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RNAV (GNSS)

Old 9th Dec 2014, 03:30
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It would be interesting to know the logic and what they think they are achieving.
VDA only provides advisory obstacle protection until the MDA. Once below the MDA, the VDA is no longer valid -- you are now flying a visual segment and thus must (visually) fly clear of obstacles.

But most pilots don't know this. Most think VDA provides obstacle clearance to the runway. So with the VDA plugged in, pilots tend to "blindly" follow the glideslope all the way down to the runway threshold. This is dangerous if there are obstacles penetrating the 34:1 (1.68 degree) protection surface.

The example commonly cited is KBHM RNAV (GPS) RWY 36 (Birmingham, AL). If you fly the 3.04 degree VDA to the MDA, then transition to land visually, no problems. But if you continue flying the VDA below the MDA, you get perilously close to a house sitting on top of a ridge, 2nm from the runway. The roof of the house penetrates the 34:1 surface, providing just 190ft clearance below the VDA path (picture below):


House penetrating 34:1 surface @ KBHM RWY 36 (from FAA briefing notes)

So the FAA will no longer publish a VDA if there are obstacles penetrating the 34:1 surface.
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Old 9th Dec 2014, 08:58
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peekay4, thank you for your response and image. Even after your explanation I still don't know what the FAA are achieving by removing the VDA.

VDA only provides advisory obstacle protection until the MDA. Once below the MDA, the VDA is no longer valid -- you are now flying a visual segment and thus must (visually) fly clear of obstacles.
This is correct, but when the aircraft is visual don't we still expect that the VDA can be maintained to the threshold?. After all as procedure designers don't we build a VDA from the threshold for the aircraft to fly? I would not expect an aircraft to have to change the descent profile to get to the threshold. This is why the VSS was invented. If the VSS is not penetrated then we expect the aircraft can fly a constant profile to the threshold. If it is penetrated then you increase the VDA or remove the straight in minima. (ICAO criteria) Removing the straight in minima forces the aircraft to get visual at a higher altitude and thus have time to visual mitigate the offending VSS penetration.

Question: When the FAA remove the VDA are they also removing the straight in minima?

To try to explain better what I am saying, lets take the Birmingham example, and lets assume that there is no VDA published....

The example commonly cited is KBHM RNAV (GPS) RWY 36 (Birmingham, AL). If you fly the 3.04 degree VDA to the MDA, then transition to land visually, no problems. But if you continue flying the VDA below the MDA, you get perilously close to a house sitting on top of a ridge, 2nm from the runway. The roof of the house penetrates the 34:1 surface, providing just 190ft clearance below the VDA path (picture below):
With no VDA published the pilot still wants to fly as stable approach as possible and now has to guess a suitable descent profile. With no guidance from the approach plate...lets assume he guesses 3 degrees (cause thats a normal profile)...he now has much less than 190ft clearance from the house.

I would suggest the approach profile for this approach is too shallow. So as I see it you have 2 choices. Increase the profile 3.1 or 3.2 should do it...or publish a circling minima only ( I would put an aeronautical light in the roof as well). This forces the aircraft to have to visually mitigate the obstacle.

I just don't see how removing the VDA mitigates this problem..
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Old 9th Dec 2014, 13:16
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Seems like the FAA didn't do their job with those houses. They appear to penetrate Part 77 surfaces for Runway 36 and should have been issued Hazard Determinations.

Also, where was the airport management when those houses were proposed?
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Old 9th Dec 2014, 15:24
  #44 (permalink)  
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Peekay4:


So the FAA will no longer publish a VDA if there are obstacles penetrating the 34:1 surface.
That is not correct.

Since I couldn't find a Terps Instruction Letter or policy memo on AFS-420's website, I asked the author of Order 8260.19. He advises me it is still somewhat a "hot potato." It is still flight inspection's call. If they feel it should be removed, then in some cases Aero Nav designers first try to raise the angle.
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Old 9th Dec 2014, 18:31
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I asked the author of Order 8260.19. He advises me it is still somewhat a "hot potato." It is still flight inspection's call.
Yes, I shouldn't have been so definitive.

I believe the actual process is as follows:

1. If the 34:1 surface isn't clear, then Flight Inspection will assess the approach
2. During assessment, the VDA guidance will be flown "one dot low"
3. If stable descent can't be maintained at "one dot low", then VDA will not be published
4. Similarly, if a EGPWS warning occurs, then VDA will not be published

Question: When the FAA remove the VDA are they also removing the straight in minima?
VSS is a (relatively new) ICAO concept. FAA uses TERPS to determine if an approach meets the straight-in criteria.

