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jgoggi

Radar tilt angle

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Hi, it's not 100% clear to me and I have been finding contrasting opinions... Is the 777 weather radar TILT angle related to the aircraft longitudinal axis or to the horizon?

Example: what tilt do I need to set if I am climbing with 10 degrees pitch up, in order to scan what is in front of my nose along my path?

Thank you.

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It is linked to the INS so it knows where the horizon is.

 

1 degree up is 1 degree above the horizon.....horizon means regardless of pitch.

 

(See PMDG777 FCOMv2 page 11.10.17 under point 3)

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Weather radar tilt is not set depending on nose attitude!

 

It is set according to your altitude....or better said according to at what height of the cloud you want to display.

 

Rule of thumb:

1 degrees up/down makes you look into the cloud 1000ft above/below your horizon/level when your ND is at a 10nm range.

2 degrees up/down.....2000ft above/below your horizon at 10nm distance.

2 degrees up/down....4000ft above/below your horizon at 20nm distance.

 

During cruise you would want to take a look into the TS clouds at a level where they have the most moisture....which is somewhere around 25.000ft.

So if I fly in FL320 and I want to see the intensity of a cloud that is 40nm ahead of my position, with 1 degrees down....I am looking into that cloud at about 32000 - 4000 = 28.000ft.

At -2 degrees I am looking at 24.000ft.

Both are good elevations to take a look at.

 

If I am at FL380 then -1 degrees is not going to do me much good.

Because a cloud at a 40nm range would be dissected at 38.000-4000 = 34.000ft

At that level you are still looking at ice christals that give hardly any radar returns.

Result....you think aaah nothing inside that thing and you will fly right through a TS!!

You need to go down to at least 3 degrees at FL380...this will give you a presentation of the cloud at 38.000 - 12.000 = 26.000ft....an elevation that actually gives a clue as to what is below you!

 

On take off....set 5 degrees (= 5 degrees above the horizon) and during flight reduce by 1 degrees for every 5000ft altitude.

So at 5000ft...reduce to 4 degrees up.

At 10.000ft....reduce to 3 degrees up

....

At 25.000ft reduce to 0 degrees up

etc

During descend you reverse the process.

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To expand on Rob's post -- the normal advice is to set the radar such that you are just seeing ground returns on the top of the ND. This means that the tilt angle will depend on both the aircraft's altitude (because the distance to the horizon increases as altitude increases) and the ND range setting (bearing in mind that beyond the horizon -- which will be c. 220NM away at FL350 -- you will not get any ground returns). Therefore, with the ND range set to, say, 160NM you will need much less tilt than if you set the ND range to 40NM.

 

This is a bit difficult in the PMDG 777 as, as I understand it, ground returns are not (yet?) modelled -- therefore Rob's rule of thumb is a good one.

 

Bear in mind that not every red return is a CB. Part of the art of using the radar is interpreting the dangerous CBs from the harmless heavy rain showers. Manipulating the tilt and gain controls can be very useful in this regard: if you're still seeing red fairly high up in to the cloud, it's probably nasty. If, on the other hand, a red return 160NM out disappears when you tilt the radar up slightly -- this is almost certainly a much more benign system.

 

Likewise the gain (which should under normal circumstances be left in AUTO/CAL) may be used to identify the strongest cells (useful if you are in the tropics, perhaps, where you are likely to see an awful lot of red). Reducing the gain slowly will result in the returns slowly fading from red to yellow to green and eventually all the way to black: the areas which stay red the longest are the strongest cells. Don't forget to set the gain back to AUTO/CAL immediately afterwards, though!

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