View Full Version : Weather diversions

Nevile Bartos
12th Dec 2003, 13:00
Had an interesting case recently when a jet wanted to divert around weather. He was close to the next sector so further coordination was required before the issuing of a diversion off track.

The next sector was busy so I waited for a few seconds before calling him back. Next thing the pilot called up and says he needs the diversion now, otherwise they will be flying into the mess ahead. This all occured within around 5 nm from when he first requested the diversion.

Anyway, my query, why do you leave these requests so late? How far ahead can you see on the radar?

In busy periods these requests may not be available immediately, although the pilots expect to be given them when requested. Of course, they are approved immediately when available. Any info would give me an appreciation of what you guys see when encountering a cell.


12th Dec 2003, 14:31
Same thing happened to me today - a jet at least 400nm outside of my airspace and not known to me at all called me up requesting Wx diversion as the comms equipment for his responsible sector were playing up in the weather. As I was trying to figure out who to coordinate with and tee up the clearance (as I did not have a direct line), again I was "told" by the aircraft (Within about 1 min) that they were diverting without the clearance.

Agree with Neville - any info would be appreciated! :rolleyes:

Sperm Bank
12th Dec 2003, 15:55
Sounds like it could have been poor planning. We can generally see wx visually 100 miles away or if in cloud, pretty clearly on radar. There are times on departure where we turn the corner and low and behold a cell pops up, however once again we generally get a look at them on the ground during taxi/line up etc. I cant and wont paint all the lads with the one brush as there are many variables. On most given days however we should be able to give you plenty of notice

Some pilots like to fly right up to cells before they divert (lack of experience) while most others try to plan ahead. The distance and direction of turn of the diversion will depend on the size of the cell, wind direction and speed etc.

That is a very basic explanation lads.

12th Dec 2003, 16:45
There could be several reasons. I fly a bit lower than jets so their view may be better. Sometimes weather ( ie rain ) is very heavy and the return from radar is just all green with slight yellow and red. As you pass thru the rain the radar can see further. In troughy weather particularly it may not be easy to "see" very far ahead at all.

Another possibility is that the radar is covered in rain as described above and then as you come out of heavy rain you visually see a huge cell a few miles ahead that radar missed and need immediate diversions.

I have also been flying towards benign looking weather according to the radar and visually looks OK and then a huge lightening strike will appear directly ahead and confirm not all is well.

One further point is this type of weather pattern produces all over mess and you need to get up close sometimes to see the best way thru. In frontal weather its much more obvious and long distant adjustments can be made.

There is only some much radar can do using gain, tilt, etc particularly in a trough system like today. We sometimes have to change close to the mark. Most pilots flying jets and regional turbo props are experienced and will alter course earlier than later if the facts present themselves. Hope this helps a little

12th Dec 2003, 18:42
On another diversonary note ...

A B747 on climb requested diversion in C airspace so that they could climb in C and avoid E.

After sorting out the traffic and providing the requested diversion, the controller suggested to the crew "no problems this time, but a little more notice would help"

Prompt response from the Captain ... "where do you want me to send my roster?"

12th Dec 2003, 18:50
On another diversionary note, working as an ATC in the middle east I find that the pilots don't ask for NM left or right of track, but instead ask to fly a particular heading to avoid the weather. Is there a reason Oz pilots do it differently??

Capt Fathom
12th Dec 2003, 19:04
Nevile & Blastoid,
In a perfect world we would all get the notice we need. Unfortunately weather is very dynamic.
Weather that looks benign 80 miles away suddenly takes on a whole new meaning when you get up close, requiring a last minute request for a diversion. The reverse is also true. If everyone asked for 20nm offtrack 'just in case', there would be no end to it.
My experience is that 80% of the time you can see it coming. The rest of the time it becomes a last minute scramble!

12th Dec 2003, 19:31
I support the comments by Capt Fathom. There are times when nothing is gunna work for you. Weather radar is good but it isn't perfect. At 80 miles, wx can seem pretty benign or, at worst, the cells seem to be well isolated. The problem with the latter observation, which always makes me a tad cautious, is that additional cells can build very quicky or, indeed, the cells that I'm seeing are shielding the cells behind!