I'm not a TERPS expert but I believe despite the obstacle, KBHM RNAV RWY 36 does meet the straight-in criteria:

- The final approach course is aligned with runway
- The computed VDA at 3.04 degrees is within limits (to MDA)
- Required Obstacle Clearance (ROC) of 250 ft is maintained when approach is flown as designed (leveling off at MDA)
- Height above touchdown (HAT) is < 1000 ft

FAA allows the 34:1 (and 20:1) protection surfaces to be penetrated. However, higher visibility limits will be imposed as a result. E.g., in this case, the minimum visibility was raised to 1 SM (5000 RVR) for Cat A

I do agree that this approach should be revisited; e.g., moving the FAF so a descent angle of 3.2 degrees can be maintained throughout.
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Old 9th Dec 2014, 19:13
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Protection all the way to the runway TDZ?

It is a hot potato specifically because both TERPS and PANS-Ops have largely ignored this issue of assuring a safe flight path below DA(H) to the TDZ since the concept of original TERPS and before before CRM was invented (and well before 1975). Hence ONLY [real] RNP protects all the way to the TDZ, and all the way back out, for a balked landing from the TDZ, considering both rare normal and selected non-normal events (e.g., the first nav failure, and the first engine failure on a multi-engine aircraft). Hence NO TERPS or PANS-Ops procedure would ever pass a serious scientifically derived SSA, FMEA, or FHA, at anything other than a largely trivially un-obstructed runway. This was one of the original and continued flaws in both TERPS and PANS-Ops, which led to RNP, and for which (other than the well configured and signed AC120-29A), still has unnecessary, counterproductive, and flawed criteria even being used by FAA in their other RNP references for their version of pseudo RNP [lite] (e.g., AC90-101A and AC90-105). This is why airlines using [real] RNP procedures typically use criteria related back to the criteria of AC120-29A, and not other more recent FAA RNP related criteria.
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Old 9th Dec 2014, 22:24
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Peekay4, again thankyou for the information (its interesting to see how the rest of the world deals with this)...and no I am not picking on you. I am really trying to understand the FAA course of action. (I owe you a beer for all the hard questions)

So, after all that,

It appears the example cited to explain the why the FAA are removing VDA's actually meets the criteria and in all likely hood was removed by a flight inspection pilot because he/she got the heeby jeebies about the house on the hill.

I don't know too much about the accident being referred to, but at the end of the day they landed short? Correct? This indicates to me that a whole bunch of things went wrong....not limited to a) following vertical guidance below the MDA b) loss of situational awareness when doing this, which leads to c) poor airmanship.

I am not surprised that they hit the ground whilst following vertical guidance....that's where vertical guidance is supposed to take you. The fact that they didn't hit anything on the way to the ground indicates to me that the other design considerations to protect to the ground are valid as well. If the aircraft had left a smoking hole at the top of the hill and wiped out the house, then we have an argument.

If they missed the house on the hill....which I assume they did....then isn't the house on the hill a bit of a red herring?

3. If stable descent can't be maintained at "one dot low", then VDA will not be published
If stable descent can't be maintained at "one dot low" then the incorrect VDA has been used to design the approach.

Question for everyone. How does removing the VDA make this situation better or safer?
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Old 10th Dec 2014, 01:00
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Tom Imrich:


The KRIL RNP based approach minima at RNP .1 could still safely be even lower, if the present obsolete and unnecessary limitations were removed that are still being placed on RNP procedure design. The fully allocated real VEB could easily provide for DA(H) down to and even somewhat below 200' HAT compared to the higher DA(H) shown for the RNAV(RNP) Z Rwy 8. The present 250' HAT floor limit was only put there for largely political reasons, not due to any technical limitation of engineering or physics. That potential additional advantage for RNP is particularly valid considering that any aircraft with RA available could also use an RA floor to additionally bound improbable to extremely improbable VNAV non-normal events. Unlike with TERPS or PANS-Ops, with [real] RNP, particular non-normal as well as rare-normal events are already suitably addressed and accommodated.
Tom, you are missing the climb gradient limitations on both runway ends at KRIL. That's a fact of RNP AR life, although you may not agree with it.

