Low fuel pressure... again

[Brichards]

For the last 18 months, I have been plagued with low fuel pressure at altitudes as low as 6000 feet. The problem usually occurs after flying at two or three thousand feet for thirty minutes or so, stopping for thirty minutes to pick up passengers then climbing out to a cruise altitude for a 3 hour flight. Three weeks ago I noticed that upon hearing the low pressure warning and switching on the electric pump, the egt's would momentarily rise sharply then return to normal. This seemed to make sense since a low pressure condition should create a lower fuel flow.

Yesterday, flying alone, I climbed to nine thousand and upon receiving the initial alarm at six thousand feet, left the electric pump off and noticed that the fuel pressure would bottom out at about 10 pounds, sit there for a few seconds then recover to 22 or 23 pounds. I also noticed that immediately before the pressure recovered, the fuel flow would increase from 10 gallons per hour to 12.5 gallons per hour which seems to blow out my conclusion above. This problem repeated itself three times at 6500, 8000 and 9500 feet on yesterdays flight within a ten minute period. The same fuel flow increase happened each time just as the mechanical pump seemed to begin recovering.

Barry Richards
N245DS

Last November, I replaced the mechanical pump during the annual inspection, however, the situation has not improved.

I am aware that Diamond recommends using the electric pump at altitude, however, I have seen this problem last as long as 2 hours and with the poor reliability record of the electric pump, I am afraid that I will find myself in a low fuel pressure situation with a failed electric pump. In my pinion, Diamond's recommendation is like a Cirrus parachute, a cheap fix to a weak design.

[Brian]

Barry,

I have had this problem too. Does switching tanks have any affect? Does it mostly happen when the tank has more than 10 gallons in it?

I blew some air through my tank vents and haven't had this problem since.

Good luck!
Brian

[Brichards]

Brian,

The tanks were full when I started the experiment, so I could not have burned more than 12 or 15 gallons by the time I landed. I intend to try this experiment again with a fresh data card in the MFD so that I have hard data to rely on.

There appear to be two vents per tank attached to the inspection panel. I have not yet looked at the schematic to determine if they are both vents but that can be easily rectified.

Thanks for the suggestion.

Barry

[Robin]

Brian

Thanks for the info.

Have you informed Diamond? We need to give them useful feedback.

Regards

Robin

[Brian]

Brian

Thanks for the info.

Have you informed Diamond? We need to give them useful feedback.

Regards

Robin

Robin,

I posted this data to the DAA (other Diamond) site a year ago but have not sent it directly to DAC.

If I had to speculate, I would say the check valve fuel vent may require just a little too much delta pressure to open (sticky??) and so the engine driven fuel pump starts to draw a vacuum on the tank. At high altitudes there is less differential pressure across the check valve due to lower ambient pressures. With full tanks there is less air volume and so the vacuum is drawn quicker and thus there is a higher frequency of occurrences with full tanks. One tank may be more prone to causing this problem than the other.

What I don't understand is why the pressure doesn't gradually decrease (unless the pump looses suction head at some point causing its discharge pressure to rapidly drop) and what would cause the check valve to pop open so soon after the fuel pressure drops--why wouldn't pressure stay low? There is another vent in addition to the check valve vent; does it not keep up with the fuel flow?

As I said, I blew some air through the vents a little over a year ago and haven't noticed any problems since.

Brian

[ThomasD]

Not much help to you I'm afraid, but I have been flying my DA40 (s/n 40.829, 300hours TT) at altitudes up to 14,000ft and regularly at 10,000ft without using the electric boost pump at all.
I know the POH says to use the electric boost at high altitude, but I watch the fuel pressure and it is well in the green (G1000), and never had any hiccups or low fuel pressure indications or warnings so never so far seen the need to use it.

Based upon my sample of one, it would seem you do have some issue as suggested by Brian?

Tom

[Kurt h]

I dont have much to add except that i occasionally experience the low press alarm when switching from the left to the right tank. Easily cured by running the elec pump for a few seconds. Cannot recall if it is only with full tanks, so I will check that.

Altitude must be at least 8000 to act up. Have not worried much since the elec pump or switching back to the left always fixes. I may try clearing the vent as described earlier.

[Brichards]

Barry,

I have had this problem too. Does switching tanks have any affect? Does it mostly happen when the tank has more than 10 gallons in it?

I blew some air through my tank vents and haven't had this problem since.

Brian,

I am going to the airport Saturday or Sunday and intend to try to clean out the vents with compressed air. I have a compressor in the hangar but I thought a better solution would be a can of compressed air such as that used for cleaning dust out of computers. The flexible tube that comes with the can is probably the right size to fit in the vent.

Any suggestions?

Thanks,
Barry

[Tim M]

This is a chronic problem with an DA40 that I regularly rent at KDTO (N204SG). There have even been occasions when the boost pump did not resolve the warning. As long as the fuel flow is normal I do not pay much attention to it anymore.

[Brichards]

Shot pressurized air through all 4 vents but suffered the same problem on my test flight. There were two failures on each tank so if the problem is a check valve, both are bad. My next step is to replace the mechanical fuel pump with a new, not rebuilt, pump. This problem did not occur for over two years so there must be a solution.

I logged the data from this flight but am not sure how to view it. I would appreciate any pointers in this regard. I plan to try looking at it in the morning.