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 Once below the MDA, the VDA is no longer valid -- you are now flying a visual segment and thus must (visually) fly clear of obstacles.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):


So the FAA will no longer publish a VDA if there are obstacles penetrating the 34:1 surface.

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.

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....

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..

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?

Peekay4:


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.

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

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.

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.

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?

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?

Tom Imrich:


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.

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.

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).

@ 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.

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 FAA has a similar policy as mentioned previously.

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.

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.

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.

Terrain and obstacles.

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.

Those are RNP AR approaches, which require a very high equipage bar. Most airframes simply do not have the equipment.

Alas, that is simply not the case.

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 ?

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.......

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.....

OK465
 

LPV (Localiser performance with Vertical guidance) Boeing aircraft are not equipped to utilise LPV minimums.

But i take your point :ok:

The FAA and Transport Canada recognize only the LPV approach as a "precision" approach.

LNAV and LNAV/VNAV are considered "non precision" approaches.

Any approach (LNAV and LNAV/VNAV) that shows its minimum as a MDA minimum must not be flown below that minimum in the event of a missed approach (usually) and that is the reason for some operations adding an increase of 50 feet to that MDA to make it a DA.

@ OK465...
 

That's because they added 77 feet to change it to a DA.

No, an LPV is not considered a precision approach.

LPV and LNAV/VNAV are considered Approach with Vertical Guidance (APV). APV minima depict DA(H).

LNAV is considered a Non-Precision approach (NPA). NPA minima depict MDA(H).

One does not "add" anything to an LPV or LNAV/VNAV procedure to "change it" into a DA. The DA is determined by obstruction clearance standards.

For definitions, ref: AIM 5-4-5, FAA Order 8260.3B, Transport Canada AC 700-023, and ICAO Annex 10.

Or see some discussion earlier in the thread.

Guess some of the procedures described here are company procedures.

That is the procedure we use for RNP AR approaches, but for normal LNAV/VNAV RNAV procedures we simply press APProach and go, there is no need for any other switching. On the bus we use a fully managed approach for normal RNAV LNAV/VNAV approaches, however in my opinion the implementation of "fully managed" is better on the boeing than on the bus.

Again, that depends. Especially to which temperature that LNAV/VNAV minimum can be used. If too low a temperature is chosen the minimum can be substantially higher than the bare LNAV minimum.

@ peekay4...
 

My mistake, I did not proof read my comment and you are correct about not adding the 50 feet to the LNAV/VNAV approach. I meant to say all other non precision approaches can be flown to a DA instead of MDA by adding 50 feet if your company is entitled to.

However I would have agreed with you on the LPV "non precision" approach status until a few days ago.

I'm just back from a recurrent training and the instructors at the training facility told us that the FAA just changed the status of the LPV approach (I assume this change happened not too long ago) and without giving us more specifics, that the LPV approach is now considered a "precision approach".

We were told the same thing in the sim by the sim instructor and our check pilot also alluded that he read something lately about LPVs from TC. His comment was that it was in the final stages of being approved by TC as a precision approach... Just passing on what I was told in class and in the sim a few days ago.

Also the aircraft manufacturer of the aircraft I fly as now changed the autopilot limitation to allow us to fly down to 80 feet on a LPV approach just like a CAT I or II approach. All other types of approaches are still restricted to 320 feet AGL with the AP engaged (which in itself means nothing on how it relates to LPV approach status).

Yes, and no. As of November (last month) the ICAO reclassification scheme went into effect and will eventually be fully incorporated by member states.

The new scheme introduces two types of approach methods (2D and 3D), two types of approach minima (Type 1 and Type 2), in addition to redefining three types of approach procedures (NPA, APV, and PA).

1. Approach methods:

2D: without vertical guidance, flown to an MDA(H)
3D: with vertical guidance, flown to a DA(H)

2. Approach minima:

Type A: minimums 250 ft (75m) or greater
Type B: minimums below 250ft (75m), and further divided into CAT I, II and III

3. Approach procedures:

Non-Precision (NPA): an instrument procedure (IAP) designed for 2D Type A
Approach with Vertical Guidance (APV): an IAP designed for 3D Type A
Precision Approach (PA): an IAP designed for 3D Type B

You can see even in the "new world" there is still a differentiation between APV and PA approach procedures, although they are both considered 3D procedures.

The beauty of all the above is I haven't had to specify any kind of equipment (ground & airborne) are required to fly an approach. The new classification better aligns with the Performance Based Navigation (PBN) concept.

With the new classification, in the future we can better "mix and match" what equipment can be used with which approach procedures (so back to the original question asked in this thread).

So how will existing LPV approaches fit into the new classification? Depends on the particular approach procedure. Many LPV approaches do meet the criteria for a Precision Approach procedure (e.g., at KAPA Rwy 35R), but other LPV approaches only meet the APV criteria (e.g., at KRIL Rwy 26).

Well I guess we just better start making pop corn and see how all of this will play out! :ok:

Why can't they just keep it simple? :ugh:

Haha, thanks for catching that. Fixed.

Life may end up being simpler with the new classification -- in the future. It's just that we're in transition now, so things seem complex since we have to live with the old and the new.

E.g., today we have so many types of approach procedures because they are based on the equipment required (NDB, Lctr, VOR, VOR/DME, ILS, LOC, MLS, GPS, GLS, RNP, etc., etc.) This situation will only get worse as new technologies come online.

In the future, we may end up with just three standardized approach procedures (NPA, APV, PA). That's it. Most aircraft will have GPS/RNAV and most airports will have at least one 3D approach.

From a pilot's perspective, all we care about if we're going to fly 2D or 3D. The FMS can automagically select the most appropriate approach technology & calculate the relevant minimums based on equipment available.

