Fly2HMO
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Fuel Selectors: Why No Both Position On Low Wings?

Sun Nov 20, 2005 6:35 am

It just came to my attention that Cirrus and Lanciar don't have a both position for the fuel selectors. Why is this? I'd figure it would be pretty annoying to switch from left and right several times.

Thx  Smile
 
Alias1024
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RE: Fuel Selectors: Why No Both Position On Low Wings?

Sun Nov 20, 2005 7:37 am

Low wing aircraft like the ones you described never have a both position because they would violate FAR 23.951.

FAR 23.951 basically says that no fuel pump can draw from more than one tank at a time, and provisions must be made to prevent air from being drawn into the fuel supply line.

In a low wing aircraft if one fuel tank ran out, the pump would suck air from an empty tank instead of fuel from the full tank.

A high wing aircraft like the C-172 can have a both position because they use gravity to feed a reservoir, and then the fuel pumps are downstream of the reservoir. If fuel runs out in one tank, fuel from the other tank still goes to the reservoir, preventing air from getting into the fuel line. Since low wing aircraft cannot use gravity to feed a reservoir, they cannot have a both position.

I hope this helps answer your question.
It is a mistake to think you can solve any major problems with just potatoes.
 
Mir
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RE: Fuel Selectors: Why No Both Position On Low Wings?

Sun Nov 20, 2005 2:14 pm

Quoting FLY2HMO (Thread starter):
I'd figure it would be pretty annoying to switch from left and right several times.

You would figure correctly.  Smile

And not only that, but plenty have planes have done unnecessary forced landings because the engine stopped due to one tank being exhausted and the pilot forgetting that there was another tankload of fuel waiting to be used.

At UND, the fleet is all Pipers (low-wing). Rumor has it that the first thing ATC says after you declare an engine failure emergency is "Have you switched tanks?"

-Mir
7 billion, one nation, imagination...it's a beautiful day
 
Fly2HMO
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RE: Fuel Selectors: Why No Both Position On Low Wings?

Sun Nov 20, 2005 5:02 pm

Duh, I never thought of that.  dopey 

Must be easy to feoget for people that are used to high wings and transition to low wings. I hope I never forget.  duck 
 
erj-145mech
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RE: Fuel Selectors: Why No Both Position On Low Wings?

Mon Nov 21, 2005 12:30 am

Quoting Alias1024 (Reply 1):
A high wing aircraft like the C-172 can have a both position because they use gravity to feed a reservoir, and then the fuel pumps are downstream of the reservoir. If fuel runs out in one tank, fuel from the other tank still goes to the reservoir, preventing air from getting into the fuel line.

Skyhawks don't have reservoirs or fuel pumps, everything else is correct. You won't get into a header or reservoir tank until the 205/206/210 series airplanes.
 
Alias1024
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RE: Fuel Selectors: Why No Both Position On Low Wings?

Mon Nov 21, 2005 6:41 am

Quoting Erj-145mech (Reply 4):
Skyhawks don't have reservoirs or fuel pumps, everything else is correct

I should have been more specific. I am referring to the newer fuel injected models like the 172S. My PIM shows a reservoir along with 2 fuel pumps. I don't know if older 172's with carburetors need the fuel pumps or not.

[Edited 2005-11-20 22:45:00]
It is a mistake to think you can solve any major problems with just potatoes.
 
Stoicescu
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RE: Fuel Selectors: Why No Both Position On Low Wings?

Mon Nov 21, 2005 8:54 am

The Piper Warrior II I fly has only L/R and Off positions. Start on the fullest tank and check it from time to time. A nice way to do it is USE the clock. Say the time says x:xx I will stay on L until the minutes show y:yy check it and if applicable switch. Hey at least you don't have to worry about hitting the flaps with your head all the time  Smile

Quoting Mir (Reply 2):
At UND, the fleet is all Pipers (low-wing). Rumor has it that the first thing ATC says after you declare an engine failure emergency is "Have you switched tanks?"

it won't help at all... it probably sucked enough air to make it unusable
 
Mir
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RE: Fuel Selectors: Why No Both Position On Low Wings?

Mon Nov 21, 2005 9:35 am

Quoting Stoicescu (Reply 6):
it won't help at all... it probably sucked enough air to make it unusable

Apparently not, because reportedly it fixes the problem most of the time.

-Mir
7 billion, one nation, imagination...it's a beautiful day
 
ShyFlyer
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RE: Fuel Selectors: Why No Both Position On Low Wings?

Mon Nov 21, 2005 9:46 am

In the TBM700, the fuel selector switches between the left and right tanks automatically.
I lift things up and put them down.
 
ajd1992
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RE: Fuel Selectors: Why No Both Position On Low Wings?

Tue Feb 19, 2008 4:52 pm

You're right, it is annoying.

Even more so, because i'm really anal about having uneven tanks  Silly Thanks for explaining why low wing aircraft can't have a dual setting - I've always wondered why.
 
askr
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RE: Fuel Selectors: Why No Both Position On Low Wings?

Sat Mar 15, 2008 10:06 pm

My first post on a-net, so please don't bite  Smile

Couldnt it be possible to place a pump bellow the lowest tank?
Obviously, there woldn't be much space left for a reserviour, but could just the plumbing, with non-return valves just under the tanks be enough to keep the pump fed?
ATC-PL Wanabe :) - 2nd application is in... 11 July...
 
Mir
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RE: Fuel Selectors: Why No Both Position On Low Wings?

Sat Mar 15, 2008 11:40 pm



Quoting Askr (Reply 10):
Couldnt it be possible to place a pump bellow the lowest tank?

I imagine it would, but then you'd have to have at least three fuel pumps in the plane (one for each tank to feed the reservoir and another to feed the engine - add another if you want a backup). That's just more things that can fail on you, as well as more weight and complexity. It isn't worth it - just remember to switch tanks every so often.

-Mir
7 billion, one nation, imagination...it's a beautiful day
 
BE77
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RE: Fuel Selectors: Why No Both Position On Low Wings?

Wed Apr 02, 2008 2:59 pm

BE77 Skippers (circa 1979-1981) are low wing, with an electric fual pump under the engine (just behind the nose wheel), and the tank options are simple...OFF and BOTH. There is no L or R option. Tanks cross feed themselves...when on a solo X-C you have to be sure to load as much of the baggage / misc stuff in the plane to the far right to counterbalance the pilot weight to keep things level (not really noticeable in flight - but the little offset is enough that by landing you have all the remaining fuel in the left tank!)
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