Avgas Specific Gravity

[RadioSaigon]

An interesting point of discussion, raised in another thread (http://www.pprune.org/forums/showthread.php?t=331480&page=2).

Ok while we are ERSA bashing. The conversion table for AVGAS of litres to kilograms is wrong. A litre of 100/130 does not weigh 0.72 kg.

How can the regulator approve a publication, for 15 years that I know of, with incorrect fuel conversions. Yes I have advised them of this in the past.

As I stated in that other thread, I (as I'm sure have many others) have been using a SG of 0.72 for Avgas 100LL and 100/130 fuel calculations for many years. It's the accepted 'norm' and is published as the conversion standard in the AIP of every country in which I've ever flown. There is a reference to a Wikipedia entry in that other thread for those that are interested.

The ensuing discussion has prompted me to a little further research on the topic. References as follows:

PNG Gas (http://www.pnggas.com/pdf/mobil/aviation_guide_tables.pdf) which appears to be Exxon Mobil Australia at SG 0.695 (provided by vans),
BP Australia from their MSDS (http://www.msds.bp.com.au/pdf/ref_4683_Avgas_100.pdf) an SG of 0.70,
BP Africa from their MSDS (http://www.bp.com/sectiongenericarticle.do?categoryId=16003553&contentId=7020867) an SG of Density @ 15°C ASTM D 1298 kg/m3 (which I'm pretty sure will be within a bulls-roar of 0.70),
The Deakin University MSDS (http://agrippina.bcs.deakin.edu.au/bcs_admin/msds/msds_docs/Avgas%201-5_files/msdsview.html) suggests an SG of 0.70, and
Chevron Global (http://www.chevronglobalaviation.com/docs/aviation_gas.doc) suggest an SG range of 0.65 to 0.75 (MS Word document)So, on the basis of those figures, I would suggest that there is an insignificant material discrepancy caused by using a generic SG of 0.72 as published in the AIP of many countries, or even an SG of 0.70 as used by many pilots (including me) on a daily basis.

Lets examine that a little more closely. We'll postulate an aircraft able to carry 500 litres of fuel at full-tanks.

In the very best case, the SG of 0.65 the weight of fuel will be 325Kg,
At the low end of a more 'realistic' SG of 0.695 that weight will be 347.5Kg,
At the ball-park figure most of us use, SG 0.70, that weight is 350Kg,
Using the published AIP SG of 0.72, the weight is 360Kg, and
The most pessimistic case SG of 0.75 yields a fuel weight of 375Kg.So across that SG range of 0.65 to 0.75 in an aircraft capable of carrying 500 litres we see a fuel weight range of 50Kg. Not even another passenger. So whilst a litre of 100/130 may not weigh exactly 0.72Kg, it is so close to that, that for most aircraft the discrepancy is immaterial.

[bentleg]

and i suspect there is variation in SG with temperature too, but again, not material

[tio540]

RadioSaigon

Hi there, glad I created some interest. First Wikapidea is not a reliable source, don't rely in it.

Anyway the SG (Specific Gravity), used by the ERSA applies to 80/87 octane, where 80 applies to octane rating for lean mixture, and 87 octane at full rich mixture. This has an SG of 0.72, at ISA or 15 deg C as an industry standard.

When Australia adopted the 100/130 octane Avgas the SG changed to 0.71, where 100 octane is the lean octane rating, and 130 octane for rich mixture octane rating. 100/130 Avgas is a higher grade of fuel, hence is lighter than 80/87 Avgas. The industry standard for 100/130 octane SG is 0.71 at ISA or 15 deg C.

This is the figure that should be used, not the 0.72 conversion that the ERSA and all other CASA publications use.

In reality, because Australia is warmer than 15 deg C the SG will be nearer to 0.69 or 0.70. But you still need to use 0.71 SG as this is the STANDARD figure for performance calculations.

So there you have it, just because you have done it for 30 years, doesn't make it correct. The difference is small but it does matter.

