Fuel stratification and varying density issues
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Fuel stratification and varying density issues
This is an article(from Aviation Week) I have edited to only include information on subject. Has anybody encountered issues that have been due to fuel stratification or varying fuel density issues.
"How can a Gulfstream V make it from San Francisco to Tokyo in a headwind routinely, but sometimes struggle to make Tokyo to San Francisco with a tailwind?
The answer has to do with fuel. And even if you’re not flying an ultra-long-range aircraft, you need to understand how fuel density impacts how much Jet-A you can load on your aircraft, how your company fuel minimums may not be enough, how your flight-planning software can lie to you, and how your aircraft fuel measurement system can trick you into taking less than you need. You also need to know what to do when it all goes wrong.
About Fuel Density and Dictates
The answer to our GV riddle perplexed early Gulfstream pilots for years, so much so it was a common complaint against the airplane’s fuel system. Every aircraft was tested for a full capacity of 6,118 U.S. gal. Aircraft attitude during refueling was found to have only a negligible impact on total fuel capacity. The fuel measurement system specification of +/-300 lb. was bested by every aircraft tested. The engineers had to dig further and came up with the answer: fuel density.
Studies reveal fuel is denser in the Western U.S. than in the Far East. Given the GV’s maximum fuel volume is fixed at 6,118 gal., the highest allowed Jet-A fuel density of 6.99 lb./gal. would permit a fuel load of 42,764 lb. The lowest allowable density of 6.46 lb./gal. would leave a fully loaded GV with only 39,522 lb. of fuel. This variation of 3,242 lb. could reduce the aircraft’s range by almost 2 hr. There’s nothing you can do to fix low fuel density except be forewarned: Never count on a full load of fuel leaving airports in Asia.
Fuel Measurement
Most aircraft fuel quantity measuring systems use fuel capacitance probes that eliminate the problems attendant to the old float designs. A capacitance probe is a metal rod surrounded by a hollow metal tube with an insulator that prevents the inner rod from contacting the outer tube. The probe is mounted vertically so fuel can flow in and out from the bottom and air flows in and out of the top. An electrical charge is applied and the capacitance is measured. The ratio of fuel to air affects the capacitance and a fuel quantity is derived.
Modern systems tend to use several probes in each tank and multiple computers to ensure the result is accurate. The design specification in a GV, for example, is +/-300 lb. Still, these modern systems can be fooled when the temperature of the fuel in the truck is appreciably warmer than the fuel in your wings. Even the best fuel quantity computers can be compromised by variations in temperature within the same tank.
Let’s say you spent several hours in your GV at high altitudes and landed with fuel tank temperatures somewhere south of -10C. If you take on a full load of fuel from a truck that has been sitting for hours at a higher ambient temperature, the fuel will become stratified: The layer on top will be less dense than the layer below. Fuel computers will tend to think the fuel quantity is higher than it really is. A 2007 Gulfstream study revealed the fuel quantity in a GV could be 800 to 1,200 lb. low under these conditions.
If the airplane sits for about 10 hr. on the ramp, the discrepancy goes away. But what if you don’t have 10 hr.? You still need to add a known quantity of fuel, but don’t stop the truck just because the gauge says you have enough. (You might not.)
Even after 10 hr., you could still find yourself unable to take on a full fuel load because of high ambient temperatures. The warmer fuel gets, the more space it occupies and you could run out of tank volume before you have the necessary fuel by weight. A 2005 study showed that every 10C increase in temperature can reduce a GV’s total fuel capacity by 200 lb. While we assume our jet engines burn fuel by volume, this isn’t true; our range is determined by the weight of fuel. Density is even more critical than temperature.
The fuel truck driver wants his “How much?” question answered by volume, be it gallons or liters. But again, your aircraft burns the fuel by weight, measured either in pounds or kilograms. And just because 3,000 U.S. gal. of fuel weighs in at 20,000 lb. most of the time in the U.S., that doesn’t mean the equation will hold everywhere else. Going from the lowest to highest density can change the weight of 3,000 U.S. gal. by over 1,500 lb. So, don’t let the fuel truck pull away until the fuel total by weight equals or exceeds that on your flight plan."
"How can a Gulfstream V make it from San Francisco to Tokyo in a headwind routinely, but sometimes struggle to make Tokyo to San Francisco with a tailwind?
