RNAV alternate requirements (AIM 1-2-1)

jason

jason

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Jason W (FlyNE)
So, let's see if I understand this correctly...

According to AIM 1-2-1, assuming that an IAP is required at the alternate, I can't use said airport as my alternate if...


  1. it only has GPS approaches OR
  2. the non-GPS approaches require equipment that is not installed (e.g. ADF, DME, etc.) as I can't plan to use GPS in lieu of these items at my alternate
In other words, the alternate must have an approach that can get me below the ceiling (at the time of arrival at the alternate) completely without the use of my GPS.

Additionally, these requirements are not applicable if the GPS is WAAS capable.

Now, if I fly my non-WAAS capable airplane to my original destination and find myself heading to the alternate, I can fly a GPS approach and/or an approach requiring equipment not installed (e.g. ADF or DME) as long as the GPS systems are fully operational (no RAIM warnings, etc.).

Is all of this correct?
 
FlyNE said:
So, let's see if I understand this correctly...

According to AIM 1-2-1, assuming that an IAP is required at the alternate, I can't use said airport as my alternate if...


  1. it only has GPS approaches OR
  2. the non-GPS approaches require equipment that is not installed (e.g. ADF, DME, etc.) as I can't plan to use GPS in lieu of these items at my alternate
In other words, the alternate must have an approach that can get me below the ceiling (at the time of arrival at the alternate) completely without the use of my GPS.

Additionally, these requirements are not applicable if the GPS is WAAS capable.

Now, if I fly my non-WAAS capable airplane to my original destination and find myself heading to the alternate, I can fly a GPS approach and/or an approach requiring equipment not installed (e.g. ADF or DME) as long as the GPS systems are fully operational (no RAIM warnings, etc.).

Is all of this correct?
Click to expand...

FLYNE,

That is a good summary.

I understand your statement "In other words, the alternate must have an approach that can get me below the ceiling (at the time of arrival at the alternate) completely without the use of my GPS", but it might be more accurately stated as: "In other words, the alternate must have an approach that the weather is forecast to meet the required alternate minimums (at the time of arrival at the alternate) completely without the use of my GPS."

A minor point that isn't covered in the AIM has to do with the fact that some versions of WAAS GPS units do not meet the full TSO requirements and they have the same restrictions in the AFMS that the non WAAS GPS units have, if they have not been updated. Bottom line, the restrictions are listed in the AFMS.
 
John Collins said:
FLYNE,

That is a good summary.

I understand your statement "In other words, the alternate must have an approach that can get me below the ceiling (at the time of arrival at the alternate) completely without the use of my GPS", but it might be more accurately stated as: "In other words, the alternate must have an approach that the weather is forecast to meet the required alternate minimums (at the time of arrival at the alternate) completely without the use of my GPS."

A minor point that isn't covered in the AIM has to do with the fact that some versions of WAAS GPS units do not meet the full TSO requirements and they have the same restrictions in the AFMS that the non WAAS GPS units have, if they have not been updated. Bottom line, the restrictions are listed in the AFMS.
Click to expand...

Thanks, John. Of course that is how I meant it, but your wording is much more accurate. And thanks for the clarification on the WAAS wording. That did sound a bit too simplistic for something coming out of the FAA.

I guess that I see these requirements as a "you need to have a plan if you lose RAIM" statement. Why does adding WAAS to the equation make them comfortable relaxing the requirements?
 
FlyNE said:
I guess that I see these requirements as a "you need to have a plan if you lose RAIM" statement. Why does adding WAAS to the equation make them comfortable relaxing the requirements?
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An IFR installed WAAS GPS is approved for sole means of IFR navigation, which means it meets the requirements of 91.205 (d)(2) that requires "navigation equipment suitable for the route to be flown". Unaugmented GPS is only approved for supplemental IFR navigation and does not meet the requirement and therefore requires that other equipment be available (VOR, ADF, DME, ...) that is suitable for the route to be flown.

RAIM is an algorithm used by unaugmented GPS navigators to determine if the position is trustworthy and to warn the pilot when it isn't. RAIM uses a fourth or fifth GPS satellite to determine if the position solution is acceptable by substituting it for each of the other satellites in the solution and making sure that any position difference remains within bounds.

WAAS doesn't use RAIM. It uses precisely surveyed ground stations to determine the accuracy of each individual satellite and provides correction information along with integrity information which is relayed to the aircraft receiver via a geostationary WAAS satellite. As long as the WAAS receiver is within the service volume of WAAS, it uses the WAAS based integrity.

