Fuel Gauge Red Arc

Robert Eilers · Post by **Robert Eilers** » Mon Jan 02, 2006 2:24 am

Has anyone actually caluculated how much fuel, total and useable, remains in the tanks at the top of the fuel gauge red arc? I estimate the top of the red arc is 1/4 of a tank, or 5.25 total and 2.75 useable gallons per tank. Of course, on my aircraft like most other 170's it is impossible to accurately determine when the needle is in the red arc due to the bouncing. If my estimates are correct, a total of 10.5 gals remain at the top of the red arc, or 5.5 useable, and that would be the minimum total for takeoff. I assume the takeoff restriction is due to possible interruption of fuel flow. However, if that is the case it seems the same precautions would exist when landing with less than 10 total gallons

. On a recent trip due to weight it was necessary to calculate fuel closely. I landed on more than one occasion with just over 5 gals total useable remaining - or 30 minutes - as planned - but not comfortable.

Post by **Bruce Fenstermacher** » Mon Jan 02, 2006 2:55 am

Robert

Regardless whether your gauge has a red area or not (some don't) there is no restriction to taking off or landing with the fuel level in that area according to the TCDS or the flight manual. Doesn't mean it's a smart thing to do.

tshort · Post by **tshort** » Mon Jan 02, 2006 5:38 am

I think mine is placarded for no takeoff with less than a specific amount (can't remember exactly what it says...?). I do have the red arc - and there is bouncing even when I'm just sitting in the plane!

Thomas

thammer · Post by **thammer** » Mon Jan 02, 2006 5:22 pm

I fly a 140 and the mention of the lack of the 1/4 tank takeoff limitation in the TCDS or operators manual got me thinking about it. I also find that there is no mention of it in the 140 operators manual or TCDS. I was looking up the part number for the guage in the IPC and found much to my surprise that there is a part number for a decal for the gauge, "No Take Off Warning". Doesn't say what the warning is though.

tye

lowNslow · Post by **lowNslow** » Mon Jan 02, 2006 6:22 pm

The warning is posted in the Owners Manual for the 170B in no uncertain terms - "Do not take off if the pointer is in the red band". I know there is some question if this a legal restriction since it is not mentioned in the flight manual or the TSDS, but I think you would still have some splanin to do if you flamed out on takeoff with that restriction in the Owners manual and on the guage.

Post by **Bruce Fenstermacher** » Mon Jan 02, 2006 7:07 pm

Yes you would Karl. You would have to explain to who ever may ask that the owners manual is not an approved source of information. Don't think that conversation would go to much further.

One of my gages has the red area the other does not.

Now lets see. At what airplane attitude are those gages calibrated, level flight or 3 point attitude? I've often found that sitting on the ground the gages read at less that in level flight. A quarter of a tank on the ground is really three eights in the air.

Listen don't get me wrong. When the gage is reading about a quarter I'm thinking I better be getting some gas. And there isn't many times I'd take off with just quarter tanks but I have and I'm absolutely sure I'll do it again depending on the mission at hand.

Here is a question. If one tank was empty and you had a half a tank on the other side, would you have more head pressure that having both tanks at a quarter?

Dave Clark · Post by **Dave Clark** » Mon Jan 02, 2006 9:12 pm

[Here is a question. If one tank was empty and you had a half a tank on the other side, would you have more head pressure that having both tanks at a quarter?[/quote]

Sure, I think even if fuel selector is on both.

johneeb · Post by **johneeb** » Mon Jan 02, 2006 9:37 pm

N9149A wrote:Here is a question. If one tank was empty and you had a half a tank on the other side, would you have more head pressure that having both tanks at a quarter?

Bruce is that assuming that I can fly wings level

Mon Jan 02, 2006 10:07 pm

That's a good point John. I of course would always fly with the empty tank low in this situation to get every bit of head pressure from the filled tank in the high position

Dave I'm thinking maximum head pressure to the carburetor would be with the selector on just the tank that was half full. Otherwise the pressure would be split between supplying the carb and the other tank.

I'm just thinking here. I have no data to back up these thoughts.

Do we have any fluid engineers out there?

blueldr · Post by **blueldr** » Tue Jan 03, 2006 5:09 am

Head pressure is a function of the height of the column of fluid above the point of measuring. Most of the height in a C-170 fuel system is in the fuel lines coming down from the wing tanks to the carburetor inlet. The tank depth is only about six inches of a total of about three feet or a little more. That six inches from full to empty would not have much effect on the head pressure. Keep in mind that we're talking pressure and not volume here.

mit · Post by **mit** » Tue Jan 03, 2006 5:42 am

I guess some folks would think, flying one with only one tank installed. Would be crazy! But I don it to get it home.

Post by **Bruce Fenstermacher** » Tue Jan 03, 2006 4:09 pm

That's what I was thinking bl. Following that logic then thou very slight, one tank with 10 gallons would yield a higher head pressure than both tanks with 5 gallons. We're talking about our 170 tanks and setup here and assuming you've figured out how to fly wings level.

cessna170bdriver · Post by **cessna170bdriver** » Tue Jan 03, 2006 4:45 pm

N9149A wrote:That's what I was thinking bl. Following that logic then thou very slight, one tank with 10 gallons would yield a higher head pressure than both tanks with 5 gallons. We're talking about our 170 tanks and setup here and assuming you've figured out how to fly wings level.

You don't necessarily need to fly wings level, just well coordinated.

(Remember Bob Hoover's demo of loops and barrel rolls with a glass of liquid on the glareshield?) Actually, the fuel pressure at the carb will be a bit higher in a coordinated turn than in level flight, due to the g-force. All that said, though, I would feel more comfortable with 10 gallons on one side, than with 5 gallons in each, since on some days the only time I get the inclinometer ball in the middle is when it is passing from one side to another.

Miles

Metal Master · Post by **Metal Master** » Wed Jan 04, 2006 3:36 pm

I have leak-checked fuel tanks on Boeing 727's and on small Cessnaâ€™s using a water monometer to check the pressure. I would use a column of 20 inches of water to test a 172 tank. This is equal to about .2 pounds per square inch. Three inches of water is equal to about .01 PSI that is Point Zero One. Three inches of water would be about .03 PSI. The three inches of fuel difference in the tank does not make much more pressure. the 30 or so inches from the carb inlet fitting to the top of the tank fuel level is all the pressure you get. Probably about .3 PSI.
Having a having a 100-gallon fuel tank 6 inches tall makes exactly the same pressure at the bottom of the tank as 1-gallon tank 6 inches tall.

Note my numbers here are not exact as I do not have my chart with me and they refer to water not fuel but the idea is the same, the weight of fuel and water being close but different

thammer · Post by **thammer** » Sat Jan 07, 2006 4:15 am

N9149A wrote:Robert

Regardless whether your gauge has a red area or not (some don't) there is no restriction to taking off or landing with the fuel level in that area according to the TCDS or the flight manual. Doesn't mean it's a smart thing to do.

I don't know about 170's but the 120/140 has a one or two page C.A.A approved "Airplane Flight Manual" (not the operators manual) that one of our eagle eye pilots noted contains the 1/4 tank take off restriction in section 1 "Limitations" "(c) Fuel Quantity Gauge - Do not take off on a fuel tank less than 1/4 full."

Maybe at least the early 170's have a similar item in their AFM.

Tye