Hope that clears up any confusion. I just wish instructors would teach the correct figure, and CASA could help by publishing the real numbers.

Hope I didn't bore you too much.:)

[RadioSaigon]

yes bentleg there is... from memory about 0.5% per 5degC change. As you say, an absolutely immaterial factor for most aircraft that burn Avgas.

[tio540]

The SG 0.71 matters for the same reason that we use 15 deg C, to use a STANDARD figure for performance calculations. It is a baseline on which calculations are made.

Shame CASA can't get it right.:)

[RadioSaigon]

hmmm... ok.

Wikapidea is not a reliable source, don't rely in it.

I know that, and I don't rely on it. It is however a quick & easy reference that will get you within the ball-park and often provides useful references to further more empirical information when necessary.

the SG (Specific Gravity), used by the ERSA applies to 80/87 octane, where 80 applies to octane rating for lean mixture, and 87 octane at full rich mixture

Are you sure? My references were all based on a search for 100/130 octane, returning an SG range of 0.65 to 0.75 as drawn from the MSDS for those products. Further, 80/87 octane was not commonly used in aircraft engines until around the time of the 1st fuel crisis in the 80's. The SG 0.72 was in use long before that. Why would regulators base their SG figures on an octane rating not commonly used???

The industry standard for 100/130 octane SG is 0.71 at ISA or 15 deg C.

So you want to make a wholesale change to documentation for 0.01??? That is a whole 10Kg per 1,000 (yes one Thousand) litres?????? Or another way 1Kg per 100 litres, which it is more likely most avgas burners are going to pick up, or multiples there-of. The change you advocate is insignificant and of interest only to pedants -which I would suggest is why CASA has not acted on your suggestion for the last 15 years!

[flying-spike]

The SG of Avgas is what it is noted as on the delivery docket to the tanker or the QA test. I worked for an operator who used.72 (or whatever was in AIP) changed our weight and balance calculations to reflect what it was supplied at (.69), Ended up meaning we could could carry more freight because of the reduced fuel weight. Net savings/incresed income per year was around $30k. For which I was rewarded with $0.

[tio540]

I believe the 0.72 SG was just carried through publications as they were updated. Possibly it even stems from mogas of the past.

I agree the difference is small, but it is a very common calculation that is simply used incorrectly. After all this is a professional pilots forum.

Flying Spike - according to CASA you would then be flying overweight, even by just a couple of kilos is still overweight. That is the risk.

Maybe I just need to get a hobby. :)

[flying-spike]

Nope, The ammendment was made to the weight and balance calculations in the operations manual and accepted by the then CAA. As an RPT organisation we kept track of all our supplier documentation and had an approved loading system i.e. We had the evidence to support our ammendment

[Nivo]

I have just received a Shell Aviation fuel guide with my new carnet. In the conversion tables at the back it states "Avgas specific gravity - 0.695". I thought it strange as I was also taught to use 0.71.

cheers,

Nivo

[bushy]

The fuel supplier will advise what the ACTUAL sg of the fuel is. It is perfectly reasonable and factual to use this for calculations.
Our esteemed regulator has been building in margins for years.
The old Australian flight manuals had two lots of take off charts. One for use in Australia, and another for use in PNG. Apparently the aeroplanes knew when they were in PNG, and performed better.
The american flight manuals and the makers manuals were different.

[ForkTailedDrKiller]

This is great news!

I can cart another 5 kg of "toys" in the Bonza! ;)

Dr :8

[flyinggit]

I am new to this but would like to know does it make that much difference to light aeroplanes weight at the end of the day? I'm starting to fly a C172 would that type of plane have any issues with weight of fuel with different SG's?

ty


Git

[RadioSaigon]

...would like to know does it make that much difference to light aeroplanes weight at the end of the day?

Precisely the point FG... short answer is no. It will not make a significant, material difference to you or your aircraft at all.

[tio540]

I will now add some more fuel to the fire. When Avgas is loaded at the fuel distributor it has a certain measurable SG, at ambient temperature.

