Wouldn't just running the aux fuel and crossfeed at lower altitude on a regular basis get any of that low lying water flushed through the system? I have done that since 1999 in my 421 and never had a problem with anything freezing at altitude. Never even though of an additive.

Is a 421B different than a 412C regarding fuel icing and pumps? I never had any issue with either with my B.

This was not an issue during Flight Safety initial training, either.

Just lucky?

Off topic... are you the Curt Brown that raced a few years at Reno?

Tj

John,

In my 12 years 1200+ hours as a 421C owner, I would turn off the aux pumps at 1000' AGL and turn back on at 2-3000' AGL.

If you wait until cruise to turn off the aux pumps, make sure to turn off the aux pumps before you lean the mixtures. There is a mysterious feedback lump that momentarily reduces fuel flow out of the engine-driven pump when the aux pumps are turned off. As long as you are running reasonably rich when you turn off the aux pumps, you shouldn't get a stumble.

With the aux pumps running, you are pressurizing the fuel lines from the wing all the way to the engine-driven pump. I think it is safer to leave these lines unpressurized (other than head pressure) - less chance for some minor leak to become something major.

Not me, TJ.

My experience in 1000hrs on the 421C was identical to yours.

With the heated fuel spiders, I only ever once noticed a slight fuel flow reduction due icing (flying in Europe), easily solved by enriching. It needs a combination of hot high humidity weather at the point of fuelling, and a climb into very cold dry air in the FLs, say from ISA+20 to ISA-10 (I am guessing). I could tell something was odd because I got a lot of frost very quickly on the cabin side windows, which normally doesn't happen, but correlates with what I described. It all normalised after 10-20mins. You don't get fuel icing just because it's very cold in the 421C, it needs an unusually high amount of water dissolved in the fuel (from the hot humid refuel on the ground) and very cold ambient in cruise. In the US, I've heard a comparable issue for someone departing a hot Florida day northbound and entering a a different cold airmass quite quickly enroute.

The 421Cs engines will be starved of fuel with only the Aux LOW pumps if an Engine driven pump fails, the LOWs are there for vapor suppression. In cruise, if your engines run fine without them, I guess it's OK (more importantly, permitted by the POH) to switch them off, but I always ran with them on, and never had a problem.