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Class II (oceanic) plotting

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Class II (oceanic) plotting

Old 12th Mar 2008, 11:13
  #1 (permalink)  
Thread Starter
Join Date: Feb 2001
Posts: 20
fish Class II (oceanic) plotting

Morning all,

I am trying to get to the bottom of when and why oceanic plotting takes place. Does anyone have, please:

1. A reference as to any rule or compulsion which requires plotting?

2. A strong reason why plotting should be done, in particular on fixed routes/airways which are contained in an FMC data base?

3. When does your company do it?

Info seems to be very thin on the ground and I am not experienced in class II navigation other than the odd Tango route.

Thanks in anticipation.
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Old 12th Mar 2008, 19:48
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I believe that the MNPS manual strongly suggests the post position plot for starters. Now that in it's self is not regulatory so you simply have to weigh your experience with the concept of good operating practice. I'm pretty sure that there are some well known airlines that probably do not plot. I know for a fact that there are airlines that do not plot when on a airway that is in the NDB. They are however required to carry a plotting chart with them in the event of a reroute that is off airway. If your operating under FAA oversight I believe that the inspectors handbook has some language regarding this issue as well.

From personal experience I can tell you that it has saved be from making an even more egregious error than I had made earlier in the flight. For the time it takes it's cheap insurance that you are where you think you are and where you think your going.

Last edited by Spooky 2; 13th Mar 2008 at 22:52.
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Old 13th Mar 2008, 13:23
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Thanks spooky.

I will look for the FAA inspector's handbook. There doesnt seem to be any other guidance for areas outside the North Atlantic, so is it airmanship the requires it? If so what are the arguments for and against?

If I am right the MNPS manual talks of it in terms of preventing errors during input into an FMS. So, with fixed routes that are stored in the NDB why bother?

Are there any areas that might be ETOPS but be still class I (within range of normal nav aids), eg. Australia maybe?

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Old 13th Mar 2008, 17:05
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The plotting rule is buried in PansOps somewhere; a Check Captain showed me the excerpt several years ago, but I don't remember the exact reference.

Unfortunately, the rule had not been updated to consider GPS equipped airplanes, so a lot of the rationale behind it is obsolete...

While I believe there is still value in plotting the NATS tracks, I believe it is useless on established routes such as the NOPAC and US-Hawaii routes when the airplane is equipped with GPS and FMS.
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Old 13th Mar 2008, 17:26
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Join Date: May 2002
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I think it is a very good crosscheck for the computers. Why not do it? We all have enough time on our hands on these long legs and we my just pick up a computer error that we may not have been aware of.

belt and braces.
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Old 13th Mar 2008, 17:30
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Here is an old Memo that I created for the company that I was working for at that time. Take it for what it's worth.

The Dreaded Post Position Plot or PPP

On XX transoceanic flights, Post Position Plots (PPPs) are accomplished for three reasons:

We have committed to complying with the North Atlantic Minimum Navigation Performance Specifications (MNPS) which strongly recommends them.

They show a difference between the plotted track and the track loaded in the FMS.

They show the FMS data base errors related to the abbreviated LAT/LONG convention (i.e., 5630N)

Post Position Plots understandably come under fire because they are not only redundant checks but because they appear to be useless since the ND does such a good job of displaying the aircraft's position and track. It is true they are redundant, but don't be deceived by the level of comfort the ND gives you. The ND can be more seductive than productive in determining position error. You know you are on the magenta line, a PPP ensures the magenta line is in the right place. A PPP ensures that the plotted track is the same as the track in the FMS. Of all the checking and rechecking we do, a PPP is the only direct comparison between the track loaded in the FMS and the track plotted on the chart. All of the other checks of the FMS that we accomplish - loading, the checking and circling during preflight, the checks of each leg enroute - reference the flight plan only.

A PPP can also detect FMS database errors. Be aware that the abbreviated LAT/LONG conventions (short conventions) are created manually at Honeywell and they are subject to human error. You would expect a "be careful" article like this one to include historical anecdotes that "it has happened before". Well it has and more than just once. There were a number of waypoint errors wherein, for instance 5630N was really 5631N. Of course errors such as these should be caught during the FMS verification using the NAV DATA page and also enroute when the track, distance and time for each leg in the FMS is checked against the flight plan. The PPP is the third and last chance for catching these errors.

The professionals amongst us appreciate the power of the redundancy in our oceanic procedures. The care that is exhibited by a PPP and other ocean crossing disciplines has served transoceanic pilots well for decades The MNPSA manual is replete with cautions that every recommended check has caught errors and even with so many checks in place, errors continue to occur. Please do yourself and XXX a favor and accomplish the required oceanic disciplines with professionalism.

What A Post Position Plot Does Not Do!

A Post Position Plot (PPP) does not check the accuracy of the navigation system. The navigation accuracy of the IRS/GPS/FMS system as a whole is checked when we accomplish a Navigation Accuracy Check while still in Class 1 airspace. We can then rely on that check for the crossing. Except for database errors, FMS or IRU navigation errors can not be detected in Class II airspace. (Of course the FMS/IRU system as a whole does internal comparisons constantly and will detect and alert you for most of its own errors, such as an errant IRU). But, in the unlikely event of a computation error that is not detected internally, you can not detect it by looking at the ND or by accomplishing a PPP. Lets assume that you are correctly using the A/P and LNAV has put you (Longitudinally) in the right place on the magenta line. If the FMS thinks it is in the right place but it is really 40 miles south of where it is supposed to be, the aircraft will indeed be 40 miles off course. But the ND will look normal because the ND is telling you where the FMS thinks it is and the FMS thinks it is in the right place! If it were aware of its error, it would show it on the map and correct it. But, if it is aware of it, it can neither display the error or correct for it.

Donít be seduced by the map display. If you are in LNAV and VNAV the map can look normal regardless of any errors that are in the system.

Okay, What'a About That Navigation Accuracy Check?

Currently there is little language in our I.O.S. regarding the application of a Nav Accuracy Check. As always one is treading on rough grounds when he makes sure statements about the myriad of FMS combinations that are in our fleets however the simplest method of doing a Nav Accuracy check would be to insert a VOR within 150 miles of your position and the hard tune the same VOR and compare the FMS position with the hard tuned VOR DME and bearing. Generally and this is where it gets a little fuzzy, a distance error of no more than 6 miles and 4 degrees is considered to be within limits. The 6 mile error figure dates way back to the days of Carousel IV equipment and is really nothing more than a situational awareness figure. Obviously if there were a significant difference in either of the values it would be a tip of that something might be amiss and the crew should remain vigilant for other navigation anomalies.
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