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I am trying to get to grips with using RNAV approaches, so will appreciate any corrections/feedback to my current understanding:

  1. Selecting an RNAV approach provides only GPS quality (equivalent to category 1 ILS) LNAV.
  2. As opposed to ILS VNAV/LNAV, current PMDG aircraft have no knowledge of WAAS signal receivers. The PMDG  GPS system is 'perfect' enough anyway!
  3. RNAV approaches currently do not support vertical guidance, so no LPV (localizer  performance with vertical guidance) approaches are supported.
  4. Vertical guidance whilst flying an RNAV approach is given only by setting QNH for the airport. So autoland is not supported.
  5. The pilot must use the decision altitiude to determine the aircraft's descent to the runway. So accurate setting of local QNH for the approach is essential.

This leaves me with one main question - what criteria should I use to choose between an ILS and an RNAV approach to an airport? 

 

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First of all, it is normally considered good practise to use all of the available navigational aids at your disposal, even when you are conducting a visual approach. Therefore, unless you are on a training flight, or the approach is offset to the landing runway, or the ILS is U/S or not installed. then in the interests of flight safety I suggest it makes perfect sense to have the correct ILS frequency and inbound course  tuned whenever possible.

Non-ILS approaches, including RNAV approaches, are not as accurate when compared to those flown using an ILS, even allowing for any GPS updating.  In the B744 the maximum RNP error allowed for during FAA certification in order to carry out a RNP/GPS type of approach is 0.3nm or smaller and, because the 744's Autopilot/Flight Director system does not provide any VNAV path warning in the event of a vertical deviation, the autopilot must be disconnected at 360ft AAL (or higher depending on the published minima for that runway).  Provided there is GPS updating then the use of the autopilot with the ND in MAP mode is used; but great care is needed in certain parts of the world where the navigation aids are less reliable and/or accurate GPS updating has not been successful. 

I am not aware of any 744 operators that use anything other than QNH when operating at or below the Transition Altitude; but this has nothing whatsoever to do with being able carrying out an autoland or not.  Rather, the autopilots rely on height information obtained from the radio altimeters after glideslope capture and all the way down to the runway. 

Setting an accurate QNH (or QFE) is essential all of the time (i.e. day or night, VFR or IFR) and not just when on an approach. The vast majority of controlled flight into terrain accidents in the past have occured within 30 miles of the airfield and, almost without exception, the lack of any visible contact with the ground has been a significant factor. 

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1 hour ago, Richard McDonald Woods said:

Selecting an RNAV approach provides only GPS quality (equivalent to category 1 ILS) LNAV.

They're difficult to compare, the ILS gets more sensitive as you approach the runway whereas an RNAV has the same accuracy the whole way down. I would suggest they're much of a muchness > 5 miles out.

1 hour ago, Richard McDonald Woods said:

As opposed to ILS VNAV/LNAV, current PMDG aircraft have no knowledge of WAAS signal receivers. The PMDG  GPS system is 'perfect' enough anyway!

Yes but none of the airliners I've flown has any type of Augmentation System on and they were all able to do RNAV approaches to a better than Non Precision minima. RNAV isn't just GPS, it covers a whole gamut of navigational equipment.

1 hour ago, Richard McDonald Woods said:

RNAV approaches currently do not support vertical guidance, so no LPV (localizer  performance with vertical guidance) approaches are supported.

They can encode a 'glideslope' into the box and it's shown in the flight deck in a similar fashion to an ILS glideslope. The autopilot can follow it too.

1 hour ago, Richard McDonald Woods said:

Vertical guidance whilst flying an RNAV approach is given only by setting QNH for the airport. So autoland is not supported.

Yes and yes but they're unrelated. The Capt's baro is used for the alt restrictions (at least on the aircraft I've flown) so you need to make sure it's right. Autoland is not supported because the accuracy of the system isn't sufficient to put the aircraft on the end of the runway, especially in the vertical profile, as you said, it's dependant on the Capts baro (setting and working).

