[N941WR]

The question was asked in another thread how many of you installed alternate air control and I thought I would make a poll out of the question.

 

[cwreeves]

alternate air

My first thought was this thing is just something else to install that will loosen up and have to be fixed but I installed it after much soul searching. It wasn't a big deal and didn't take much time. I just figured Van's had a good idea and there really isn't much can go wrong with it.
Charlie, Tucson AZ

 

[Greg Arehart]

There were some posts here in the last year or so indicating some potential weak points in the standard installation, plus, once pulled you have to get on the ground and take off the cowl to close it up again. So I manufactured my own system, which is basically a sliding door system that can be opened AND closed from inside the cockpit.

greg

 

[hgerhardt]

Spring-loaded flapper door on the inboard side of the FF snorkel. Made the whole thing out of fiberglass. Made it long before Van's came out with their kit.

Never tested it with a clogged filter. I suppose I should block the filter one of these days and see if the engine will run WOT with the tail tied down.

Heinrich Gerhardt

 

[L.Adamson]

Of course I did. The more weight the better! And it's just one more knob to go with my blue knob...

L.Adamson ---RV6A

 

[Bob Brown]

It's a one way pull

I always figured if I ever have to pull it open, I won't care about closing it in flight, It'll be a bona fide emergency...in fact, I'll probably be so excited I'll pull so hard it will rip the the cable off the door...

 

[Bill Wightman]

Nope - nada

No alt-air control here. IO360 with snorkel of my own design and filter in front of the left front cylinder.

I'm hoping that the DAR who does my airworthiness inspection doesn't have any problem with the lack of alt-air.

Have any of you had problems with FAA-DAR's squawking your plane for NOT having alt-air???? I hope not!

 

[Greg Arehart]

I recently had the inspection and was told by the inspector that he has "nothing to do with design" that "you could put a jet engine on a barn door with a shovel" and he could not disapprove of it from a design standpoint. Seems that the main thing he was looking for was whether there were enough threads on the bolts past the nuts and stuff like that. So my take is that the presence or absence of an alt air door would probably be irrelevant (at least from the perspective of the FAA inspector). Depending on your inspector/DAR, they may see this issue differently.

My nickel's worth (used to be 2c but up on account of inflation...)

greg

 

[asav8tor]

This comes up from time to time and there are a whole lot of opinions on having an alt air door. Sometimes posters will come up with an idea or improvement for increasing the safety factor for a problem that has never happened in the RV fleet. Then the alt air door subject comes up and there are many negative comments about the need. Fact is several RVs have gone down and fatalities have resulted from engine stoppage when an alt air source could have saved the airplane and pilot/pax. I witnessed one of them.

By the way, I can reach up from the lower cowl outlet and reach the alt door to close it without taking the cowl off on the 4.

 

[kiwipete]

Things DO sometimes happen!

Having had an engine stop through a blocked filter in a twin, I wouldn't dream of not fitting one. (One of the engines Alt air didn't work )
An hour or two's work over the whole project is nothing and is worth it for the peace of mind.

Peter

 

[Jamie]

I did not have alternate air during my airworthiness inspection. My DAR signed it off, but encouraged me to install alternate air.

 

[Brantel]

A freind of mine was working on moving his FAB over and after ~100hrs found the Van's supplied cable to actuate the alternate air was broken inside the sheath.

He would have never known until he tried to use it and the cable pulled free in his hand.

 

[roee]

A freind of mine was working on moving his FAB over and after ~100hrs found the Van's supplied cable to actuate the alternate air was broken inside the sheath.

He would have never known until he tried to use it and the cable pulled free in his hand.

Seems like that should be a pre-takeoff checklist item.

 

[Brantel]

Seems like that should be a pre-takeoff checklist item.

Not unless you want to decowl the plane every preflight.

You must climb under the engine and reset it by hand if you have the standard Vans alt air and you pull it.

