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A suggestion re CFIT

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A suggestion re CFIT

Old 31st May 2001, 21:20
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Lightbulb A suggestion re CFIT

I was watching my recently aquired copy of ITVV's B757 video last night. The Capt showed how they overlaid the weather radar on the Nav display in the 757's "Glass Cockpit".

However, it was a sunny day(!) with no thunderheads in sight, so he pointed the radar dish down and got a display of the Alps mountains in glorious technicolour. Contoured - peaks in red etc.

The thought occurs:

Could the crew of a "Glass Cockpit" jet do this when approaching in bad weather to a runway surrounded by high ground? Then they would 'see' where the high ground was, and could steer around it or abort the approach.

Could this have prevented the 757 crash at Cali in '95, for instance?

Is this possible? Discuss....

Live long and Prosper.....

[This message has been edited by swashplate (edited 31 May 2001).]
Old 1st Jun 2001, 18:08
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Something very like this is on the go in Alaska now, called Project Capstone. http://www.alaska.faa.gov/capstone/
Old 1st Jun 2001, 19:26
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Its called EGPWS which is fitted to many aircraft already, and is not wx radar based. It relies on the aircraft knowing its position, and comparing that to a terrain database. The terrain can be displayed on the map on one side of the cockpit, with the wx radar on the other.

Old 1st Jun 2001, 19:54
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Thumbs up

Yes we have this in the Avro RJ100.

The weather radar display is in the central area between the pilots. This can show terrain or weather as selected.

We also have a weather radar on our own, independent range scales overlaid on each Navigation Display (lower EFIS screen). This one can show only weather, not terrain.

So to get what you're saying, you can set the weather radar "terrain" mode on - it will then show the terrain (as cough says, it's not radar, just a display of where we are in relation to its terrain database) - that's on the centre screen, with the radar layered as bright or dim as we like along with the localiser or whatever on our NDs.

If you don't have EGPWS, you can angle the radar down so terrain fringes appear in about the top 10% of the screen and the rest of the screen will then show a combination of peaks and weather. It takes a lot of practice to interpret, but plenty of people have been doing this for many years.

I liek to use this kit every chance I get in the mountains even in fair weather to get the hang of how to use it.
Old 1st Jun 2001, 21:46
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Red face

If the 757 Wx radar is the same as the 767, I would be very cautious in using it as a means or aid to avoid terrain.

I was never able to obtain any usefull information from this wx radar in ground mapping. Example: over water, I an a ship but with the wx radar unable to pick it up. Another example coming into Montreal, where the St-Lawrence River and its bridges are easyly seen and discrimated from the river, same result, unable to see them on radar.

I agree that with some wx radar ( I have seen some) you can do what has been said. But, make sure that you have tried it on a CAVU day to learn how to interpret the info.
Old 2nd Jun 2001, 05:16
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Weather radars cannot detect terrain covered with ice or dry snow (they detect water not ice). The Air NZ DC-10 hitting Mt Erebus is a catastrophic example of this.

With pitch stabilization on the weather radar you do not know what terrain it is looking at with respect to the aircraft attitude.

Old 2nd Jun 2001, 09:51
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I fly in Australia, so I don't get the opportunity to point my radar at snow fields, however you are not quite correct in saying that weather radar will not detect snow covered terrain. The Bendix corporation says that rock covered with ice and snow (particularly powdered snow) can be unreliable, but previous crew on the ANZ Mt. Erebus flights had reported that radar terrain contours matched closely with the actual terrain.

Weather radar will not detect dry snow in the air (correct) but it is not a "water detector" in the sense that it always detects water. Radar will not detect bodies of water on the ground (lakes, rivers etc) as the beam is reflected from the flat surface away from the aircraft. Terrain is detected as is scatters the beam, and the back scatter can be seen on the display - I would think that snow covered terrain would react the same way.

The fact that weather radar in transport jets is pitch stabilised is what makes it a good terrain awareness (note, not avoidance) device. Most jets descend on an angle of about 4 to the terrain, so setting your radar with the bottom of the beam at 4 nose down (and it will maintain this on descent as it is stabilised) will show anything in the way of your descent on the way down - terrain or weather.