1 hour ago, Richard McDonald Woods said:

The pilot must use the decision altitiude to determine the aircraft's descent to the runway. So accurate setting of local QNH for the approach is essential.

This is true of most approaches, only some peculiar CAT1 approaches, Cat II and Cat III use Decision Height (DH) and thus are independent of baro. Everything else uses Minimum Descent Altitude (MDA). In about 15 years flying commercially I've used a DH about 10 times.

1 hour ago, Richard McDonald Woods said:

This leaves me with one main question - what criteria should I use to choose between an ILS and an RNAV approach to an airport?

Choose the one you're most comfortable with using your knowledge and experience. Generally I'd choose an ILS as the act of flying one is easier, on the jumbos I fly, RNAV approachs have been shoehorned into the box and are a bit clumsey to execute, although very effective.

The main advantage of an RNAV approach is that it doesn't need to be a straight line or a continuous angle descent. You can design and fly an RNAV that guides you around hills and deposits you in the right place, facing the right direction at the right height and speed to take over and land. This can be very advantagous in mountainous areas or areas with a lot of airspace restrictions. In the past these approaches were made up of convoluted radio aids and bearing/distances that were a pain to fly, took up an awful lot of capacity and if you got them wrong, tended to kill everyone.

I've crossed with Bert but what he said too.....

Hope this helps,
Ian Webber

 

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2 hours ago, Richard McDonald Woods said:

This leaves me with one main question - what criteria should I use to choose between an ILS and an RNAV approach to an airport? 

For me?

  1. Barring any real reasons otherwise, take what's assigned (ILS is available on 13L at JFK, but they won't use it unless they're really desperate, as it then forces LGA onto 13, which then forces TEB and EWR into airspace sharing and N90 just goes down the drain, traffic-flow-wise - situations like that will mean I pull up an RNAV overlay for a visual approach to 13L, or similar)
  2. Is ILS available?
    Yes > Go with that.
    No > Go with RNAV with a fallback to something like a LOC, VOR, etc.

At all times: have some sort of supplemental information displayed as an aid, as Bertie mentioned.

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Not much to add other than to say that most operators will have a policy along the lines of "use the most accurate type of approach available", which generally means ILS, RNAV, VOR, NDB, visual, in that order of preference.

As Kyle says on occasion for noise or other air traffic management reasons ATC may be using an approach other than ILS even when one is installed, and in this case you're obviously going to fly what ATC give you unless there is some overriding reason not to.

Re: vertical guidance: as Ian and Bertie have explained, the issue with RNAV approaches is that the vertical profile (in the 744 at least, and most commercial jets) is synthesised in the box and driven entirely by the barometric altimeter. 

This means that it is essential to have the correct QNH set, and QFE is certainly not an option unless you have a special navigation database with the crossing heights on the procedure coded for QFE.

The other issue with this (in real life) is that temperature will have an effect on the true vertical path flown by the FMC when following a baro-VNAV glidepath. This is because the barometric altimeter is subject to temperature error - it is calibrated to read correctly in ISA conditions. However, when the OAT is colder than standard the altimeter will over-read - that is to say your true altitude will be lower than indicated. The aerodrome QNH value takes temperature in to account so the altimeter will read correctly on the ground, but the higher you are above the aerodrome the greater the error will become.

Geometrically this is a problem because although the 'end' of the glidepath (at the aerodrome) will be in the right place, at the FAF - several thousand feet higher - the aeroplane will actually be a lot lower than indicated. Not only does this compromise the terrain clearance at the FAF itself, it also means that the actual glidepath flown will be shallower than that depicted in the chart which will have implications for obstacle clearance in the final segment as a whole. This is in contrast to an ILS approach where the glideslope is a fixed path in space defined by a radio beam and therefore is unaffected by any of this.

For this reason, RNAV approaches will normally have a minimum temperature associated with them. As long as the temperature at the aerodrome is above the charted minimum temperature you are fine - you can fly the procedure as published, changing nothing in the box, and you will be protected.