The answer has to do with fuel. And even if you’re not flying an ultra-long-range aircraft, you need to understand how fuel density impacts how much Jet-A you can load on your aircraft, how your company fuel minimums may not be enough, how your flight-planning software can lie to you, and how your aircraft fuel measurement system can trick you into taking less than you need. You also need to know what to do when it all goes wrong.
About Fuel Density and Dictates
The answer to our GV riddle perplexed early Gulfstream pilots for years, so much so it was a common complaint against the airplane’s fuel system. Every aircraft was tested for a full capacity of 6,118 U.S. gal. Aircraft attitude during refueling was found to have only a negligible impact on total fuel capacity. The fuel measurement system specification of +/-300 lb. was bested by every aircraft tested. The engineers had to dig further and came up with the answer: fuel density.
Studies reveal fuel is denser in the Western U.S. than in the Far East. Given the GV’s maximum fuel volume is fixed at 6,118 gal., the highest allowed Jet-A fuel density of 6.99 lb./gal. would permit a fuel load of 42,764 lb. The lowest allowable density of 6.46 lb./gal. would leave a fully loaded GV with only 39,522 lb. of fuel. This variation of 3,242 lb. could reduce the aircraft’s range by almost 2 hr. There’s nothing you can do to fix low fuel density except be forewarned: Never count on a full load of fuel leaving airports in Asia.
Fuel Measurement
Most aircraft fuel quantity measuring systems use fuel capacitance probes that eliminate the problems attendant to the old float designs. A capacitance probe is a metal rod surrounded by a hollow metal tube with an insulator that prevents the inner rod from contacting the outer tube. The probe is mounted vertically so fuel can flow in and out from the bottom and air flows in and out of the top. An electrical charge is applied and the capacitance is measured. The ratio of fuel to air affects the capacitance and a fuel quantity is derived.
Modern systems tend to use several probes in each tank and multiple computers to ensure the result is accurate. The design specification in a GV, for example, is +/-300 lb. Still, these modern systems can be fooled when the temperature of the fuel in the truck is appreciably warmer than the fuel in your wings. Even the best fuel quantity computers can be compromised by variations in temperature within the same tank.
Let’s say you spent several hours in your GV at high altitudes and landed with fuel tank temperatures somewhere south of -10C. If you take on a full load of fuel from a truck that has been sitting for hours at a higher ambient temperature, the fuel will become stratified: The layer on top will be less dense than the layer below. Fuel computers will tend to think the fuel quantity is higher than it really is. A 2007 Gulfstream study revealed the fuel quantity in a GV could be 800 to 1,200 lb. low under these conditions.
If the airplane sits for about 10 hr. on the ramp, the discrepancy goes away. But what if you don’t have 10 hr.? You still need to add a known quantity of fuel, but don’t stop the truck just because the gauge says you have enough. (You might not.)
Even after 10 hr., you could still find yourself unable to take on a full fuel load because of high ambient temperatures. The warmer fuel gets, the more space it occupies and you could run out of tank volume before you have the necessary fuel by weight. A 2005 study showed that every 10C increase in temperature can reduce a GV’s total fuel capacity by 200 lb. While we assume our jet engines burn fuel by volume, this isn’t true; our range is determined by the weight of fuel. Density is even more critical than temperature.
The fuel truck driver wants his “How much?” question answered by volume, be it gallons or liters. But again, your aircraft burns the fuel by weight, measured either in pounds or kilograms. And just because 3,000 U.S. gal. of fuel weighs in at 20,000 lb. most of the time in the U.S., that doesn’t mean the equation will hold everywhere else. Going from the lowest to highest density can change the weight of 3,000 U.S. gal. by over 1,500 lb. So, don’t let the fuel truck pull away until the fuel total by weight equals or exceeds that on your flight plan."
If you don't have a fuel densitometer* on board, you only have half a fuel gauging system.
*A densitometer is typically a cylinder which is electrically oscillated in the fuel; the frequency of oscillation being proportional to fuel density.
*A densitometer is typically a cylinder which is electrically oscillated in the fuel; the frequency of oscillation being proportional to fuel density.
Last edited by Mechta; 14th Apr 2015 at 17:03.
A good article. I have been teaching my co-pilots about this for years. I am glad to see it is getting exposure to a wider audience. It is important to pay attention to the density.
I have flown round the world with an old fashioned glass hydrometer to test uplifted fuel, and it was useful.
MM
I have flown round the world with an old fashioned glass hydrometer to test uplifted fuel, and it was useful.
MM
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I assume the other Boeing types have the same or similar. Does your typical high end bizjet have this?
