Airway width/wx deviation

[Yani Yani]

Input on the following query would be appreciated :

Assuming I am on an e.g. Oceanic airway of an 8 NM width, 4 NM either side of me and I need to deviate around a small CB, lets say two miles right of course, do I need ATC approval ? Will a say 30 degree heading change for that CB trigger bells and whistles on your screen ?

And what if I am closer to a terminal area, lets say on a STAR where the airway,or tolerances,are considerbly tighter, can I still sneak left or right in the same fashion without ATC approval ?

The above statements assume I am aware of the width of the airway I am on. My airplane's FMC will inform me what the required RNP is of the airspace/airway I am on and it narrows down as I transition from say Oceanic airspace of 4 NM to Domestic airspace of maybe 2 NM down to 0.3 NM when I am on the approach.

Do ATC have a similar concept of "narrowing" airway width or navigational tolerances as I approach a terminal area ?

Regards

[HEATHROW DIRECTOR]

<<And what if I am closer to a terminal area, lets say on a STAR where the airway,or tolerances,are considerbly tighter, can I still sneak left or right in the same fashion without ATC approval ? ..

I suppose alot depends on the airspace you are in. Where I used to work abroad you could sneak left or right 90 degrees without any hassle as there were few aircraft. In, or approaching, a busy TMA it would be immensely dangerous to even think about such action. You should ALWAYS seek ATC approval before leaving any designated route, especially a STAR or SID, because you cannot possibly be aware of all other traffic around you.

[Slo Moe]

Absolutely ALL deviations need an approval from the ATC.

Only an emergency or a TCAS RA would allow something else.

Example: There might be another aircraft on an opposite track
6NM offset of the airway centreline and if you deviate 2NM just
on the right spot you would be almost head on for a moment.
You might not even hear the other traffic on frequency, it might
be coordinated over the phone, or via the radar screens
(silent coordination).

At least the ATCO wants to keep his/her separation, if the minimum is 5NM
at that airspace. As a pilot you most likely will not know which is the
separation minima in different parts of airspaces. They vary
a lot depending on many factors.RNP does NOT tell the separation minima.

Do you want to try to guess which
separation minimum the ATC is using this time?

3NM
or
5NM
or
8NM
or
10NM
or
17NM
or
20 NM
or
2 minutes
or
3 minutes
or
10 minutes
or
80 NM

or something else, just to name a few.
I would rather stick to the clearance.

[Slo Moe]

As a pilot you would not know, if the ATC is a procedural approach.

There are such separations, approved by national CAA:s (Civil aviation authorities)
that allow airplanes to fly approx 4.5 NM nearest from each other when
flying on certain radials.

(One aircraft flying VOR radial inbound and the other flying a VOR radial outbound)

2NM deviation from that, and the other deviating because of equipment tolerances
and you might be quite much wanting to adhere to the clearance.

So be proactive,
look at the weather radar display
and ask in good time.

The rules for separation might change some day in oceanic control also.
It would be good for you, if you had "healthy habits and procedures"
at that time also.

Think, if there is another pilot deviating
"silently without approval" then it is
up to the allmighty what is the
distance between the other.

[Jagohu]

I can only agree with the previous posts - please always ask for the approval of the controller if you would like to deviate. You might not know that you're flying through a military airspace on a designated and therefore rather narrow corridor - you might eventually get shot down or hit by a fighter even(not squawking so no TCAS)...
Take those 10 seconds extra, it will pay off both for you and for us! Thanks :)

[Yani Yani]

Thanks for input guys.

So I wonder why airways have a varying width then ? Oceanic airways have width of e.g. 8NM and STARs down to 0.3NM RNP, Required Navigational Performance. Many fellow pilot collegues out there argue that if an airway is xx miles wide, then as long as I am within the confines of that airway, I am complying with the ATC clearance and hence could do a slight heading change/deviation for e.g. a small CB. This does not pertain to radar vectors or if on approach of course.

Also, if my navigational equipment onboard is based on GPS versus e.g. VOR/DME based systems, I believe I would appear to ATC as being much more "accurate" or more on the center of the airway yes ? So that means you guys must be faced with aircraft stating they are smack center of the airway while in reality, based on their nav equipment, they are off center by some distance ? I assume ATC must have some tolerances in instances like this. How do these aircraft appear on your radar screens ?

Or perhaps my thinking on airway width is immaterial as far as ATC goes ? Is it correct to state that ATC assumes that if an aircraft is supposed to be on a specific airway then that aircraft will attempt to stay on that airway as accurately as possible, with tolerances allowed based on nav equipment onboard ?

[HEATHROW DIRECTOR]

<<How do these aircraft appear on your radar screens ? >>

Surely you have seen a radar screen? Aircraft appear as tiny points of light, or "targets", sometimes with labels containing information on callsign, altitude, speed, etc., "attached" to the targets.

