Gliding Descent to Save Fuel
Constant Descent Approaches (CDAs)
CDAs are nothing new; we have been doing them for years. They are no more than descents from TOD at idle power to the start of the approach. And while we are at it, let’s kill stone dead the idea of glide approaches to touch down in commercial operations! The CDAs that SAS and others have been doing all require the aircraft to be stable (Vref+ the correct increment, gear down, landing flap, and power set) at or before 1000ft agl. The problem is how do you do this in a complex and busy ATC environment without reducing the overall capacity of the system?
Recently, several new technologies have been tested in simulations and line operations. The first is ADS-B which uses the Mode S transponder to broadcast information on the aircraft’s position and intent. ADS-B ‘Out’ can provide ATC with better information than current radar. If ADS-B ‘In’ is fitted as well, the aircraft can receive this same information and display it in the cockpit. Page 37 to 41 in the Flight Safety Document on this website will explain more:-
http://www.flightsafety.org/asw/jul09/asw_jul09.pdf
The FAA in their work on NextGen and the Europeans in the SESAR Joint Undertaking are working towards the same ends. The current problem is that ATC do not know what the aircraft is doing and the pilot does not know where he/she is fitting into the traffic pattern. Modern FMS has much better information of the aircraft’s position and projected trajectory than anything ATC can conjure up. So, if this can be data-linked to ATC it would become possible for them to fit aircraft much more elegantly into the overall system and thereby improve safety, reduce workload and improve economy. Equally, if the pilots could see where they are in relation to other traffic in the descent and approach pattern, they could have certain tasks delegated to them by ATC, e.g. to follow in the stream at a constant distance or time interval from preceding aircraft. Eurocontrol and other research bodies have done extensive simulations using active line pilots and active controllers and have proved it can work. Also, UPS at Louisville in Kentucky have been using the system in line operations. See the presentation 6 at the ASAS Global Network Forum in this website:-
http://groups.google.com/group/asas-global-network?hl=en
I have also flown similar systems in simulations at NLR’s facilities in Amsterdam and can vouch for their potential. So, let’s remove the concern about glide approaches and see how the whole ATM system could be improved to increase safety, capacity, economy, and reduce environmental emissions.
CDAs are nothing new; we have been doing them for years. They are no more than descents from TOD at idle power to the start of the approach. And while we are at it, let’s kill stone dead the idea of glide approaches to touch down in commercial operations! The CDAs that SAS and others have been doing all require the aircraft to be stable (Vref+ the correct increment, gear down, landing flap, and power set) at or before 1000ft agl. The problem is how do you do this in a complex and busy ATC environment without reducing the overall capacity of the system?
Recently, several new technologies have been tested in simulations and line operations. The first is ADS-B which uses the Mode S transponder to broadcast information on the aircraft’s position and intent. ADS-B ‘Out’ can provide ATC with better information than current radar. If ADS-B ‘In’ is fitted as well, the aircraft can receive this same information and display it in the cockpit. Page 37 to 41 in the Flight Safety Document on this website will explain more:-
http://www.flightsafety.org/asw/jul09/asw_jul09.pdf
The FAA in their work on NextGen and the Europeans in the SESAR Joint Undertaking are working towards the same ends. The current problem is that ATC do not know what the aircraft is doing and the pilot does not know where he/she is fitting into the traffic pattern. Modern FMS has much better information of the aircraft’s position and projected trajectory than anything ATC can conjure up. So, if this can be data-linked to ATC it would become possible for them to fit aircraft much more elegantly into the overall system and thereby improve safety, reduce workload and improve economy. Equally, if the pilots could see where they are in relation to other traffic in the descent and approach pattern, they could have certain tasks delegated to them by ATC, e.g. to follow in the stream at a constant distance or time interval from preceding aircraft. Eurocontrol and other research bodies have done extensive simulations using active line pilots and active controllers and have proved it can work. Also, UPS at Louisville in Kentucky have been using the system in line operations. See the presentation 6 at the ASAS Global Network Forum in this website:-
http://groups.google.com/group/asas-global-network?hl=en
I have also flown similar systems in simulations at NLR’s facilities in Amsterdam and can vouch for their potential. So, let’s remove the concern about glide approaches and see how the whole ATM system could be improved to increase safety, capacity, economy, and reduce environmental emissions.
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Microburst,
you say "Both airplanes are equally away from stall but they have not the same ability to manoeuvre, to change trayectory"
Not sure I agree with that. Both aircraft have a 1.69 G capability to maneouvre. If they are at the same tas, that's the same turn radius
I don't see what higher coefficents of lift, flatter aoa curves, more aoa change required to add lift etc have to do with this argument. Your points about that may be true, but a 1.3 x V stall factor should equate to same maneouvrability, flare capability etc as long as the aircraft are at the same tas.
