Continuous Descent Approaches
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Continuous Descent Approaches
I am looking for some information on CDAs. From the Flight Deck, what effect do they have in terms of workload, fuel saving (if any), ease or difficulty of the procedures etc etc.
Any first hand info would be much appreciated, including gripes etc. Also if anyone has knowledge of any other websites that might have useful gen.
Many thanks in advance.
Any first hand info would be much appreciated, including gripes etc. Also if anyone has knowledge of any other websites that might have useful gen.
Many thanks in advance.
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Negligible increase in workload. You have to frequently compute estimated track miles to touchdown and make sensible adjustments to rate of descent to constantly watch you don't end up flying level. Very difficult to quantify any fuel savings (if any), but the environment benefits of not levelling off at 3000' over a city and pouring on power are too important to be dismissed. They are merely an extension of proper airmanship. I have sat and watched pilots cleared to 4000' 25 miles out descend 1000 or 2000' with idle power and then level off again! I feel disgust when I see flying like that, from a passenger comfort point of view and noise perspective. They should be mandatory.
There are different regulations in the US where continuous descents should not necessarily to be followed.
There are different regulations in the US where continuous descents should not necessarily to be followed.
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For Jet Operations into controlled airspace -
For my airline, and I suspect most others, what you describe is the optimum, sought after procedure. Ideally, the thrust levers are brought to idle at Top of Descent, and not advanced again until a pre-determined point on final approach to stabilise the approach speed / thrust / configuration. For my airline, stabilisation point is 1000 feet AGL in VMC, or ILS commencement in IMC.
To address your various points -
(1) Workload is reduced. Step descents significantly increase workload.
(2) Fuel saving is considerable, having to spool up to maintain various step altitudes on descent at low, fuel inneficient altudes can throw away all of the fuel savings from carefully optimised flight of the last 10 hours.
(3) Ease or Difficulty? - much easier, see No. (1) above.
Additionally, use of the 'Low Drag Approach', which implies idle thrust down to about 1000 feet (in VMC) is much desired in noise sensitive areas. It is also a considerable fuel saver. Of course, to some nameless operators, 'Low Drag Approach' means get it low, and drag it in.
It is well understood that the desired continuous descent approach will not always be possible. Operations at uncontrolled aerodromes requires correct, level flight on the downwind leg, OR, ATC may require an interim level-off because of other traffic, OR, Transiting other airspace may require a level-off segment, etc.
In this pilot's opinion, the Continuous Descent Approach is (1) Easier, (2) Less workload, (3) Fuel saving, (4) Environmentally friendly, and most importantly, (5) Normal!
For my airline, and I suspect most others, what you describe is the optimum, sought after procedure. Ideally, the thrust levers are brought to idle at Top of Descent, and not advanced again until a pre-determined point on final approach to stabilise the approach speed / thrust / configuration. For my airline, stabilisation point is 1000 feet AGL in VMC, or ILS commencement in IMC.
To address your various points -
(1) Workload is reduced. Step descents significantly increase workload.
(2) Fuel saving is considerable, having to spool up to maintain various step altitudes on descent at low, fuel inneficient altudes can throw away all of the fuel savings from carefully optimised flight of the last 10 hours.
(3) Ease or Difficulty? - much easier, see No. (1) above.
Additionally, use of the 'Low Drag Approach', which implies idle thrust down to about 1000 feet (in VMC) is much desired in noise sensitive areas. It is also a considerable fuel saver. Of course, to some nameless operators, 'Low Drag Approach' means get it low, and drag it in.
It is well understood that the desired continuous descent approach will not always be possible. Operations at uncontrolled aerodromes requires correct, level flight on the downwind leg, OR, ATC may require an interim level-off because of other traffic, OR, Transiting other airspace may require a level-off segment, etc.
In this pilot's opinion, the Continuous Descent Approach is (1) Easier, (2) Less workload, (3) Fuel saving, (4) Environmentally friendly, and most importantly, (5) Normal!
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For a modern FMS-driven airplane, the VNAV function attempts a continuous descent anyhow. Ideally that descent is at idle thrust, but that doesn't always happen.
