Gas gauge calibration

[N3243A]

It would be great if people wouldn't put so many of their value judgements on a simple request for information.

Well said Shane,

I should have followed my own instincts and avoided posting in the first place. I find these go-arounds tedious at best and mostly just frustrating.

Bruce

[GAHorn]

Shane originally posted: ...Anyone tried doing this? Any ideas?...
With that in mind, I feel it's a little harsh to later complain when someone offers their ideas. Ideas almost always contain judgments and opinions. My intent was to impart the concept that unuseable fuel is fuel we can never plan on. Conservatively, that means that any attempt to include it in our fuel/flight planning is useless and hazardous.
If being "scientific" then I suggest we adhere to the standard terminology in use such as "unuseable fuel", etc., as a conscientious scientist would.
The factory, their test pilots, and the certification authority all "scientifically" determined and committed to writing the useable fuel in this aircraft...42 gallons total, 37 useable. That determination was made after the aircraft was demonstrated to suffer fuel starvation in normal flight attitudes. The (anecdotal) story in The 170 Book in which an owner described a special technique he found useful in accessing the unuseable fuel does not fit the definition, and as such cannot be relied upon.
With regard to NTSB reports, keep in mind that accidents are investigated after the event. There's hardly a "standard" fuel exhaustion accident. The position of and damage to the aircraft can affect the amount of fuel remaining to be sampled several days after the event when the wreckage is examined. When the engine arrives at the scene of the accident first, and the carb and it's fuel lines are damaged in the impact, their integrity is compromised and it's unlikely that unuseable fuel will fail to find it's way onto the ground.
Yes, it's fun, educational and a worthy endeavor to investigate the many issues involved in our hobby, and there's certainly no intent to discourage curiosity for whatever purpose. I also like to fool around with such topics of discussion. But if you ask for "ideas" then please don't be disappointed if you get them in discussion.

[N3243A]

gahorn wrote = My intent was to impart the concept that unuseable fuel is fuel we can never plan on. Conservatively, that means that any attempt to include it in our fuel/flight planning is useless and hazardous.

Shane needs no defending but I will say this due to the fact that I have been in several of these go-arounds now, not the least of which was the famous "mogas wars" on the old board and understand his position. Come on George, when you give ideas, you sometimes throw in inflammatory remarks and subtle negative implications, like the one above. Where did he mention that he wanted to use unuseable fuel in his flight planning? I didn't get that at all from any of his posts, why did you?. That also implies he wanted to operate in an unsafe manner, hence the (negative) judgement value. I think we all know what unuseable fuel is, how much are birds have, etc. and we don't need a big sermon on it. You also sort of minimized the whole deal by saying "if a gallon or so is all that's important to you"...."and it seems we're discussing an inherently risky operation." No, it was 2 gallons per side, 4 gallons total. And the "risky operation" talk is just plain inflammatory and is not an expression of an "idea". Not very flattering implications really and sort of mean spirited. About the only explanation I can think up is that you secretly enjoy a little controversy on the boards and from time to time throw in barbs for amusement. Having said that, I appreciate your hard work in moderating these boards and even agree with most of your advice but sometimes.....well.... let's just move on. (I can hardly wait for the reply on this, I'm sure it will be full of "ideas".)

The man simply wanted to know why his 21 gallon tanks only hold 19 gallons? The unuseable fuel really has nothing to do with it and is there regardless. I think it's a reasonable question. The best answers so far are trapped air above the filler cap in the 3 point position or refill measurement error. Trapped air above the filler cap due to the tank angle is an interesting concept. That would mean even less useable fuel to deal with when flight planning. Let's try to solve the problem instead of this other crap.

Bruce Christie

[N170BP]

Um... what he said.... and the thought occured to me (with
relation to this thread) is I wonder if the air cavity in the forward
part of the fuel tanks (in the 3-point attitude) is in any way affected
by:

1. 180 (taller) gear legs?
2. Larger (than 600x6) tires?
3. Less than a "perfectly" (or newly) arched tailwheel spring?

Seems to me the above are just a few variables that would affect
how much fuel one could get into the tanks in a 3-point attitude.
Admittedly, there probably isn't a *great* deal of difference, but surely there must be *some* difference (?).