However, if the temperature is lower than that on the chart then if you are going to fly the procedure you must apply corrections to all the relevant altitudes and therefore can only fly it to LNAV minima (rather than LNAV/VNAV).

This is because to fly to LNAV/VNAV minima you are not permitted to make any changes to the procedure in the box once you have loaded it - you must load it directly from the navigation database and check that it matches the chart.

In any event, if the procedure is not in the database you cannot 'cook your own' by entering the waypoints from the chart, for instance.

The good news is that temperature error is not currently simulated in FSX or P3D so this is largely academic! 

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6 hours ago, skelsey said:

However, if the temperature is lower than that on the chart then if you are going to fly the procedure you must apply corrections to all the relevant altitudes and therefore can only fly it to LNAV minima (rather than LNAV/VNAV).

I checked a few RNAV approach charts for various US airports, and most have a note specifying the minimum temperature for LNAV/VNAV. At San Diego, the note for the RNAV (GPS) Runway 09 approach states: “For uncompensated Baro-VNAV systems, LNAV/VNAV NA below 6°C (43°F) or above  47°C (117°F).”

Interestingly, at my home airport KELM, in the chilly northeast, the minimum temperature is -20 C (-4 F), I assume due to a more favorable terrain profile. At San Diego, even though the initial part of the approach is over the ocean, the final segment has the aircraft descending over the rapidly rising terrain of Point Loma.

Another curious thing: although temperature may restrict when LNAV/VNAV minima can be used, many RNAV approaches appear to have lower minima for a straight LNAV approach than for LNAV/VNAV in any case.

In the case of the runway 24 RNAV (GPS) at KELM, the LNAV/VNAV DA is 1870 feet with minimum visibility of 2 1/2 sm, while the LNAV MDA permits descent down to 1740 feet, with visibility of 1/2 sm (or RVR of 2400). 

Would this be because in an LNAV approach, the pilot is directly controlling/monitoring the descent path, rather than the autopilot?

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10 hours ago, JRBarrett said:

Would this be because in an LNAV approach, the pilot is directly controlling/monitoring the descent path, rather than the autopilot?

The simple answer is no.  In the case of the B744 it is recommended that all Non-ILS Instrument Approaches (i.e. non-precision, including RNP, VOR, NDB, RNAV visual) are flown using the autopilot. On every approach the pilots should continuously monitor the horizontal as well as the vertical descent profile and not simply rely on the autopilot(s) to control the descent path.  It is good airmanship to be prepared for the unexpected - just in case something does go wrong!

Simon has already mentioned the Final Approach Fix (FAF) in his earlier post. This position is particularly important on a non-ILS (i.e. non-precision) approach, because it is often the last opportunity a pilot has of ensuring their altimeter subscale reading is set correctly.  Assuming it is set correctly then the aircraft's altitude should agree with that shown on the published procedure.  However, if the altimeter reading disagrees by a significant amount then it could be potentially very dangerous to continue the approach, because you could be starting the final descent at a much lower (or higher) altitude than you think.  If you are still in cloud when this happens then there is a real risk of early ground contact, so the safest course of action will inevitably be to discontinue the approach and climb to at least the airfield safety altitiude before trying to determine the cause for the false altimeter reading.

Assuming the aircraft passes over the FAF at the correct height, then the stopwatch should be started and the appropriate ROD for the aircraft's groundspeed initiated.  The timings can then be used to adjust the ROD and determine when the Minimum Descent Altitude (MDA) or Decision Altitude (DA) is reached, so that at this point a safe landing can be accomplished visually with the aircraft correctly aligned horzontally to the runway and descending on the correct profile; otherwise a missed approach should be flown.

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11 hours ago, JRBarrett said:

In the case of the runway 24 RNAV (GPS) at KELM, the LNAV/VNAV DA is 1870 feet with minimum visibility of 2 1/2 sm, while the LNAV MDA permits descent down to 1740 feet, with visibility of 1/2 sm (or RVR of 2400). 