If planned in advance, a continuous descent approach works as well or better than a series of steps.
If planned in advance, a continuous descent approach works as well or better than a series of steps.
As said above it makes a huge difference on the ground in terms of noise which makes you wonder why one of the most noise sensitve airports on Europe, Amsterdam, does not use it. They change runways all the time because of noise complaints but I suspect a lot of the complaints are due to aicraft dragging overhead at 2000ft from miles out having been descended by ATC.
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PPP=PPR. As previously said, I am also amazed at the number of 20k+ hour pilots I fly with that cannot come up with a sensible plan when left to their own discretion. Minimum vectoring altitude 30 miles from the airport or 5000agl on 5m finals!
piss poor planning=piss poor results!
piss poor planning=piss poor results!
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I heard also that descending at idle can cause shock cooling depending on OAT, some airliners actually keep higher power setting and speed brakes deployed to keep the engines warm during descent. Not the most fuel efficient but the cost of keeping the engine in good condition far outweighs the fuel savings in that case.
Anyone have experience with that?
Anyone have experience with that?
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I heard also that descending at idle can cause shock cooling depending on OAT
most jet engines have 'flight idle' where when you pull all the power back at certain conditions of flight, the engine will 'idle' at a higher speed
The FMS will adjust for approach idle if it is told when icing conditions may be expected.
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so it's impossible to shock cool a turbine? or the FADEC or FMS is taking care of that, which will prevent it?
Limitations I've seen pertain to heat-soaking the turbine (e.g., run at idle for X minutes after running at high power, before shutdown).
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CDAs
Guys,
in designing ATC procedures for CDAs, what are the traps to look out for. Are all FMS/FMC capable of accepting input that will allow for a descent profile to be flown automatically? Do all FMC's handle the profile in the same manner? Can an FMS "recapture"a CDA should there be an interupt to the profile - due to ATC intervention? Is there any need for a levelling off segment due to decellaration? Can a "hard"waypoint be configured, such as "cross wpt xxx between 8 & 10 thousnad between 230 and 210 KIAS? What impact does any other ATC constraints have in the calculation, and how can they be overcome? What affect would ATC restriction to TOD have, or any intermediate descent clearance - say not below 10000'until 30nm from touchdown? How often should a 'distance to go' update be provided?
Thanks good people. So many questions, but hopefully some answers, and hopefully, any other tips that may be considered appropriate gladly accepted.
in designing ATC procedures for CDAs, what are the traps to look out for. Are all FMS/FMC capable of accepting input that will allow for a descent profile to be flown automatically? Do all FMC's handle the profile in the same manner? Can an FMS "recapture"a CDA should there be an interupt to the profile - due to ATC intervention? Is there any need for a levelling off segment due to decellaration? Can a "hard"waypoint be configured, such as "cross wpt xxx between 8 & 10 thousnad between 230 and 210 KIAS? What impact does any other ATC constraints have in the calculation, and how can they be overcome? What affect would ATC restriction to TOD have, or any intermediate descent clearance - say not below 10000'until 30nm from touchdown? How often should a 'distance to go' update be provided?
Thanks good people. So many questions, but hopefully some answers, and hopefully, any other tips that may be considered appropriate gladly accepted.
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If you have RNAV procedures, you need tight speed control to keep the aeroplanes coming out at the other end at the right time. However. most airfields in practice use radar approach procedures rather than RNAV which cuts out the FMS system altogether. The best way for pilots to fly them are with autopilot engaged with Heading and Vertical speed mode in use- trying to handfly makes the procedure much harder to assess. This gives the tightest following of ATC instructions. At some stage an indication of miles to touchdown is useful, though it seems to be very much a guess by the controller and is only an indication of whether an extended downwind leg is in prospect. The pilots can then start making pretty good estimates of distance to go themselves and allow control of Vertical Speed. As long as level decceleration/flap extension legs are flown at idle power, they still count as 'continuous descent'. What has to be avoided is level flight with power on below transition level.
What is the basis of your interest? Is it professional? Some indication of your level of knowledge in your profile would be helpful.
What is the basis of your interest? Is it professional? Some indication of your level of knowledge in your profile would be helpful.
