H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.
OK. Dauber Wasp nests removed from hangar... appeared to be unoccupied.
I never saw any wasps buzzing around the hangar.
Visually inspected the vents thoroughly with LED light... clear.
Got the attached syringe ($5 at PepBoys)... blew air into the vents... clear.
Inserted a doctor style Q-tip swab 5.5"... apparently clear.
Again the low fuel pressure indication occurs on both tanks... and continues with banking source tank high.
Unlikely its a vent problem.
Had to delete mine because Designrs deleted his, rendering my reply nonsensical. Which is not unlike most of my other posts, now that I think about it.
Last edited by Merlinspop on Thu Oct 22, 2015 10:38 am, edited 1 time in total.
The Rotax mechanical will likely come with ne fittings. If the electric doesn't come with fittings I would use the old, unless I thought there was something wrong with them.
Guys, I need help and moral support. Not feeling good at all here.
Have been battling the indicated fuel pressure issue for months.
Just replaced the fuel pump.
Removed Gascolator, disassembled fittings and thoroughly cleaned, connected and reinstalled.
Replaced two fuel hoses on the supply side.
Still have low fuel pressure warnings...
It is severely impacting enjoyment of the aircraft. It could be Dynon. It could be a hose. It could be a wire / harness. I feel as if I'm shooting in the dark against logic... like the cause could be related to air in the tires.
I just can't take it anymore. I could be tearing into my fuel system over bullsh*t Dynon warnings, or I could have a legitimate fuel pressure issue.
** The biggest thing is that I am not capable of dealing with ongoing persistent non-specific troubleshooting. **
Maintence I can handle. A problem I can handle. A this or that situation I can handle... but the "could be anything... can't find it..." this I can not deal with. If something is not right, and can't be fixed, it totally kills all enjoyment completely.
On top of that my annual is coming up.
There's a new SB that just came out for firewall nose gear reinforcement (add support brackets, rivets, drilling... 8 hours.)
Rubber replacement is coming up soon, depending upon how I count the 5-year start date... (First drop of fuel in Europe, or airworthy date in the USA?) I'd like to do the rubber incrementally.
Everything at once... I've got to spread out the maintence.
On top of that I feel like the indicated fuel pressure issue will not even be resolved.
So I can't even enjoy the aircraft at all.
I need help so bad. It's really getting me down.
Suggestions? Thanks.
The only thing you can do at this point is put a mechanical pressure gauge in parallel with the electric sensor, and fly a bunch with it, until you start getting low pressure alarms. I know of no other way to determine if the problem is real, or a false indication.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV [email protected]
AvSport.org
facebook.com/SportFlying
SportPilotExaminer.US
I know exactly how you feel, I've had the same problem intermittently for years. And so have a lot of other pilots. The interesting thing is, I'm not aware of a single forced landing or crash due to this "low fuel pressure" phenomenon. Anybody?
Like you, I've done everything practical to isolate the problem. New fuel pressure sensor, new pump, new fuel pickup assembly, careful inspection of the fuel system. I have NOT installed a separate mechanical gauge but I trust my Grand Rapids EIS because all the other readings (oil pressure and temp, CHT, manifold pressure, EGT, system voltage) are stable and in the ball park. Why would your Dynon and my GR EIS and a lot of other engine monitors give a false fuel pressure indication when everything else works properly??? Note that it isn't just Dynons. I think we have to look elsewhere than the EIS, whatever make you happen to have.
Installing a separate mechanical fuel pressure gauge would require penetrating the firewall with a hose and reworking the plumbing downstream of the fuel pump. I suppose you could disconnect the hose from the fuel pressure sensor and connect the hose from the mechanical gauge there, but you still have to get through the firewall. My opinion, it's not worth it. Personally, I'm willing to bet that you would see the same sort of indications on the mechanical gauge.
Another observation: the fuel pressure sensor is connected to that 5-way "octopus" that sits on top of the engine, NOT to a carb inlet. It's possible (but unlikely) that this causes the erratic fuel pressure indication. The octopus incorporates a restrictor which bypasses fuel back to the main tank to prevent vapor lock... I've seen automobile applications where this restrictor is thermostatically controlled and opens up when it gets hot... that could cause a fuel pressure drop, but the restrictor on the Rotax is just a fixed orifice (I've heard it's really a main jet out of some carburetor, don't know this for a fact). Whatever, the Rotax has a "different" setup than most fuel systems but why it would cause low pressure indications on low-wing airplanes and not on high-wing airplanes is a mystery. However, there is one significant difference between high-wing and low-wing configurations: on a high-wing the fuel is under constant pressure from the pressure head in the fuel tank, where on a low-wing the fuel is "sucked" out of the main tank by the pump. Just suppose that in a high-wing, the constant pressure in the fuel plumbing prevents vapor lock, while in a low-wing you get an occasional slug of vapor going through the octopus. This could conceivably be reflected in the line that goes from the octopus to the fuel pressure sensor, without impacting the fuel pressure at the carb inlets. That's what the octopus is for, after all. Just a guess.