Is it fair to assume the figures Cessna published (with respect
to useable fuel) was with a brand new airplane with stock gear
legs, a brand-new tailspring assy. and perhaps 600x6 tires? You
can bet your back-side the performance figures were no doubt
achieved/published with 600x6 tires mounted for maximum "advertised" speeds, etc. What's the difference in height (in radius) between fully inflated 600x6 tires and 800x6 tires? And how does that effect the amount of fuel you can pack on-board before it runs out of the back sides of the filler necks? Maybe I'm delving into pedantics here, but at least
my brain is still working after all them beers (some of course might argue with that assesment!) <grins>

For the record, I'm a chicken sh_t when it comes to fuel planning....
(during a cross-country, I put as much on-board as I can afford
and/or think I can haul aloft and I simply land 3 hours later....). 8^)

Bela P. Havasreti
'54 C-170B N170BP

[GAHorn]

(Hey Bruce, There's no reason to bring up another old, unrelated grudge, in which you tried to tell me what I think. If this is personal, then chew on me privately and spare everyone.)
Shane needs no defending because he's not being attacked. If I came across that way, then I'll have to plead Internet clumsiness and ask for forgiveness.
Apparently unuseable fuel isn't as well understood as one might suppose. I felt this thread and The 170 Book article demonstrate that. (There's a lot of really good info in The 170 Book, but there's also a lot that should be taken with a grain of salt.)
I didn't bring up useable/unuseable fuel. Another member did. But it's certainly germane to the topic.
"Where did he mention that he wanted to use unuseable fuel in his flight planning? I didn't get that at all from any of his posts, why did you?"
I got it from his post on April 10 and again on April 9 when he specifically stated "Actually, it is very important whether that unuseable fuel amount (3 gallons) was still in my tanks ..."
He certainly was considering it's presence when he was making his available fuel calculations during flight planning.
I have not tried to minimize anything, especially with regard to safety. Since when is safety a taboo subject? It was his statement (twice) that he was dealing with operating in/near the "margins". I'm not criticizing him. I was simply answering him as truthfully and frankly as possible. And if we are talking about operating "in the margins" then I don't think it's untruthful or too critical to opine that may be risky. I can assure everyone there's no mean spiritedness in it.
The thread supposedly is about calibrating a dipstick. The airplane has 37 gals useable. Just because the tank fillers claim 21 gal capacity, does that mean the tank alone? Or does that mean the tank and it's related half of the entire system? (including air pockets.) I believe it's a moot point. If a full tank registers the maximum amount of useable fuel on the dipstick, and if an empty tank takes the same amount to refill it, then what more can one hope for?
(As an aside, the '53 Owner's Manual states the airplane has 20 gal. tanks in the Description chapter. It still adheres to the 37 gals useable, and 42 gals total.)

[zero.one.victor]

You guys just quit being mad at old George--he can't help being the way he is!
And quit trying to kill yersef in them flyin'machines!

Eric

[N3243A]

It's not personal and I absolutely don't want' another chat board war. I only mention the "mogas wars" because this is not the first time I have seen inflammatory remarks. The moderator's keyboard can sometimes be pretty "sharp" and he shoudn't be surprised if people take offense from time to time. It's not that it is percieved as an attack so much as lecture where one isn't desired or warranted. Where the moderator percieved an individual as "wanting to operate in the margins" I percieved it as someone wanting to know where the margins are so he can avoid them. A subtle but important distinction. Of course safety is not a "taboo" topic. It's the negative implication of "operating in the margins" where it was never intended in the first place is what this is all about. Why the implication it all? Whether the topic is mogas or unuseable gas or passing gas, do we all need the full sermon on every topic as a CYA measure to insure all bases are covered? Nuff said, Let's move on.

[zero.one.victor]

I must confess to having felt (or imagined that I felt!) the sharp point of George's keyboard a time or two myself!
I heaqrd a comment about that kind of thing once: "she's got a tongue that could clip a hedge!"
A lot of times,the way a post comes across is not necesarily how the poster meant it. (But sometimes--it is!)
Hopefully we can all agree to disagree,without anybody taking offense at innocently intended comments (or as we call them--cracks!).

