huv wrote:
But as an instructor, I find that IFR students and pilots spend (too) much time and energy trying to understand this – because before CDFA, everyone understood the difference between DA and MDA.
It becomes clear if you consider that the difference has to do with flight technique. When you use a DA, you go missed when that altitude is reached unless you have the necessary visual references. When you use a MDA, you level off at that altitude and go missed at the MAPt unless you have the necessary visual references.
Before the advent of CDFA, the flight technique depended on the approach type so that precision approaches were always flown with a DA and non-precision approaches were always flown with a MDA. With CDFA you can choose the flight technique for a non-precision approach.
(Technically speaking you have to check for the MAPt also when flying a CDFA, but in practise you will reach the DA well before reaching the MAPt unless you seriously mishandle your descent.)
bookworm wrote:
Thanks for the pointer to the FSIMS search page — OpSpec easily found. Do you have a reference to the MITRE study you mention please? A quick search of their tech pubs came up blank.
No, but i know the individual who managed the reports and I have a copy. Send me a PM and I will locate them and send to you directly.
NCYankee wrote:
There are two ways the CDFA is typically flown, one using CDFA as a method to arrive at the MDA, and then fly level at the MDA to the MAP.
This would not fall within the CDFA definition used in Europe:
‘continuous descent final approach (CDFA)’ means a technique, consistent with stabilised approach procedures, for flying the final-approach segment of a non-precision instrument approach procedure as a continuous descent, without level-off, from an altitude/height at or above the final approach fix altitude/height to a point approximately 15 m (50 ft) above the landing runway threshold or the point where the flare manoeuvre shall begin for the type of aircraft flown;
Personally I think it’s the worst of both worlds — there’s a greater chance of arriving late at the decision point than with an expeditious descent on a conventional NPA (“dive and drive”, though the “dive” might just be 4 degrees rather than 3 degrees) and it has all the disadvantages of a level segment when it comes to approach stability.
Thanks for the pointer to the FSIMS search page — OpSpec easily found. Do you have a reference to the MITRE study you mention please? A quick search of their tech pubs came up blank.
When I teach the CDFA method, the MDA is still the MDA and one needs to add a buffer (often called a derived DA), typically 50 feet above the MDA so that the MDA is not busted. There are advantages and disadvantages when using this method verses the drive and dive for a GA piston aircraft. There are two ways the CDFA is typically flown, one using CDFA as a method to arrive at the MDA, and then fly level at the MDA to the MAP. The other is to treat the MDA+50 as a DA, make the decision at the adjusted DA, and begin the missed approach climb at that point if the runway environment is not seen, continue climb along the FAC until the MAP, then follow the balance of the missed approach procedure.
When visibility is good and ceilings are above the MDA, the CDFA is a good choice. When visibilities are low or close to the minimums or ceilings are close to minimums, CDFA is much more likely to result in a missed approach than dive and drive. Although a MITRE study provided good evidence that using a CDFA is safer than DND for turbojet aircraft, there study did not reach the same conclusions for piston aircraft where the accident rate was slightly favoring DND, although not statistically significant.
bookworm wrote:
The latest version of the OpSpec C073 policy note I can find seems to have a “cancellation date” in 2014. Is there a more recent version please?
In my previous post, I quoted the current OpSpec. Best place to find the current OpSpec is on the FAA FSIMS website at http://fsims.faa.gov/home.aspx
Search on C073.
Cobalt wrote:
I get that it needs to be approved for the approach. Operator approvals still make no sense, or am I missing something? What makes an operator special so they deserve an approval the other operator – or us GA types – does not have?
Certificated operators in the US have to be approved for Opspec C073 in order to be allowed to use a DA in Lieu of an MDA in the US. The certificated operators have the training, operational control, and standards to follow the guidance in the Opspec to mitigate any risks with using the MDA as a DA. General aviation pilots simply do not have an equivalent level of training or compliance and undoubtedly would apply an MDA as a DA where the risks are in no way mitigated. Note not all procedures are compliant with the requirements of the Opspec and pilots must be trained to follow the approved procedures. See section e which I have highlighted. I have copied the most current version (effective 2/10/17, Cancellation 2/10/2018) of the wording in the Opspec below. Note how conservative the application of this Opspec is, it eliminates most approaches in the US as for all practical purposes, the runway needs to already be served by a vertically guided approach or VGSI system that eliminates any hazard.