Would this be because in an LNAV approach, the pilot is directly controlling/monitoring the descent path, rather than the autopilot?

Excellent question, and one that I had to have a look at the KELM RNAV 24 plate and do some searching.  An article that explains this well is https://www.boldmethod.com/learn-to-fly/navigation/when-lnav-minimums-are-lower-than-vnav-which-should-you-use/

The TERPS experts can explain this better perhaps, but it boils down to the differences between determining a DH vs MDA and in this case how obstacles will influence the outcome.  As mentioned by Bertie and discussed in the article the choice of which one to choose is part of good piloting, and one must consider the weather, equipment available and ones own piloting experience.  The LNAV MDA will have more pilot workload and if the weather is close to minimums I probably would not select the non precision LNAV over the LNAV/VNAV to get that extra 130 foot of ceiling if there were other options, unless I was familiar with that approach and the area.

Edited by downscc

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1 hour ago, downscc said:

Excellent question, and one that I had to have a look at the KELM RNAV 24 plate and do some searching.  An article that explains this well is https://www.boldmethod.com/learn-to-fly/navigation/when-lnav-minimums-are-lower-than-vnav-which-should-you-use/

The TERPS experts can explain this better perhaps, but it boils down to the differences between determining a DH vs MDA and in this case how obstacles will influence the outcome.  As mentioned by Bertie and discussed in the article the choice of which one to choose is part of good piloting, and one must consider the weather, equipment available and ones own piloting experience.  The LNAV MDA will have more pilot workload and if the weather is close to minimums I probably would not select the non precision LNAV over the LNAV/VNAV to get that extra 130 foot of ceiling if there were other options, unless I was familiar with that approach and the area.

 

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Great article Dan, thanks.

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4 hours ago, downscc said:

if the weather is close to minimums I probably would not select the non precision LNAV over the LNAV/VNAV to get that extra 130 foot of ceiling if there were other options, unless I was familiar with that approach and the area.

Good article and I tend to agree with you, Dan, although I think your Chief Pilot wouldn't be too happy if you had to divert because you didn't fly to the lowest published minima for a particular runway.  

Professional pilots are trained to operate to the lowest published minima for their aircraft type at any airport they fly into and this includes a sound knowledge of the airline's route structure as well as all normal and alternate airfields. Some airfields and approaches are more demanding than others and this can sometimes result in a pilot having to undergo a special briefing, a simulator detail and even a visit under supervision with a Training Captain before being allowed to operate there on their own.

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Agreed -- a very good article!

One thing which is worth noting is that the use of "LNAV/VNAV" in the minimums section of an approach chart is slightly misleading in that it refers to the "3D" aspect of the approach design, which is distinct from the autopilot modes LNAV and VNAV; in theory, if permitted by your airline, you could temperature-correct the altitudes for the FAF and MDA and fly the profile using LNAV and VNAV modes to the LNAV (2D) minimums. However, most airlines do not permit changing anything (including altitudes) in the FMC from the FAF onward and so this effectively rules that out.

In practice, in the 744 VNAV is only permitted where there is a published glidepath angle. If the temperature is below the minimum published on the chart AND LNAV minima are available then the approach may be flown in LNAV and V/S using temperature-corrected minima and a temperature-corrected vertical profile.

One other aspect I was reminded of when checking the books -- in the 747-400, the FMC RNP value defaults to RNP 0.5; therefore all RNAV approach procedures will require the RNP value to be manually amended to 0.3 on POS REF page 2 prior to the approach.

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13 minutes ago, berts said:

Dan, although I think your Chief Pilot wouldn't be too happy if you had to divert because you didn't fly to the lowest published minima for a particular runway.  