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It is possible to shock-cool a turbine, but not in the context discussed here...
My one was an in-flight shut-down from 105% N1, enforced due to a mechanical failure in the governor. The resultant thermal gradients trashed the engine, which had to be taken out of service (RB199)
My one was an in-flight shut-down from 105% N1, enforced due to a mechanical failure in the governor. The resultant thermal gradients trashed the engine, which had to be taken out of service (RB199)
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CDA
Thanks. My interest is professional, in researching the issues that need to be considered when undertaking such a procedure design. There is currently such research and trialling being undertaken in Oz, US and shortly in various European destinations as well. Maybe other places are also undertaking such effort, which if so, sharing of information/experiences will be of a distinct advantage. As LHR currently seem to be the only "regular" user at this end, there would seem to be scope for a wider application, especially during hours of low demand, such as AMS provide. So, any answers to the specifics raised will help in building the knowledge base, and yield something that works, and can be used at a wider cross section of aerodromes.
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Disagree a bit Notso F.
In my opinion a CDA approach is an idle descent without a level segment.
So : one flys it as a glider,
which means speed is a pitch mode ! (and thrust is at idle)
So don't fly them in V/S because then speed is a thrust mode,
(and V/S the pitch mode with thrust whatever req. by the speed.
and thrust on your engine is what you don't want in a CDA
cheers.
In my opinion a CDA approach is an idle descent without a level segment.
So : one flys it as a glider,
which means speed is a pitch mode ! (and thrust is at idle)
So don't fly them in V/S because then speed is a thrust mode,
(and V/S the pitch mode with thrust whatever req. by the speed.
and thrust on your engine is what you don't want in a CDA
cheers.
There are mixtures of ideas in this thread possibly because there are alternative CDAs. Procedures designed to transition from cruise altitude into the TMA and thence onto the final approach, differ from those used during final approach for NPAs (Constant Angle NPA).
In the first case the initial procedure designs were to reduce workload and increase efficiency / economics; now days they are more biased towards flow management and environment / noise. Whilst the most efficient descent may be a glide, such a procedure cannot accommodate different aircraft types and give ATC full control of speed. Thus descents into the TMA are compromises, but even for a standard profile the variability of wind, power requirements (configuration / icing), wt, etc, results in a range of operating techniques from power-on to flight idle + airbrake. An FMS (VNAV) based procedure gives the crew the big picture and a ‘how goes it’, whether flying on autos or manually. Non VNAV aircraft require information of track distance to touchdown so the crew may calculate the profile and also judge a high or low energy state.
A CDA on final approach has been promoted to reduce CFIT on NPAs; this avoids step downs during the procedure, and normally mandates a GA at MDA if nothing seen. A difficulty for the industry is to train crews to adapt from a decelerating approach on an ILS (where the vertical path is constrained), to an open vertical path (unconstrained) on NPAs. Recent CFIT accidents and incidents recorded crews flying VOR/HDG and VS during a decelerating NPA; they forgot the basic flight mechanics that a deceleration at constant VS will increase the flight path angle and aim the aircraft short of the runway; thus during a CDA on a NPA, the VS must be reduced as speed is reduced. To combat these problems some operators now fly constant speed / configuration NPAs. Flight crews should carefully brief NPAs, use the tables of range vs distance now on the charts, and treat all NPAs as something very different and potentially hazardous.
In non VNAV aircraft I preferred to fly manually, this gave a better feel for the aircraft both in control force, trim (speed), and deceleration (energy management). Of course if the workload increased and/or the monitoring pilot was tasked elsewhere then I used the autos, but preferably in HDG with speed on pitch, the rate of descent being controlled with power / airbrake.
Thrust provides energy, the elevator distributes it.
In the first case the initial procedure designs were to reduce workload and increase efficiency / economics; now days they are more biased towards flow management and environment / noise. Whilst the most efficient descent may be a glide, such a procedure cannot accommodate different aircraft types and give ATC full control of speed. Thus descents into the TMA are compromises, but even for a standard profile the variability of wind, power requirements (configuration / icing), wt, etc, results in a range of operating techniques from power-on to flight idle + airbrake. An FMS (VNAV) based procedure gives the crew the big picture and a ‘how goes it’, whether flying on autos or manually. Non VNAV aircraft require information of track distance to touchdown so the crew may calculate the profile and also judge a high or low energy state.
