I’m about to get my Aztec weighed for a fresh new W&B. From what I can tell it’s never been weighed since it was mfg and even then it was probably a rubber stamped weight taken from a comparably equipped model. My current W&B is the product of a 1” stack of adds deductions taken over half a century.

Questions for those that have performed this process. Mine is the standard 4 tank configuration. Poh says 36 gallons / tank. 35 usable / 1 unusable. (144/140). Are those numbers fairly accurate for real world measurements? Also, and this question isn’t unique to Aztecs. What is “full” when filling the tanks. The thick rubber caps are about 2” thick (deep). I’m not sure if it matters but I try to fill the tanks to where the fuel is close but wouldn’t be touching the bottom of the rubber stopper (if I’m filling the tank and not planning to immediately fly).

I know these questions aren’t likely of a lot of practical value but I can’t help but ask.

If you want the most accurate weight & CG, drain all the fuel you can including rocking the wings with the drains open then add the POH specified "unusable fuel". If there's a spec for "undrainable" fuel you could subtract that from the added unusable fuel although the undrainable fuel probably isn't significant enough to bother with.

After doing the weighing, top each tank with an accurately metered pump, stopping at the POH spec'd quantity for that tank accounting for the unusable amounts. If the tank won't take the full amount you'd need to choose between using the actual capacity or fixing whatever is causing the loss of capacity. But if you can add the correct quantity, the level you see looking into the filler is the "full" level.

DRAIN THE FUEL!

It is pretty easy to drain all the fuel from an Aztec. With the plane leveled, every drop of fuel will come out through the sump drains.

There is no way to get an accurate weight by filling the tanks and subtracting the fuel. You can be off by as much as 80 pounds using that method. And the CG will be incorrect.

Once empty, you add one gallon per tank, that is the unusable fuel. Then weigh the plane.

DRAIN THE FUEL!

It is pretty easy to drain all the fuel from an Aztec. With the plane leveled, every drop of fuel will come out through the sump drains.

There is no way to get an accurate weight by filling the tanks and subtracting the fuel. You can be off by as much as 80 pounds using that method. And the CG will be incorrect.

Once empty, you add one gallon per tank, that is the unusable fuel. Then weigh the plane.

The Eagle drains only drain unusable fuel when the plane is tail low. Very low. I think they're a only helpful for improper fuel cap maintenance, and folks who don't keep the struts at the correct extension.

I measured the difference in height between the fuel cell sump, and the aft corner of the fuel cell. You have to have at least two inches of differential ( the nose strut one inch higher, and main strut one inch lower) to get any fuel to an Eagle drain when the main sump is at the minimum level. And then another inch of differential to get any water out the Eagle drain that the main sump couldn't get.

I tried to sell myself on Eagle drains on a previous Apache, but after careful consideration, I decided it was better just to operate the plane as Piper intended.

I respectfully disagree and I think we have had this discussion before.

Water contamination in PA 23s has been a common and widespread problem and has caused many crashes. The fact is that when almost all PA23s are tied down properly they do hold undrainable fluid in the aft inboard corners. Most of the time when you exit an Aztec the nose strut overextends and stays there. A small wrinkle, which is very common, in the bottom of the cell can hold back quite a bit of fuel even when level. One gallon of unuseable and undrainable fuel in each tank seems about right to me plus what is in the strainer reservoirs and crossfeed line.

The fuel cap access door seal with a single camloc fastener is prone to leaking as are the rubber thermos bottle type caps. Even if maintained properly. And to add to that mud daubers can plug the scupper drains that double as fuel vents in a very short time. When this happens the water can back up in the scupper and overflow through the open vent hole in the top of the fuel cap. The early models have shorter cap receptacles and unless the plane is parked tail high and exactly level water can enter the tank around the thermos caps before reaching the level of the scupper drain. Unless these expensive caps are brand new and adjusted properly they do not seal well at all.

The FAA has recognized the problem and issued the below referenced AD to address the known issue by installing wedges in the aft inboard corners under the fuel cell to act as ramps The AD was later rescinded because it caused the vertical corner of the fuel cells to kink and many eventually leaked. The Eagle drain STC is a superior solution to the problem.

https://www.govinfo.gov/content/pkg/FR- ... -23215.htm

I recently picked up an Aztec that had been sitting outside for a bit. I drained all of the quick drains and the pressure crossfeed. On takeoff roll one engine quit and I aborted. I was able to restart it and return to the ramp where I drained additional water. I shook it and taxied it some and got more water out before flying it to my place and installing the Eagle drain kit. I later learned from the line guy that the owner had previously hired someone else to take it for an annual and an engine quit on takeoff and he had to be towed back to the ramp where he left it.

I have ferried many many PA23s and find water in almost all of them. Even ones that are hangared. One of the first few was an Apache. I kept getting water and drained the carb bowls a couple of times after taxiing and shaking it. When I thought I finally got it all I took off from Leesburg across the Ocala National Forest. It was a little bumpy and some trapped water made it to one engine and it quit in the middle of nowhere. I was able to pump the water out using the accelerator pump while descending. When it finally purged and caught I got it back to a safe altitude and the other did the same. Had I feathered it I would have been in the trees.

After that I developed a technique to syphon the bottom undrainable corners with aquarium tubing secured to a welding rod. I have had to replace split fuel lines and collapsed carb floats from freezing and have found triangles of ice in the tanks before. To date I have parted out about 175 PA23s and quite a few were crashed due to undrainable water in the fuel.

A couple that I have sold were later lost due to water in the fuel. Used caps are never in good enough condition to sell and good fuel selector valves, fuel strainer bowls and crossfeed drains are frequently ruined from water contamination and resulting corrosion.

I have retried and salvaged many other models and the PA23s are by far the most prone to water contamination. I am surprised that there is not an AD mandating fuel drains in the inboard aft corners of the tanks.