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Weather avoidance and ATC

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Weather avoidance and ATC

Old 30th Aug 2010, 02:12
  #21 (permalink)  
 
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I think a point needs to be made about the original question in the thread;

Weather avoidance "in THE TMA." Not at altitude. That is a different set of circumstances and I think the regs already cover most contingencies in thas regard.

At altitude our TAS is very high, therefore it behoves all of us to ensure timely weather avoidance. Transport aeroplanes will come apart inside these Cb walls at 8 miles a minute! So do whatever you have to do, get off track, climb, descend away from traffic, broadcast intentions, etc, etc. (All things that have been rightly noted in this thread.)

However, in the TMA, I think a little more mental latitude is called for. Our speed is lower, our airspace is extremely limited, and we are in close proximity to both other aircraft, all trying to avoid, and of course, terrain.

ATC have a job to do, so try and make it easier for them... give them options."I require heading xx for xxmiles to avoid" That helps him plan. If he can't help you he may give you an alternative (then again, he may not, so be ready for it) If all fails, you may have to be prepared to ride it out, or hold clear. With only minor exceptions, worldwide, I've yet to see weather below the freezing level that will bend a transport aeroplane. Sure it'll shake you up a bit, so batten down the crew early if you know you're going to be in the thick of it and be prepared to ride it out at minimum penetration speed.
If it is really that bad, and you dn't want to push on, then hold clear of the weather (at least ATC can plan around you as a fixed point in space then) The Final approach requires more caution of course, as you are configured with a lower g-tolerance and windshear is your biggest threat, so by all means AVOID./ Hold/whatever.

But for goodness sake, don't make a bad situation worse by self vectoring in proximity to other traffic...who may be 'self vectoring' unannounced themselves! ATC will hardly be able to rescue you then: Imagine a multiple TCAS RA event going on WITH weather avoidance by other traffic! As to the A7700 option.... I'm not convinced that'd be a good idea. Certainly NOT in most 2nd and 3rd world ATC environs.( I stand to be corrected, happily if it becomes ICAO promulgated practice, of course) It'd cause total confusion and almost certainly would not get you the result you need. Remember this is a TMA situation here... you simply don't have the space.
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Old 30th Aug 2010, 07:03
  #22 (permalink)  
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Originally Posted by Honkozzie
At altitude our TAS is very high, therefore it behoves all of us to ensure timely weather avoidance. Transport aeroplanes will come apart inside these Cb walls at 8 miles a minute!
The certification regulations don't quite agree with you here. It is not your TAS which counts here, it's the dynamic pressure. At high altitude, at cruise airspeeds, the dynamic pressure is not as high as at medium altitudes. The gust limits which the aircraft structure must adequately resist (plus a safety factor) actually decrease for higher flight levels. Here is the current CS 25.341 (a) 5:

Originally Posted by CS 25.34 (a) 5
(i) VC: Positive and negative gusts with reference gust velocities of 17.07 m/s (56·0 ft/s) EAS must be considered at sea level. The reference gust velocity may be reduced linearly from 17.07 m/s (56·0 ft/s) EAS at sea level to 13.41 m/s (44·0 ft/s) EAS at 4572 m (15 000 fet). The reference gust velocity may be further reduced linearly from 13.41m/s (44·0 ft/s) EAS at 4572 m (15 000 fet) to 7.92 m/sec (26·0 ft/sc) EAS at 15240m (50 000 ft).
(ii) At the aeroplane design speed VD: The reference gust velocity must be 0·5 times the value obtained under CS 25.341(a)(5)(i).
PBL
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Old 30th Aug 2010, 16:13
  #23 (permalink)  
 
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Considering the lack of space for us pilots and ATCOs, my practice has been to avoid the problem by having and briefing a clear route free of weather in or out of the terminal area. I have have bad memories of holding much too close to weather on the arrivals to KATL, too many times. One particularly bad afternoon at SINCA where the pax had all their window shades down after we landed. The TMA is just too compact.