Upon arrival at your preferred fuel supplier the SG is again measured, and will be at a different temperature. This is because the distributor tanks will be at a different temperature to the tanker.

If the standardised SG between the two test samples the entire 25,000 litres or so is outside recognized tolerances it is rejected.

So even the three hour trip from Shell or whoever, to your refueller, the SG will change.

Hence the use of the standard SG of 0.71. I really need to take up golf or something else.

Hey Nivo, did you train in Melbourne by any chance?

[desmotronic]

Damn thats 30 min holding up my sleeve

[Maggott17]

I do not know what you modern jet jockeys are whinging about.

I remember getting the good old Mirage up to above 40 grand, roaring around at the speed of heat, then throwing the jet at the ground and ending up with more fuel in the gauges than when I took off.

[Peter Fanelli]

I can cart another 5 kg of "toys" in the Bonza!


That's gotta be what, at least 7 portable GPS's :}

[john_tullamarine]

Several points ..

(a) most, if not all, pilots don't carry an hydrometer ... you can always check the SG from the local fuel chap .. certainly accurate enough for government work .

(b) since the (general if not total) demise of big bird petrol burners ... for most small aircraft the error associated with SG is not significant but does represent a payload penalty. For the former, the quantity of fuel on board made it useful/mandatory to check SG (... I still use an hydrometer which fell off the back of an ex-QF DC4 decades ago)

(c) over the past 30-odd years of weighing aircraft, I have always run an SG check (one of the few weight control folk who bother to do so). Unless things have changed in more recent times (have only been playing with kero-burners for quite some time now) the typical SG for 100LL used to run at around 0.710-0.715 and 100/130 (can you still get that as a general rule?) at around 0.69-0.70 as I recall.

(d) CASA documentation reflects the 0.71/0.72 spec split .. as I recall, Pub 17 includes those values although I misplaced my file copy quite some years ago so I can't check that claim right now ...

(e) there is no mandatory requirement to use the "standard" 0.71/0.72 SG value ... provided you can show evidence of the validity of a lower value (and the refuel values will be fine) the chap doing the ramp check would be obliged to accept your calculations

(f) temperature affects the SG but not so much that one would lose sleep over it for small aircraft

[tio540]

(e) there is no mandatory requirement to use the "standard" 0.71/0.72 SG value ... provided you can show evidence of the validity of a lower value (and the refuel value will be fine) the chap doing the ramp check would be obliged to accept your calculations

All good stuff, but in practical terms the refueller has three figures. The tank SG, any fuel added to his stock SG, and the calculated SG between the first two.

Rather than changing the weight for each refuel, there is an standard figure, which brings me to my earlier posts. 100/130 has a standard SG of 0.71, and not 0.72 commonly used.

If there is a correct figure it should be used. Conversions in the ERSA are in two decimal places for a reason, surely.:) I'm off to the golf course now.

[john_tullamarine]

.. one needs to keep track of refuel history .. and occasionally run an hydrometer reading .. but, for most small aircraft, at the end of the day .. it really doesn't amount to enough delta to worry.

[bushy]

In Alice Springs the sg of 100LL is usually .69 something.

[Sunfish]

In designing mechanical things, tolerances always accumulate the wrong way.

I will continue to use 0.72, round my times, weights and fuel loads up against the day when every little bit counts.

[john_tullamarine]

In Alice Springs the sg of 100LL is usually .69 something

I've never seen LL down that low ?

against the day when every little bit counts.

nothing wrong with conservatism if one is paying the bills ... otherwise company SOP takes a significant seat re policy.

[bushy]

I've never seen it higher than that.

[tio540]

I doubt that Alice Springs has 100LL. The only Avgas shipped there is 100/130.

[Creampuff]

Very interesting thread

According to the Tech Data Sheet published by one very large manufacturer of petrochemicals that I could check in a hurry, the SG of its 100LL AVGAS @ 15degC is .718. (See: 100 LL TDS (http://http://www.shell.com/static/aviation-en/downloads/safety_data_sheets/outside_usa/avgas100ll_pds_australia.pdf)). That’s very close to .72.

