Fuel question for the Aztec pilots.

[Jdm]

I know a few on here with thousands of Aztec hours. Please tell me your tank strategy. It’s a PA-23-250 F model.
In particular, which tanks are you using during takeoff? Assuming all tanks are full that is. The question comes up because of slight contradictions between the POH and popular after market checklists, including the ones given out at Aztec specific training schools. The POH simply says FUEL-ON, CROSSFEED OFF, which implies the fuel could be ON either inboards, or outboards. The aftermarket checklist says to takeoff on the inboards and then switch to the OBs in cruise.
My preference (assuming full fuel) has been to takeoff on the OB’s and then switch to the IB’s late in the flight when the OB fuel is close to empty, or prior to landing, whichever comes first. The reason I like this strategy is there’s less transferring required, less wear and tear on the leaver mechanisms, and less opportunity for mistakes.

 

[mondtster]

There were a few different fuel tank configurations on Aztecs, so some of the suggestions may create confusion unless that is considered.

However, with that said, the most common configuration that I am aware of is the Aztecs that have 4 fuel tanks, two in each wing. 36 gallons each, 144 total. All 4 tanks are main tanks so you can take off on either inboards or outboards. For a normal trip I take off on the inboards and run them to the top of the climb. Once in cruise, I switch to the outboards and run them to empty or until I'm in the terminal area getting ready for an approach. Then I'd switch back to the inboards for landing.

I don't see anything wrong with your method. I've done as you suggest as well, in some circumstances.

 

[AggieMike88]

In the Aztec I have been flying for my multi engine time, I start, take off, climb, and initial cruise on outboard tanks. Switch to inboards when the fuel indication is showing about 25%.

Switch method is one side at a time: electric fuel pump on, move the tank lever, fuel pump off, verify engine remains happy. Repeat for other side.

Cross feed remains in off position entire flight.

 

[Jdm]

Mondster’s method is apparently the way they teach it in some of the flight schools. Problem is I can’t figure out exactly why people do this if they don’t have to. Seems like a lot of switching. I’ve heard a few theories but nothing I can document.
AngieMikes method is the way I’ve evolved into managing the fuel. Except I run the OBs a whole lot lower than 25% but we do some very long flights and I don’t need to leave anymore in the OB than I have to.

 

[Jdm]

Btw: This Aztec F holds 56gal in the OB’s and 36gal in the IB’s Total 184gal, 177 usable.

 

[mondtster]

Mondster’s method is apparently the way they teach it in some of the flight schools. Problem is I can’t figure out exactly why people do this if they don’t have to. Seems like a lot of switching. I’ve heard a few theories but nothing I can document.

The simple answer on why I’ve always done it the way I do is simply a carryover of procedure from the Baron, which had main and aux tanks. The Aztec is an oddity among light airplanes in that all the tanks are mains rather than the outboards being aux tanks.

I was under the impression that the F models with the bigger tanks had the outboards as aux only but perhaps this is not correct. I haven’t seen one in that configuration in ages.

 

[Jdm]

I was under the impression that the F models with the bigger tanks had the outboards as aux only but perhaps this is not correct. I haven’t seen one in that configuration in ages.

Thanks, that is a good point. I’ll have to check the actual POH on my next flight.

 

[Warren Dunes]

Why?

Like many questions this one goes to history.

The original Apache fuel system was two 36 gallon bladders, one per wing ouboard of the nacelles. Each engine had a mechanical pump, each sump had an electric pump. With the fuel selector you could send fuel from either bladder to either engine or the opposite bladder. Fuel return from engine was to the same side (main) bladder regardless of its source.

The only time to have cross feed on is when actively moving fuel side to side.

Then the two 18 gallon auxiliary bladders were added. They were plumbed directly to their respective mains.

Piper was frugal shall we say, never changing anything they didn't have to. There was no need to change the fuel selector, so Piper did not.