I do agree with A7700, it's better than keeping your maneuver a a secret. BTW, I fly a Global, so I have more flex than you airline types

GF
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Old 31st Aug 2010, 02:38
  #24 (permalink)  
 
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PBL

I think you're missing my point somewhat! The main thrust of my post was to do with the philosophy of assessing the criticality of weather avoidance at all costs, as opposed to looking for the path of least resistance, and mitigating further threats to the operation.

The other point I was making with regards to TAS was purely one of increased momentum at altitude. For a given gust vector, an increase in TAS (for a given IAS/EAS)will result in a greater resultant displacement vector from the flight path. And we all (hopefully)know what that can result in...

Cheers
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Old 31st Aug 2010, 07:04
  #25 (permalink)  
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Originally Posted by Honkozzie
I think you're missing my point somewhat!
If you are trying to make a point using mistaken ideas about aerodynamics, then you are failing to make your point. If your point is well taken, you should be able to make it without using mistaken reasoning. So best to try, no?

Originally Posted by Honkozzie
The other point I was making with regards to TAS was purely one of increased momentum at altitude. For a given gust vector, an increase in TAS (for a given IAS/EAS)will result in a greater resultant displacement vector from the flight path. And we all (hopefully)know what that can result in...
As far as I can interpret it, this statement seems to be misleading as well.
You talk about a "resultant displacement vector". "Resultant" is a technical term in dynamics for what you get by summing two other vectors, in this case they must be distance vectors. But I don't see those in your story so far. There is a velocity vector in the direction of flight path, and an acceleration vector, caused by the gust, normal to it. To get a resultant, you need to sum distance vectors, or velocity vectors, or acceleration vectors, but you have so far only adduced one of each, so there is no "resultant". Maybe you mean "resulting"?

Here is an argument to the exact contrary of your conclusion.

A displacement from the flight path is a function of acceleration normal to the flight path, and duration of the force causing the acceleration (the lift engendered by the gust). The lift engendered by the gust will be dependent upon the coefficient of lift, which itself is functional on the angle of attack. And those goings on are described by EAS, with two exceptions. One exception is the duration of the force, which, for a fixed gust domain, is less when TAS is higher. The other exception is the AoA in the gust, which is the angle of the resultant vector of the velocity of the airplane and the gust velocity normal to the airplane velocity. (Let us ignore the difference between geometric angle of attack and effective angle of attack for the purposes of this discussion.)

For purposes of aerodynamics, gust velocities are taken to be EAS (see CS 25 for examples). Let us fix the EAS of the airplane and the gust. The resultant velocity vector will have a specific angle to the velocity vector of the airplane, and this is AoA. This will generate a specific lift, which will accelerate the airplane upwards, and of course this acceleration will change the airplane velocity vector, so the AoA will be continually changing during the encounter. The divergence from flight path is a function of this, which one can obtain by double-integrating the acceleration, which is functional upon the continually-changing AoA. The salient point here is that, for a given EAS of airplane and gust, and given initial AoA of the airplane, the displacement from initial FP is a function of the duration of the gust.

Now, for a given lateral gust size, if your TAS is higher, you will be through the gust faster, that is, less duration of exposure. That means, according to the reasoning above, your displacement will be less than if your TAS is lower.

This is the exact opposite of the situation you want us apparently to assent to: you claim the displacement is greater.

So which is right? Is the displacement greater, or less, for a higher TAS but all EAS's the same?

To summarise, I am not yet convinced that
Originally Posted by Honkozzie
we all (hopefully)know what that can result in...
.

BTW, if your "main point" is that weather avoidance is a good thing, I agree, for the paying passengers as well as the pilot. But weather avoidance is a more complicated issue than just not going where the red shows. Consider ice-particle icing, for example. That can ruin your day also.

PBL
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Old 31st Aug 2010, 09:05
  #26 (permalink)  
 
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Hmmm...

How to take a simple question about weather in and around the TMA and turn it into something extremely dull..

Thanks.
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Old 31st Aug 2010, 10:38
  #27 (permalink)  
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This debate has been round before.