 

[roee]

Oh, I get it now

Not unless you want to decowl the plane every preflight.

You must climb under the engine and reset by hand the standard Vans alt air if you pull it.

Sorry, I misunderstood. I'm not there yet on my project, and I didn't realize that even with the cable setup that you're talking about you still need to get into the cowl to reset it.

I don't like that at all... if for no other reason than because you can't conveniently test it on a regular basis (as in during your pre-takeoff checks). I think I'll opt for a push-pull setup on my plane, so I can switch back and forth at will from the cockpit.

 

[LifeofReiley]

Hmmm...

I installed the alt air, per my install the cable will fully open and close the door. I don't think I would want to close the door during engine on flight, but on the ground it really should not be an issue. Also, if the cable is broken in the sheath, it will pull out on your hand.

 

[Bill Wightman]

Filter blockage???

Greg - Thanks for your reply... your explanation of the DAR's perspective really helps.

BUT, a few of you have mentioned possible filter blockage which of course would stop the engine. I'm having a bit of trouble envisioning how the filter could get blocked in the first place. Its a preflight item, and very easy to see and inspect when placed in front of the #2 cylinder. Its happened, so its possible, but could anyone help to explain what happened in a bit more detail?

Y'all have me thinking of adding an alt air source; just want a bit more info.

 

[asav8tor]

Greg - Thanks for your reply... your explanation of the DAR's perspective really helps.

BUT, a few of you have mentioned possible filter blockage which of course would stop the engine. I'm having a bit of trouble envisioning how the filter could get blocked in the first place. Its a preflight item, and very easy to see and inspect when placed in front of the #2 cylinder. Its happened, so its possible, but could anyone help to explain what happened in a bit more detail?

Y'all have me thinking of adding an alt air source; just want a bit more info.

Snow, Ice, snorkle collapse, bird, balloon, mechanical ( something comes loose and blocks the airflow), filter media issue.

If the air stops the motor stops. This is a REAL threat that has happened as opposed to IMAGINARY threats that have never happened.

 

[L.Adamson]

Greg - Thanks for your reply... your explanation of the DAR's perspective really helps.

BUT, a few of you have mentioned possible filter blockage which of course would stop the engine. I'm having a bit of trouble envisioning how the filter could get blocked in the first place. Its a preflight item, and very easy to see and inspect when placed in front of the #2 cylinder. Its happened, so its possible, but could anyone help to explain what happened in a bit more detail?

Y'all have me thinking of adding an alt air source; just want a bit more info.

As I remember...............

An RV flying over the Sierra/Nevada's in California injested some wet snow, which built up around the filter & effectively killed the engine.

I went to look this up on the NTSB reports, but the website is having problems.

L.Adamson --- RV6A

 

[kiwipete]

In my case it was flying through sleet and wet snow which blocked and froze on the filter.

Peter

 

[zav6a]

Alt air

The alt air design I have seen on vans would not help in most of the instances folks have mentioned here. When the little door onthe top of the snorkle opens, it does not bypas the filter or block off the incoming air. If you are flying through heavy snow, it will pack in just as tight (and on your wings!) without with out the alt air door open. That leaves a bird strike or balloon that leaves the opening blocked. Incredibly unlikely.

I ripped mine out and riveted a cover over the hole. It was constantly vibrating itself to pieces and it was hard to get a definitive seal to prevent loss of intake air into the lower cowl. If I had a carb I would obviously have it but I would figure something out other than what Vans sells.

 

[Scott Hersha]

Alt Air

Van's alternate air door goes on the bottom of the FAB and opens inside the filter, bypassing the filter - that's why it's called alternate air. On my RV-8, I can preflight it and almost close it with the cable. The bent over piece on the bracket has a little too much 'squeeze' to allow me to close it completely with the limited force applied with the little cable. I need to manually close it the last 1/2 inch by hand by reaching up in the cowl exit opening and sliding it closed the last little bit. This is not a difficult reach and can be done in just a few seconds. I built mine per plans.