I don't pretend to represent myself as a professional pilot thoroughly trained on a route before allowed to Captain that route.  My comments regarding familiarization with the approach and airport environment were based on my own experience.  In my four decades of flying in the GA environment, I've had many many experiences with approaches I've never seen before and airspace that is totally new and in this environment the familiar saying, 'There are old pilots and bold pilots but no old bold pilots,' has served me well.  To be sure I've flown many approaches to mins (LOL my first instrument cross country in a PA-28R Arrow ended as an ILS into Keesler AFB at minimums, coming in over the back bay it was the blackest darkness I've ever seen until over the approach lights, whew). As a rule, I tend to be conservative when assessing my own skills, and prefer to rely on those skills rather than luck.

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22 hours ago, downscc said:

As a rule, I tend to be conservative when assessing my own skills, and prefer to rely on those skills rather than luck.

That's great advice Dan - and having read a little of your aviation background in your other posts my comment about your Chief Pilot was meant as a joke.  Anyway, I agree we shouldn't take things for granted or rely on luck when we are flying.

I remember seeing a rather clever message on this subject some years ago.  It was in the form of a simple cartoon drawing with two single engine light aircraft flying directly towards each other,  They were at the same height and entering into the opposite edges of the same large cumulus cloud, so obviously the two pilots couldn't see each other's aircraft.  The caption underneath had them both saying to their passengers "It's alright, I've got an IMC rating!"  

Those two pilots might have had the skills to fly in cloud, but I think you will agree that if they had any hope of avoiding a collision they would definitiely have needed a lot of luck!  Reading about your PA-28R Arrow experience reminded me of that cartoon drawing, because it was another example of  'I learnt about flying from that' - so thanks!  

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On 2/17/2019 at 12:11 PM, berts said:

Simon has already mentioned the Final Approach Fix (FAF) in his earlier post. This position is particularly important on a non-ILS (i.e. non-precision) approach, because it is often the last opportunity a pilot has of ensuring their altimeter subscale reading is set correctly.  Assuming it is set correctly then the aircraft's altitude should agree with that shown on the published procedure.  However, if the altimeter reading disagrees by a significant amount then it could be potentially very dangerous to continue the approach, because you could be starting the final descent at a much lower (or higher) altitude than you think. 

Sorry if I'm a little dense, but I know nothing about RNAV approaches yet. Are you talking about comparing the Radar Altimeter to the baro Altimeter here? How does one "ensur[e] their altimeter subscale reading is set correctly."?

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Matt, by comparing your altimeter reading to the charted altitude on say, a Jeppesen chart.

The faf (final approach fix) is shown as a maltese cross. At the faf, you can compare pressure altitude or radio altitude to the chart as usually both are given on charts.

Basically making sure you’re at the right place, at the right height, at the right speed by comparing your lateral position, height and speed to the requirements on the chart.

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On ‎2‎/‎17‎/‎2019 at 12:25 PM, downscc said:

Excellent question, and one that I had to have a look at the KELM RNAV 24 plate and do some searching.  An article that explains this well is https://www.boldmethod.com/learn-to-fly/navigation/when-lnav-minimums-are-lower-than-vnav-which-should-you-use/

The TERPS experts can explain this better perhaps, but it boils down to the differences between determining a DH vs MDA and in this case how obstacles will influence the outcome.  As mentioned by Bertie and discussed in the article the choice of which one to choose is part of good piloting, and one must consider the weather, equipment available and ones own piloting experience.  The LNAV MDA will have more pilot workload and if the weather is close to minimums I probably would not select the non precision LNAV over the LNAV/VNAV to get that extra 130 foot of ceiling if there were other options, unless I was familiar with that approach and the area.

2

Hi Dan,

The question of why LNAV minima are lower than LNAV/VNAV minima comes up quite often, so much so that FAA decided they need to explain it in the AIM:

 

3. Vertical guidance (LNAV/VNAV). A line is

drawn horizontal at obstacle height until reaching the

obstacle clearance surface (OCS). At the OCS, a

vertical line is drawn until reaching the glide path.