The pilot in command has final responsibity for the safety of the flight. No one else. It is the pilot in command alone who decides what is safe and what is not and if they make an informed decision that avoiding a CB before being able to get clearance is the safe option then the law backs up that decision 100%.

Yes if one can ask or advise ATC about what is happening then that is the preferred option.

However, I am worried that ATC are saying no-no-no to the idea of (when necessary) avoid first and tell later simply to avoid having 2.5nm between two blips when there should be 3 and this pressure to do something unsafe could lead the less experienced into a lot of trouble.

Thinking of the LTMA situation, there is far too much pressure on people to maintain departure rates when lots of CB's about and that is what causes most of the fear.

If there was traffic avoiding weather all over the place and the avoiding traffic could not get a word in and they subsequently had to rely on TCAS to avoid a collision (not simply a loss of separation) then the biggest question would be why the departure was permitted?

The question always comes round about weatehr being displayed on ATC displays. Every time it is asked of NATS the answer "we have the weather filtered out" comes back..........which was a great answer in the 1970's when it was good not to have aircraft disappearing into the weather.

However, they seem to have not realised that today it is possible to have a colour real time feed from the local weather radar as a selectable overlay on the display when required. The information is fed as far as the ops room but that is all. and the reason why they won't do it - money.

So when I can't get a word in edgeways and use my display of the dangerous area of weather ahead to avoid it in the interests of safety don't complain because if money was not put before safety you could have the same (if not better) information when you need it.

As for the 7700 squawk all I can say is this suits the statisticians perfectly and the printouts will not have an airprox due to weather avoidance wearing two standard (orcam) squwaks.

Next we will have -

ABC123 turning right to avoid.

ATC - Negative continue heading 360 maintain 2000ft

ABC123 is now heading 090 at 3000ft

ATC - You turned and climbed without clearance. I am going to send a report and your stupid captain ass is toast.

ABC123 - can you please include in your report the fact that we avoided opposite direction traffic at our level.
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Old 31st Aug 2010, 11:00
  #28 (permalink)  
 
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At last, some sense.

DFC is spot on, remember the ATCO does not have his backside strapped to that aircraft, we do.
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Old 31st Aug 2010, 12:09
  #29 (permalink)  
 
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<<However, they seem to have not realised that today it is possible to have a colour real time feed from the local weather radar as a selectable overlay on the display when required. The information is fed as far as the ops room but that is all. and the reason why they won't do it - money.>>

It's 8 years since I retired from LATCC. At that time there was a weather radar display in the ops room connected direct to the Met Office. I was told that they charged NATS Ģ50,000 per year for the privilege of seeing a picture which anyone can now bring up for free on the internet!!

<<ATC - You turned and climbed without clearance. I am going to send a report and your stupid captain ass is toast.>>

Thankfully, Air Traffic Controllers are professional people who would never use such language.

Also meant to add something I've said many times.. When I was working the interpretation of patterns on airborne weather radar varied dramatically between types of aircraft and airlines; sometimes even with the same type and same airline. One would continue as if there was no weather whilst the one 3nm behind wanted to gyrate all over the sky.

Last edited by HEATHROW DIRECTOR; 31st Aug 2010 at 14:28.
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Old 2nd Sep 2010, 17:22
  #30 (permalink)  
 
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Well, in my oppinion you must avoid CBs telling ATC your intentions in order that the service providing to the rest of the traffic by ATC is safe and correct. If you canīt contact to ATC, then I wouldnīt wait and I would make my decision, I mean to avoid CBs keeping in mind my TCAS.

I donīt agree to select SQ 7700, It is not a good idea because in that case everyone would be performing a missed approach instructed by ATC as you would be in an urgent situation.

I think the ATC should have given clearance to enter holding pattern until the mess is solved. I donīt understand why ATC allowed that stress situation as there are a lot of IF or IAF inside TMA or even you can say: 360š OVER HERE

If your endurance let you a hold over a point... I would make that decission until weather improves.

BR.
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