It is not to do with improving radar signature, it's because leaving the WXR on allows military units to look at it's frequency band, PRF and scan patterns and therefore identify your aircraft as an airliner.

Flew the BAe146 for several years, Wx Radar only turned on when needed, or in the dark, when unsure if it was needed. Lots of Wx radar problems.

For the last 2 years I've been flying the Douglas B717 :} where the radar is turned on entering the runway for take off and stays on until the after landing scans. I've yet to have an unserviceable radar in the 717.

SmokeAnd Noise,
EGPWS does radiate, it uses the radio altimeters.
Re the safety: the 50W to 100W quoted goes into the pencil beam in the front of the aerial. All you get behind the aerial is a few milliwatt spillover.
And if you're inside the aircraft, all of that gets reflected by the aircraft structure, so it doesn't even get to you.
Bears absolutely no relation to the 1W to 2W that your cellphone pumps out right next to your ear and brain.
Re switching it on and off:
My experience with all kinds of electronic equipment is that most failures are caused by the transients when switching it on and off, not by keeping it running.
The only reason I can see for switching it off might be to reduce wear and tear on the mechanical bits (antenna pedestal), and in old models that still use tubes to prolong the tube life (although they don't like switching off and on either).

I like to leave it on as a TCAS back-up in clear skies. There are planes up there with TCAS turned off due to defects. I first noticed how useful it was when we had a no TCAS one day.

Took off a few years ago in the 757 on a dark night heading out over the water and flew straight into a rather bumpy CB.

Had not seen any lightning prior to departure, (hadn't seen anything for that matter)

After turning on the Radar we could see cells everywhere and dodged the rest.

I turn it on and leave it on for the entire flight now, regardless of conditions, day or night, you never know when there might be something out there that you would have thought unlikely.

stilton,
I thought that was the mentality of every professional pilot on here in the first place, so I was rather astonished to even find a topic about turning off WX radar in flight.

I leave mine (helicopter wx radar) at STBY after start checks and also if not using it in flight, rather than OFF. When switched off, the antenna can vibrate against the end of the gimbals, which can't do it any good. At STBY it perks up off the stops.

ShyTorque,
Dead right about the reason or not leaving it at OFF.
But what about the argument for the STBY?
Just pedestal wear, or something else?

EGPWS
 

ChristiaanJ

...now I don't claim to be an engineer so correct me if I'm wrong. To my understanding the EGPWS uses an internal database of terrain in combination with a GPS position and therefore does not radiate. Basic GPWS (that modern aircraft still have in addition to the EGPWS), like you say, uses the radio altimeters. The difference is that we're sitting closer to the radar than the rad alt antenna - depending of aircraft type of course.
If you say that there's no radiation on the backside of the antenna and that the nose cone doesn't reflect any radiation back in our direction I suppose I'll just have to take your word - even if it sounds quite unlikely to me.

SAN

75/76

You can only have terrain or weather displayed, CAVOK, mountainous area, I know which Id choose. :ugh:

SAN,
You're right about the EGPWS.
Right again.
It's not somebody with an alleged radar background on an internet forum, like myself, who can convince you the 'backscatter' from the wx radar into the cockpit is negligeable.

Why not drop in on your radio/radar workshop, which almost certainly will have a field strength meter sitting on a shelf, and ask them to bring it along next time they have to do a check on the wx radar, do some measurements in the cockpit, and explain them to you?

And if they have a suitable wide-band field strength meter, they can even compare it with your cell phone.

Far better that way to explain and comprehend the issues, rather than taking my word for it.

DooblerChina,Can somebody here elucidate an ancient?
Do the latest weather radars still have a MAP mode?
Older wx radars had a spoiler plate or similar arrangement to create a rough approximation to a cosine² beam in mapping mode, but recent ones seem to use a pencil beam even for mapping, and just let you play with the tilt.
True or false?

Think about it, if there's backscatter your tea won't get cold. :O

I woudn't know... never touch the stuff.

Not really true you can only have wx or terrain displayed on the 75/6.

One Pilot can select terrain on their side with wx on the other.

Standard practice with us.

Day light...If necessary;

Night...ALWAYS...

Chritian J

Pencil beam only as far as I understand your question.

For others:

Some modern displays overlay wx on map, others as has been mentioned can have wx one side of the panel, map on t'other, or even wx and map side by side on the same screen.
The best (to my mind), EGWPS map and 'Target' flashing if any unfriendlies detected.
The previous poster pointing out condensation causing gimble and other probs is correct. Some types suffer badly from Radar location. Not such a problem with newer units that do not utilise moving antenna, but why on earth would you want to take heat away from electronics when operating at -56C.
Even on pre-landing, STBY is prefered to OFF.

standby??
 

our type on the 738 only has "test, WX, WX/TURB/MAP positions on the unit box--

"TEST-
tests weather radar system operation-
allows test pattern and any fault messages on navigation display MAP, center MAP,VOR and APP modes, with WXR selected.

Note: if the airplane is on the ground and the thrust levers are not advanced for takeoff, WXR tests the predictive windshear system (PWS) indications...deactivating WXR on the EFIS control panel will NOT discontinue the test and can result in automatic WXR activation on both pilot displays. test last approximately 15 seconds."

only other switch is the EFIS Control Panel-WXR-"energizes weather radar 'transmitter' and displays weather radar returns--limited to 320 nm"

sounds as if some parts of the unit remain powered regardless of the various switch positions-and only the display or 'transmitter' itself is controlled by switches.

standby
 

stator vane,Deactivating WXR on the EFIS control panel is the equivalent of the STBY position of the selector switch on older models. The transmitter is switched off, so the system no longer radiates.
The rest remains powered up.

I believe the A330/A340 FCOM says turn it on when entering the r/w then off when leaving the r/w after landing. So I guess there is no discussion there at least. Also it is a requirement to have it on when flying in the Arabian Gulf region. Never heard of a wx radar failing coz it was left on too long!