This is the DA for the approach. This method places

the offending obstacle in front of the LNAV/VNAV

DA so it can be seen and avoided. In some situations,

this may result in the LNAV/VNAV DA being higher

                           than the LNAV and/or Circling MDA

The AIM has nice picture in it describing the difference, but you are correct.  LNAV/VNAV uses a sloping obstacle clearance surface (OCS), while LNAV uses a level, flat surface OCS.  As described above, this can result in LNAV/VNAV DA and visibility minima being higher than just LNAV only minima.  Nothing prevents the pilot from using VNAV to continue the descent to the LNAV MDA; however, the pilot cannot descend below LNAV MDA without the runway environment in sight. This requires the pilot to sight the runway environment prior to reaching the LNAV MDA to either initiate the level off at MDA or a missed approach some height above MDA to avoid going below, a practice the airlines call "derived decision altitude".  In Europe, the application of a heigh loss additive is mandatory when CDFA is used to an MDA. Leveling at MDA will cause you to destabilize an approach, not a good thing in a jet. 

The decision to use the lower LNAV MDA or the higher LNAV/VNAV DA depends a lot on the type of airplane involved and the runway.  For jets, most pilots and operators would elect to use the LNAV/VNAV DA because of the greater risk of destabilizing the approach.  In a light single engine aircraft landing a longer runway, it might be beneficial to continue down the extra 100 feet, level at the MDA and perhaps sight the runway. 

A few years back, we looked at raising LNAV only MDA to that of the LNAV/VNAV DA in these cases, but decided it was not in everyone's best interest. We did; however, decide to ensure that Circling MDA was always no lower than the straight-in LNAV only MDA, which can happen in very rare occasions when the controlling obstacle is just outside of the circling protected airspace but penetrates the LNAV only OCS.  There's an airport in Texas where this can be found.  It creates a strange case where the circling MDA for one approach is higher than the other approach and the LNAV MDA for that same approach, which could lead a pilot to try to circle from a different approach to achieve a lower landing minima.  Perfectly legal, but we're trusting pilot common sense not to do it. 

One of these days, we need to have lunch at Waterstreet!

Rich Boll

 

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On ‎2‎/‎15‎/‎2019 at 8:07 AM, Richard McDonald Woods said:

I am trying to get to grips with using RNAV approaches, so will appreciate any corrections/feedback to my current understanding:

  1. Selecting an RNAV approach provides only GPS quality (equivalent to category 1 ILS) LNAV.
  2. As opposed to ILS VNAV/LNAV, current PMDG aircraft have no knowledge of WAAS signal receivers. The PMDG  GPS system is 'perfect' enough anyway!
  3. RNAV approaches currently do not support vertical guidance, so no LPV (localizer  performance with vertical guidance) approaches are supported.
  4. Vertical guidance whilst flying an RNAV approach is given only by setting QNH for the airport. So autoland is not supported.
  5. The pilot must use the decision altitiude to determine the aircraft's descent to the runway. So accurate setting of local QNH for the approach is essential.

This leaves me with one main question - what criteria should I use to choose between an ILS and an RNAV approach to an airport? 

 

3

Richard, 

Please allow me to take a hack at your questions.

1.  An RNAV approach (RNP APCH in ICAO parlance - and soon to be FAA PBN parlance - and not to be confused with RNP AR approaches) is based on a Global Navigation Satelite System (GNSS). In the US, only GPS is allowed for approaches, although, that's about to change.  FAA will soon begin accepting other systems, like Europe's Gallielo.  Category 1 approaches cover everything from ILS, to RNP APCH, to VOR and NDB approaches. CAT II and CAT III approaches cover only ILS and GLS down below CAT I landing minima.

An approved GNSS (in the US - GPS), provides the position solution for the RNAV system when flying an RNP APCH and provides accuracy for basic containment within the initial, intermediate, final, and missed approach segments. 

