Inertial Drift

[Allflight57 0]

Someone posted a question about this a few months ago and got no answer. Going to try again...I am currently going IAH-LHR and have been monitoring my inertial position closely, and by the time I get near Boston, the inertial position has over 4NM of drift.That seems rather high (I could be wrong). I've also seen screenshots of people doing similar long haul flights without that amount of drift. The only thing I can think of is I guess as long as ANP remains less than RNP, I am in the clear and shouldn't worry about it?

 

Thanks,

 

Tyler Mason

[scandinavian13 5,192]

Someone posted a question about this a few months ago and got no answer. Going to try again...I am currently going IAH-LHR and have been monitoring my inertial position closely, and by the time I get near Boston, the inertial position has over 4NM of drift.That seems rather high (I could be wrong). I've also seen screenshots of people doing similar long haul flights without that amount of drift. The only thing I can think of is I guess as long as ANP remains less than RNP, I am in the clear and shouldn't worry about it?

 

Thanks,

 

Tyler Mason

 

Acceptable drift on a 10 hour flight can be up to 22nm. These systems are being updated by reference to GPS position and DME/DME updating. Your primary concern is ANP, which is the aircraft's ability to understand where it is in the world using all of the sources.

 

[skelsey 765]

That does feel relatively high by modern standards, though as much as a couple of nautical miles per hour of IRS drift is not impossible. What happens when you look at the individual IRS positions rather than the triple mix? Is one particularly higher than the other two?

 

The main question for you heading over the North Atlantic MNPS area is your navigation accuracy and redundancy. You need two fully serviceable long-range navigation systems to operate in the MNPS area, of which your GPS is one and your IRS is another.

 

Precise definitions of 'how much drift is too much' are generally spelt out in individual operators' manuals. However, as a general guide if the difference between two systems (i.e. GPS vs IRS mix) is more than 25NM then one of the systems should be regarded as failed, in which case ATC should be notified and you should consider your options as you will not be allowed in to the MNPS area: i.e. either divert or re-route outside the MNPS airspace (i.e. below FL280/above FL420 or route via one of the 'Blue Spruce' routes).

 

As always, prior to coasting out on the track you should verify your navigation accuracy with a bearing and distance check against a radio aid, and likewise when you come back in to radio aid coverage on the other side.

[Allflight57 0]

Thanks guys, that clears it right up. Simon, I can only see the triple mix. Is there a way to view each of the three individual IRUs position?

 

Tyler Mason

[skelsey 765]

Should be on one of the POS REF pages (3/3 I think -- at least it is on the 747 -- I assume the 777 is the same in this respect).

[Allflight57 0]

I do remember that from the 747. 777's is different, its showing GPS L/R and FMC L/R on POS REF 3/3. The way I understand it is the ADIRU consists of 6 ring laser gyros and a bunch of other equipment that give it position data among other things, maybe thats why it just displays  an average position on the POS REF page...who knows haha

 

Tyler Mason

[skelsey 765]

Ah -- that is probably the reason!

 

Still on steam-power and four engines over here ;-)

[skelsey 765]

Incidentally, did you do a full IRS alignment before departure or just a quick alignment?

 

The reason I ask is that a quick alignment does not remove all the sources of error in the way that a full alignment does -- it more or less just resets the position, but the drift error from the accelerometers remains and is likely to continue to increase -- basically the system will drift more quickly than if you do a full alignment, which fully resets the system.