A CDA on final approach has been promoted to reduce CFIT on NPAs; this avoids step downs during the procedure, and normally mandates a GA at MDA if nothing seen. A difficulty for the industry is to train crews to adapt from a decelerating approach on an ILS (where the vertical path is constrained), to an open vertical path (unconstrained) on NPAs. Recent CFIT accidents and incidents recorded crews flying VOR/HDG and VS during a decelerating NPA; they forgot the basic flight mechanics that a deceleration at constant VS will increase the flight path angle and aim the aircraft short of the runway; thus during a CDA on a NPA, the VS must be reduced as speed is reduced. To combat these problems some operators now fly constant speed / configuration NPAs. Flight crews should carefully brief NPAs, use the tables of range vs distance now on the charts, and treat all NPAs as something very different and potentially hazardous.
In non VNAV aircraft I preferred to fly manually, this gave a better feel for the aircraft both in control force, trim (speed), and deceleration (energy management). Of course if the workload increased and/or the monitoring pilot was tasked elsewhere then I used the autos, but preferably in HDG with speed on pitch, the rate of descent being controlled with power / airbrake.
Thrust provides energy, the elevator distributes it.
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CDAs seem mostly to be considered applicable to precision approaches, but the NPA and visual approach should also be able to benefit from the advantages provided, whilst the differing techniques pointed out have to be properly tested and evaluated, and a proper safety assessment must be undertaken.
To assist in defining what a CDA is, does the following capture the necessary elements:
Continuous Descent Arrival(CDA) is a technique used by arriving aircraft which provides for continuous descent, to a position that allows it to join a glide path to an arriving runway at an altitude and distance, commensurate with environmental and operational limitations and requirements
Edited this after the comments of Blue heaven, re differences between Arrival and Approach
To assist in defining what a CDA is, does the following capture the necessary elements:
Continuous Descent Arrival(CDA) is a technique used by arriving aircraft which provides for continuous descent, to a position that allows it to join a glide path to an arriving runway at an altitude and distance, commensurate with environmental and operational limitations and requirements
Edited this after the comments of Blue heaven, re differences between Arrival and Approach
Last edited by themwasthe days; 25th Aug 2004 at 07:50.
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I think that the opinions of CDAs will rather depend on where a pilot has experienced them! The flight-idle-to-1000ft proponents are obviously not regulars at Heathrow, which mandates CDAs but effectively imposes gate heights and speeds which demand speed/vs autopilot modes thereafter (the gates are too low and slow to allow a flight idle descent).
CDAs of any variety are fairly easy to achieve as long as the pilot is given a reasonable degree of freedom over descent points and/or speed. To achieve the FI to 1000ft profile at an airfield with arrivals from all directions is going to be difficult without gates to focus the arrivals, but those gates must correspond with a power-off profile for the majority of likely arriving aircraft. Radar vectoring at any point in the arrival makes it more difficult for the pilot to judge the CDA profile, as he's effectively got to guess how far the controller's going to take him, so CDA arrivals should follow the published arrival track wherever possible (almost never at LHR!). At LHR, with the variables introduced by the controlling methods used (which I'm not complaining about), it's amazing so many pilots manage to achieve a more-or-less CDA (from the hold fix) at all!
CDAs of any variety are fairly easy to achieve as long as the pilot is given a reasonable degree of freedom over descent points and/or speed. To achieve the FI to 1000ft profile at an airfield with arrivals from all directions is going to be difficult without gates to focus the arrivals, but those gates must correspond with a power-off profile for the majority of likely arriving aircraft. Radar vectoring at any point in the arrival makes it more difficult for the pilot to judge the CDA profile, as he's effectively got to guess how far the controller's going to take him, so CDA arrivals should follow the published arrival track wherever possible (almost never at LHR!). At LHR, with the variables introduced by the controlling methods used (which I'm not complaining about), it's amazing so many pilots manage to achieve a more-or-less CDA (from the hold fix) at all!