Scott

 

[L.Adamson]

Van's alternate air door goes on the bottom of the FAB and opens inside the filter, bypassing the filter - that's why it's called alternate air. On my RV-8, I can preflight it and almost close it with the cable. The bent over piece on the bracket has a little too much 'squeeze' to allow me to close it completely with the limited force applied with the little cable. I need to manually close it the last 1/2 inch by hand by reaching up in the cowl exit opening and sliding it closed the last little bit. This is not a difficult reach and can be done in just a few seconds. I built mine per plans.

Scott

And my carbed 6A is the same. And it's all from Van's.

L.Adamson

 

[N53LW]

Snorkle

Installed a spring loaded round door, on the outboard side of the snorkle. It will automatically open if the filter clogs. Used the same spring used on the Bonanza.

 

[IowaRV9Dreamer]

Is there room in the FAB to plumb carb heat to the inside of the filter and use the hot air as the alternate source? I'd like to do that.

 

[Bob Miller]

alt air

I finished the airplane without the altrenate air option. Neither the FAA inspector nor the DAR mentioned (or noticed) its absence. Nevertheless, shortly after the first flights I reworked the snorkle and installed a cable pull system. I've never used it, but it's there generating a cetain amount of peace of mind.

RV-8, IO-390
Dakota Boy, N127M

 

[N208ET]

I just installed mine

However, the alt air door doesn't completely seal when closed. There is a 1/32" by 1/2" inch space where I can see light between the door and the mounting bracket. Does anybody elses not completely seal?

 

[osxuser]

If you have a chance of flying with impact icing, then sure. Otherwise, day/vfr plane, I wouldn't... one more thing to wear out and get sucked into the engine... and it could happen, i've seen some worn out enough that the door was coming off the hinge.

 

[asav8tor]

Spark stops, engine stops. Risk mitigation solution: (2) independent sources of spark.

Fuel stops, engine stops. Risk mitigation solution: (2) sources (tanks) of fuel, (2) fuel boost pumps.

Air stops, engine stops. Risk mitigation solution: (2) sources of air.

Go with one if you like just realize RVs have come down due to loss of air. I am very familiar with two of them. One was day VFR. I was at the accident site and drove the pilot back to his home airfield.

 

[Rick6a]

A couple of thoughts. I had always thought the alternate air for the carburated version as a marginal design at best, certainly in my case observing no perceived RPM drop during routine checks. If required in an emergency, I have held to some real doubts about its true capability. Following a recent and unrelated FAB failure, I took the time to replace Van's arrangement with a far more efficient heat muff. I now see at least a 30-40 RPM drop when switching to alternate air. See post #4:
http://www.vansairforce.com/community/showthread.php?t=38341

Christmas 2002: A transient RV-6A pilot on his way to visit family happened to be passing through our area when he found himself flying through light snow and experienced in-flight icing and engine stoppage. God was with him because as he broke out of the low level clouds with little room to manuever, he was presented with an uncommon gift....an open field. Within the course of a few heartstopping moments, he set his RV down upon it. Luckily, he was spared injury. In fact his RV suffered only minor wheel pant damage as the plane rolled to a stop on the frozen bumpy surface. His words, not mine: "It didn't have enough heat to melt the snow/ice that was starving my engine of air, and it doesn't provide an alternate air path if the filter is blocked. I'm going to change that design (alternate air) when I get home."

As for my fuel injected -8 with its filter mounted to the air inlet ramp per plans, I installed an alternate air source as a backup, just in case. Who knows? The payoff could be profound.

