Top of Descent/ glide angle calculations
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For the descent, especially in Airbus aircraft it really depends what mode you are in as to what ROD and pitch angle you get as in a managed profile speed is the key driver, so therefore your pitch angle will vary through the various levels to meet constraints and speed targets. For least fuel, in a low aircraft congestion area pitch and speed will remain fairly constant. However in London, you rarely use a managed profile as headings and constraints will change what the FMGC has predicted. If you descend at flight idle with a selected speed, again pitch will vary to maintain the speed and roughly ROD of about 2000 - 2500fpm. I use the 3 x rule for height you need to lose plus a mile for any speed you need to lose per kt. I guess what I am trying to say is that you rarely achieve a constant descent glide angle - except for a long approach to LGW / LHR!! Best to expect the unexpected. Better to be slighty under the profile and if your going down you cant slow down!!!!
Just to complicate things further, A good technique I use to accelerate and climb, is to do so with a selected ias then you of course also increase your mach as you climb, however there will be some trade off with ROC (well for airbus aircraft anyway - fuel hungry boeings can do what ever they like!) It works for me.
Just to complicate things further, A good technique I use to accelerate and climb, is to do so with a selected ias then you of course also increase your mach as you climb, however there will be some trade off with ROC (well for airbus aircraft anyway - fuel hungry boeings can do what ever they like!) It works for me.
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Gentlemen (and Ladies?)
All I can say is “Wow!” There is some really interesting and valuable information floating around on this thread, and I’m going to copy it and keep for my future reference. Having said that, I am hoping that I might be able to ask a question of those here, specifically Smokey and Bubba, however any of the rest of you certainly are welcome to throw your comments in as well. I’d much rather have too much information than not enough.
I have been watching the results of the Pinnacle Airlines accident unfold with great interest. A CRJ200 on a repositioning flight was dispatched using, I think, FL370. But for some reason the crew decided to climb to the service ceiling of the airplane, FL410. Records show that temperature that night was ISA+10; the crew elected to climb at a constant ROC, and upon reaching 410, they were very slow with an uncommonly high pitch attitude. They remained at 410 for about 3 minutes (with decaying airspeed and increasing pitch attitude) and lost an engine, autopilot disconnected, lost the second engine, entered stick shaker, got stick pusher, never were able to get either engine back on line (even though the ADG did deploy and did provide some electricity for some things), and wound up crashing, killing both crew members.
As you can imagine, there have been a lot of questions generated about glide angle, rate of descent, distance capability, etc., etc. I have been on a campaign for some time now, trying to spur interest in having direct reading AOA indicators in airline cockpits, and it seems that this accident may be enough of a catalyst to make that argument. I recognize that several things change throughout the descent with respect to the descent angle and descent rate; but I would like to know the following:
With a power off situation such as this, if a direct reading AOA indicator is available, is there an AOA that might be used (even if it had to be adjusted a couple of times) during the descent that might be used, instead of recalling and applying all the very good information posted in this thread, that would allow the crew in such a harrowing and difficult circumstance to get the best return from the only source of energy they have available to them, height?
Thanks in advance.
_______
AirRabbit
All I can say is “Wow!” There is some really interesting and valuable information floating around on this thread, and I’m going to copy it and keep for my future reference. Having said that, I am hoping that I might be able to ask a question of those here, specifically Smokey and Bubba, however any of the rest of you certainly are welcome to throw your comments in as well. I’d much rather have too much information than not enough.
I have been watching the results of the Pinnacle Airlines accident unfold with great interest. A CRJ200 on a repositioning flight was dispatched using, I think, FL370. But for some reason the crew decided to climb to the service ceiling of the airplane, FL410. Records show that temperature that night was ISA+10; the crew elected to climb at a constant ROC, and upon reaching 410, they were very slow with an uncommonly high pitch attitude. They remained at 410 for about 3 minutes (with decaying airspeed and increasing pitch attitude) and lost an engine, autopilot disconnected, lost the second engine, entered stick shaker, got stick pusher, never were able to get either engine back on line (even though the ADG did deploy and did provide some electricity for some things), and wound up crashing, killing both crew members.
As you can imagine, there have been a lot of questions generated about glide angle, rate of descent, distance capability, etc., etc. I have been on a campaign for some time now, trying to spur interest in having direct reading AOA indicators in airline cockpits, and it seems that this accident may be enough of a catalyst to make that argument. I recognize that several things change throughout the descent with respect to the descent angle and descent rate; but I would like to know the following:
With a power off situation such as this, if a direct reading AOA indicator is available, is there an AOA that might be used (even if it had to be adjusted a couple of times) during the descent that might be used, instead of recalling and applying all the very good information posted in this thread, that would allow the crew in such a harrowing and difficult circumstance to get the best return from the only source of energy they have available to them, height?
Thanks in advance.
_______
AirRabbit
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The book that comes with the shiney bird I drive (B757) gives the figure of 3.5 miles per 1000 feet of altitude loss, this is at the economy Mach/ then IAS calculated by the FMS.
Interestingly...but not surprisingly..the whole thing comes down to energy management i.e. a heavier aircraft has more potential than a lighter aircraft, so at light weights we use 3 miles per 1000 feet, and at heavy weights we use 3.5 - 4.0 miles per 1000 feet. In practical terms, trying to match the desent profile is often impractical due to ATC constraints.
Interestingly...but not surprisingly..the whole thing comes down to energy management i.e. a heavier aircraft has more potential than a lighter aircraft, so at light weights we use 3 miles per 1000 feet, and at heavy weights we use 3.5 - 4.0 miles per 1000 feet. In practical terms, trying to match the desent profile is often impractical due to ATC constraints.
Thread Starter
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AirRabbit,
I started this thread as more of a curiosity, than as something really useful. But it has made me think about things more carefully. Your suggestion of using an AOA indicator has been suggested by others, and I often wonder why we don't have them in the civil aviation fleet, but this would digress into another thread subject (as an aside Boeing had a tech report about AOA use for the airline fleet).
In the interest of simplicity, maybe I should just remember what I was first told: "when you pull back on the yoke things on the ground get smaller; if you keep pulling back they will get larger again". Also, I was once asked to explain by an examiner what keeps airplanes in the air. After launching into the usual explanations, the examiner just snickered and pulled out his wallet. He then replied "it's this green stuff that keeps airplanes in the air, don't ever forget that".
regards,
flybubba
I started this thread as more of a curiosity, than as something really useful. But it has made me think about things more carefully. Your suggestion of using an AOA indicator has been suggested by others, and I often wonder why we don't have them in the civil aviation fleet, but this would digress into another thread subject (as an aside Boeing had a tech report about AOA use for the airline fleet).
In the interest of simplicity, maybe I should just remember what I was first told: "when you pull back on the yoke things on the ground get smaller; if you keep pulling back they will get larger again". Also, I was once asked to explain by an examiner what keeps airplanes in the air. After launching into the usual explanations, the examiner just snickered and pulled out his wallet. He then replied "it's this green stuff that keeps airplanes in the air, don't ever forget that".
regards,
flybubba
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AirRabbit
As a rough approximation, 1.5 * V stall, about a normalized aoa of .44 gives best L/D for todays jets for low level ops. I'd think for above 30,000 feet, use 1.6 *V stall at sea level, .39 normalized aoa, to factor in the rise in stall CAS due to compressibility.
Hawk
As a rough approximation, 1.5 * V stall, about a normalized aoa of .44 gives best L/D for todays jets for low level ops. I'd think for above 30,000 feet, use 1.6 *V stall at sea level, .39 normalized aoa, to factor in the rise in stall CAS due to compressibility.
Hawk
