During ... en route ....

[SRover40]

When you follow FLT plan route, you meet some CB ahead of you..
How many NM can you deviate without advising ATC ?

Think I studies back in my instrument training.
but think I forgot.... :{



.....:ok:

[I-2021]

Hi,

I think it depends on the RNP of the airway. If you have RNP 5 , then you have a 5 NM buffer left or right of the airway axis. So if you will deviate more you must advice ATC.

[flyboyike]

^^^^^^^^^^^^^^^^^^

Not a bad answer, but what if you're not on an airway?

[DBate]

You should always advise ATC if you divert off an airway or off track, even if you stay within airway boundaries.

First, there might be some other traffic in the vicinity - maybe on an off-airway route - wich will be coming close to you, and your deviation could cause a proximity alert on the controllers radar screen, since minimum seperation could then be below limits. This is especially true in high density areas like all of central europe (I was given this info by an ATC controller).

Secondly, if you inform ATC, you keep them in the picture. Often ATC WX-radar is not as good as ours, and shows just the approximate position of a CB. Telling ATC about your diversion gives them the possibility to anticipate request for diversions by other aircraft, and they might start to vector everybody around the CB in general.

Just my thoughts...

[Spitoon]

A practical answer will be that i depends whether you're getting a radar service from AC (or, more correctly, whether you are in a radar controlled environment). If you are not, firstly other aircraft will be separated from you by much greater distances and so a few miles will not normally bring you into proximity to other aircraft and, secondly, the controller will have no way of knowing that you have deviated.

In a high density radar environment where the controller may be using as little as 5 NM separation between aircraft, well, if you're clever enough to have got an IR, do I really need to go on?

Real answer, if you are going to deviate from the track that ATC has assigned or that you've told ATC you will follow - then tell them! You may only have one aircraft to worry about, ATC looks after the big picture.

Slight rant over!

[ft]

"I think it depends on the RNP of the airway. If you have RNP 5 , then you have a 5 NM buffer left or right of the airway axis. So if you will deviate more you must advice ATC."

That is a dangerous answer. If you are yourself RNP 5, this means you will keep within 5 nm of the track you think you are on 95%. This means every intentional deviation will rapidly increase the percent of time you spend outside of these 5 nm.

If you have RNP 3, you will be within 3 nm of the track you think you are on 95% of the time. Very little time will be spent outside of the 5 nm boundary. However, if you deviate 2 nm you will spend 2.5% of your time outside the 5 nm boundary on the side to which you deviated. Any further deviation and you are back to the rapidly increasing percentage of time you'd have seen had you been on RNP 5 and deviated.

Assuming a distribution (e g gaussian) you can crunch the numbers and figure out the odds of the gamble it is to start deviating without advising ATC. You're playing the statistics which are intended to keep your aircraft and other aircraft safely separated.

With any RNP, if your equipment tells you you are on the edge of the airway you are in fact outside of the airway 50% of the time.

You may (or may not) understand this, but you never know about the guy with a fresh instrument rating reading your answer. He may take it as gospel and start spending 30% of his time in the air right at the edge of the glossy purple corridor depicted on the GPS screen - thus being outside the airway 15 percent of the time in the air.

[flite idol]

Its easy to confuse RNP (required navigation performance) with ANP (actual navigation performance). Your FMC should be able to tell you the RNP for the airspace you are in and the ANP of your aircraft!

[Dan Winterland]

Bearing in mind that SLOPS (Strategic Lateral Operations Procedures) allow for a maximum of 2NM offset without informing ATC where allowed, I would suggest that any deviation should be requested, unless under SLOPS - then it's 2NM.