Gliding Descent to Save Fuel
 

New 'gliding' landing method for planes to slash fuel consumption
I am curious as to how this really works in practice, and what limitations there are in using this technique. Any advice appreciated.

Hell, the most economic descent would be so but unless they have a commercial glider in mind it will never happen.

Having said that, for ATC to calculate descent into a straight in approach "engines idle" (complex calculations given aircraft type and weight) would be ideal but unprecedented. As aircraft enter the approach phase thrust is required to offset the increased drag normally to safely land at design speeds.

All that is is a Continuous Descent Approach. No big deal at all. Every modern jet can do them at near idle power. In the 717, a small amount of "fat" (too much, in my opinion) is built in so that if unexpected tailwinds are encountered, the aircraft can still stay on/get back to the ideal descent profile and to provide a safe approach. Spinning up from Flight Idle just before touchdown is not desirable.:eek:

The big problem is ATC and other traffic getting in the way, requiring either lower-than-desired profiles, non-optimum speed or hold-ups.

If one operator does successful CDAs, then another is being unfairly penalised because they are being kept out of the way.

I believe all this bluster about CDAs and RNP is just a smokescreen for an ATC system which cannot cope with the traffic levels.

Referred to here : http://www.pprune.org/jet-blast/3846...d-neutral.html

as 'landing in neutral'!

Read on a magazine months ago
 

Since a cuple of years Airbus and Boeing in cooperation with some airlines are testing in the USA at SFO a sistem granting a CDA in congestionated areas also.
ATC send an "approach profile proposal" to the aircraft by CPDLC, the crew evaluate the proposal and, if they accept, download and activate it.

Text reference here.

ReverseFlight, you can read some more here:

Arlanda and Brisbane Airports Pursue RNP and 4D Trajectories | AVIATION WEEK

SAS Performs First Managed 4D Trajectory Flight in Revenue Service

Can i just point out the reason we go down final app with power on is not to offset the effect of drag in fact the opposite is true the drag is to offset the thrust.

Reason for this is jet engines require time to come up to power, if you do not have them stablised the response time in the event of a go around(Idle thrust to go around power in the event of a "glide app") would be unacceptable at least with decision heights that are normal/required for all wx airline transport ops.

Todders

To perform a CDA does not mean to land at idle thrust.
Each company has an altitude gate at which acft MUST be stabilized accordingly to different wx environment. And one of the stabilizing parameter to be respected is thrust out of idle.

When I was flying the 727 we had a contest between the pilots on this subject. The contest, when the weather was VMC and ATC traffic load low, was two fold, first that when cleared from for descent at pilots discretion from the mid altitude, around FL 240, you closed the throttles, and timed the descent to hit 10,000ft. and 250 kts IAS at the same time. Then, again with the weather and ATC cooperating, you would continue the descent to arrive at the OM, or if no ILS a five mile final, at glide slope intercept altitude, start extending the LEDs/flaps/gear and then at 500 feet in landing configuration you would touch the throttles bring up the power for landing.

After a while it became quite easy. Oh, you were allowed to use the boards, but if you did, you only won one beer.

I landed the 727 only one time never touching the throttles from 10,000ft and it was not intentional. I think I was set up by the guy in the right seat and the FE. We were landing at NAS Miramar (of 'Top Gun' fame), I had coasted in from the west for a right hand downwind. I was at flaps 15 still descending at the midfield point when I was distracted looking for a pair of F-14s that were called out to us as traffic. After I finally located the F-14s I realized that I had passed the end of the runway, how far I didn't know. So I ask my good buddy in the right seat how it looked for turning base, this was the following conversation, sort of;

Guy in right seat=RS

FE=FE (duh)

Me=Me (of course)

RS, "It looks fine, go ahead and turn in."

Me as I look at him, "You sure?"

RS, "Oh yeah, it's fine."

FE peering out the right side cockpit window, "Oh yeah, you can make it."

Me, I start the turn and call for 'gear down'. (Still at flight idle, where the engines had been since we left 10,000ft.)

Me when I finally see the runway, "Jesus Christ, we're too close, flaps 20 then on speed flaps 30, before landing checklist at flaps 30."

RS, "Huh, yeah, I just might have called that just a mite too soon. Flaps moving."

Me, "No sh!t.'

At this point I place my hand on the throttles, more out of habit in this case instead of need.

Me, "Okay, we will touch down in the touchdown zone, but at what airspeed I've not a clue, but we have 12,000ft of runway so stopping will not be a problem, you two a$$holes."

We landed without me ever moving the throttles, stopped with moderate braking with 5,000ft of runway left. That night in Phoenix where we RON my two 'buddies' bought all my beers that night.

So the point of this post is, this idea can and will work with weather and ATC cooperating. However, I don't have clue if an fly-by-wire Airbus can do total flight idle descent from altitude to landing.

And, of course, "Slashing" fuel aqnd CO2 is a total load of carp. If we could engineer CDAs on every approach we would make a very usful saving of a few hundred kilos per flight- maybe .5% of total fuel burn, and well worth doing.

But hardly "Slashing":rolleyes:

Long time ago there was a 727 accident caused by a idle landing.

