GPS?Steve Bell

Hi Subin,An aircraft has to be certified as being able to operate within the margins set for both RNP and RVSM. An aircraft declared as RNP/RVSM capable which cannot subsequently operate within these margins would put the operator in a heap of trouble.The biggest reported error, after investigation, of aircraft operating to these margins is GNE (gross navigational error) and this is nearly always attributed to the failure of the flight crew to take all necessary precautions. It is easy for us sim pilots to set up a route and expect the aircraft to follow it in, usually, not very busy skies. And using software like ServInfo, we get the impression on trans-oceanic flights, that we know where we are because the software shows us. In real life trans-oceanic flights, before the availability of GPS, pilots and ATC had to rely on calculated position reports for the correct maintenance of their separations. Now, even with GPS, pilots are required to regularly cross check their positions along a route by reference to times elapsed, navigation aids passed, etc.Next time you are on a long sector, try to regularly prove to yourself that you are where you believe you are. This is an excellent way to learn improved navigation techniques.Happy navigating,Richard

It's easy to crosscheck the accuracy of your track, the information is provided by the FMC (go to pg 12-55 AOM) from the PROGRESS page select POS REF... you can see the differences between IRS and GPS, which is a good indicator of the quality of your estimated position. Depending on how many and where the satellites are in the sky, the GPS can usually put you within 0.1 nm (600 ft). The advantage to also having IRS is these systems are not suseptable to the occasional degradations in service that GPS has, and it is a truely redundant system.

Strategic Lateral Offset Procedurehttp://en.wikipedia.org/wiki/Strategic_Lat...ffset_ProcedureDavid "jumbojock" RobertsonPS Ooops didn't read page 2 of the thread...sorry. How do you delete a post?

Hello,Thanks for clarifying things about SLOP.Now one more question : on the PROGRESS page 2/3 LSK2L, it says "XTK ERROR" ...Can these figures been changed, say 1NM, so you fly 1nm L or R from track ?Or is this just for information ?

The crosstrack error cannot be set as you suggested, it is a measurement of where the aircraft is with respect to where the navigation computer would like to be. Offsets are set on the RTE page.

Hi DanThanks for your reply. I thought it was something like that.But can you plaese point me in the right direction of to where exactly (what RTE page and/or in what format) the offset can be set?Thanks for the effort.

William,For SLOP, you are only allowed to choose no offset, 1 mile right or 2 miles right offsets. So enter 'R1' or 'R2' at LSK 6R on the ACT RTE page 1 and press Exec. Dashes are initially displayed at LSK 6R when the offset option becomes available. On the ground, or on an inactive route page, or during a SID, STAR or approach procedure, an offset cannot be entered. The end point of an offset route is the first waypoint of an arrival or approach procedure, or a track change more than 90 degrees, or a route discontinuity.Hope this helps, Cheers, Richard

Richard,Thanks or the update, it works.Never seen it before, always focused on LEGS-page!Again learnt something through this forum!

Holy crap, you learn something new every day! Just when I thought I had the Queen down pat, along comes another trick. I just input 2R in the offset and it works like a champ. I didn't even have to use HDG, I just left LNAV enabled and it corrected, and followed the deviation perfectly. Was this proper if it followed the new path without issue?