For the purposes of weight and balance calcs, I'd err on the side of simplicity and caution and round up from .718 to .72.

The more interesting question seems to me to be: what is the real effect of differing SGs on engine power?

Let’s say I fill up with 100LL that’s been warmed so that the SG is down around .69 per Alice Springs and Bushy’s measurements. I do the W/B calcs on the basis that it weighs .72. I’m sure not going to be overweight. But how much difference will that make to engine power while I’m rolling down the runway? (Let’s assume it’s 15degC and I’m at MSL/1013 and someone warmed the fuel in storage for fun.)

[RadioSaigon]

Thanks Creampuff, interesting -although your link doesn't work. I think I found what you refer to here (http://www.shell.com/home/content/au-en/shell_for_businesses/aviation/aviation_msds_pds_0822.html). Interesting to note that whilst the TDS reports an SG of 0.718 as you mentioned (I too would use 0.72 in preference) the MSDS which is the next link below shows a density range of:
690 to 760 kg/m3 at 15°C.

From that one could extrapolate that 1m3=1,000 litres, therefore the SG range is 0.69 to 0.76

What does this mean in the circumstances of this discussion? Again, IMO absolutely nothing. The effect on any current avgas burners W&B is negligible and for what might be considered a pretty 'normal' fuel uplift of around say 200 litres for a single -or 'per engine' for a light twin- is only going to make an effective difference of a couple of Kg.

The more interesting question seems to me to be: what is the real effect of differing SGs on engine power?

Indeed an interesting question! At 1st glance, I believe the density is not what's going to effect engine power. If your engine burns 60 litres an hour at a density of 0.69, I would think it would still burn 60 litres an hour at a density of 0.72. I think the RON of the fuel will be what effectively influences your engine output, which we control with the mixture. In the case of 100/130, 100RON at a rich mixture or 130RON lean. This is heading in a direction now which is more in the purview of Chimbu Chuckles... hopefully he'll pop in here soon to enlighten us, without baffling us with bull**** ;)

[tio540]

There is no 100 LL in Alice Springs, and the 100/130 octane has a lower SG than 0.72.

Not all fuel in Aus is 100 LL, in fact many sites don't ever supply it.

I would also suggest half the piston fleet where I am carry 3 times your 200 litres.

I think it comes down to being able to furnish satisfactory evidence to the authority that you are compliant in all respects, weight and balance. An approximation because I want to do it that way doesn't really cut it.

[john_tullamarine]

An approximation because I want to do it that way doesn't really cut it.

concur .. approximation isn't going to win the discussion during the ramp check .. one has to be able to quantify and justify the numbers .. equally well (and similarly trivial) using the higher "standard" SG can, for some circumstances, create a minor error with things such as CG limits. At the end of the day it is more useful for folk to be aware of the problems, advantages and disadvantages than to worry too much about the fifth decimal point of the calculations.

If, however, you are flying a large piston aircraft .. then the actual SG is the way you would go as the numbers and the potential errors are more significant. Guess there are not all that many DC4s/DC6s etc still flying in Oz .. the DC3s would be a case in question, though, as there are a few still actively involved in the game.

[RadioSaigon]

An approximation because I want to do it that way doesn't really cut it.

Nowhere have I suggested that anything such is the case at all. I still see you offering no supporting evidence either that your 0.71 is more accurate, or that it will make any significant difference to any current avgas burners' W&B.

My contention that your 0.71 is simple and pure pedantry stands.

Further, although your GA airframes may be capable of holding 600 litres or more, how often do they??? In my experience, it is usual for most GA aircraft to operate well under full-tanks to maximise pax/freight payload, as appropriate to the route flown. This further minimises any perceived advantage to using a non-standard SG.

And what about refuelling at remote stations where batch-SG figures are unlikely to be available? Makes a further nonsense of using a non-standard SG.

