month 3 weeks 6 days 11 hours ago) and read 3123 times:

Quoting Pihero (Reply 10):
Yes, but AFAIK, the selection of engine A/I triggers an *idle * setting that is in fact higher than *Flight Idle*. It may be transparent to you but the EEC as a matter of fact sets up a thrust that could deal with the bleed requirements of your configuration.
That happens with all airplanes I know that are equipped with an EECU / FADEC ...

Same on the E170/190.

The thrust levers themselves will go to the idle position but the N1 is actually about 44% instead of 21-23%.


User currently offlinebristolflyer From United Kingdom, joined May 2004, 2290 posts, RR: 0
Reply 12, posted (1 year 1 month 3 weeks 5 days 11 hours ago) and read 2915 times:

Thanks for all of your answers - very interesting.


Fortune favours the brave
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 13, posted (1 year 1 month 3 weeks 5 days 3 hours ago) and read 2854 times:

long ago we used to drive up to the descent point at .85 pull the throttles to idle and zoom down at redline thinking this was cool but a few yrs ago they decided that the 290 kt descent was the most economical for fuel and that became the standard. This means that rarely were the throttles at idle anymore.

User currently offlineDashTrash From United States of America, joined Aug 2006, 1522 posts, RR: 2
Reply 14, posted (1 year 1 month 3 weeks 4 days 2 hours ago) and read 2698 times:

Quoting AA737-823 (Reply 9):
That's an alarming reality.

Something like the CJ610 bleeds off 4 stages, but only one is tapped for environmentals. The other three bleed overboard for stall / surge protection. Since there is no HP / LP crossover, you're bleed availability will be based on power settings.

Want some fun? Misconfigure the environmentals in that POS and advance the power. Feels like your head is in a vice.

I also flew an old Sabreliner that also had a single bleed source out of the JT12s, but incorporated bleed straps that opened below 70 something %Ng. When they opened cabin altitude rises sharply. It allowed you to pull more power off higher than the CJ-610 powered aircraft, but you were still limited in what you could pull out.

My understanding is the older JT8Ds are similar. Never flown anything powered by them however, so that statement could be inaccurate.


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