I'm not suggesting the final approach/landing be done at idle power, and I appreciate the spool up times required, and perhaps the added drag of swept wing aircraft at high aoa. My inquiry was about the ability to arrest rates of descent, as I believe you put it in one of your earlier posts, of swept wing aircraft versus straight wing. I don't see why they are different.
you say "Both airplanes are equally away from stall but they have not the same ability to manoeuvre, to change trayectory"
Not sure I agree with that. Both aircraft have a 1.69 G capability to maneouvre. If they are at the same tas, that's the same turn radius
I don't see what higher coefficents of lift, flatter aoa curves, more aoa change required to add lift etc have to do with this argument. Your points about that may be true, but a 1.3 x V stall factor should equate to same maneouvrability, flare capability etc as long as the aircraft are at the same tas.
I'm not suggesting the final approach/landing be done at idle power, and I appreciate the spool up times required, and perhaps the added drag of swept wing aircraft at high aoa. My inquiry was about the ability to arrest rates of descent, as I believe you put it in one of your earlier posts, of swept wing aircraft versus straight wing. I don't see why they are different.
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Hello Hawk
The truth is I don't know where that 1.96g comes from.
As far as I know, gs come from Lift. If one airplane has less ability to increase its lift than other airplane, all other factors remaining constant (weight, speed...) it follows that this airplane has less manoeuverability.
Please I would like to see how you get the 1.96 g in detail.
Regarding the increased drag, that is another reason not to advise idle landings in jets, and most swept wing airplanes are jets. Jets Vref is closer to the reverse command region than props' Vref. This means that we have less speed stability and we can easily fall into that region if we raise the pitch too much without adding thrust positively. Haven't you ever been a few knots below Vref and have added a little bit of thrust to recover, but it didn´t suffice, and then added some more, with little result, and then more and more thrust until you realized that the thrust was well above the normal final approach thrust? That is the speed instability. If only you had added more thrust the first time you would have gone out of the limits of the unstable speed region, but little by little you remained there a long time.
Imagine an idle landing in that situation, a few knots below Vref (it can happen if actual weight is above computed weight, among other reasons).
As I said (well, as the 727 accident report said) it is no good idea to do an idle landing in a swept wing airplane.
Summing up, I think the reasons for discouraging idle landings in jets (with swept wings) are:
- higher sink rate
- steeper path requiring more manouverability for the flare
- reduced manoeuverability (here we disagree)
- speed instability (or little speed stability)
- possibly excessive time to spool up jet engines
Nice to review and discuss about this subject! I have to take a look at a few books I don't have right now to check what happens to AoA, g and speed when speed is 1,3 Vs.
The truth is I don't know where that 1.96g comes from.
As far as I know, gs come from Lift. If one airplane has less ability to increase its lift than other airplane, all other factors remaining constant (weight, speed...) it follows that this airplane has less manoeuverability.
Please I would like to see how you get the 1.96 g in detail.
Regarding the increased drag, that is another reason not to advise idle landings in jets, and most swept wing airplanes are jets. Jets Vref is closer to the reverse command region than props' Vref. This means that we have less speed stability and we can easily fall into that region if we raise the pitch too much without adding thrust positively. Haven't you ever been a few knots below Vref and have added a little bit of thrust to recover, but it didn´t suffice, and then added some more, with little result, and then more and more thrust until you realized that the thrust was well above the normal final approach thrust? That is the speed instability. If only you had added more thrust the first time you would have gone out of the limits of the unstable speed region, but little by little you remained there a long time.
Imagine an idle landing in that situation, a few knots below Vref (it can happen if actual weight is above computed weight, among other reasons).
As I said (well, as the 727 accident report said) it is no good idea to do an idle landing in a swept wing airplane.
Summing up, I think the reasons for discouraging idle landings in jets (with swept wings) are:
- higher sink rate
- steeper path requiring more manouverability for the flare
- reduced manoeuverability (here we disagree)
- speed instability (or little speed stability)
- possibly excessive time to spool up jet engines
Nice to review and discuss about this subject! I have to take a look at a few books I don't have right now to check what happens to AoA, g and speed when speed is 1,3 Vs.
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Hi again, Hawk
I have been working with a pen, a paper and my old fx-82 (no books, unfortunately) and damnit! I think you are right
If at stall speed (1g) we have a given lift coefficient then at 1,3 that speed (1g) we have 0,59 times that lift coefficient. Irrespective of the type of wing. Which means that if at approach speed (1g) we have a given CL , increasing CL to CLmax we will achieve 1,69 times that coefficient: 1,69 g capability, as you said!