There are two measures of integrity used for WAAS, HPL (Horizontal Protection Level) and VPL (Vertical Protection Level). Depending on the navigational operation being performed: enroute, terminal, LNAV, LNAV/VNAV, LP, LPV or LPV-200, different values of HPL and VPL are required. In any case, if the HPL or VPL does not meet the criteria, the WAAS GPS will either downgrade the service level (for example to LNAV) or flag the GPS receiver so the pilot knows that the receiver can't be used.

The VPL values for vertical guidance are very stringent and the reliability of vertical guidance can't always be assured, even though it has an availability greater than 99% in most of the USA excepting parts of Alaska and all of Hawaii. However, the HPL for lateral guidance is extremely high and one can count on the system providing adequate integrity to support a LNAV approach. The bottom line is that this results in the requirement for the pilot to plan the alternate GPS approach weather based on the LNAV only minimums which are usually 800-2.

There are two basic requirements for an approach to be able to be used as a planned alternate, there must be local weather reporting (for example ASOS) and the approach navigation facility must be monitored. Since WAAS is monitored at the national level, all RNAV (GPS) approaches satisfy that condition. At our airport, Rock Hill, SC (KUZA), the ILS can't be used for filing as an alternate because it is unmonitored, but both RNAV (GPS) approaches can be used as WAAS is always monitored.
 
John Collins said:
There are two basic requirements for an approach to be able to be used as a planned alternate, there must be local weather reporting (for example ASOS) and the approach navigation facility must be monitored. Since WAAS is monitored at the national level, all RNAV (GPS) approaches satisfy that condition. At our airport, Rock Hill, SC (KUZA), the ILS can't be used for filing as an alternate because it is unmonitored, but both RNAV (GPS) approaches can be used as WAAS is always monitored.
Click to expand...

This is excellent stuff, I am learning every day.
 
This is an excellent post! Thanks for the wisdom. I am curious about the future and I hope I word this correctly:

Right now, I can fly my approaches using the WAAS-GPS or ground based equipment. Many procedures still show overlay approaches for both, although the stand alone GPS-RNAV approaches are growing in number as we know.

I also have heard that as with NDB's, the plan is to phase out many ground based navaids (VOR's) as the nextgen process takes hold.

I'm comfortable working an approach as either precision or non-precision based on the structure and navaids available. I also know that should I attempt a GPS approach there are now ground based navaids I can switch to and use for the same approach should the satellite based equipment fail.

What happens when the ground based navaids are removed in the nextgen world and we lose GPS RNAV approach capability in the situation where satellite navigation becomes unusable? What's the backup for the approach?

Just curious.....
 
All this is very good stuff--thanks for posting!

In the wake of LORAN decommissioning and with the prospect of future VOR decommissioning, folks are always asking, "But what about when the GPS network goes down? What then?"

While there are certainly scenarios that result in the loss of the GPS network, it hasn't happened yet. However, the first flight away from home I ever took as a new pilot in my "new" plane, my 430W went dark and didn't come back. (I used VORs and pilotage to complete the VFR flight.)

So my personal concern is not, "What if terrorists or gubmint ineptitude wipe out the GPS network one day," it's, "What do I do the next time my sole GPS receiver conks out and I'm in IMC with no VORs?"

That's far more likely, and I'm *not* likely to spend the ten+ AMUs to get panel redundancy. But I haven't seen a bunch of VORs falling off the map, so I'm not too worried...yet.
 
So let's reformulate the scenario: I have WAAS, I can legally select an alternate where the only available approach is an RNAV (GPS), as long as I don't plan for better than the LNAV minimums. I can't get in at my destination, I fly to the alternate and try to land there but my sole GPS receiver dies and now I not only can't complete the approach, I can't even fly the published missed. Hopefully I'm in a radar environment and can get vectors to maintain separation and to shoot an ILS, LOC, or VOR approach somewhere else (I don't have an ADF). But if not, all I can do is climb, hope I don't hit something, and hopefully get somewhere else by VOR navigation (if they haven't decommissioned all the nearby VORs by then). It seems this is the nightmare scenario for which my airplane's previous owner installed a secondary, non-WAAS GPS. But I can't afford to keep two GPS databases AND my MFD updated and legal for navigation (in fact, I've heard that updates are no longer available for the Trimble). So in that situation, I declare and use my second GPS with its obsolete database anyway.