Appendix A. Sample OpSpec C073, Vertical Navigation (VNAV) Instrument Approach Procedures (IAP) Using Minimum Descent Altitude (MDA) as a Decision Altitude (DA)/Decision Height (DH): 14 CFR Part 121
a. The certificate holder is authorized to use minimum descent altitude (MDA) as a decision altitude (DA)/DH with vertical navigation (VNAV) on a Nonprecision Approach (NPA). The certificate holder will use operations specification C073 in conjunction with Operations Specification C052, Straight-In Nonprecision, APV, and Category I Precision Approach and Landing Minima—All Airports. The certificate holder is authorized to conduct instrument approach operations using the following airplanes and RNAV systems certified for these VNAV operations as listed in Table 1 below.
Table 1 – Authorized Airplanes and Equipment
Airplane Type
(M/M/S)
Area Navigation System
(Model/Version)
Remarks
NOTE: New software versions do not have to be updated in the table if inspectors confirm an advisory vertical guidance capability remains after the software update. The confirmation should be confirmed by the updated Service Bulletin (SB), a manufacturer/Original Equipment Manufacturer (OEM) statement, or any other FAA-approved method.
b. Public Vertically Guided IAP Assessment. Obstacle clearance surface (OCS) assessments protect the instrument procedure, including the missed approach. Glidepath Qualification Surface (GQS) assessments protect the landing area and are accomplished on 14 CFR part 97 IAPs with a published DA/DH. These approaches conform to the U.S. standard for Terminal Instrument Procedures (TERPS) and include the ILS, Ground Based Augmentation System (GBAS) Landing System (GLS), RNAV RNP, RNAV GPS IAPs with a localizer performance with vertical guidance (LPV) DA, and/or lateral navigation (LNAV)/VNAV DA.
NOTE: C073 provides protection for the temporary altitude loss below the MDA when performing a missed approach at an MDA used as a DA/DH. The use of an MDA as a DA/DH does not ensure obstacle clearance when continuing the approach from the MDA to the landing runway. Operators must see and avoid obstacles between the MDA and the runway when 14 CFR part 91, § 91.175 requirements are met and the approach is continued below the MDA for landing.
c. Authorized Approaches. The certificate holder may fly all part 97 nonprecision straight-in IAPs listed as authorized in operations specification C052, Table 1, columns 1 and 2, using an MDA as a DA/DH if the approach meets one of the following requirements and all subcomponents.
(1) Serves a runway that has a published RNAV IAP (“RNAV (GPS),” “RNAV (RNP),” or “GPS” in the title) with a published LNAV/VNAV or RNP DA and:
(a) Is selected from an approved and current database.
(b) Has the exact published final approach course as the RNAV IAP.
(c) Has a published Vertical Descent Angle (VDA) coincident with or higher than the barometric vertical guidance (glideslope (GS)) on the published RNAV IAP.
(i) A published VDA is not required when using the LNAV minima line on an RNAV approach that also has a published LPV and/or LNAV/VNAV DA.
NOTE: The VNAV path must cross at or above all stepdown fix altitudes. The stepdown fix crossing altitudes must be referenced on the barometric altimeter.
NOTE: The VDA is advisory guidance only. Flying the published VDA below the MDA does not guarantee obstacle clearance.
(2) Serves a runway that has a published ILS, GLS, or RNP IAP with LPV minima and:
(a) Is selected from an approved and current database.
(b) Has the exact published final approach course as the ILS, GLS, or RNP IAP.
(c) Has a published VDA coincident with or higher than the electronic GS on the published ILS, GLS, or RNP IAP.
(i) A published VDA is not required on an ILS/Localizer (LOC) approach when the ILS GS is out of service and the approach is flown using LOC‑only procedures.
(ii) A published VDA is not required when using LNAV minima on an RNAV approach that also has a published LPV or LNAV/VNAV DA.