With this latter scenario, it is often impossible to see the shielded cells until within 20 miles or less. That's when sh!ts are trumps, of course! :ugh: Given a choice, I'm sure that I'm not the only one who is happy to thread between cells that are well separated. The problem is that, while they might appear to be well separated when you're 80-100 miles away, the buggas can grow and gang up on ya real quick.

That's the other case where sh!ts are trumps on short notice. In general, I always try to give ATC as much notice as possible as I'm keenly aware that I'm unlikely to be the only aircraft in that particular bit of airspace at that particular time.

Oh yeah... and thanks for the prompt response and clearance to divert, guys and guyesses! I always appreciate it! :ok:

12th Dec 2003, 20:00
A bit more from an ATC perspective.

When diversions are happening I'm usually asked for a distance off track, but sometimes for a heading.

For weather diversions I clear aircraft for a specified maximum distance off track. I usually try to approve (or prepare for) a greater distance off track than actually requested. For instance, a pilot requesting 10 degrees off track might get 20NM instead. It amounts to double what she/he asked for at 60 miles from the commencement of the diversion, and allows a bit up my sleeve for planning purposes. If they want more they get more, subject to the usual delays for coordination with adjoining units or level changes for separation. Like most things, the earlier I'm asked the the better I can do it. It's also a lot easier in the radar environment (where I work) than it is non-radar.

I never approve a heading for weather diversions. It is usually less flexible, and often leads to more discussion when the requested heading doesn't work (for instance, aircraft clears the first cell only to find another one to dodge). When everyone's diverting around weather the last thing we need is more chat.

If asked for 50NM or more off track I know this is gunna be fun (?) for everyone for several hours hence.

I also think that assigned headings should remain a radar tool used by ATC for separation, sequencing and terrain clearance - certainly not for avoidance of weather that I (ATC) can't see.

As an aside, I'm reminded of one of the best human-machine interfaces I've seen - the pilot of a multi million dollar 100+ seat passenger jet using the distance between the tip of his finger and the top of the dashboard to measure change in azimuth, in order to decide if he needed to climb above or divert around weather ahead. Simple, certainly, and common among pilots I'm sure, but a poignant reminder to me that the simple human is indeed master of the complex machine.

Anyway, it's all my own opinion/technique, of course, and is only provisional until I've sought the approval of Dick, Snarek, the Two Ronnies, Bill and Bill and the rest of AOPA!



13th Dec 2003, 04:08
I use nm L/R in cruise and headings on climb descent when on approach/departures.

I teach guys to ask for more miles than they need as many ask for not enough/barely enough.

You can see the weather at 80nm but it is usually to early to make a decision until 40nm or even 20nm.

If I can't get a clearance I am going to divert around it anyway. Fortunately a clearance has always been forthcoming so I mustn't be leaving it to late to ask.

13th Dec 2003, 18:20
I agree with Kooka. When diverting around WX, one is obligated to make the best possible decision. The best possible decision can only be made with the best possible information. The best possible information, offered by both radar and visual is gained inside 40nm.

Sperm bank writes in brakets, "lack of experience" as a reason why pilots divert inside 40nm. What a fool!! In fact its quite the reverse, its idiots like sperm bank that panic at 200miles when they see WX ahead and then go ahead and conduct non essential diversions.

When your looking for a street sign whilst driving, one usually waits until you can see it before turning!


Sperm Bank
13th Dec 2003, 19:07
To all the ATCO's reading this thread. I will not use the quintessential vernacular Mr powerbeater uses nor his derogatory tone. I will suggest to you however , that Mr powerbeater is the type that is causing you guys last minute hassles. There is virtually NO circumstance, particularly at high altitude where a a last minute diversion should be necessary, it is as I say generally mis-management by the pilot(s).