Of course a far likelier scenario is alternator failure. I already know that when that happens, I'm going to lose the Sandel almost immediately. So it's really important to have both COM and NAV backups that don't depend on ship's electrical power. All this shows that what's legal isn't necessarily safe, and keeping my butt out of trouble depends above all on having options.
 
azure said:
All this shows that what's legal isn't necessarily safe, and keeping my butt out of trouble depends above all on having options.
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I don't think this regulation is necessarily in place to govern actual receiver/hardware failure. In the scenario you posed, I would consider that a very unlikely but certainly a plausible emergency situation.

In the case of a non-WAAS'd GPS, the regs restrict alternate airport selection (no GPS approaches or ADF/DME substitution using GPS at the alternate for planning purposes) since RAIM could become unavailable. With WAAS, adequate signal is guaranteed unless NOTAM'd (monitored on a national level) and thus the lifted restriction on alternate airport selection.

All that said, your last statement is absolutely correct. It's extremely important to have options or "outs."
 
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HPNPilot1200 said:
I don't think this regulation is necessarily in place to govern actual receiver/hardware failure.
Click to expand...
I know -- I was just carrying the discussion that Bruce and Jim were having a little further.

In the case of a non-WAAS'd GPS, the regs restrict alternate airport selection (no GPS approaches or ADF/DME substitution using GPS at the alternate for planning purposes) since RAIM could become unavailable. With WAAS, adequate signal is guaranteed unless NOTAM'd (monitored on a national level) and thus the lifted restriction on alternate airport selection.
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My understanding is that the restriction on alternate planning even with WAAS is because even when there is no NOTAM'd WAAS outage, it's very possible for the WAAS-enhanced vertical precision to be insufficient for acceptable vertical guidance on an approach. When this happens, the unit downgrades the approach to LNAV-only. I suspect this happened to me one night last December when I was shooting the RNAV (GPS) 18 @ KFNT with my instructor. That approach has LPV minimums but for some reason I had no glideslope. What confused the picture is that I also had no LNAV+V glideslope on the RNAV into home base, where there had always been one before. So that led to a couple of weeks of confusion and consulting with my avionics tech, thinking that I'd had a component failure in my CNX-80. But I had just updated to the latest Jepp cycle and it turns out that's when they mistakenly decided to turn off WAAS on a bunch of RNAV approaches with LNAV+V to force them to annunciate as LNAV only. But LPV approaches weren't affected by that decision, so I still don't have any other explanation for why I had no glideslope at FNT -- except that it was probably downgraded due to inadequate vertical precision.

So bottom line, I think it's reasonable to have to plan on not being able to shoot an LPV approach at your alternate, even if there are a lot of other, likelier, more distressing scenarios that you should be taking into account as well.
 
I am not particularly concerned about the GPS system failing; I am very concerned about GPS integrity being compromised in large areas by terrestrial interference, which we already know can very easily occur, by accident, or as a result of widespread deployment of RF-interfering equipment (aka "LightSquared").
 
SCCutler said:
I am not particularly concerned about the GPS system failing; I am very concerned about GPS integrity being compromised in large areas by terrestrial interference, which we already know can very easily occur, by accident, or as a result of widespread deployment of RF-interfering equipment (aka "LightSquared").
Click to expand...
Not sure I agree with "very easily". Has there ever been an episode of "compromised GPS in a large area by terrestrial interference" that came as a surprise...i.e., wasn't some planned event that was NOTAMed in advance? As for LS, that bullet appears to have been dodged, and I think a similar threat in the future is unlikely after this episode.

I think inadvertent disruption is very unlikely. I also think intentional disruption over a large area or for a long duration is also unlikely. Any system small enough to be portable will have limited effect; any system not easily portable will be quickly tracked and neutralized.
 
SCCutler said:
I am not particularly concerned about the GPS system failing; I am very concerned about GPS integrity being compromised in large areas by terrestrial interference,
Click to expand...
But this is why WAAS was invented in the first place - to assure integrity. In other words either GPS signal is valid for navigation or it is invalid and you know about it. But there can't be situation it is invalid and you are fooled into believing it is valid. I think WAAS guarantees integrity with probability like 99.99999%
 
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Perhaps we need to look at the issue from the perspective of failure of GPS as an in-plane event rather than a system-wide one. A well, nothing is fail-safe, so the notion of 'never-happening' may be wishful thinking...

Right now, should the GPS-WAAS unit in my plane fail, I have backup in ground based navaids. When the ground based navaids disappear, what's the backup?
 
My understanding of the FAA Nextgen plan is to keep about half of the VOR's as a backup plan. VOR navigation, ILS, and radar will be around for a long time.
 
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