Fuel return line

by in 912 / 914 Technical Questions 13 years ago

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Re: Fuel return line

by Roger Lee » 12 years ago

Roger Lee

127 5.2k 4.78k

Hi Jay,

Quote:

"Lastly, I am also replaceing the 12 year old rubber vibration isolator system on my plane. Their capacity to minimize vibration is, by the book, obsolete and may be a major cause of my venting problem. Rubber mounts are supposed to be replaced every 5 years. The bottom line is, Rotax reccomends that fuel return line for a reason."

Now if we can get everyone else to follow your lead.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Fuel return line

by Roger Lee » 12 years ago

Roger Lee

127 5.2k 4.78k

Hi Jay,

Quote:

"Lastly, I am also replaceing the 12 year old rubber vibration isolator system on my plane. Their capacity to minimize vibration is, by the book, obsolete and may be a major cause of my venting problem. Rubber mounts are supposed to be replaced every 5 years. The bottom line is, Rotax reccomends that fuel return line for a reason."

I hope many others will follow your lead.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Report
Re: Fuel return line

by Ivor Floyd » 12 years ago

Ivor Floyd

0 11 0

I have a SportCruiser and have had a fuel pressure problem these last few months. The following changes have been made:
1. New fuel pump fitted
2. New pressure sender
3. Position of pressure sender changed to the fire wall on the side of the engine and as high as possible
4. Fuel return line connected to the gascolator

I am still experiencing low fuel pressures at times and suspect that it may be fuel vaporization. The fact that the fuel line connects to the gascolator and not to the fuel tank may, in my opinion, not resolve the issue of fuel vapor because the fuel remains in a closed circuit and is not released to normal atmospheric pressure as would happen if connected to the fuel tank. Does this make sense?

Thank you said by: Michael Saunders

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Re: Fuel return line

by Roger Lee » 12 years ago

Roger Lee

127 5.2k 4.78k

Hi Ivor,

The return line was a good addition. Back to the tank is a good choice, but many do go back to the gascolator. The entire fleet of Flight Design CT's entire fleet as well as others have return lines to the gascolator and do not have an issue with vaporization. It possible to be vaporization, but not a high suspect on the list of possibilities unless your fuel line is unprotected and not near a major heat source. A couple of things here. One make sure the fuel line is in fire sleeve. This is for fire protection, but it also helps insulate the line. Second you need to put a dependable mechanical gauge in line and check it against the other fuel pressure gauge. When Rotax tested the new fuel pumps they also discovered how inaccurate most pressure gauges and senders where right from a the factory (i.e. VDO and others). When you are talking about 2.2 - 5.8 psi then a pound or two inaccuracy is a lot. Last is that the senders and gauges are seriously affected by poor grounds on the plane. This is a must check to make sure this isn't an issue. Gauges that bounce all over are usually a poor ground or bad sender and the sender can be bad out of the box. You need to check it against that mechanical gauge. If an oil pressure or fuel pressure is bouncing around on a gauge it usually isn't the engine, but a bad component.

Here is why we notice fuel pressure issues sooner than other higher pressure and even higher temp problems.

The VDO fuel pressure sender that many of us use has a working range of 0-30 psi. If you had it in a service that read 20 psi then if it was off by 2-4 psi high or low you would never really know or even notice. If that same sender is used in a service with really low pressures like our fuel pressure of 2.2 - 5.8 psi then a 2-4 psi variance may be very noticeable and even unsettling. This now becomes important to do a check with an independent mechanical test gauge for a comparison to see if it is the engine system or a component. Usually it is a component or poor ground.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Fuel return line

by Ivor Floyd » 12 years ago

Ivor Floyd

0 11 0

Hi Rodger,
Thank you for your reply.
Firstly, the fuel lines in my aircraft are all protected with fire sleeves and the new pressure sender readings compare with the one that was replaced. The reading is steady under most circumstances and increases when the electric pump is switched on.
After the sender unit was changed and relocated to the firewall, I took the aircraft for a run-up and high speed taxi test, then returned to my hangar. Fuel pressure readings were 0.35 bar and a bit higher with the electric pump on.
At the hangar I switched off the engine for 5 or 10 minutes, restarted without problems and taxied back to the runway. on the way I noticed the fuel pressure drop and again returned to the hangar. The engine was run at 4000 rpm and the fuel pressure eventually fell to zero and the engine started missing from what I assume was fuel starvation. I think the fuel flow readings were also eratic but I did not raelly take notice of this.
Two days later I again took the aircraft and took off immediately after run-up checks, electric fuel pump on. The pump was later switched off without problems. The flight time was only about 20 minutes, but the fuel pressure was again about 0.35 bar and slightly higher with the electric pump on. It was an early morning flight and outside temperatures were mild. I have not had the opportunuty to fly again but am nervous to undertake a long flight.
Any thoughts on the problem and why do you seem hesitant to consider fuel vaporization.
Regards,
Ivor

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