2. ILS is technically not LNAV/VNAV.  It's localizer and glideslope, two ground-based navigation systems providing lateral and vertical guidance to the runway threshold. The localizer is a VHF transmitter, while the glideslope is UHF, but all that is transparent to the pilot as the ILS receiver in the airplane handles it. ILS is more accurate than any un-augmented RNAV system.  WAAS provides a ground and satellite augmentation system that removes errors inherent with GPS by providing correction signals to the aircraft.  WAAS approach capable RNAV systems can use the LPV line of minima, which are based on the lateral and vertical obstacle clearance surfaces and containment areas as an ILS approach.  Provided the airport lighting system supports it, LPV minima can be as low as 200' and 1800 RVR. 

WAAS is a four letter word for US airlines.  Airbus offers an option for their A350 line,  Boeing is offing an option for their B737MAX line. The airlines are looking to have three types of approaches: ILS, GLS (GBAS or ground-based augmentation where the satellite corrections signals are sent directly to the airplane over VHF ground stations), and RNP AR.  They grudgingly accept RNAV approaches to LNAV or LNAV/VNAV minima until something better comes along. 

3. Not correct.  RNAV approaches do support vertical guidance, either in the form of Baro-VNAV or a WAAS generated glidepath. These approaches are classified as "APV" or Approaches with Vertical Guidance.  These do not meet the ICAO definition of "precision approaches", so they had to invent a new term. The LPV line of minima is based on using a WAAS generated glidepath that is not subject to barometric or temperature altimetry errors since it is electronicly generated by the WAAS system and provides equivalent to an ILS glideslope without inherent distortions with the later. That allows use to put RNAV approches with LPV minima where we might not be able to put an ILS approach. 

4.  Autoland is a function only of an ILS approach.  No RNAV approach supports autoland.  GLS will in the future, assuming we get past the GPS jamming issue.  

5.  accurate QNH is essential.  For an ILS, it does not affect the electronic glideslope.  It does however affect the height above the touchdown zone where you will reach DA.  For Baro-VNAV on an RNAV approache to LNAV/VNAV minima, a wrong QNH can fly you into the ground. A nasty simulator instructor would sometimes give a transposed altimeter setting of 29.96 when he actually set the airport's QNH to 29.69.  The way low RA indication on final is the only clue that something was wrong and something very bad was about to happen.  BTW, PMDG's VNAV Will simulate this very nicely. 🙂

Last Queston is easy.  ATC will usually assign the approach to use.  Don't upset their apple cart until there is a a real reason to do so.  For example, at Teterboro NJ (KTEB), the RNAV 19 approach has lower LPV minima than the ILS 19 approaches (quirk because these two approaches were designed at two different times to differing sets of criteria).  NY only uses the ILS 19 because the missed approach supports overhead KEWR traffic.  However, if the weather drops low enough, then you may need to request the RNAV 19.  ATC will provide alternate missed approach instructions. 

Hope this helps,

Rich Boll

 

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On 2/25/2019 at 12:03 AM, richjb2 said:

Nothing prevents the pilot from using VNAV to continue the descent to the LNAV MDA

 Hello Rich

Hate to nit pick at what was a good post, but -

are you sure this statement is right? It sounds a bit off because I think LNAV only procedures are predicated on another vertical mode being used (ie. not vnav).

The lnav/vnav vertical minimum being more restrictive (higher) because of the possibility of a miss-set altimeter in VNAV could cause collision with obstacle/s in the approach splay.

All other vertical modes are not susceptible to a miss-set qnh, so the minima can be lowered.

At least, the above is my understanding between lnav/vnav and lnav only minima in the vertical control context during the respective procedures.

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1 hour ago, Copper. said:

are you sure this statement is right? It sounds a bit off because I think LNAV only procedures are predicated on another vertical mode being used (ie. not vnav).

I'm not Rich but yes, it is correct. 

The article Dan linked explains it well - the difference is that the LNAV is designed as a '2D' approach to an MDA in the same way as a VOR or NDB, whilst the LNAV/VNAV is designed as a '3D' approach to a DA.

As you will be using your barometric altimeter to descend along the published vertical profile then either is equally at risk from a mis-set QNH, regardless of whether you V/S in to the ground or let the autopilot do it for you in VNAV :).