The swept wing only means we need more angle of attack to achieve the CLmax. This fact could contribute to the unstable speed regime risk that I mentioned earlier, as a large increase in AoA means a large increase in drag (but how swept wing affects CD as compared with a straigh wing one I don't know. Argh!).
Maybe the risk is tal strike in swept wing airplanes when idle landing, rather than inability to flare
It also means that, since CLmax is lower, Vref in a swept wing airplane nees to be higher than in a straight wing one, but that only contributes to require more distance to land.
Still, we have some reasons why we should not attempt idle landing in jet airplanes. And I swear on god that the report I read showed the damn swept wing CL-AoA curves!
Thanks. I have enjoyed learning!
I have been working with a pen, a paper and my old fx-82 (no books, unfortunately) and damnit! I think you are right
If at stall speed (1g) we have a given lift coefficient then at 1,3 that speed (1g) we have 0,59 times that lift coefficient. Irrespective of the type of wing. Which means that if at approach speed (1g) we have a given CL , increasing CL to CLmax we will achieve 1,69 times that coefficient: 1,69 g capability, as you said!
The swept wing only means we need more angle of attack to achieve the CLmax. This fact could contribute to the unstable speed regime risk that I mentioned earlier, as a large increase in AoA means a large increase in drag (but how swept wing affects CD as compared with a straigh wing one I don't know. Argh!).
Maybe the risk is tal strike in swept wing airplanes when idle landing, rather than inability to flare
It also means that, since CLmax is lower, Vref in a swept wing airplane nees to be higher than in a straight wing one, but that only contributes to require more distance to land.
Still, we have some reasons why we should not attempt idle landing in jet airplanes. And I swear on god that the report I read showed the damn swept wing CL-AoA curves!
Thanks. I have enjoyed learning!
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If we are talking about gliding all the way to the ground then we will end up with a glide slope that is considerably greater than the usual 3 degrees.
Going back to the basics,
Glide range = Lift to Drag ratio x height
Rearranging this gives Lift to Drag ratio equals the cotangent of the glide slope.
For a glide slope of 3 degrees we need a Lift to Drag ratio of about 19 to 1.
Not many commercial transport aircraft will achieve this in the landing configuration.
Going back to the basics,
Glide range = Lift to Drag ratio x height
Rearranging this gives Lift to Drag ratio equals the cotangent of the glide slope.
For a glide slope of 3 degrees we need a Lift to Drag ratio of about 19 to 1.
Not many commercial transport aircraft will achieve this in the landing configuration.
The Kabaka,
It is not much use only to be able to do CDAs from 6000ft. What is needed is for all aircraft to be able to do them from TOD to around 1500ft to 1000ft without intermediate level outs and without overall loss of airport capacity. The systems I gave references for in my last post may be able to achieve that one day. In fact London Heathrow has nothing to boast about; according to the Eurocontrol Performance Review Report for 2008 (www.eurocontrol.int/prc) on page 59 in Fig 72 it is shown to have the worst performance, in terms of additional time spent on the approach, of any major airport in Europe!
It is not much use only to be able to do CDAs from 6000ft. What is needed is for all aircraft to be able to do them from TOD to around 1500ft to 1000ft without intermediate level outs and without overall loss of airport capacity. The systems I gave references for in my last post may be able to achieve that one day. In fact London Heathrow has nothing to boast about; according to the Eurocontrol Performance Review Report for 2008 (www.eurocontrol.int/prc) on page 59 in Fig 72 it is shown to have the worst performance, in terms of additional time spent on the approach, of any major airport in Europe!
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London Heathrow has nothing to boast about; according to the Eurocontrol Performance Review Report for 2008 (www.eurocontrol.int/prc) on page 59 in Fig 72 it is shown to have the worst performance, in terms of additional time spent on the approach, of any major airport in Europe!
Opps...another LHR hurrah down the drain.
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The reason the 727 had high sinkrate accidents early in its existence had nothing to do with the sweep of it's wings and everything to do with the huge amount of drag generated by the LEDs and flaps in the landing configuration. Basically, as soon as the nose comes up, the speed goes away....fast, and without really changing the descent path of the airplane. If the engines aren't spooled there is no margin for error and even then a late application of power can take what seems to be eons to change the descent path. Being below a couple hundred feet, unspooled, at Vref is a dangerous place in a 727. A fantastic airplane, but it had to be flown like a 727, not just a "swept wing airplane".
The DC8, on the other hand also had a swept wing, was much larger and heavier, and had no unusual high sinkrate tendencies. It also had a different wing section and no LEDs, as well as much smaller (relatively) flaps. It had a much larger margin of error.
The DC8, on the other hand also had a swept wing, was much larger and heavier, and had no unusual high sinkrate tendencies. It also had a different wing section and no LEDs, as well as much smaller (relatively) flaps. It had a much larger margin of error.