[AirNoServicesAustralia]

Yani, the reason different airways have varying widths is based on the required navigation performance of the aircraft able to use that route. Unfortunately these days pilots think that because the aircraft says they are at point X they are exactly at point X when really they could actually be a mile south of point X.

For the most part these days, yes with GPS etc, aircraft are where the nav system says they are, but sometimes (even with new aircraft) they wander a bit and are up to a couple of miles of track.

So when you think you need 3 NMs left of track to get around a CB and the air route is RNP5, that doesn't mean you can help yourself and not tell ATC. The RNP5 tolerance is there to account for errors inherent in normal operations, that is, nav errors with aircraft, radar tolerance errors from our side etc. It is not to allow for intentional diversions from the published ATS route. By you choosing to intentionally go 3NMs off track you are adding an additional 3NM to the "error" for that route and that it where the problems can happen.

As someone else said, due to the congestion around Dubai these days we have no choice sometimes but to run aircraft close (not much above the minimum 5NMs). If both aircraft decide that they can wander even a mile without telling us, we have a reduction and get suspended (making the staffing situation even worse than it is, resulting in even more holding for the pilots than there already is).

Bottom line is, follow the rules and don't take shortcuts. Sorry but the end of the day it's your backside and all those people behind you that is on the line when you don't do as you should, so always ask before diverting. And to some of the pilots in the Middle East, that is BEFORE, not after having already turned 30 degrees, and then telling us that you are diverting. As I said with the number of due regard Military aircraft around who aren't talking and you may not know about, it is insane to go off your cleared clearance under any circumstance out here at the moment.

Cheers.

[Slo Moe]

Yani Yani,

Thank you for showing a healthy and positive attitude about
your profession.

Apologies, if I was a bit direct on my earlier postings.
In other sense I might say that DCT is OK...:D

I quite much second the previous two opinions
from ANSA and H.D.

With respect,

Slo Moe

[Slo Moe]

BTW the term I used "procedural approach" can also be "procedural ACC".
(Area Control Centre)

Those facilities usually do not say "radar contact", they base their separations
(The minumum distance that the aircraft are kept from each other, in plain
language)
on certain procedures. (The word procedure does not refer to the
checklists etc.)
The procedures are methods that are based on using the navigational
instruments or such (VOR, VOR/DME, ADF, altimeter, time etc.) and the
reports that pilots give. (Report passing 10 miles DME, report passing four
thousand feet etc)

They are just those clearances that are quite a task to write down...

Oceanic conrol has been based quite much on procedural separations.
That's almost all that I know about that subject.

[Slo Moe]

In the ACC a typical procedural clearance could be:
"Follow airway UN472, maintain FL360, report passing (fix) QWERTY"
And although they might not give radar service, (No "Radar contact" or "identified" etc.)
they can have radar to monitor your flight.

As a pilot you could write the above (to help you write it down a bit faster)
"UN472, F360, RP QWERTY". These kinds of abbreviations the ATCOs use,
if they are not putting all your flight details on the label of the radar screen.

In a procedural approach a typical departure clearance could be:

After departure expect left turn. Follow radial 175 from XYZ VOR.
Maintain 3 thousand feet. At distance 10 DME XYZ VOR climb to FL 140.
When passing 6000 feet cleared direct to YUIOP.

So if you want to write this down a bit faster, you could do it like this:

LT,
V175XYZ,
3.0/D10XYZ,
´I` F140 (The character ´I` here represents an "UP" arrow, my keyboard does not have that)
6.0/DCT YUIOP

And you are supposed to fly from departure a left turn, climb to 3000 feet
following radial 175. On that radial you climb to FL140. When passing 6000
feet you are "cleared to leave the radial" direct to YUIOP.

And although they might not give radar service, (No "Radar contact" or "identified" etc.)
they can have radar to monitor your flight.

[tired-flyboy]

OK so your in a different part of the world to me, but what about bidirectional airways.

airway 10 miles wide (rules state that aircraft should be positioned 2 miles form airway edge) leaves 6 miles to play with.

we're allowed to use 3 miles separation between aircraft. if you were one of these aircraft you'd very very quickly have a bad day if the opposite aircraft turned without a word. (even 10deg could cause an awful lot of problems!)

best rule - ALWAYS ASK! can't be said often enough

also most radars today are very good at noticing your path (we use trail dots to give history of where the aircraft has been)

[jadaz]

Hello guys,

I would like to append a question to Yani's original post, what if the Airplane in question had com. failure?

Special procedures for inflight contingencies in the Oceanic Airspace allow a 10NM/19Km deviation maintaining level due to weather in case a revised ATC clearance cannot be obtained, if it is more than 10NM you have to climb or descend 300ft/90m.

Ref. jeppesen text/Air Traffic Control/Air Traffic Managment (DOC 4444)/ Chapter 15.2.3.3

What is the case for European/Middle Eastern (NonOceanic) Airspace? Could the answer be stated in the Emergency section of jepptext?

Best Regards
Jadaz