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Recently replaced the fuel transmitter in my 1968 PA28-180 with Hershey Bar wing. While performing the calibration, it got me to thinking about unusable fuel and oil.
Type Certificate 2A13 identifies unusable oil to be 1.8 lbs (~1 qt) and unusable fuel 2.2 lbs. (~.4 gal) for weight & balance purposes.
My questions are:
(1) If the oil dip stick registers 6 quarts in the engine, does that include the 1.8 lbs. (1 qt.) of oil deemed unusable, or are the markings on the dipstick calibrated to include the unusable oil? I've not incrementally checked when filling the engine so hoping others have?
(2) If one makes a dip stick for the fuel tanks or purchases one, does it include the 2.2 lbs. of unusable fuel or not? Actually, I'm more interested in where does the 0.4 gal unusable fuel remain and how can our fuel systems be so good at sucking out all that fuel?
I believe I can answer question 2 easily enough as noted below. Not sure about question 1.
A previous thread had some discussion on unusable fuel.
https://www.piperforum.com/threads/fill-to-the-tabs.24945/This thread adds to that discussion, as there may be some additional clarification required for the previous thread discussion.
When one wants to create a fuel dip stick, the process is to simply drain the tank and per the Piper Maintenance Manual (PMM), fill in 5 gallon increments and mark your stick accordingly and check your gauges. Keep in mind. I believe that one is draining some unusable fuel in the process. The previous thread was prompted by the OP noting variations in his fuel readings and fillings. What is important to understand, I think, is the variability allowed in marking your dip stick and correlating the fuel gauges. Per the PMM, the following applies when calibrating:
The other important factor to consider is that when you are calibrating the fuel transmitter, the airplane needs to be level (horizontal and lateral), which for the most part means it is leveled using jacks to replicate inflight level flight. It is essential to level the aircraft when calibrating your fuel gauges.
However, if you calibrate your fuel stick (I used a paint stir stick) to the level position, your markings may exceed the variance allowed when your aircraft is sitting on the ground, because most Cherokees are going to sit differently given the nose and wing strut extensions. Leveling the aircraft and marking the dipstick in that position is going to result in a different indication when the aircraft is sitting on the ground.
So rather than mark my dip stick with the aircraft level, I marked them using the sitting-on-the-ground position (noting my strut extensions on nose and wings).
I use the fuel gauges (despite the variance allowed) for determining fuel on board. I generally fuel to either the TABS (18 gal, 36 total) or top off (50 gal) and subtract the 0.4 gallons for when I set the fuel totalizer. Actually, I subtract a full gallon to allow an additional margin of safety. I use the fuel dip stick as a cross check, recognizing that the dip stick is inserted into the filler opening until it hits the bottom of the tank. Because of the wing dihedral, I do not register any fuel on the dip stick when it gets down to about 3 gallons. Also because of the wing dihedral and lateral strut variances, filling to the TABS can provide much variance as well (given the tank design, to the bottom of the TAB, even with the TAB, or just covering the TAB can result in a gallon or more difference in reading).
As a further note on creating a fuel dip stick, if one just drains the tank using the existing fuel sampling drain plug, it should empty the tank completely and some in the outlet line as well. When you start to fill it back up, some of the fuel will go to replenish the unusable fuel as the tank outlet hose fitting sits slightly above the bottom of the tank and the outlet hose may have a hump in it as it goes toward the landing gear.
I am a little surprised that unusable fuel is only 0.4 gallons on my model and year, given the plumbing arrangement and location of the fuel selector, gascolator, and carburetor float bowl. I would think there would be more fuel left in the tanks, lines and bowls (hence my additional margin). The 1973 and after Cherokees have 12 pounds (2 gals.) of unusable fuel stated in their TC. Not sure what changed in the fuel system design, but that is why I set my totalizer for 48 gallons total usable despite the Type Certificate saying I have 49.6 gallons available
At any rate, any variations in fuel stick and fuel gauge readings are going to be affected by the stance of the aircraft on the ground and fuel unless the aircraft is level. The dip stick and fuel gauges really can't discern the amount of unusable fuel, so it appears to be a benign question. The first question remains a curiosity however.