On the other hand operators have to establish procedures that will work in every situation with every craft they posses. Inefficiency will be embraced to help prevent disaster.

 

[Kristin]

Neither the Apache nor Aztec ever had any fuel return to the tanks. That is only on Continental engines as their fuel injection system works that way. The Lycoming/Bendix system has no fuel return.

 

[kgruber]

Structurally, it is better to have weight in the outer tanks.

 

[mondtster]

Neither the Apache nor Aztec ever had any fuel return to the tanks. That is only on Continental engines as their fuel injection system works that way. The Lycoming/Bendix system has no fuel return.

On Aztecs that is true, but for the record, there are some Lycoming/Bendix systems on other airframes that do return fuel.

 

[mondtster]

Structurally, it is better to have weight in the outer tanks.

Ive wondered if that was part of the reason Piper added larger outboards later on in the run of the Aztec. The flight manual may allude to this through their recommended procedures.

Almost all my Aztec flight time and maintenance experience is with D and E models so I’m not real familiar with the F model.

 

[Warren Dunes]

Structurally, it is better to have weight in the outer tanks.

I've never flown an Apache/Aztec with the big tanks, but in the Comanche, "outboard" fuel is essentially "free" (does not count against payload).

 

[Kristin]

On Aztecs that is true, but for the record, there are some Lycoming/Bendix systems on other airframes that do return fuel.

Which ones? I have never run into one. The system is just not designed to allow that from the fuel metering servo.

 

[Kristin]

I've never flown an Apache/Aztec with the big tanks, but in the Comanche, "outboard" fuel is essentially "free" (does not count against payload).

That is only true for the early Comanches that have the STC'ed tip tanks. Those do boost the gross weight, though you don't have to have fuel in them to get the increase. In the Twin Comanches, tip tanks boost the GW by 125 lbs, but that extra 125 must be carried in fuel.

 

[mondtster]

Which ones? I have never run into one. The system is just not designed to allow that from the fuel metering servo.

Some of the servos do. Specifically the Bendix RS servos. The more common RSA servos do not, which is what everyone thinks of.

I can tell you with certainty that the IO-540-A1A5 and IO-540-K1C5 have fuel servos that most definitely return fuel. Neither of which are obviously found on an Aztec, but still interesting and worth mentioning.

 

[Jdm]

I was under the impression that the F models with the bigger tanks had the outboards as aux only but perhaps this is not correct. I haven’t seen one in that configuration in ages.

I checked today. They are not designated as “aux only” tanks. The poh simply refers to them as “fuel” tanks.

 

[Kristin]

Some of the servos do. Specifically the Bendix RS servos. The more common RSA servos do not, which is what everyone thinks of.

I can tell you with certainty that the IO-540-A1A5 and IO-540-K1C5 have fuel servos that most definitely return fuel. Neither of which are obviously found on an Aztec, but still interesting and worth mentioning.

Thanks! Learned something! Pretty rare. Never mentioned in A&P school, which was almost 40 years ago, and never ran into them in any of the common products I have worked in, particularly Pipers. Which airplanes got these systems?

 

[mondtster]

Thanks! Learned something! Pretty rare. Never mentioned in A&P school, which was almost 40 years ago, and never ran into them in any of the common products I have worked in, particularly Pipers. Which airplanes got these systems?

Yeah, they're fairly uncommon. I've ran across a few airplanes that had this style fuel system but the only one I can think of off the top of my head is the Dornier Do-28-B1.

 

[Kristin]

Yeah, they're fairly uncommon. I've ran across a few airplanes that had this style fuel system but the only one I can think of off the top of my head is the Dornier Do-28-B1.

That is an understatement. I have never seen one of those puppies, let alone worked on one. It seems that the RSA was an improved version of the RS and must pretty well have taken over by the early 60's. As best I can tell, there may be versions of the RS servo that return fuel and versions that don't, but am not 100% on that. I have enjoyed the digging though. Always fun to learn new things.