Continuous Descent Approaches
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Continuous Descent Approaches
Hi all,
I'd be very interested in hearing from pilots and ATCos on their experiences of flying/controlling CDAs. I'm particularly interested in idle power descents from cruise to final approach.
What are the energy management considerations?
What scope exists to modify the descent trajectory for sequencing arrival streams?
What about different aircraft types (e.g. B757 vs A320)?
How do CDAs affect decent planning?
Do you have to increase spacing between aircraft to account for errors in predicting the CDA trajectory?
What is the effect on throughput?
Any other comments or experiences?
Many thanks!
I'd be very interested in hearing from pilots and ATCos on their experiences of flying/controlling CDAs. I'm particularly interested in idle power descents from cruise to final approach.
What are the energy management considerations?
What scope exists to modify the descent trajectory for sequencing arrival streams?
What about different aircraft types (e.g. B757 vs A320)?
How do CDAs affect decent planning?
Do you have to increase spacing between aircraft to account for errors in predicting the CDA trajectory?
What is the effect on throughput?
Any other comments or experiences?
Many thanks!
Join Date: Mar 2000
Location: Arizona USA
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I used to fly with a co-pilot who, upon reaching top of climb, would twist his fancy Breitling watch bezel, and proudly announce to all in earshot that...we will descend at precisely 116 DME.
Fine.
Except that ATC would throw a wrench into the mix, and state that descent should start at 100 DME.
Whoops, descent profile out the window, so it was max speed brake for this guy, followed by added power at FL060, simply because he could not understand energy management.
When I suggested...close the taps at 100 DME and adjust the VS so as to arrive at 30 DME at FL100, 250 knots, it all worked out much better.
Some folks listen (and learn)...some don't.
Aeroplane...Lockheed tri-motor.
A fine machine.
Fine.
Except that ATC would throw a wrench into the mix, and state that descent should start at 100 DME.
Whoops, descent profile out the window, so it was max speed brake for this guy, followed by added power at FL060, simply because he could not understand energy management.
When I suggested...close the taps at 100 DME and adjust the VS so as to arrive at 30 DME at FL100, 250 knots, it all worked out much better.
Some folks listen (and learn)...some don't.
Aeroplane...Lockheed tri-motor.
A fine machine.
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On our old 146 you can lock the VS, lock the IAS or lock the altitude. You can also capture a selected altitude, but for CDA, all we can do is now take our little booklet supplied by Gatwick (thanks Claire) and look at the chart on the back to get a fpm from the range/altitude.
Remembering to fly level for 20 seconds first is a problem as we get bored by then, but trying to keep a speed and VS is a nightmare as the old autopilot is very poor at locking on and holding a VS. The RJ is supposed to be better, but they never let us fly those and now never will (BACON!!).
Generally, it would be a lot easier if ATC gave us a couple more range checks along the way, as our -100, G-MABR, doesn't even have a nav system that helps at all. Others with GNX-XLS fitted can help if the pilot is clever enough to put a couple of self generated waypoints in, but that takes time and familiarity.
There are times when 500fpm will result in a level segment so you simply cannot win. However we do try our best!
Also, can the ATC on director please tell us to reduce to 160kts before we capture the glide, not just after! it's a lot easier especially if we are just leveling at the capture altitude.
Remembering to fly level for 20 seconds first is a problem as we get bored by then, but trying to keep a speed and VS is a nightmare as the old autopilot is very poor at locking on and holding a VS. The RJ is supposed to be better, but they never let us fly those and now never will (BACON!!).
Generally, it would be a lot easier if ATC gave us a couple more range checks along the way, as our -100, G-MABR, doesn't even have a nav system that helps at all. Others with GNX-XLS fitted can help if the pilot is clever enough to put a couple of self generated waypoints in, but that takes time and familiarity.
There are times when 500fpm will result in a level segment so you simply cannot win. However we do try our best!
Also, can the ATC on director please tell us to reduce to 160kts before we capture the glide, not just after! it's a lot easier especially if we are just leveling at the capture altitude.
