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# CDAP approach

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In the Boeings you can do a CDAP approach by using V/S and verifying your descent rate by watching the green arc. How is it done in the MD-11, does anyone know? Also, can you legally use FPA instead of V/S?Tom

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What is a CDAP approach?

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CDAP = Constant Descent Approach ProceduresMike Ray writes: This is defined as flying that part of the approach from .3 miles outside the FAF (Final Approach Fix) to MDA +50 using a predetermined Vertical Rate of Descent.http://www.skybrary.aero/index.php/Constan..._Angle_Approach

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Thanks, Daryl. Sounds interesting but I wouldn't try that here in Corpus Christ where the winds vary significantly from surface to 1000 agl, especially when a front is moving through the midwest North of us. Then by that definition is it a VFR approach or a technique used on non-precision approaches?If non-precision approach, then IFR minima apply. I wouldn't understand how to use the FPA has an indicator in this case... the PROF allows for a min height for non-precision approaches.

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In the Boeings you can do a CDAP approach by using V/S and verifying your descent rate by watching the green arc. How is it done in the MD-11, does anyone know? Also, can you legally use FPA instead of V/S?Tom
I think what you mean is an approach with engines in idle from Top of descent until FAF/FAP with a glidepath of 3 degrees. In the MD11 there is no green arc indeed. So you have to calculate your required descent rate, use the 1:3 rule or just follow PROF-path. You can legally use FPA, but FPA is mainly used during Non-precision/visual approaches. Not during descents.Regards, Floris

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I think what you mean is an approach with engines in idle from Top of descent until FAF/FAP with a glidepath of 3 degrees.
No, that's not what I mean. CDAP is for non-precision approaches, constant rate of descent inside the FAF, typically following a 3-degree glide angle. As opposed to descending to the minimum altitude, then flying a level segment.Tom

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No, that's not what I mean. CDAP is for non-precision approaches, constant rate of descent inside the FAF, typically following a 3-degree glide angle.Tom
Ok, in that case, you can use FPA. 0.3NM prior FAF, select FPA -3.0 degrees.I only know Continious Descent approach like explained here http://en.wikipedia.org/wiki/Continuous_Descent_Approach

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I wouldn't understand how to use the FPA has an indicator in this case... the PROF allows for a min height for non-precision approaches.
Well, instead of calculating the vertical speed that would give a 3-degree glide path (or whatever is required), I'd just dial the required descent angle directly. I'm just not sure if you are supposed to or not.Tom

Here

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Maybe. I believe (but could be wrong) that the FPA indication is based on aircraft performance in the air without regard to windage. With a typical head wind, a 3 deg descent indicated on the FPA will undershoot the desired glide path with respect to the ground. In other words, the FPA indication has no ground reference.The whole concept has it's real world limitations. I was taught in the 70's to descend to the MDA, then the concept of the constant descent gained popularity a decade later during my commercial training. The tradeoff is once you get to the MDA and see the airport environment you can legally go below the MDA whereas you may still be in the cloud at MDA if you delay getting down there if the clouds have ragged bottoms. I've experienced this first hand in the low stratocumulus that will seasonally occur along the gulf coast.Both procedures are good but pilot judgement should dictate how to shoot the approach.EDIT: Of course, this is irrelevant if my assumption regarding FPA and windage is wrong.

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Maybe. I believe (but could be wrong) that the FPA indication is based on aircraft performance in the air without regard to windage. With a typical head wind, a 3 deg descent indicated on the FPA will undershoot the desired glide path with respect to the ground. In other words, the FPA indication has no ground reference.The whole concept has it's real world limitations. I was taught in the 70's to descend to the MDA, then the concept of the constant descent gained popularity a decade later during my commercial training. The tradeoff is once you get to the MDA and see the airport environment you can legally go below the MDA whereas you may still be in the cloud at MDA if you delay getting down there if the clouds have ragged bottoms. I've experienced this first hand in the low stratocumulus that will seasonally occur along the gulf coast.Both procedures are good but pilot judgement should dictate how to shoot the approach.EDIT: Of course, this is irrelevant if my assumption regarding FPA and windage is wrong.
FPA is calculated by IRS and is very accurate. V/S is adjusted automatically as the groundspeed changes due to wind or when airspeed changes during approach. Regards Floris

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Thanks very much for that tidbit... wow, pretty nice tool, which I have not taken the time to understand but definitely will not ignore it in the future. However, the strategy of getting down early to have a look is still valid, right? Thanks Floris.

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Downscc, I suspect the main advantage of CDAP is that there are less changes in attitude and power during the final approach which I

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Thanks very much for that tidbit... wow, pretty nice tool, which I have not taken the time to understand but definitely will not ignore it in the future. However, the strategy of getting down early to have a look is still valid, right? Thanks Floris.
Where FPA is much preferred above V/S to fly non-precision approaches to control the vertical flightpath, have a look at another MD11 feature as well.To fly an inbound course from the FAF to the RWY theshold, using TRACK Mode iso HDG, will correct for any (changing) crosswind conditions during approach and also correct DriftAngle for changing approach speeds.Regards,Harry

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Where FPA is much preferred above V/S to fly non-precision approaches to control the vertical flightpath, have a look at another MD11 feature as well.To fly an inbound course from the FAF to the RWY theshold, using TRACK Mode iso HDG, will correct for any (changing) crosswind conditions during approach and also correct DriftAngle for changing approach speeds.Regards,Harry
One more note,You could fly a CDAP in V/S if you change the MCP to meters instead of feet. Some Navigraph nDAC charts show the MDA in both meters and feet, or you'll just have to make those caculations on your own.

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One more note,You could fly a CDAP in V/S if you change the MCP to meters instead of feet. Some Navigraph nDAC charts show the MDA in both meters and feet, or you'll just have to make those caculations on your own.
Kevin,Missing a point here I'm afraid.The MD-11 is different from other Boeings (except 777) in it's ability to fly non-precision approaches, using AP/FD coupled FPA and TRACK.Once you get used and understand these features, the choice what modes to use for this kind of approaches will become very clear.Regards,Harry

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I've flown CDAP's in V/S with the MD11. It's not that hard to do.

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I've flown CDAP's in V/S with the MD11. It's not that hard to do.
Sure, nothing against using V/S for this.My experience is that FPA and TRK modes make it even less hard to do.So I use it for simulation purposes also.Regards,Harry

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Boeing's also have a FPA and TRK. The only differences I've found between the two is not being able to adjust your TRK PTH via the MCP on a Boeing. But TRK PTH is still visible on the EHSI in MAP mode. Also the fact that I tend to use meters more often in the MD11 vs FT on a Boeing. But it's relatively the same thing.

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I imagine each will have his preference for doing a CDAP. I prefer FPA, but as long as you stay on profile, you're safe, so it doesn't matter.The one thing that annoys me about the MD is that PROF really sucks because it will level off at MDA. A Boeing will allow you to stay in VNAV until just before touchdown. Another thing I miss is the "green arc". A superior tool for sure. I spend many sleepless nights wondering why Airbus and MD decided not to put it in.

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