bristolflyer
Topic Author
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### How Much Power Used During Descent?

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)?

Fortune favours the brave

N243NW
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### RE: How Much Power Used During Descent?

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.
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vikkyvik
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### RE: How Much Power Used During Descent?

 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.
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goboeing
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### RE: How Much Power Used During Descent?

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.

Woodreau
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### RE: How Much Power Used During Descent?

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.
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Francoflier
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### RE: How Much Power Used During Descent?

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.
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N243NW
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Joined: Thu Jul 24, 2003 4:29 am

### RE: How Much Power Used During Descent?

 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.

wilco737
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Joined: Sun Jun 20, 2004 12:21 am

### RE: How Much Power Used During Descent?

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

DashTrash
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### RE: How Much Power Used During Descent?

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.

AA737-823
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### RE: How Much Power Used During Descent?

 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.

Pihero
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### RE: How Much Power Used During Descent?

 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 ...
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goboeing
Posts: 2477
Joined: Mon Jun 05, 2000 5:31 am

### RE: How Much Power Used During Descent?

 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%.

bristolflyer
Topic Author
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Joined: Fri May 14, 2004 1:35 am

### RE: How Much Power Used During Descent?

Fortune favours the brave

CosmicCruiser
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### RE: How Much Power Used During Descent?

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.

DashTrash
Posts: 1271
Joined: Mon Aug 21, 2006 8:44 am

### RE: How Much Power Used During Descent?

 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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