Simple. Maybe? Depends on how the transition goes, I guess.

200' ROC, 50' momentary descent.....until they get over that, you are stuck.

peekay4:

If it were only so. But look at all the LP approaches in FAA-dom now, and they are being added quite rapidly.

Yeah.

In 2010, ICAO resolution A37-11 set the goal that all instrument runway ends world-wide shall have APV (3D) approach procedures by 2016, with 70% to be completed by this year. Under some exceptions, straight-in 2D LNAV approaches may be substituted.

While this lofty goal will be hard to reach, the transition to RNAV/GNSS/PBN as the primary method for approaches seem inevitable.

Here's the 2012 snapshot of the progress towards that goal, with a number of member states already achieving much, from the ICAO database:

State (% PBN Runways completed, 2012)

Finland -- 96%
United States -- 94%
New Zealand -- 94%
Kenya -- 93%
Australia -- 91%

Austria -- 79%
Canada -- 75%
Egypt -- 71%

Brazil -- 62%
Germany -- 54%
Czech Republic -- 53%
Chile -- 52%

Costa Rica -- 40%
France -- 38%
Denmark -- 37%
Norway -- 36%
Switzerland -- 35%
Singapore -- 33%

Netherlands -- 26%
Thailand -- 24%
Japan -- 23%
Indonesia -- 21%
Russian Federation -- 20%
United Kingdom -- 19%
Spain -- 14%

China -- 9%
Ecuador -- 9%
Italy -- 8%
Sweden -- 5%
Turkey -- 3%

Mexico -- 2%
India -- 1%
Belgium -- 0%
Greece -- 0%
Ireland -- 0%
Israel -- 0%

(*) A number of small states with few instrument runways are either at 100% or 0% completion.
(**) These are 2012 numbers (latest in the published database), many states have made considerable progress since.

No doubt about it for IFR airports. (That leaves a whole lot of VFR airports without anything.)

And, the GNSS/GPS/RNAV IAP may be nothing more than LNAV to circling minimums and perhaps at a 90 degree angle to the runway. (example KEMT).

Make no mistake, the FAA didn't pay for WASS unless they wanted LPV to "every runway end." But, that was the high-level movers and shakers. Not those who faced with the hard realities of terrain and close-in runway environment.

Further, ILS at the principal U.S. airline airports will remain primary. As well as in the lofty ICAO world. :)

Well the intent (both FAA and ICAO) is to eventually replace the majority of ILS installations with WAAS or GBAS approaches (i.e., LPV and GLS). GLS CAT I is already operational in the US, and the FAA is currently testing GLS CAT III prototypes. (Only about 100 of the 1,200 ILS systems in the US are CAT II/III).

The FAA had planned to start phasing out some ILS installations by 2015, however the timing may be under review.

I believe the FAA is still committed to decommission the vast majority of VORs by 2020. A huge number of existing ILS procedures must be revised as a result. I wouldn't be surprised if many of the ILS procedures will be retired along with the VORs.

Politics and economics always trump the FAA. GLS is slowly progressing because one airline is willing to dabble with it.

But, think if you were a fleet manager at say, DAL or AAL, where Boeing wants a million dollars per airplane in your 150 bird fleet to change out your FMS and related gear to give you WAAS/LAAS and GLS auto-land capability. I don't know the exact count of CAT III ILS's but your number sounds correct. Those 100, or so, CAT III systems are at airports that get weather on a frequent enough basis that fail-active autoland is essential.

I those CAT III ILS's will be around for a lot of years.

As to VORs, the FAA is already pruning them, but rather slowly. They agreed with ICAO to not decommission any VORs that are the end point of oceanic routes. Also, few, if any, in the intermountain west.

If the politics of existing avionics hadn't driven the equipage of RNP AR, that system would transition from LNAV/VNAV to GLS and much lower minimums than we see today with RNP AR.

Well, DAL just performed their first GLS approach into Houston a couple of weeks ago (December 1). They already have 34 GLS-capable B737s, out of a total of 112 GLS-capable B737s on order. According to the FAA, DAL is also considering GLS for 45 A321 future deliveries.

AAL will have GLS on all of their B787s on order (GLS is a standard feature on new 787s).

From an economics perspective, these new technologies will allow more fuel-efficient operations at higher traffic densities.

Not to mention, the FAA currently spends $100 million each year for ILS maintenance. It would be a big win if this number can be cut in half through decommissioning. Maybe keep one ILS per airport for backup (as opposed to one per runway end), in addition to the CAT II/III ones.

I do expect to see CAT III ILS around for quite a long time. Regulatory progress is extremely slow. We have done GLS autoland trials five years ago with excellent results, however there is no apparent movement to allow lower than CAT I minima in europe. Even though we have been certified for operational use of GLS since 2009 there was only one airport in my country with an operational GLS until a few months ago, when FRA introduced the second GLS station and GLS approaches to all landing runways.

Getting approval was helped by the fact that boeing essentialy offered us GLS free of charge on all 737s since 2006. Sadly a retrofit on our A320 fleet would cost around a quarter million per aircraft which makes it economically unfeasible. Since we are now phasing out the 737 in favor of A320s we will lose our GLS capability. If we (or our on this site unmentionable major shareholder) orders new Airbii it might be included.

Didn't take Boeing very long. :)

Don't worry...

FAA FY2015 budget: $15 billion.

DoD FY2015 budget: $560 billion. Cost of satellite upgrades amortized over service life: $366 million per year.

I think the DoD can afford it. :E

As a comparison the FAA spends ~ $220 million a year to maintain "legacy" navaids (VOR, NDB, ILS, etc.).

GBAS appears to be a no cost option on both A and B.The benefits are immense, cost is low, I cannot understand why it is not on every ac and installed at virtually every airport.
Something just doesn't make sense here.