Eric

[2723D]

Well, how much gas in my tanks makes a difference, and whether the unuseable was in there when I started to calibrate the dipstick matters - it matters because I will either have 3 gallons useable or zero gallons useable per side when I get to a given (albeit low) point. 3 gallons useable per side is 6, thats 45 minutes of flight time, which might just be what I "think" is my safety fuel, which might not actually be there . Bottom line is that nobody else really worries about this, so I will figure it out myself. Even if I never get that low in the tanks, I would rather know that I have 1.5 hours of fuel instead of 2.0 left, in Alaska, where airports with gas can be a long ways apart, with a lot of unfriendly terrain inbetween, in matters. I also like monitorring my fuel burn, keep an eye on it to make sure it doesn't creep up, so I would like my calibration system as good as possible, off a couple of gallons really throws things off when you plan a 3 hour flight with multiple landings and no re-fuel.

It doesn't surprise me that some people posting here are quick to react, they spend most of their time in books, and less time out actually flying the plane over terrain and in situations that really push the 170, to most people in the lower 48 a 2000 foot strip is REALLY short, and having gas more than an hours flight away is also a long way. Up here, where a lot of people die every year from every possible type of mistake and bad luck, the better a pilot you are, the more questions you ask and more you know how your plane really performs, the more times you get to come home and park your airplane. For those people who react to questions like mine with a condescending view point, my guess is that books take precedent over actual, learned knowledge. I'll start with the manual, but when it comes right down to it, everyone of our planes is different, and if you believe everything in your manual, with a 50 year old plane that could have had any number of modifications without a logbook entry (happens all the time up here) then I think that YOU are unsafe. The best information I find on the website are the folks actually finding things out about their specific airplanes. I posted this question here because I thought other folks might be interrested in the question of how to safely calibrate a dip stick so that it actually works, even at the low end (even if we would be unlucky enough to be flying with only 3 gallons useable left, it is still good to know you got only 3 and not the 6 or 9 you thought you had). I think we'll end there, happy flying to you, I'm off to the Knik gravel bars to practice some more short field work with full tanks.

[blueldr]

For what it's worth on this topic, my '52 with standard tanks will accept 41.2 gallons on one side and 40.6 gallons on the other side after the tank has been run dry in level flight. This has been done on two separate occasions consisting of separate flights for each tank.

BL

[N1478D]

Most always, the plane is filled in the three point attitude. Most of the time the fuel is used in the straight and level attitude. So, here is what I would do if I wanted to know the amout of fuel available to the carb. Fill the tanks in the normal 3 point attitude. Raise the tail and drain the fuel at the CARB measuring the amount. At this point you would know fairly well the total amount available. You could work from that point, calibrating a fuel stick.

[zero.one.victor]

This is what I did recently when W&B'ing my 170. I did lower it back to 3-point attitude and pulled the sump drains--I only got maybe another quarter gallon out of each tank. I did calibrate my dippin' stick from this empty condition.

Eric

[Koop]

I recently pulled my left fuel tank on the 1955 170 B to check on a spar repair that had been done some time ago. I put a pillow on a ladder and put the back end of the plane up to get the fuel tank level so I could drain all the fuel out. I must have had it about right because when I pulled the tank out there was no fuel left. So my feeling is that usable fuel, depending on what landing gear and size of tires you have, is dependent on what attitude your in. If your in a three point attitude your going to have 2.5 gal in each tank of unuseable fuel. If your in straight and level flight you would have 5 more gallons of fuel to burn. The only time you are normally in a three point attitude is when you are taxing, when you are starting your take off run and when you are flaring before your landing, hopfully a few inches above the runway. Oh one more thing for what it is worth. I learned a long time ago that when I go to the airport I put my ego in my back pocket. Koop

[zero.one.victor]

It just occurs to me that I recall reading a definition of usable fuel as being that amount of fuel which is usable in all possible flight attitudes. In other words,the fuel has to be available to the engine even if the airplane is severely nose-up,nose-down,or whatever. Just about all of it is availble in straight & level flight,not so for more extreme attitudes.
That reminds me of the private pilot applicant who was asked by the examiner to demonstrate straight & level flight. "OK" he said,"which one do you want to see first?"

Eric

[Koop]

For me it is about reality and common sense. But I do think the defintion you gave is a very good one, Eric. koop