Multiscan radars and gain control
 

In my previous airline some of the multiscan radars on our Airbus fleet required the gain to be set at +4 or +8 at lower levels. Would anybody have a reference as why the gain had to be set in this way and as to which radars are affected?

Below 20000ft if I remember correctly. The anomaly was modded out of our fleet about a year ago.

RTFM.
It's in the FCOM.

Not in my fcom it isn’t

It was in our SOPS which are in the Fcom these days.

Had that on some aircraft. Those with +4 could be set to auto gain above FL200, those with +8 above FL150. But it was in the FCOM (TDU section).

Not a policy in my airline, but in dodgy looking skies, I do like to whack the manual gain up to see what it shows. If there is absolutely nothing there, it just displays black or random grain, if there is anything vaguely there, it will draw it. That's on an RDR-4000, our old manual system seemed much better at finding the tiniest but of weather on the automatic setting.

Multiscan seems to dismiss weather that would better be avoided. With the cabin secure, as an experiment, we flew through a cloud we were visual with in day VMC that we would have probably dodged around normally. It showed nothing more than occasional green on the radar display. It wasn't horrendous, but it presented with some decent turbulence that was getting on for moderate, which we probably would have rather avoided. At night we wouldn't have bother dodging around it according to the radar display. It seems pretty shy to call out a TCU in my opinion when our previous completely manual weather radar would have told us to avoid using the standard settings/guidance.

Additionally, the RDR-4000 from Honeywell should draw areas of hail, active thunderstorms etc. Recently, we flew past a pretty violent thunderstorm over Cork in Ireland, whilst it was fairly low level (<FL300) it was still was very active, with lightning flashes at <1 second intervals. It didn't draw the thunderstorm symbol at the normal gain settings. If it only draws that symbol with a 200nm CB extending to FL500, then that's not a lot of use as we'd avoid that area based on the normal radar returns anyway!

gain changes the focus of the beam....this changes the return, hence affecting the color calibration you see.

in addition, the issue of air density, and backscatter from the sidelobes.

think of it as a distant similar to driving with your low/high beams on in fog.

as noted by several posters, attempt to correct this with software can lead to missed detection.