It is not known whether the Mt. Erebus crew used their radar for terrain mapping during the accident flight - certainly nothing on the CVR mentions it. As they were flying "visually" at the time it is doubtful that they would have been referencing it in any case.

[This message has been edited by Checkboard (edited 02 June 2001).]
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Old 2nd Jun 2001, 14:53
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In my experience weather radars do not detect dry snow, ice, or hail.

Correct me if I am incorrect, my understanding for this is that the return on radar is due to the water being excited by the radar energy and reflecting the incoming beam.

I have used weather radar to navigate at night in the past, using the coastline depicted on the screen.

The method you suggest works for the descent, but when flying S&L or with a reduced RoD in the terminal area it is providing useless information with regard to weather avoidance, and is displaying "too much" terrain information (terrain that aircraft will clear). For it to be effective, you need to be clear of weather, and changing the tilt every time to match the aircrafts flight path. It does draw a pretty picture of built up areas and high rise buildings around the airport.

Old 2nd Jun 2001, 17:15
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FE Hoppy
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A couple of years ago I was flying regular trips from northern europe to the far east. This included crossing the hindu cursh and an apropriate decompression escape route due to the very high terrian. The weather radar in terrian mode was in this case very useful in mapping the peaks and valleys ahead and when vmc you could see just how good the picture was.
nb all the "hills" were covered in snow but painted well.
Old 2nd Jun 2001, 19:35
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Code Blue
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<font face="Verdana, Arial, Helvetica" size="2">Radar will not detect bodies of water on the ground (lakes, rivers etc) as the beam is reflected from the flat surface away from the aircraft. Terrain is detected as is scatters the beam, and the back scatter can be seen on the display</font>
The first "radar guided" bombing raid of WWII using H2S was chosen for Hamburg precisely because of this fact. The Coastline was about all they could discern and apparently ('cos I haven't been there) the coast near Hamburg has a shape that is easy to spot.

Enuf trivia - need to get a life

-.-- --.- -..-
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Old 4th Jun 2001, 13:07
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<font face="Verdana, Arial, Helvetica" size="2">
Radar will not detect bodies of water on the ground (lakes, rivers etc) as the beam is reflected from the flat surface away from the
aircraft. Terrain is detected as is scatters the beam, and the back scatter can be seen on the display
You're assuming that the water is flat. If there are ripples on the order of the radar wavelength (typically centimetre or millimetre) then you will get backscatter from those. What wavelength is weather radar using??

Interestingly (at least, to me) Radar makes a very good way to detect ships, especially when the seastate is calm, as the wake is rougher than the rest of the sea surface. You get a nice bright trail leading straight to your target. Take a look at


for example. That one is a photo, but you get just the same effects in RADARSAT images - I just dont have any to show. There's a shuttle-based SAR image of the gulf stream, however, just to show that radar can detect bodies of water on the ground pretty darn well




[This message has been edited by Evo7 (edited 04 June 2001).]
Old 5th Jun 2001, 11:32
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HF (3 to 30 MHz)
VHF (30 to 300 MHz)
UHF (300 to 1,000 MHz)
L band (1,000 to 2,000 MHz)
S band (2 to 4 GHz)
C band (4 to 8 GHz)
X band (8 to 12 GHz)
K band (12 to 40 GHz)

C band weather radar thus has a wavelength of around 3 to 7½ cm, and X-band around 2 to 2½ cm.

I was really only talking about what can be expected from airbourne weather radar, although I expected someone to mention that ocean waves can be shown - interesting if you want to know the swell direction. That is why I only mentioned lakes & rivers above.
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Old 6th Jun 2001, 14:48
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Makes sense - the backscatter is due to resonant Bragg scattering, so you'll pick up water waves of twice the wavelength of the radar (assuming you are looking pretty much downwards - there's a sin (alpha) term too, where alpha is the angle of incidence).
Means C-band will pick up 6 - 15 cm waves, which is probably a bit big for lakes. Should probably see them given a decent surface wind though? Or no? Most of the work I did was K-band, so I'm not sure.

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