We can either see visually or pick up on radar (most times) exactly where wx is and know which way it is going. On the more modern a/c, the wind vector is pretty much a dead give away of the direction of the wx. On a/c without wind vector info, the troops will have some idea of the wind direction from their planning stage. At medium levels between 15000 and 25000 buried in cloud (which rarely happens in Oz) the guys probably don't get the same picture as we do up a little higher. However they should never have to fly within 40nm of a cell before they need to make a decision.

Weather detection and diversion is a VERY basic fundamental in airline flying. We all get it wrong sometimes by turning the wrong way etc, but a last minute split ar$e turn is both not necessary and more to the point pretty unprofessional.

I would imagine powerbeater that you fly freight as no airline check captain would accept your ludicrous assertion above. There are some very good books on radar use and weather avoidance. Get some and read them. You might learn something.

14th Dec 2003, 06:54

You've got private mail

Cactus Jack
14th Dec 2003, 12:37
Sperm, I get the feeling from your posts that you are a 737NG pilot for VB?

That being the case, you are probably only used to a very sophisticated WX Radar system, operating in Australia, which we all understand to be a very benign weather environment, in comparison to the USA or Europe.

Try using the older style radars, and you may give just alittle more understanding to why these calls do sometimes come late.

Sometimes you can blame late requests on inexperience. Sometimes it's stupidity or ignorance or inattention. But sometimes it's due to operating limitations of the WX Radar or the dynamic weather situation.

You can't just say it's always due to inexperience and there is no excuse. Maybe sometimes there are excuses.

And with all due respect to you power12, calling another pilot an"idiot" without actually knowing them is probably quite unprofessional in itself.

14th Dec 2003, 13:41
Hello Airey, remember the "Rogue", always seemed to need direct inside a turning point to avoid a "cell".

Noticed during last weeks wholesale diversions a few requests for a particular heading, and a few others who requested L/R then explained what headings they would be on and for how long. Hadn't noticed it before.

Rabbit 1
14th Dec 2003, 14:03
NG Wx Radar. That's a science all on it's own. This has been covered in the Tech forum. Well worth reading.

14th Dec 2003, 19:37
When a wx diversion appears necessary I'll always ask for so many nm off track in preference to a heading. Once approved this gives a band of airspace in which to negotiate the wx, which is almost always dynamic and subject to interpretation as you get closer.

This also reduces the need for further RT chatter should a heading need to be changed, particularly in a busy RT environment or the dreaded HF.

Hope this is of some use to the ATCO's reading this thread.

15th Dec 2003, 07:21
I'm not sure that Australian wx is so benign. I recall reading a report some years back that the area from Perth clockwise to Qld was one of the world's worst for thunderstorms. The rainfall out of thunderstorms at Perth has, in microbursts, been as heavy as measured anywhere.

I had a pilot flying in the DLQ area back in the '80's, surrounded by thunderstorms, tell me that it was worse than he had encountered in New Guinea.

15th Dec 2003, 10:12
Just a quick point about weather diversions from both a radar and non radar envioronment. When i worked procedural non radar airspace, we needed to know how far left or right of track you will go so we have some sort of basis to establish a lateral separation standard. If we pin you down to a certain distance from track we can confirm sep from other nearby routes.
In radar airspace i can see you the whole time so i dont really care where you go, so i clear A-C to deviate as required and let me know when clear. We try to stick with vertical separation as much as possible in wx diversions.
To thunderbox, yes headings are pretty much useless, would much rather a distance left or right of track.

compressor stall
15th Dec 2003, 13:15
Sometimes particuarly in the tropical areas and with ISOL and SCT wx, repeated changes of heading to pick you way left and right around storms or TWR CU would be a pain in the arse for you and RT chatter, particularly if everyine else is doing it.

If seems much more sensible to ask for 20nm left and/or right of track and give yourself a corridor in which you can operate.

Sure in terminal areas, a heading works well somtimes, but in cruise at FLs, it seems better to give yourself "Horizontal Block Levels".


15th Dec 2003, 15:43
last Friday from BN CN after dep from SYD:

"XXX, every man and his dog is diverting at the moment, I'll get back to you"!!

Heard many calls for 60nm off track.