Enroute RNAV and RAIM questions

Alchemy

Alchemy

Well-Known Member
Today our company was filing our flights out of O'hare on non-rnav routes due to "inability to assure RAIM coverage" or something to that effect. Of course, ATC was clearing us on RNAV routes either via amended full route clearance before takeoff or once airborne. Just in case, we were checking predictive RAIM with our FMS's for the various waypoints we were assigned, and it all came up good.

I'm pretty ashamed at how little I actually know about RAIM requirments for enroute nav and sid's/star's. Who is responsible for checking this for enroute nav? The pilots are only required to verify RAIM prior to an RNAV approach, as far as I know (I'm not RNAV approach qualified). Why would our company claim that the RAIM was no good when our FMS's and ATC all seemed to indicate it was working fine? Could it just be that they were given bad info?

Thanks.
 
I had the same thing on my flightplan today from TYS-EWR. I did predictive RAIM checks for most of the VORs on our route and EWR. All showed RAIM "yes." I have no idea why they filed non-RNAV.
 
FlyChicaga said:
I had the same thing on my flightplan today from TYS-EWR. I did predictive RAIM checks for most of the VORs on our route and EWR. All showed RAIM "yes." I have no idea why they filed non-RNAV.
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Did ATC actually go along with the VOR-based route in your case or did they clear you onto the Dylin at some point?
 
They re-cleared us via the PHLBO2 arrival shortly after passing GVE. I checked RAIM for most of the waypoints on the route before accepting the clearance. All said "yes."
 
I'll try to hopefully clear it up for you as much as I can, but AIM 1-1-19 goes pretty in depth.

As far as who's responsible for determining RAIM predictions, I'm not gonna insult you with the whole "PIC"/"All Available Information." I'm going to guess that your company/dispatch filed non-RNAV as a result of the common NOTAM's that say "GPS may be unreliable within area XYZ" or on many approaches "WAAS outages may occur daily." I see more than a few of those here on the West Coast, but typically never have a problem. I'd rather have an "FYI this may happen" instead of a "WTF?! 'INTEG'-flag!" pop up on my display out of no-where...not that it couldn't happen anyways.

Since you ran a RAIM prediction and it came up good, I'm gonna guess that was the situation. And ATC, being familiar with the route conditions, cleared you accordingly.

It is always a good idea to do the RAIM check for any phase of flight, but GPS does it automatically when it goes into APPROACH mode (approx 2 nm prior to FAF). If there will not be sufficient RAIM it will not go from TERMINAL => APPROACH.

The FAA just published new guidance REQUIRING pilots filing RNAV SIDS/STARS to conduct a RAIM check before the proposed flight, which makes sense since on such a procedure you're on relying on it as your sole means of navigation (although good situational awareness and asking "what if" is good practice). There is no requirement though for the enroute phase.

Integrity monitoring in a normal GPS is provided by either the 5th satellite or a barometric altimeter (Baro-Aiding). However in many of the newer ones it is provided by WAAS.

WHEW! I hope that helps you out! I certainly don't know it all, and if I missed/messed anything, I apologise in advance! Anything else feel free to ask!

Check 6,
Tommy
 
Well we only get the NOTAMs that are provided to us by the company in our weather packets for each flight, I certainly didn't see any regarding GPS coverage issues yesterday. That's not to say I couldn't have missed one accidentally, when you have 5 pages of notams to look through, sometimes it happens. Interesting about the requirement for pilots to check coverage prior to conducting an RNAV SID or STAR. Either that hasn't made it into our manuals yet, or dispatch is allowed to do that for us.
 
Air Pirate said:
Integrity monitoring in a normal GPS is provided by either the 5th satellite or a barometric altimeter (Baro-Aiding). However in many of the newer ones it is provided by WAAS.
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Just wanted to ask here, as I'm sort of confused by how you worded this part. Are you saying the WAAS still requires the use of RAIM? Because it's my understanding that you don't have to deal with RAIM, when operating under a GPS certified under TSO146, due to the WAAS.
 
KSCessnaDriver said:
Just wanted to ask here, as I'm sort of confused by how you worded this part. Are you saying the WAAS still requires the use of RAIM? Because it's my understanding that you don't have to deal with RAIM, when operating under a GPS certified under TSO146, due to the WAAS.
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Yah, I guess I didn't word that very well. My apologies. When you have a WAAS reciever AND are recieving a WAAS signal, then that signal is providing the integrity and signal corrections, and thus do not need the receiver's RAIM function. That is why you dont have to specifically do a RAIM check. Always still a good idea though in the event of a WAAS outage. That's why the G1000 still has a RAIM prediction function.

Also, here's the AOPA brief on the new rule on preflite RAIM checks before flying RNAV SID/STARS: http://www.aopa.org/advocacy/articles/2009/raim_issue_brief.html

Hope that clears things up! :)
 
Air Pirate said:
Yah, I guess I didn't word that very well. My apologies. When you have a WAAS reciever AND are recieving a WAAS signal, then that signal is providing the integrity and signal corrections, and thus do not need the receiver's RAIM function. That is why you dont have to specifically do a RAIM check. Always still a good idea though in the event of a WAAS outage. That's why the G1000 still has a RAIM prediction function.

Also, here's the AOPA brief on the new rule on preflite RAIM checks before flying RNAV SID/STARS: http://www.aopa.org/advocacy/articles/2009/raim_issue_brief.html

Hope that clears things up! :)
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Thanks for that. Makes more sense to me now. I've got tons of time in the G1000 environment, but I've never flown a SID/STAR in one. ATC doesn't seem to want to give them to us just flying along in a 172.
 
Alchemy said:
Today our company was filing our flights out of O'hare on non-rnav routes due to "inability to assure RAIM coverage" or something to that effect. Of course, ATC was clearing us on RNAV routes either via amended full route clearance before takeoff or once airborne. Just in case, we were checking predictive RAIM with our FMS's for the various waypoints we were assigned, and it all came up good.

I'm pretty ashamed at how little I actually know about RAIM requirments for enroute nav and sid's/star's. Who is responsible for checking this for enroute nav? The pilots are only required to verify RAIM prior to an RNAV approach, as far as I know (I'm not RNAV approach qualified). Why would our company claim that the RAIM was no good when our FMS's and ATC all seemed to indicate it was working fine? Could it just be that they were given bad info?

Thanks.
Click to expand...

My guess is the service that checks the RAIM for the dispatchers was out of service, therefore the dispatcher couldn't check the RAIM and were just forced to go old school on you.
 
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