Type Certificate 2A13 identifies unusable oil to be 1.8 lbs (~1 qt) and unusable fuel 2.2 lbs. (~.4 gal) for weight & balance purposes.
My questions are:
(1) If the oil dip stick registers 6 quarts in the engine, does that include the 1.8 lbs. (1 qt.) of oil deemed unusable, or are the markings on the dipstick calibrated to include the unusable oil? I've not incrementally checked when filling the engine so hoping others have?
(2) If one makes a dip stick for the fuel tanks or purchases one, does it include the 2.2 lbs. of unusable fuel or not? Actually, I'm more interested in where does the 0.4 gal unusable fuel remain and how can our fuel systems be so good at sucking out all that fuel?
I believe I can answer question 2 easily enough as noted below. Not sure about question 1.
A previous thread had some discussion on unusable fuel.
https://www.piperforum.com/threads/fill-to-the-tabs.24945/This thread adds to that discussion, as there may be some additional clarification required for the previous thread discussion.
When one wants to create a fuel dip stick, the process is to simply drain the tank and per the Piper Maintenance Manual (PMM), fill in 5 gallon increments and mark your stick accordingly and check your gauges. Keep in mind. I believe that one is draining some unusable fuel in the process. The previous thread was prompted by the OP noting variations in his fuel readings and fillings. What is important to understand, I think, is the variability allowed in marking your dip stick and correlating the fuel gauges. Per the PMM, the following applies when calibrating:
The other important factor to consider is that when you are calibrating the fuel transmitter, the airplane needs to be level (horizontal and lateral), which for the most part means it is leveled using jacks to replicate inflight level flight. It is essential to level the aircraft when calibrating your fuel gauges.
However, if you calibrate your fuel stick (I used a paint stir stick) to the level position, your markings may exceed the variance allowed when your aircraft is sitting on the ground, because most Cherokees are going to sit differently given the nose and wing strut extensions. Leveling the aircraft and marking the dipstick in that position is going to result in a different indication when the aircraft is sitting on the ground.
So rather than mark my dip stick with the aircraft level, I marked them using the sitting-on-the-ground position (noting my strut extensions on nose and wings).
I use the fuel gauges (despite the variance allowed) for determining fuel on board. I generally fuel to either the TABS (18 gal, 36 total) or top off (50 gal) and subtract the 0.4 gallons for when I set the fuel totalizer. Actually, I subtract a full gallon to allow an additional margin of safety. I use the fuel dip stick as a cross check, recognizing that the dip stick is inserted into the filler opening until it hits the bottom of the tank. Because of the wing dihedral, I do not register any fuel on the dip stick when it gets down to about 3 gallons. Also because of the wing dihedral and lateral strut variances, filling to the TABS can provide much variance as well (given the tank design, to the bottom of the TAB, even with the TAB, or just covering the TAB can result in a gallon or more difference in reading).
As a further note on creating a fuel dip stick, if one just drains the tank using the existing fuel sampling drain plug, it should empty the tank completely and some in the outlet line as well. When you start to fill it back up, some of the fuel will go to replenish the unusable fuel as the tank outlet hose fitting sits slightly above the bottom of the tank and the outlet hose may have a hump in it as it goes toward the landing gear.
I am a little surprised that unusable fuel is only 0.4 gallons on my model and year, given the plumbing arrangement and location of the fuel selector, gascolator, and carburetor float bowl. I would think there would be more fuel left in the tanks, lines and bowls (hence my additional margin). The 1973 and after Cherokees have 12 pounds (2 gals.) of unusable fuel stated in their TC. Not sure what changed in the fuel system design, but that is why I set my totalizer for 48 gallons total usable despite the Type Certificate saying I have 49.6 gallons available
At any rate, any variations in fuel stick and fuel gauge readings are going to be affected by the stance of the aircraft on the ground and fuel unless the aircraft is level. The dip stick and fuel gauges really can't discern the amount of unusable fuel, so it appears to be a benign question. The first question remains a curiosity however.