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RYR STN->BTS, two years ago (737-800NG, august, very nice weather, no wind, excellent visibility, ac nearly empty) - the pilot made a descent from (just guessing) 2-3 km, and the engines didn't spool up till we were taxiing. Extremely smooth approach and landing, no reverse thrust and very little braking. The best landing I ever experienced!
(I fully understand it is not possible for a slf to be sure about engine RPM, and I am not saying they were idling, just that I couldn't hear them)
(I fully understand it is not possible for a slf to be sure about engine RPM, and I am not saying they were idling, just that I couldn't hear them)
the pilot made a descent from (just guessing) 2-3 km, and the engines didn't spool up till we were taxiing. Extremely smooth approach and landing, no reverse thrust and very little braking. The best landing I ever experienced!
Don't judge an approach and landing by the touchdown...
Thanks Bergeriel, you have it in a nut shell. Kieth williams as an ATPL Instructor you surely must know gliding to touch-down will never be part of the equation due to 'stable approach' requirements. As Bergeriel states CDA is and has been a standard method of descent since jets were invented. What stops a 'perfect' idle descent being ATC requirements. The only thing that is changing is trying to get ATC more on board. With the traffic density the way it is I have my doubts, but it's worth a try. As for the 727, if any aeroplane needs power to land, thats the one. You never closed the thrust levers until you were finished with the wings.
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Dear Captain Bloggs,
I appreciate fully that a "smooth landing" from a slf perspective isn't necessarily a "technically clean" landing. I just wanted to point out that it happens (probably a very rare occasion) - and that in fact you can glide to landing on a 737 (in ideal conditions). We may also consider the fact that BTS isn't a big or busy airport. Also, arriving from STN, the ac didn't come directly to landing, but made a wide turn before aligning and coming to land (already on idle). I am guessing the pilot had a clearance for a visual approach, and did this intentionally. He may have flown a bit faster to the landing, but as the ac was nearly empty, it didn't lead to some heavy braking.
I am eager to see if i'll ever have the chance to experience this kind of "quiet landing" again. It felt like being in an oversized glider!
(humbly going back to "read-only mode" ;-)
I appreciate fully that a "smooth landing" from a slf perspective isn't necessarily a "technically clean" landing. I just wanted to point out that it happens (probably a very rare occasion) - and that in fact you can glide to landing on a 737 (in ideal conditions). We may also consider the fact that BTS isn't a big or busy airport. Also, arriving from STN, the ac didn't come directly to landing, but made a wide turn before aligning and coming to land (already on idle). I am guessing the pilot had a clearance for a visual approach, and did this intentionally. He may have flown a bit faster to the landing, but as the ac was nearly empty, it didn't lead to some heavy braking.
I am eager to see if i'll ever have the chance to experience this kind of "quiet landing" again. It felt like being in an oversized glider!
(humbly going back to "read-only mode" ;-)
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Personally, I'll sacrifice a little fuel to be fully configured, with all checklists completed, etc, before 1000 feet AGL. Just feel safer that way.
But, for interest, have a read of the Hong Kong Noise Abatement procedures for late night - early morning arrivals; "...all operators...CDA approach....thrust idle....until touch down..."
But, for interest, have a read of the Hong Kong Noise Abatement procedures for late night - early morning arrivals; "...all operators...CDA approach....thrust idle....until touch down..."
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swept wing/spool up
If you watch closely the landings of the (swept wing) Space Shuttle as it lands I suppose you'll see the engines spool up @ around 500'........? Guess that's why it doesn't crash into the ground each time it lands???!
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FreqSLF,
I would surmise that the additional airstream noise and vibration from having landing gear hanging and flaps at full deflection for landing was enough to mask the sound of the engines at approach power. For the jets I've flown, the engine's rotor speed has been in the 60 - 70 % N1 for the approaches, and this is usually much quieter than the up to 100 % one might hear on takeoff or even climb.
I would surmise that the additional airstream noise and vibration from having landing gear hanging and flaps at full deflection for landing was enough to mask the sound of the engines at approach power. For the jets I've flown, the engine's rotor speed has been in the 60 - 70 % N1 for the approaches, and this is usually much quieter than the up to 100 % one might hear on takeoff or even climb.
The space shuttle does not have any operative engines during re-entry and landing. That's why it's descent angle is so steep; it IS gliding - like a brick. The 500ft thingee is the commencement of the flare for landing.
Black Nat, have you any idea on the profile of the Shuttle? 1500 feet/second initially and a final profile of 10,000 feet at five miles. Sure we can all fly like that and land like that, with training, but its dangerous and we don't do anything dangerous with the general public down the back. Your 'Space Shuttle' theory also has a few holes in it with a Go-Around.