This problem or situation seems to be endemic to low-wing airplanes with Rotax 912ULS engines. Rotax engines have Bing carburetors. Bing carburetors are designed for motorcycles, where the fuel tank is above the carb and fuel is gravity fed to the carb. I have that setup on MY motorcycle... 3 gallon fuel tank feeding two Bing carbs by gravity. I have no way of measuring fuel pressure on the bike, but I've put over 30,000 miles on it with NO fuel starvation issues (including some time at 10,000 feet, up in the mountains east of here). I've never heard of a low fuel pressure indication on a Flight Design CT, and there are a lot of them flying around. High wing airplane (or motorcycle), gravity feed to the Bing carbs, no problemo.
A suggestion: be comfortable with dead stick landings, then go up over a convenient airport, get some altitude, cruise til you get a low fuel pressure warning, and just LET IT BE and see what happens. If the engine stops, glide down and land. If the fuel pressure comes back up by itself, go on your way. I've done this twice, and both times the fuel pressure came back up in a few seconds with no help from the aux pump.
I did have one flight where the fuel pressure would come back up immediately with the aux pump, then go right back down as soon as I turned the aux pump off. This went on for about 20 minutes, then the fuel pressure went back up and stayed up. I was over the Grand Canyon and didn't want to experiment, but the point is, whatever was going on, the aux pump brought the pressure back up above 2.2 PSI every time. I ALWAYS check the aux pump as part of every preflight. Facet doesn't publish a MTBF on these pumps, but I doubt mine has 10 hours on it total. If it works when you take off, it will likely be available during a normal duration flight.
Note that the Vans RV-12 (low wing, Rotax power) wires the aux pump to the master switch so it is on all the time. If my aux pump were wired that way, I would never have seen a low fuel pressure indication. I don't know what the reliability of the modern Facet pump is... in the old days, they used mechanical points which would eventually burn and wear out and the pump would fail. The modern ones use a big transistor to switch the solenoid that does the pumping. Transistors fail too, but nowhere near as often as those points did. I would guess Vans did some research and are satisfied that the Facet pump can be left on all the time without compromising its reliabilty.
Here's my bottom line: This low fuel pressure business is a serious annoyance that can drive you nuts, but in my opinion it is NOT a safety-of-flight issue. It happens on too many airplanes, and the fix or bandaid or whatever is to turn on the aux pump as needed. As I mentioned, I am not convinced the low fuel pressure situation is a problem... if I let my airplane go for however long it takes it to bring the fuel pressure back up by itself (a few seconds, 2 out of 2 tries), the engine does not hiccup or falter. The REAL test is fuel flow. If fuel flow continues at a normal rate with "low" fuel pressure, then low fuel pressure is a non-issue. We have no way of measuring fuel flow but it would be possible to install a fuel flow meter. They are available, and I'd rather penetrate the firewall with a couple of wires (or use an existing penetrator) than try to run a hose with gasoline under pressure into the cockpit.
All the above is strictly the opinion of one amateur. I'm not a PE or an A&P/IA and I take no responsibility for anything I've said. Sorry for the lawyerese, but this is the Age of Litigation.
bstrachan wrote:I know exactly how you feel, I've had the same problem intermittently for years. And so have a lot of other pilots. The interesting thing is, I'm not aware of a single forced landing or crash due to this "low fuel pressure" phenomenon. Anybody?
All the above is strictly the opinion of one amateur. I'm not a PE or an A&P/IA and I take no responsibility for anything I've said. Sorry for the lawyerese, but this is the Age of Litigation.
Barrie Strachan
2006 StingSport N2772N
I started this thread, with no idea how prevalent the issue was ... I thought it was just my issue.
As you say Barrie, I'm convinced it's an anomaly that's not going to be solved soon. My 2007 Sting with a TruTrak PFD, gives me the exact same indications. If I flip on the Electric Fuel Pump, the pressure usually comes right up, but no always.
With the usual disclaimers about not being an expert of any sort ... I've flown for 10 minutes with the fuel pressure showing "0". I can assure you, there was some pressure there, because that big fan looking thing on the front of my airplane, kept turning.
