Residual Fuel - 701 - with Downward Pitch

I am about to have my 701 finally inspected - next week I hope and have been calibrating fuel tanks, fuel flow etc and am a bit worried about the amount of residual fuel left in my tanks when simulating a downward pitch.

I have built a Czech Kit with the supplied fuel tanks - a bit larger than the American ones being longer at the "thin" trailing wing edge end. They hold 45 litres full and have the fuel outlet at the back inboard corner as with the "normal" plans.


I jacked my plane up on blocks and took the air out of the front tyre simulating a 10degree nose down pitch, then again at 8 degrees and 5.5degrees. My results are concerning! I allowed the fuel to be pumped out at each level (with my electric pump) until air came through, then returned the plane to level and measured the fuel remaining on my calibrated dip stick - results as follows:

At 10 degrees nose down, air is introduced with 33 litres of fuel remaining - ie at 70% full tank
At 8 degrees nose down, 22 litres remain - ie at almost half full
At 5.5 degrees nose down, 14 litres remain - ie at one third full

This was of course in the static situation with no wing movement / changing gravitational forces etc and one tank at a time.

My question is to you guys experienced flying 701's - Is this a real problem?? Can I safely make an approach with less than half tanks?

 

What should I be writing in my aircraft logbook as far as unusable fuel is concerned??

Would appreciate your collective wisdom on this

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Comment by Normand Lambert on November 3, 2011 at 7:35am

Hi Jock,

Since I was also concerned with pitch/fuel starvation issues, I did two things :

1) added a Boost pump for extreme pitch conditions (takeoff, landing). So, I turn it ON prior to takeoff and landing. That is just to be safe. For me, that is a non-Issue because I own a Piper Cherokee (low wing) and this is standard procedure.

2) My fuel tanks have 2 fuel outlets : one in the front inboard lower corner and one in the back lower inboard corner. I also run 2 fuel lines (one runs to the front with the "V" 4130 tube, the back one follows the door to the backrest) per side with a Tee fitting at the seat level.

So, for the downward pitch, the front outlets always get fuel and, for takeoff, the back outlets get the fuel.

By the way, I read somewhere that in takeoff mode (VERY STEEP attitude) it is possible to starve the engine because the fuel tanks could become LOWER than the carburator. So carburator should be as low as possible if a fuel pump isn't available. Just watch for that !  

My 2 cents.

Norm

Comment by Randy Owen on October 1, 2011 at 5:15pm

I agree with Gary. Your pitch attitude is not that low for approach and landing. That being said, I've tried some pretty steep descents at times, and never had a fuel flow issue doing it. I have the factory Zenith tanks. I hope your inspection goes well! Keep us posted.

Randy

Comment by Jock Struthers on September 29, 2011 at 3:10pm

Thanks for your comments Ralph and Gary - hopefully I can fly somewhere to meet up with one day soon.

I am sure I am not the only 701 builder to think about this.  The CZAW tanks are essentially the same as ZAC's but lengthed to fit against the rear channel in the wing to give more capacity rather than a seperate channel a little further in.  Both tanks drain from the back inboard corner.  The question arose when my engineer asked me what fuel calbrations I had done and referred me to a CAA microlight AD requiring calibration of unseable fuel at critcal angles.

 

I've seen a Savannah with a 10 litre collector tank attached to the back of the passenger's seat, a neat "reserve" tank which if adequately vented should solve this problem - if it is indeed a problem. I am heartened to hear from those already airborne such as yourselves that steeper nose down attitudes are not usual, however I keep thinking about that video by Niol Lockington simulating a steep approach over imaginary 100ft trees near the threshold.  With low tanks it would have been "land and park" all in one!

Comment by Gary Eady on September 29, 2011 at 2:56am

My Pegastol wing kit came with polyethelene wing tanks that feed from the wider front portion. This is great for descent but creates problems for steep climbouts at low fuel levels (under 20 litres). To cope with this I had a 15 litre stainless header tank welded up and that is mounted behind the firewall much as in the earlier 701s. No problems with fuel flow now at any attitude.

I did the same as Ralph for my unuseable fuel in the log books - calculated it at cruise attitude!! You will find that there isn't much of a nose down attitude in the landing configuration in any case. I would have thought less than 5 degrees.

Good luck with your inspection. You will find that it's a relatively painless excercise.

 

Cheers

Gary

 

 

Comment by Ralph Sanson on September 29, 2011 at 12:01am

I have the CZAW tanks and have had occasions during unusually steep descents when the engine failed if I was feeding from one tank with 15 litres or lower remaining and this was the wing-down tank at that time. Switching tanks or levelling off to the glide attitude restores power after a short wait. It has never been a problem on approach to landing, the nose down attitude is not that steep. Nowdays I do most flights feeding from "both" and be sure to feed from both for landing. I would like to fit a 5 litre header tank to each side which would give a sustained reserve when the outlets become unported.

 

My take on the CZAW fuel system design, it would have been less susceptible if the ZAC original plans were followed, where both tanks permanently feed a rather large belly drain unit. For me it was easier to use the supplied Andair selector valve. I know you are supposed to measure the unuseable fuel level and normally this would be at cruise attitude with some sloshing in the tanks?. In climb attitude there isn't a problem.

 

Ralph

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