Alternate air inlet thoughts

Studying the instructions with my FAB320 air box, I have a question for those with carbs. As a VFR and light IFR flyer I am debating the need for the alternate air source that is installed in the bottom of the FAB and bypasses the filter. I suspect that if you fly into icing bad enough to clog the filter you have far worse problems and are a lot less prudent than I am. Having said that, I am prone to error on the side of safety and am wondering how many have installed the alternate air feature on a carbed engine? Thoughts?

It is not just ice, any blockage and your toast. I know of one example where snow was the culprit. There is also the possibility of a precise bird strike, or any object that hazards into the scoop.

I do not fly IFR. I do not fly in snow. There where a lot of problems with the alt. air system when I was building so I elected to not put it in my airbox.
I believe people have come up with a lot better systems now however.

redundancy is always a good thing!?!?!

I can't say I'd fly with or without one, but if there's a chance I suck a plastic bag into the intake on the takeoff roll, or the filter element decides to collapse over some rocky terrain, I think it is a fairly simple way to have an option to ensure airflow to the carb.
.......perhaps in a risk mamagement assessment there are more critical things, but fuel, air and spark are pretty high on the list I'd think.

just my $.02

flyboy1963 said:

I can't say I'd fly with or without one, but if there's a chance I suck a plastic bag into the intake on the takeoff roll, or the filter element decides to collapse over some rocky terrain, I think it is a fairly simple way to have an option to ensure airflow to the carb.
.......perhaps in a risk mamagement assessment there are more critical things, but fuel, air and spark are pretty high on the list I'd think.

just my $.02

Click to expand...

Reduncy may be good but it is not always, as you add one more thing to break, which was what was going on when I was building. Alt. air systems vibrating apart with the potential of parts being sucked into intakes.

Again, I think people have re-designed the systems to be a lot better, but I am not rushing to retro one into my airbox. If I was building new, I would rethink it.

I am open to changing my mind, so if anybody here knows of a recent instance where alterate air saved the day, please share it.

PS - if you collapse an air filter, alt. air probably won't help you, at least on the vertical induction airboxes. I have never heard of one collapsing, but....
In regard to the plastic bag, I am not sure I would have the where with all to pull alternate air on a take off roll or recognize that as the first step to take. I will keep an open mind however.

what is 'alternate'?

agreed on all those points.....if you flew into a flock of birds and lost power, perhaps only then would you think 'air starvation' or something.
if I didn't have the alt air plate on the bottom of the airbox, I'd probably just treat my carb heat valve as my 'alt air' option, and leave it at that....not addressing the air filter clogging or breakdown though is it?

Not installed

I fly without one. Been through light ice once nothing negative to report. If you have time and like tinkering install the alternate.

flyboy1963 said:

agreed on all those points.....if you flew into a flock of birds and lost power, perhaps only then would you think 'air starvation' or something.
if I didn't have the alt air plate on the bottom of the airbox, I'd probably just treat my carb heat valve as my 'alt air' option, and leave it at that....not addressing the air filter clogging or breakdown though is it?

Click to expand...

When I was looking for a plane I think I spoke to the guy that was the catalyst for the whole alternate air inlet subject. He was flying an RV-6A through light snow, IFR, and had the engine quit. The cause was determined to be a snow clogged air filter and inlet. Everyone was OK in the forced landing.

I'm sort of with flyboy, and consider my carb heat as a poor mans alternate air with the same concerns he mentions. I also pull carb heat 100% of the time I am IMC.

alternate, alternate air source

I built a sliding gate which can be opened and closed. Van's can't be reclosed. I'm not sure how to past a photo here but I can email them if you would like.

alt air/carb heat, Lyc 0320 with MA4SPA carb

FWIW: For a long time, I flew with an alt air capability, but not carb heat. The carb placement (on the hot oil pan of the 0320) is in my case particularly resistant to carb ice. I fly in the Northwest where it gets cold/damp/wet often, and I've only noted the effect of carb ice twice, on the ground during run-up on wet days. The alt air would induce a 50RPM drop. Good enough, I thought.

Then I installed a temperature sensor in the carb throat, just below the butterfly valve.

In winter, carb throat temp is often in the teens (degF); in summer, in the 20's. The alt air (which was just engine-side air) produced very little temp rise. A bit worrysome, even if I had not experienced in-air carb ice. But even once would be too much! So I put a small muff on an exhaust pipe (stacks and muff from Vetterman). Now when I apply alt air, the carb throat temp zooms from the teens to over 60-degF, with a 100RPM drop. If the primary filtered intake clogs or gets frozen over (hasn't happened yet), the heated alt-air source will always work, since it delivers air directly to the carb, bypassing the filter.

- Steven
700+ RV hours
(another hour last Sunday!)

Yep, do it ... it's not that difficult ....

At one time in my build I was going to forget the alt air/carb heat option, since I have FI. ( the alt air for the Sam James cowl kit is actually carb heat). But the forward facing air inlet, with copious amounts of ram air, got me thinking about ice or snow plugging the cone-shaped air filter. I eventually changed my mind and fab'd/installed it. It gives me one more dimension of peace-of-mind flying in the winter.