NOTE: The VNAV path must cross at or above all stepdown fix altitudes. The stepdown fix crossing altitudes must be referenced on the barometric altimeter.
NOTE: The VDA is advisory guidance only. Flying the published VDA below the MDA does not guarantee obstacle clearance.
(3) Serves a runway to an airport operating under 14 CFR part 139 with a Visual Glide Slope Indicator (VGSI).
(a) The VDA or GS on the published final approach course must be coincident with or higher than the published VGSI descent angle.
(b) The published final approach course is within plus or minus 4 degrees of the runway centerline (RCL) course.
NOTE: Operators should refer to the FAA Chart Supplement (formerly the Airport/Facility Directory) to verify that there are no VGSI restrictions if the final approach course is offset from the extended RCL.
d. VNAV Path Angle. The VNAV path angle must be greater than or equal to 2.75 and equal to or less than 3.77 degrees for Category A, B, and C airplanes; and greater than or equal to 2.75 and equal to or less than 3.50 degrees for Category D airplanes.
e. Operational Restriction. The certificate holder will not use an MDA as a DA/DH if the requirements specified in this operations specification are not met. The certificate holder may use a continuous descent final approach (CDFA), but will begin the missed approach at an altitude above the MDA that will not allow the airplane to descend below the MDA.
f. Required Training. Flightcrews must be trained in accordance with the certificate holder’s approved training program, to include VNAV procedures and the instrument procedures listed in C052.
Airborne_Again wrote:
huv wrote:
No, my impression is that Jeppesen plates for 2D approaches show MDH and 3D approaches show DH.
My impression is that Jepp show DA for every 2D approach that can be flown CDFA — which is virtually all of them.
I get that. The difference between the statements is which approaches are 2D but can be flown CDFA.
I understand that the whole CDFA business originated somewhere away from GA, perhaps driven by the big jets (and their passengers) being unsuitable for dive-and-drive like maneuvers. I also imagine that big, commercial aircraft usually fly CDFA maneuvers using their FMS system – to actually give the (auto)pilot some kind of vertical guidance. Isn’t that getting very close to 3D operation by definition? In any case, using DA in lieu of MDA when descending ILS-like guided by an FMS seems to make sense from an operational point of view.
Then I come along in the club Cherokee, trying to shoot the glide-inop-ILS, or the NDB, flying CDFA using raw data and a very manual pilot. Which means that on final approach I am quite busy establishing the correct rate-of-descent corresponding to my ground speed, and then checking altitude to make vertical nav corrections frequently. Quite a non-precision vertical profile in the true sense of the word. Chances are that I will not hit the VDP head-on, but go either a little above (will reach DA after VDP and, if visual, be tempted to dive for the threshold), or hit the (M)DA a little before the VDP (I would likely level out at DA for a short while to see if I get visual at VDP, or I can even continue to MAPt, although that is against CDFA philosophy).
This is where I get the question: what is the implication of descending for a DA instead of for an MDA?
Very often those DA numbers are exactly the same as the MDA numbers on the previous version of the plate, from before CDFA. That makes the question even more likely. “Why have they changed it from MDA to DA? Does it mean I cannot level out on the DA and fly level to the VDP? Or does it mean that I may actually go slightly below DA when going missed, just as when I fly the ILS?”
I agree that operationally this is not particularly critical. I do not give this much thought when just flying for myself. But as an instructor, I find that IFR students and pilots spend (too) much time and energy trying to understand this – because before CDFA, everyone understood the difference between DA and MDA.
NCYankee wrote:
The US Jepp charts show MDA(H) for 2D approaches.
Ok, but in Europe they (generally) don’t.
@NCYYankee,
I get that it needs to be approved for the approach. Operator approvals still make no sense, or am I missing something? What makes an operator special so they deserve an approval the other operator – or us GA types – does not have?
NCYankee wrote:
The runway and obstacles have to be compliant with the guidance in C073 and the operator must be approved.
The latest version of the OpSpec C073 policy note I can find seems to have a “cancellation date” in 2014. Is there a more recent version please?
