Airways

[SkilledMusician]

Hello,

I've recently come to understand that there are airways and that there is a set of airways for high altitude and airways for low altitude. A few questions I have:

1. I would like to know how airliners transition from takeoff to get to their airway. Do they have to follow a set of low altitude airways to get to the higher airways, or do they just go straight there?

2. Can one waypoint exist at both high and low altitude airways

3. What are the height and width of airways? Seeing as atc can ask pilots to follow various headings or altitudes, I assume there is room for manoeuvre

4. On a slightly different note, how many waypoints are selected in a typical flight plan for short, mid and long haul flights?

I appreciate any answers and also happy if there are any links that give more insight.

SM

[AerocatS2A]

1. The flight plan will typically be from the navigation aid associated with the airport direct to the first waypoint on the airway. Often the airport navaid and first waypoint of the airway will be part of a bigger airway. The actual transition from the runway to the airway will be either via radar vectors (headings from the controller to clear you from traffic until he/she can give you a direct track to one of the waypoints on the route) or it will be a SID (standard instrument departure) which is a set of waypoints that transitions you from the runway to the airway.

2. Waypoints can be part of the low and high airways. Radio beacons such as a VOR or NDB (where they still exist) will typically be part of multiple high and low routes while RNAV (GPS) waypoints are more likely to be associated only with a specific route.

3. There is no specific height or width for airways. The width is nominally 0nm, that is you are supposed to do your best to be smack bang on the centre of the route, but there is a tracking tolerance that is to allow for navigation inaccuracies. The tolerance is not to be used to meander around the airway at will though. When you are given headings from ATC you are being taken off the airway. There is no particular requirement to always be on an airway and you may go off it to avoid weather, for more direct routing, or if the controller needs you to, however whatever you do must be in accordance with a clearance from ATC.

Low routes are below flight level 245 around here (Australia) and high routes are above FL245 but as long as you have a clearance there may be times when you are on a high route at a lower altitude or a low route at a higher altitude.

4. It varies. Waypoints are normally no further apart than 200NM but they can be a lot closer, particularly close to the airport.

[BOAC]

Re 3) it is not simple How wide is an airway ? [Archive] - PPRuNe Forums (http://www.pprune.org/archive/index.php/t-307259.html)

[SkilledMusician]

Thank you Aerocat. Re answer 1, does ATC actively follow the flight plan that is filed by the pilots?

This also brings up another question. If pilots decide to change their flight plan or divert during flight, do they need to file another and send it to atc somehow? Do they need to create a whole new flight plan? And is this rather taxing work for the pilot or is it fairly easy to do?

[SkilledMusician]

Thank BOAC, that's a good link.

[kcockayne]

SkilledMusician

ATC will accommodate the route, levels & destination shown on the FPL - as far as it can. There may be a need to allocate a slightly different track to that requested because of traffic conditions, airspace restrictions, slot time restrictions etc.; & FLS will often be changed because of separation requirements. Otherwise, pilots generally get what they want - standby for examples otherwise !
If the aircraft has to divert en-route that will be dealt with without a new FPL needing to be filed. However, ATC will need to be given details of the new route & destination etc. (over the radio). If a new FPL needs to be filed, for some reason, that can be done by the airline company or, details can be taken on the air (on a "quiet frequency").

[AerocatS2A]

If you NEED to change the route then you tell ATC what you need to do and they will facilitate it. If you merely WANT to change something for convenience then you request it and you may or may not get it.

Airways width is a funny one. The airway is allowed a certain width but the assumption is that the pilot will maintain as close to the centreline as possible. The pilot is to treat the airway as 0NM wide but it is designed with some tolerances applied. These tolerances are to account for navigation errors, they are not to allow you to deviate around weather, apply lateral offsets, or anything else unless specified in the applicable country's rule set.

[SkilledMusician]

Thanks guys. Do modern autopilots align well with airways? Because if I understand, following a track or heading, doesn't necessarily lign up with a designated route.

Sorry for the amount of questions. I'm quite geeky, so when I get into a particular subject I want to know all the ins and out, even though it's not my profession.

[glendalegoon]

airway width

I'm still half asleep this morning, but last time I checked, in the good old USA, Victor Airways are 4 nautical miles from centerline, but at greater distances from navaid can get wider.

let me wake up and think about it some more.

[wiggy]

Thanks guys. Do modern autopilots align well with airways?



On a modern GPS equipped type if you're actually trying to "follow" an airway using the autopilot you are quite probably within 50 metres or so of the airway centreline.....


Because if I understand, following a track or heading, doesn't necessarily lign up with a designated route.


That's a different issue/different mode of the autopilot....If your clearance (from ATC) is to fly a heading (e.g. for weather/traffic avoidance) or fly a track direct to a point (e.g. a downroute waypoint) then you do exactly that and the airway isn't relevant.

In reality over many parts of the world we rarely follow the published/flight planned airways for any great length of time. Out of the likes of London you'll be climbing out on ATC headings initially ( traffic avoidance in the climb), then probably told to go "direct" to a waypoint on the border between say Irish or Scottish Airspace, get handed over to the next controller, who in turn will quite probably give a "direct to" somewhere on the far side of his/her area, and so on...

[SkilledMusician]

Thanks wiggy. That explains things well.

[ZOOKER]

Skilled Musician,
If you google NATS AIP,-IAIP-En-Route. (ENR), you can find all the details of airways and upper ATS routes. The AIP is one of the documents used by pilots and air traffic controllers and covers the whole of the U.K. in great detail.

[NacelleStrake]

Also, skyvector.com have some detailed, Jeppesen-like charts on their world-hi section. They also show current VFR charts.....

[Liobian]

Musician - I feel the need to tell you that the responses from our Oz cousin do not quite fit the UK scenario. Given that UKAIP reference, you'll find that UK lower airways are usually 10nm wide, but will broaden out into much wider chunks of airspace around the major cities where traffic volumes necessitate more room to manoeuvre.
And, as you surmise, a 10-nm wide airway does allow opposite-direction tfc to be radar vectored when one a/c is climbed or descended through the level of the other. The ATCO will provide radar headings to achieve at least 5nm lateral separation, whilst keeping both a/c within the airway. We are taught to keep a/c away from the edge of the airway too - you never know who is liable to pop up out there !
ATC would not normally take an a/c outside controlled airspace since that airspace provides the protection the pilot has asked (and pays) for. If traffic does leave controlled airspace, then the services usually change and pilots are advised accordingly. More about that in the AIP too.
The upper air routes are deemed to be 10nm wide but the upper airspace is vast and permits greater tactical use. That said, a/c may still need to be kept close to centrelines to avoid areas delegated to military use.