When you were at the FAA TERPs was in the dark ages compared to today. You know, I was involved then as well.

We have the GQS, which we did not have circa 1975. We have the visual segment, which we did not have back then. And, as tentative as it is on some NPAs, we have the VDA, which we did not have back then.

As you well know, there are runway ends that you can't make a silk purse out of a sow's ear.

Speaking of RNP AR, let's get away from baro in the final segment by having the avionics switch from RNP to LPV for the last 2, or so, miles to the runway end. Or, would Boeing not like that idea?

As we all know, the original model of RNP AR was predicated upon the equipage of Boeing transports at the time.
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Old 10th Dec 2014, 11:55
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If stable descent can't be maintained at "one dot low" then the incorrect VDA has been used to design the approach.
Yes and no.

Non-precision CDFA is also a relatively new concept. There are literally thousands of non-precision approaches in the US alone -- developed long before the CDFA concept -- which have obstructions in the 34:1 and 20:1 protection surfaces.

These old approaches were never designed for continuous descent. What mattered in the past was meeting the required clearance at the MDA floor after a series of step-downs.

So the VDAs for these approaches are constrained by the existing designs. The VDA isn't "selected". By rule, the VDA is simply the angle from the existing FAF to the TCH.

I.e., for these thousands of approaches, the VDA is calculated "after the fact".

This "after the fact" calculation is ok if the VDA is only used to the MDA. Below the MDA, however, there's no obstacle clearance guarantee if the 34:1 surface is penetrated.

Question for everyone. How does removing the VDA make this situation better or safer?
FAA did some tests. With the computed VDA, predictably every approach even when flown perfectly got too close "to the house" (~190 ft) and triggered EGPWS warnings. This is an accident waiting to happen.

Without the VDA, these old approaches get flown "as they were designed" (dive-and-drive). Meaning, pilots would "correctly" level off at the MDA and pass well clear of the house at a protected altitude, before making a visual landing.

So until the approach can be redesigned, we have to do a risk analysis: Which option is worse? a) using CDFA but getting perilously close to obstacles; b) reverting back to dive-and-drive but remaining well clear of obstacles.

FAA picked option b).
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Old 10th Dec 2014, 13:24
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@ peekay4...

I agree with your interpretation as to why the FAA is "taking out" the VDA info off the approach plates on "non precision" approaches.

It seems to be a fact that some pilots think that on a non precision LNAV or LNAV/VNAV approach they are protected from obstacles on that last segment of the approach and the question is why?

There as not been one recurrent ground school I have been too that this is not brought up and explained. It is also brought up in the sim and also during your initial type rating when in the FMS and approach selections are talked about.

Now I believe someone asked about if 1 or 2 GPS are required for an approach.

For non precision approaches like LNAV and LNAV/VNAV, only one GPS is required but for a precision approach like a LPV with their lower minimums, approaching the FAF you need to see dual independant LNAV/GPS sources, in other words 2 GPS are required (like in a CAT II ILS requires 2 ILS receivers) to continue down to LPV minimums.


Now where I'm not sure I agree with you is the descent below MDA statement you brought up.

Again in class this subject was brought up and not for the first time the instructors are clear on this and this is valid for boththe FAA and Transport Canada... Anyone that goes below a MDA altitude on an approach (before being visual) will be busted thus the reason and option (if granted) to add 50 feet to the MDA altitude and to now treat it as a DA.
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Old 10th Dec 2014, 13:57
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Hi Jet Jockey,

For the last part, have you read the Transport Canada exemption cited previously?

EXEMPTION FROM PARAGRAPH 602.128(2)(b) OF THE CANADIAN AVIATION REGULATIONS - Transport Canada

This CAR exemption allows descent below MDA during a missed approach incorporating "stabilized constant descent angle" (Canada's term for CDFA) . The exemption is valid for qualified operators until April 30, 2015.

Under this exemption, you don't need to add "50 feet" to the MDA (aka Derived Decision Altitude -- DDA) unless:

- there is a failure of an aircraft system;
- the aircraft is above normal maximum landing weight;
- the aircraft landing weight is limited by aborted landing climb performance; or
- height loss could be expected to be larger than normal.