 

[Scott Hersha]

Rick,
It sounds like you are talking about carb heat, not alternate air. The FAB design from Van's has both. The heat comes in at the front of the snorkel after passing through a heat exchanger of sorts. It doesn't heat the air very much and I only get about a 40 RPM drop when testing it. The alternate air door is on the bottom in the middle of the filter - bypassing it - and has no heat by design. There may be a slight increase in carb temp because of the slightly warmer ambient air laying in the bottom of the cowl, but that is not it's purpose. The only time I think you may need alternate air is if you ingested something solid, like a bird, balloon, or something that chokes off the snorkel or completely clogs the filter. Even if ice were the culprit, I doubt the carb heat as designed would do much good. Hence, the alternate air door. Engage it open, and it stays there until you land.... and you probably should land soon...

Scott

 

[gmcjetpilot]

Van says

It all depends on your application (Air box) and type of flying.
There are two air-boxes and two kinds of induction.
The standard FAB (filtered air box) is for Carb or FI mounted vertically and can utilize carb heat or alternate air.
The FAB-HORIZ (filtered air box horz) is for FWD FACING (horz) FI (fuel injection).
The FAB-HORIZ filter in the right cowl inlet is out in the open all the time and only has alternate air option.
There is no more magnetic alternate air.

For FAB with a Carb you (MUST) put in carb heat. Not to is stupid, end of story. This of course does not bypass the air-filter but does protect it from outside contaminates (sleet and snow) when the carb heat door is closed. Carb Heat just closes off the cowl air-scoop and takes air from the cowl area, which is hotter or at least warmer, but it is still filtered.

The FAB alternate air is optional per Vans instructions and needed mostly for flying in snow and ice. Why would you fly in snow and ice I don't know, the RV is not known ice approved or very smart in a small plane. People bite it in icing conditions. You can always close the carb heat to avoid "impact ice or snow" from blocking the filter if you get caught.

The FAB magnetic latch alternate air was optional for the FAB per vans instruction. It was designed to open automatically but is OBSOLETE. Van changed the FAB alternate air from magnetic latch to cable operated a few years ago. The problem with the magnetic alternate air latch was it was open all the time. Also the small rivets that attached it could come lose and go into your engine.

Once you open the cable operated alternate air, you can not close it, at least easily with the cable from what I see. (I have the parts but not install them). You have to get on the ground and manually latch it back into the holder which is tight. The alternate air draws unfiltered air under the FAB of FAB-HORIZ.

For FAB-HORIZ for the fwd face fuel injection, also has a cable operated alternate air that opens a door on the side of the snorkel/airbox that goes from the filter to FI body. It is also unfiltered air. Since there is no Carb Heat with the FAB-HORIZ and the air filter is out in the open air, I think alternate air would be a good idea with the FAB-HORIZ. Your call.

I have a FAB and installed carb heat (I have a Carb). I did not install alternate air (magnetic automatic or cable operated). The reason is I doubt the filter will be blocked with my flying and knowing the limitation. I will have to remember to use carb heat in heavy (freezing?) precipitation. Once you activate the carb heat the filter is protected from sleet and snow.

IF YOU FLY IFR IN SNOW OR SLEET (bad idea anyway) THAN YOU COULD BLOCK THE FILTER. I recall this is the Van made this part and option because it happened at least once.

Summary my suggestions

FAB - Install at least, Carb heat for Carb and for FI Carb Alternate Air or carb heat, either one. If you have a Carb I would say Carb Heat is a MUST!

FAB-HORIZ - Install the Alt Air. Why not. If flying VFR and do Arco only, you can leave it off but that's up to you. It is unlikely the air filter will be totally blocked.

If you are fair weather flyer you don't need Alternate Air in my opinion, but I would put it on the FAB-HORIZ since the filter is right out there to be blocked and there is no carb heat or "Alt Air" with the FAB-HORIZ. For CARBURETORS, CARB HEAT IS A MUST! TRUST ME I WOULD NOT LIE. THE CARB HEAT MAY NOT BE GREAT BUT NONE WILL BITE YOU SOME DAY. RV'S ARE NOT IMPERVIOUS TO CARB ICE.

 

[captainron]

It all depends on your application (Air box) and type of flying.