There are enough confusing conversions and calculations in aviation already, to safely & legally launch a flight, without muddying the waters more with unnecessary and nonsensical ones.

[tio540]

"Nowhere have I suggested that anything such is the case at all. I still see you offering no supporting evidence either that your 0.71 is more accurate, or that it will make any significant difference to any current avgas burners' W&B."

I agree completely. My only point is that the commonly used figure of 0.72 applies to an extinct 80/87 grade of fuel, no longer used here. The current grade of 100/130 has a standard weight of 0.71 SG.

"My contention that your 0.71 is simple and pure pedantry stands."

It just happens to be correct.

"Further, although your GA airframes may be capable of holding 600 litres or more, how often do they??? In my experience, it is usual for most GA aircraft to operate well under full-tanks to maximise pax/freight payload, as appropriate to the route flown. This further minimises any perceived advantage to using a non-standard SG."

Outside of the training area, many do need to fly 300 nm, and return.

"And what about refuelling at remote stations where batch-SG figures are unlikely to be available? Makes a further nonsense of using a non-standard SG."

Exactly my point, the standard SG of 0.71 must be applied, 0.72 no longer exists at the bowser. You should not use a batch figure at all, unless approved by the regulator.

"There are enough confusing conversions and calculations in aviation already, to safely & legally launch a flight, without muddying the waters more with unnecessary and nonsensical ones."

I agree again, but when you start flying a turbine you will need to adjust to 0.8 SG, so get used to it.

[RadioSaigon]

We only have your contention that 0.72 relates to the 80/87 fuel spec. No evidence has been presented. Again 80/87 only came into common use after the fuel-crisis of the 80's, so that contention appears insupportable, given that 0.72 has been in use much longer.

[tio540]

"Again 80/87 only came into common use after the fuel-crisis of the 80's"

Are you certain about this?

[RadioSaigon]

nah mate, I'm not certain of it all... to be honest, I can't even be bothered searching for further info now. It's been fun & all, but too much of a good thing... ;)

I admit your contention has some merit... but whether the benefits are worth the effort, expense and confusion... for me at least, the jury's still out on that one. Maybe 0.72 is less relevant now than it was in times past, but on the evidence I've seen to date, your 0.71 could well be just as wide of the mark. One thing I know for sure, the regulators won't be swayed by anything they see in here -and that just might not be a bad thing :E

To be honest, I reckon the time, effort and money necessary to make any change could be far better spent on things that might just bring a real, tangible improvement to the way we do things -i just might start a thread on something sometime soon :oh:

[john_tullamarine]

some comments ..

We only have your contention that 0.72 relates to the 80/87 fuel spec

I can recall various regulator documents going back to the 60s which adopt the 0.71/0.72 split

Are you certain about this?

going back a few decades we had a range of fuels .. memory may be scratching a tad here but, as I recall, the following were, at various times, in vogue - 71/73, 80/87, 100/130, 115/145 and, I think, there was another around 91/? although I just can't bring it to mind. Can't say I've ever seen the extremes or the 91 in the flesh but the other two were both very common when I started out in the 60s

0.72 applies to an extinct 80/87 grade of fuel

Used to be the regulator's approach in Australia that 0.72 was relevant for fuels below 100 and 0.71 for those above

Certainly, having taken numerous (= lots of) readings for 100/130 and 100LL over quite a few years (since the early 70s but not since, probably, the late 90s) in Australia (all out of the aircraft fuel tank at a wide range of OAT) 100 generally ran at around 0.69 - 0.70 and LL 0.71 - 0.72

FWIW, if I were to go Oz flying in a piston aircraft these days,

(a) I would enquire of a couple of fuel agencies what their recent SG readings have been (measurements are taken very regularly as SG comes into the supply chain dollar calculations I have been told in the past) and, provided that the earlier figures still apply reasonably well,

(b) I would use 0.70 for 100 and 0.72 for LL and cheerfully argue the toss at the ramp check .. but I emphasise that, for smaller aircraft, it doesn't really represent a case of mountains .. definitely molehills are the go, here.