Idle landing is dangerous for all swept wing airplanes.

Microburst,

What is it about swept wing airplanes that make an idle landing dangerous, compared to non swept wing airplanes?

Used to be this way on older designs, however....if one has three shaft Rollers installed, these engines spool-up mighty fast.
Example.
Lockheed during certification flying for the L1011, a flight idle approach to 200 feet AGL was tried several times, yet the engines spooled-up just fine for a go-around.
And yes, I know the guys that 'done it.'
Cannot be used in commercial service, of course, but demonstrates the adaptability of the Roller's three shaft design.

RollsRoyce...builds good turbine engines:ok:
A superb design...quiet, too.

hawk 37

It has to do with the slope of the CL / AoA curve. In a 727 it is much shallower than in an ATR. Swept wings are good for flying high and fast without having too much shock wave drag, but they are bad for flying low and slow because they "waste" some lifting ability.
If you compare swept and non-swept wings lift versus angle of attack curves you will see that for a given change in angle of attack, a 727 would have less increment in lift than an ATR.
You are in a 727 at 50 ft above the threshold at idle with too high a rate of descent and, because of the idle condition, with a steeper than usual angle of descend. You will have difficulties in arresting that rate and that angle with elevators only, as a change in pitch will not have much effect in lift, and therefore in the flight path. Besides, drag increases a lot during a pitch up at landing speed. So instead of a normal flare and landing the result can be a hard landing or even an accident.

On top of that, accelerating the engine to get the thrust you need to solve the problem can take too long, so you may be unable to avoid the plane from falling on the runway or even you could instinctively and frenetically pull the nose till the stall, only worsening things.

In an ATR, pulling up the nose can quickly change the flight path from steep to shallow. You can also increase thrust faster.

Well coming from south for rwy 01 works great, can keeep em at idle until ya gottabe stabilized.

19 is worse at least from south.

Guess SAS suddenly figured out that their NG's have GPS input and got procedures to deal with it.


(journos must have a dull time)

Have more landings at ENTC than i like to think of.

One have to wonder, why is that thread moved to or created in
jet blast? :}:cool:

Microburst,
If we can, lets keep the jet engine vs turbprop discussion aside. Swept wings was the specific topic.
I understand what you have said about the CL/aoa curve, just trying to relate that to an aircraft at Vref, for old times sake lets say 1.3 x Vso.
Now, it doesnt matter if we're a swept wing aircraft or straight wing, in the situation I asked, the aircraft is at Vref presumably, at dh descending to 50 feet or so for the landing.
You've got a better grasp of this than I have, so bear with me for this...
Regardless of whether it's straight wing or swept wing, there's still this 1.3 margin in speed over the power off stall speed. So...either aircraft at that speed can generate 1.69 times it's lift (L proportional to V squared). Now I know MFS is going to chime in here and say it's really 1.23 squared, but it's the concept I'm trying to demonstrate.
Basically, this means at the extreme, swept or non swept wing aircraft both have the same 1.69 G capability. So, why is hard to land a swept wing aircraft vs non swept at idle thrust?
If it's really then because of the added drag, as you say, of the swept wing aircraft at higher aoa? If so, push up the thrust levers!!
Beer number 4. Hope I'm making sense

Glide approaches
 

Nothing wrong with landing with engines back at idle if speed is correct. Doesn't matter whether it is a 'straight' wing or a 'swept' wing, they both need the approach speed to be correct. As others have said, the danger in being back at idle can be the time required to accelerate the engine if needed.

Hawk 37

If I had not read about that accident and its causes I would have doubts now, because what you say makes sense.
Let me think while writing.
Let us assume the airplanes (swept and straight wing) are at 50 ft above the threshold, idle power, and Vref. So they both are at 1,3 Vs, they have the same stall speed margin.
However, the angle of attack still plays a role in lift procuction. In a swept wing airplane, for each extra degree of angle of attack you get less extra lift than in a straight wing airplane. Both airplanes are equally away from stall but they have not the same ability to manoeuvre, to change trayectory. The swept wing CL-AoA curve is flatter. Margin to stall is one thing, ability to change flight path is another. By the way, the swept wing has a higher stall angle of attack (you can see it comparing both curves) I am not sure is this has an influence, too but I don' think so.
If you can take a look at the curves, the swept one has a lower CL max but a higher CL max AoA. Less CL means less lift for a given speed. So the conclusion is that the swept wing airplane has less CL available than the straight wing plane.
As a matter of fact, if you are making an idle landing in a swept wing airplane you better have a fiew extra knots (LDA permitting). Usually, the reason for an idle landing is that you haven been able to decelerate to VREF. But if you just reach that speed when about to flare still in idle...

Old Fella,
If you mean doing the approach and landing at idle, you're mistaken. Every jet operator I know of has a strict stabilised approach SOP which requires the aircraft to be between Vapp-5 to Vapp+15 (or less) and stay that way from 500ft AGL (some 1000ft AGL) to approx 50ft on the 3° slope. There is no way in a modern jet one could be stable and remain so at Idle.

Anybody who seriously thinks idle descents to touchdown in a jet is feasible doesn't fly jets. "Gliding in to land" means idle power until 500ft at the very latest.