In practice, however, as I mentioned, it may not be possible to use VNAV because you're probably using the LNAV minima because the temperature is below that required for the VNAV and an airline probably won't let you use VNAV after the FAF if you've messed with the altitudes on the FMC.

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Aah yes, thanks Simon. That was an embarrassing mistake.

thanks for the corrections

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On ‎2‎/‎25‎/‎2019 at 8:34 PM, Copper. said:

 Hello Rich

Hate to nit pick at what was a good post, but -

are you sure this statement is right? It sounds a bit off because I think LNAV only procedures are predicated on another vertical mode being used (ie. not vnav).

The lnav/vnav vertical minimum being more restrictive (higher) because of the possibility of a miss-set altimeter in VNAV could cause collision with obstacle/s in the approach splay.

All other vertical modes are not susceptible to a miss-set qnh, so the minima can be lowered.

At least, the above is my understanding between lnav/vnav and lnav only minima in the vertical control context during the respective procedures.

11

It is perfectly acceptable to use VNAV down to LNAV only MDA.  

By US TERPS, any MDA is based on a level obstacle clearance surface. That surface provides between 250 to 300 of required obstacle clearance (ROC) above the controlling obstacle (determined by the navigation source, e.g., RNAV, LOC, VOR, NDB, etc.). As long as you don't descend below the published MDA, you are guaranteed the ROC for the type of approach.  That's why when using Continous Descent Final Approach (CDFA) techniques to an "MDA" using any vertical mode (typically VNAV based on Baro-VNAV, VS, etc.), the pilot must add an additive to the MDA so that they do not penetrate through MDA while executing the missed approach if the runway environment is not in sight.  How the pilots descends to MDA doesn't matter. 

RNAV approaches in the US that only have LNAV minima usually have a VNAV angle provided on the IAP chart as well. In this U.S. this angle is provided on the source document by the FAA, the 8260-3 Form.  The database providers then code this angle with the procedure to support the use of VNAV (Baro VNAV or WAAS generated, doesn't matter).  This VNAV angle is advisory only!!!  It provides NO OBSTACLE PROTECTION BELOW MDA.  Recently, we have had to beat that message into the real pilots once again! 

The LNAV/VNAV line of minima is based on a sloping OCS.  That OCS is based on a slope of 102/GPA.  For a typical 3 degree GPA, that is a 34:1 surface,   The 3 degree path itself is a 20:1 surface. The difference between the two surfaces is the ROC for the LNAV/VNAV line of minima.   An obstacle may penetrate the LNAV/VNAV OCS, and if so, the procedure designer draws a horizontal line back from the top of the penetration to the OCS.  He/She then draws a vertical line from that intersection upwards until it reaches the GPA.  That position on the GPA establishes the DA for the LNAV/VNAV line of minima.  The visibility associated with the DA that would allow the pilot to also see the runway environment from the DA, will also allow the pilot to see the obstacle.  

That same obstacle may not necessarily penetrate the level OCS used by the LNAV line of minima, or if it penetrates, it may allow for a lower LNAV MDA than the LNAV/VNAV DA.  Obstacle protection is still assured if the pilot descends on the VNAV path but does not go below the LNAV MDA.

Let's see if this works.  Here's an illustration from US AIM of what I just described:

6-1-2015-8-19-43-PM-1024x683.png

This situation doesn't happen often but happens enough that the FAA decided they needed to explain it to pilots. These quirks in TERPS can sometimes lead to strange things. A while back, there was a quirk in the LPV criteria on the initial missed approach lateral obstacle clearance area and OCS that would occasionally result in LNAV/VNAV minima being lower than LPV minima.  That made no sense at all since LPV is inherently more accurate. That glitch has been fixed.

We are currently working on a fix to the RNP AR criteria that results in the RNP AR DA being higher than the RNAV (GPS) LNAV/VNAV minima even though RNP AR is a higher performance standard than just plain old RNAV.  

Rich 

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