Alex,

To be more exact using standard atmospheric conditions at 25,000 ft the ambient pressure is 5.46psi. With the 340 pressurization max differential of 4.2psi the cabin will be at 9.66psi which is about 11,110 ft. (Using linear interpolation between 10,000 and 15,000 ft.). Certainly not 8,000 ft.

Bill

Alex,

Whoops sorry, delete my last sentence..... Bill

Not necessarily a good idea. What you need to do is understand the system before doing this.

Here's why - if you do this with nearly full mains and/or before the mains get below 33 gallons per side at 38gph, you'll end up porting fuel out of the sniffle valves. When you run off of the Aux tanks the return fuel goes to the mains. On most 340s, you'll run the Aux tanks dry after about 45 minutes. That is you will have consumed 28.5 gallons in that time, and the rest of the 63 gallons on the aux tanks will have gone to the mains (34.5 gallons or 17.25 per side). That means that if you have more than 32.75 gallons on the mains, the excess will be dumped once you reach 50 gallons on each main (full capacity).

That also means you now have less fuel than you planned for which could become a bigger problem than the fact that you just dumped $$$ overboard.

This is fuel flow dependent, but you get the idea and can do the math depending on what your FF is.

Fair enough - I was going from memory, but you are right.

Alex,

I should have said more about the switch from mains to aux tanks..... The auxs are 31.5 usable, and the return from the engines is less than 50%, so you need to be down about 15 gallons on the mains. You just need to insure the TO, climb to altitude, and initial cruise take 15g or a bit more. As you said that depends on fuel flow, and certainly time to get to the cruise altitude.

I owned a 340 for 17 years and never had any problem with my fuel management procedure.

Bill

Bill - I think we are talking about the same thing - just wanted to run the numbers as this technique works when cruise is in the flight levels but could lead to trouble if leveling off low and there is not enough space on the mains to take in the excess fuel.

If you record the fuel periodically during the transfer you'll catch this as well as any other issues that might arise.

Just to play devils advocate, you could have never had any problems and also blown 5 gallons out the tank on occasion. It would not have caused a problem unless you had to fly well into your reserve fuel.

Hi again Alex,

I agree, we're saying the same thing, namely that there has to be enough room in the mains for the fuel return when running on the auxs. To be sure of not dumping fuel you could up the initial use from the mains, to say 20gal.

BTW, the time to run the auxs dry is much longer than 45 minutes. I would run conservative cruise settings of 30"MP, 2100rpm, peak EGT (low 60s% power) which gave 15gph on each engine. It then took about two hours to empty the auxs. Even at 75% it would approach 1 1/2 hours.

Bill

Sorry, that did not read how I wanted it to. The 8,000 foot cabin is at 20k. That is the "sweet spot." The cabin climbs with you above 20k.

Hi Charles,

I have had that thought but I don't think it happened. Although I never ran anywhere close to really low reserves. Dumping as much as 5 gal might leave fuel stains on the sniffle valves......? (which I never saw)

Anyway as I added, perhaps it would be smart to increase the main tank use to 20 gal to be sure.

Thanks for your comment......

Bill

I don't agree with this. I actually ran them dry once, (which is not an approved procedure by the wife). Right at about 50 minutes the way I run it per RAM. Jerry Temple says 50 minutes as well.

Yea, I was taught the mixture off and throttle full running fuel with prime and have not done that yet. I just didn't have the balls to run the fuel through it like that. But, obviously that is the way to go.

Congrats on the new airplane! A friend had one many years ago and enjoyed it.

Great to see you and Bill land in Nebraska a week or two ago.

Enjoy the new ride and try not to think about the bottom line too hard.

Hi Tom,

I completely understand the wifely approved procedures. I was usually flying with my two sons who are both pilots (older is a multi engine CFII).

Also mine were stock engines, not RAM, although I'm not sure how that affects low power settings. ??

Bill

I'm sure you know this but have someone watch you run the pumps with the mixture off sometime just to make sure they are actually all the way off and not leaking anywhere. Makes me feel much better about doing it than just hoping I'm not seriously flooding the engines.

Also so glad to hear your experience! The view out of the pilot seat is very nice huh? Especially on the ground.