There are two air-boxes and two kinds of induction.

The standard FAB (filtered air box) is for Carb or FI mounted vertically.

The VAB (vertical air box) is for FWD FACING (horz) FI (fuel injection). The filter in the right cowl inlet is out in the open all the time.


For FAB if you have carb you (MUST) put in carb heat. This of course does not bypass the air-filter but does protect it from outside contaminates (sleet and snow). Carb Heat just closes off the cowl air-scoop and takes air from the cowl area, which is hotter or at least warmer.

The FAB alternate air is optional per Vans instructions and needed mostly for flying in snow and ice. Why you would fly in snow and ice I don't know, the RV is not known ice approved or very smart in a small plane. You can always close the carb heat to avoid "impact ice or snow" from blocking the filter.

The FAB magnetic alternate air was optional for the FAB per vans instruction. It was designed to open automatically but is OBSOLETE. Van changed the FAB alternate air from magnetic latch to cable operated a few years ago. The problem with the magnetic alternate air latch was it was open all the time. Also the small rivets that attached it could come lose and go into your engine.

Once you open the cable alternate air, you can not close it, at least easily with the cable from what I see of the design (I have the parts but not installed). You have to get on the ground and manually latch it back into the holder which is tight. The alternate air draws unfiltered air under the FAB.

For VAB (vertical air box) for the fwd face injection, alternate air is cable operated and opens a door on the side of the snorkel/airbox that goes from the filter to FI body. Since there is no Carb Heat and the filter is out in the open air, I think alternate air would be a good idea with the VAB.

I have a FAB and installed carb heat (I have a Carb) but did not install alternate air (magnetic automatic or cable operated). The reason is I doubt the filter will be blocked with my flying and knowing the limitation. I will have to remember to use carb heat in heavy (freezing?) precipitation.

IF YOU HAVE VAB OR FLY IFR IN SNOW OR SLEET (bad idea anyway) THAN YOU COULD BLOCK THE FILTER. I recall this is the reason for the option.

Summary my suggestions

FAB - Install at least, Carb heat for Carb and for FI Carb Alternate Air or carb heat, either one. If you have a Carb I would say Carb Heat is a MUST!

VAB - Install the Alt Air. Why not. If flying VFR and do Arco only, you can leave it off but that is up to you. It is unlikely the air filter will be totally blocked.

If you are fair weather flyer you don't need Alternate Air in my opinion, but I would put it on the VAB since the filter is right out there to be blocked and there is no carb heat or "alt air" with the VAB.

THAT!....was an extended vacation, George!

 

[asav8tor]

Summary my suggestions

FAB - Install at least, Carb heat for Carb and for FI Carb Alternate Air or carb heat, either one. If you have a Carb I would say Carb Heat is a MUST!

FAB-HORIZ - Install the Alt Air. Why not. If flying VFR and do Arco only, you can leave it off but that's up to you. It is unlikely the air filter will be totally blocked.

If you are fair weather flyer you don't need Alternate Air in my opinion..............

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20070228X00239&key=1

I am VERY familiar with the event/installation/and what happened. If the FAB Alt air would have been installed and used the airplane would not have been lost. The carb heat install would have done nothing in this situation. This was Day VFR.

Sometimes I see threads debating the merits of a safety mod for a problem that has NEVER happened. Great way to be proactive but hey lets take care of the real threats that have happened first, then go fight the fantasy threats.

Since I was at the scene of this event I have a whole lot more perspective on this issue. I have quite a bit more info on this one but I'm not going to debate it anymore or go into detail on the board. I'll simply finish by saying I would NOT fly without alt air. You can do whatever you like. Take one mag out and fly with one ignition system if you like.

 

[szicree]

If a bird goes into my intake scoop, the carb heat door will be jammed in the up position and my engine will be without air. For this reason I have the bypass. Built per plans, it can be closed (barely) with the cable.