CDA's into LGW in the 757 are generally not a problem, 300-330kts in descent till around FL80, then VS-100 for a bit to slow back too 220kts....then FLCH down too about 1000-1500ft above platform, then its VS to keep you below the glide until you want to intercept. It gets tough when the director gives you a 45 degree cut for the LOC, you are already level with the glide, and you have 60kts tail/X wind component....and the grumpy cow won't give you a speed reduction
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We did for about a year or so ,some trials for BCDA for the eurocontrol ,at our home base. Approaching t/d, as computed by the FMC, we asked for descent,and we got clearance ,complete with the STAR.Approaching FL100,we were cleared for the ILS,with no further involvement of the ATC.
All descent was done in VNAV ,and the STAR included some mandatory gates.
Those gates were calculated with a mandatory speed ,and a minimum/maximum FL (altitude).
By doing this trials with different types,at different airports around europe,it was possible to establish the necessary gate dimension ,in order to be able to squeze in it a 737/ 320 and also bigger planes (like 747 ).
All descent was done in VNAV, with engines at idle thrust ,with FMC computed speeds untill FL100, and with gate mandatory speeds bellow FL100. No thrust was required till G/S intercept. We did few hundred flights,and it was ok,we also saved some fuel.
One good thing is less interference from the ATC, also the SID's can also be planned (as a continuous climb..) so they won't interfere with the STAR's at any point. Just imagine different tubes ,not intersecting eachother.
Due to the gates ,it was easy to space very well the aircrafts,and it worked even in very busy airports.Only problem was that the trials were done only by the base airline,so it worked only at off peak hours. But when will become normal practise for every one,my bet is it will work fine.
And just think,at FL 390 to receive clearance :Cleared to descent, Star XXX ,ILS rwy YY...' and at FL100..."cleared ILS YY..." and that's all.
The main problem that rended the BCDA inoperable was weather, when CB existed in the aircraft path,the BCDA was discontinued and the ATC was informed about this.
Also ,in order to get the necessary spacing, the ATC may ask for a specific descent speed from t/d to FL100 ,but the descent was also done in VNAV with idle thrust ,at that required speed.
Bellow FL100 ,the speeds were specified on the BCDA STAR.(the trial ones we had )
All descent was done in VNAV ,and the STAR included some mandatory gates.
Those gates were calculated with a mandatory speed ,and a minimum/maximum FL (altitude).
By doing this trials with different types,at different airports around europe,it was possible to establish the necessary gate dimension ,in order to be able to squeze in it a 737/ 320 and also bigger planes (like 747 ).
All descent was done in VNAV, with engines at idle thrust ,with FMC computed speeds untill FL100, and with gate mandatory speeds bellow FL100. No thrust was required till G/S intercept. We did few hundred flights,and it was ok,we also saved some fuel.
One good thing is less interference from the ATC, also the SID's can also be planned (as a continuous climb..) so they won't interfere with the STAR's at any point. Just imagine different tubes ,not intersecting eachother.
Due to the gates ,it was easy to space very well the aircrafts,and it worked even in very busy airports.Only problem was that the trials were done only by the base airline,so it worked only at off peak hours. But when will become normal practise for every one,my bet is it will work fine.
And just think,at FL 390 to receive clearance :Cleared to descent, Star XXX ,ILS rwy YY...' and at FL100..."cleared ILS YY..." and that's all.
The main problem that rended the BCDA inoperable was weather, when CB existed in the aircraft path,the BCDA was discontinued and the ATC was informed about this.
Also ,in order to get the necessary spacing, the ATC may ask for a specific descent speed from t/d to FL100 ,but the descent was also done in VNAV with idle thrust ,at that required speed.
Bellow FL100 ,the speeds were specified on the BCDA STAR.(the trial ones we had )
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Rule of thumb - Acft slow down 10 KIAS every nm in level flight.
Read somewhere (maybe Av & Tech) ATC is testing new software sys in SFO. They send a CPDLC msg to cruising acft with a suggested lateral & vertical path in order to maintain a CDA at idle thrust setting, considering all the expected inbound traffics. If accepted it becomes a clearence. Seems it's working good.
HVIII
Read somewhere (maybe Av & Tech) ATC is testing new software sys in SFO. They send a CPDLC msg to cruising acft with a suggested lateral & vertical path in order to maintain a CDA at idle thrust setting, considering all the expected inbound traffics. If accepted it becomes a clearence. Seems it's working good.
HVIII