The main text of the exemption is quoted below:

The purpose of this exemption is to permit pilots-in-command of IFR aircraft operated by holders of an air operator certificate or a temporary private operator certificate to descend below the minimum descent altitude (MDA), when conducting a non-precision approach, even if the required visual reference necessary to continue the approach to land has not been established. This exemption is required in order to accommodate the altitude loss below MDA that will likely occur during a missed approach, following a stabilized constant descent angle (SCDA) non-precision approach.
The FAA has a similar policy as mentioned previously.
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Old 10th Dec 2014, 14:08
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Thanks for the clarification... I had missed the point about the CDFA although I don't get its point.

The whole purpose of adding the 50 feet to a MDA approach when flying a "VPATH" or one as to assume a CDFA is to not go below the MDA in the event of a missed approach.

I personally don't understand the FAA and TC on that one.
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Old 10th Dec 2014, 14:24
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Without the exemption in place:

- If you do a "dive-and-drive", you're allowed to descend to the MDA, have a look around, and stay there until the MAP in hopes of seeing the runway environment.

- If you fly CDFA, then you're only allowed to descend to the DDA (MDA + 50ft) before a mandatory missed approach at that point, even if you haven't reached the MAP.

This gives a "disincentive" from flying CDFA. You are essentially being penalized both in terms of minimum altitude and missed approach position for "doing the right thing."

Of course, some ICAO states simply say "tough luck, if you're an air carrier we're going to require you to fly CDFA until a DDA, by regulations".

TC and FAA have taken a different approach by removing the disincentive.
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Old 10th Dec 2014, 14:51
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Which is totaly stupid IMHO.

The whole point of the constant descent approach is get to a DA and if you see nothing, get the hell out by doing a missed approach.

For the last 15+ years I have been told by TC that the dive and drive concept is a thing of the past, that constant descent approaches are the thing to do for safety and now they allow this? The only exception to this is a circling approach which is another subject and one that I personally don't think should be allowed for jet operations. I'm glad to see that there is a move to increase the MDAs because of the increase distances per category of aircraft speeds for circlings.

My last airline (a while back) had 1000' and 3 miles as its circling minimums for our jet operation and today with GPS there is no reason not to have some sort of approaches to all runways.

In any case I much prefer the European view which is IMO, is the safer alternative. If an approach to a runway as a higher minimum so be it... A missed approach at its DA, regardless of height above the airport or distance from runway end is a must if the criteria for the approach are such that obstacles require a high MDA/ DA.
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Old 10th Dec 2014, 16:57
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and today with GPS there is no reason not to have some sort of approaches to all runways.
Terrain and obstacles.
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Old 10th Dec 2014, 19:02
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Terrain and obstacles are a possibility but IMO this could/would be in a minority of approaches at certain airport and when you look at what is capable with some GNSS approaches like the one in Rifle it is hard to believe some sort of GPS approach couls not be made available to most runways.

Besides if with much higher minimums a circling approach to a runway is available I'm pretty sure a GPS approach to that runway could be designed.
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Old 10th Dec 2014, 20:25
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Terrain and obstacles are a possibility but IMO this could/would be in a minority of approaches at certain airport and when you look at what is capable with some GNSS approaches like the one in Rifle it is hard to believe some sort of GPS approach couls not be made available to most runways.
Those are RNP AR approaches, which require a very high equipage bar. Most airframes simply do not have the equipment.

Besides if with much higher minimums a circling approach to a runway is available I'm pretty sure a GPS approach to that runway could be designed.
Alas, that is simply not the case.
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Old 11th Dec 2014, 19:22
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Sorry guys, I'm referrig back to post #18 relative to RNAV GPS 8 Y:
Is it allow to fly down to LNAV(only) minima using LNAV and VS or is mandatory to use LNAV and VNAV ?
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Old 11th Dec 2014, 19:51
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LNAV minima will be higher than LNAV/VNAV minima, the latter is for all practical purposes a precision approach and in the case of Jepp's there is no requirement to add 50'

You can fly an LNAV down to LNAV minima in either V/S or VNAV but again in Jepp's case its a non precision approach, it is normally recommended that it is flown in VNAV but may be flown in the alternative method using V/S, but why would you??

If it is a LNAV/VNAV RNP (BARO VNAV) it must be flown in VNAV and for the 737NG VOR update must be switched off and vertical RNP must be changed from 400' to 125'

All of the above not to be confused with an RNP-AR approach

simples.......
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Old 11th Dec 2014, 20:41
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To LNIDA: I agrre with you but in my Boeing FCTM I found:" RNAV approaches require LNAV and VNAV modes to be used regardless of the minima publihed" and this confused me.....
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