Fuel System Questions

[Nev]

I am wondering where the fuel pressure sensor is in your system.

Ray, I need to check but I'm fairly sure that my fuel pressure sensor is plumbed in the vicinity of the engine driven pump. For that reason I'm not 100% certain that the low pressure reading was directly correlated with what we found. The data download showed the exact sequence of events was a decrease in fuel flow, followed by decrease in fuel pressure. Then increased EGT's which appear to have been accompanying the flow "bump" as everything returned to normal and caught up again. I think having the pressure sender read from the downstream side of the fuel servo would make a lot of sense.

[Ray Strickland]

Nev,
I am wondering where the fuel pressure sensor is in your system. There is a port in the side of the fuel servo screen housing. If that is where your sensor is plumbed, then any flow restriction in the servo screen could cause a low pressure reading. Glad you found the debris prior to an engine stoppage. Hopefully, all of the debris has been cleaned out such that the problem will not occur again.

[svyolo]

Having a "last chance" filter in the Bendix type system gives makes me more confident in that system, but I never heard of it before.

[Nev]

I always kind of questioned aircraft fuel system design where the gascolator/filter is upstream of fuel pump(s).

You've raised an interesting point because I'm not sure if gascolators and fuel pumps were ever designed to go together in this way. My limited understanding is that gascolators were originally designed as a combined filter/water trap in carbureted aircraft. Bob probably had this in mind, and those of us that have added fuel injection systems have then added fuel pumps downstream of the filter/gascolator. With a carbureted gravity fed system there's usually nothing after the gascolator to produce swarf.

[svyolo]

I didn't know that filter existed. I always kind of questioned aircraft fuel system design where the gascolator/filter is upstream of fuel pump(s). I have added a second, upstream filter on a couple of vehicles and boats over the years, but as far as I recall all of them had the factory filter downstream of the fuel pump.

[rodsmith]

That is great info, thanks for sharing it.

[Nev]

Fuel Servo Filter

When I did my fuel system inflight testing I decided to set a fuel pressure warning at 18 psi. My engine driven pump maintains around 22-25psi depending on the flow rate and if it ever dropped below that I figured I'd want to know about it.

Last week while cruising at 9500ft, LOP and admiring the view I got a "Fuel pressure low" warning in my headset.
I saw the fuel pressure briefly at 16 psi, then it returned to normal. I had immediately turned the fuel pump on and set full rich mixture (probably didn't help). Over the next 7 minutes I observed the fuel pressure briefly decrease again, then return to normal. Overall it did this 4 times. Following that I didn't see any fluctuations on subsequent flights. I didn't notice any other symptoms and the engine appeared to be running normally in all other respects. I kept an eye on it over the next few hours but everything seemed normal.

This week I downloaded the data for the flight in question, which confirmed a reduction in fuel flow each time, concurrent with increasing EGT's (slightly delayed), and reducing fuel pressure. Due to the CSU I hadn't noticed any change in engine sound. Each episode had lasted around 5-7 seconds. I decided it warranted looking into further and I contacted Mark who in turn spoke to Bob who suggested I checked the gascolator. I did, and as previously, it was clean.

I had read a number of times about the fuel servo "last chance" filter, but I hadn't ever seen any schedule to clean it. I finally found a Lycoming reference today that does recommend cleaning it at the annual/100 hour check. So we removed and checked it. Incidentally, it's located where the fuel "in" hose attaches on a Lycoming Bendix fuel servo and is very quick and simple to remove.

It contained quite an amount of swarf and microscopic debris and although we won't know for sure if it caused the pressure fluctuations (though it's probably likely), I'm very glad that we found and removed the contamination.

Prior to final assembly I had cleaned all my fuel lines with the air compressor, and flushed the tanks. However it looks like this swarf may have originated in the electric fuel pump as it was running in. Not certain about this, but if so then the usual fuel filter wouldn't have caught it. Same for the gascolator as they are both prior to the electric pump. It does make me wonder if the fuel flow transducer has been affected in any way, though so far it has been accurate when comparing "fuel in" on refueling.

Had I not have had the fuel pressure warning set I would have missed the whole thing and been none the wiser, which made me wonder how many times previously in small aircraft this has happened. What it amounts to is 4 small pressure fluctuations in 65 hours of flying, that I would just as easily have missed completely if not for the first fuel pressure warning.

I'm going to make cleaning the fuel servo filter part of the 50 hour check (and hopefully observe a reduction in contamination) - hopefully it's just a "one off" occurrence. Otherwise I'll be revisiting filter options.

Having now seen this, if I was building again I would clean this filter at around 20 hours during the test flying. If running an engine monitoring system I would also definitely set a fuel low pressure warning again.

[Battson]

That’s interesting Jim. My EFII System came with a spare filter, to be installed at the first annual with instructions to buy another and replace at each annual thereafter.

They do not need to be changed this often based on my experience, unless you are pouring dust and sand into your tanks! Once every 5 or so years would be sufficient.

As mentioned, I changed mine after the first 100 hrs and it was clean as a whistle. Gascolator caught everything.

[svyolo]

Fuel temp limitations have been known for a long time, and polar routes have been flown pretty routinely for 20+ years. Dispatch software has also taken it into account for a long time. I thought both Trent engine rollbacks (and maybe a couple of more) came out of China.

[zkelley2]

We fly more polar routes than probably anyone. The software works with dispatch's planning tools and different routes are chosen. This means we never really have to divert because of fuel temp because it's pretty much spot on. The company would rather burn a lot more gas on a slightly not great circle route than a ton of gas plus time on a divert.

The software is tailor made for the airframe, the surface area of the tanks, etc. Gound fuel temps and speed(tat) are all put in to get the prediction. Our other airframes have the same software but for that airframe.

Our company also doesnt seem to care about fuel costs at all either. They'll have us do .86 to save 20mins.

[Nev]

Thanks Whee, very helpful.

[Nev]

That’s interesting, we didn’t use software to predict and often ended up dealing with cold fuel. We often flew polar routes with the lower tropopause. I suspect software would have always said “don’t go over the pole”. Some aircraft are more susceptible that others depending on the configuration of the tanks. Speeding up pre-emptively to slow the cooling worked pretty well too - though we did have software that tracked individual people’s monthly and per flight fuel burn !

[zkelley2]

We have software predictive fuel temperatures. We're not allowed to flight plan into a -37C(or lower) fuel temp. As a result, I've only seen -37 once and we simply went a bit faster to get the TAT up.
I don't know how long ago this was but, I'd be amazed if all other carriers did have the same software and limitations. You can always go down and then divert if that burns too much fuel.

[whee]

Thought some pictures of the EFII pre-filter might be helpful. Like I said before, it is just a screen so complete blockage isn’t likely unless you never replace it.

[Nev]

Just for interest, aircraft using Jet A and Jet A1 suffer from fuel waxing at very low temps, around -40 to -45c as the fuel cools and trends towards TAT and in this case they don’t need water in the fuel for it to happen - it’s a different cause to what happens in light aircraft with fuel contamination that can block carb jets etc. From memory the B777 had already flown from Heathrow to Beijing on a polar route with a low fuel temp, then refuelled with a very low uplift fuel temp, followed by another polar route back to Heathrow. The fuel temp ended up extremely low and caused waxing on the fuel filters of both engines - despite oil/fuel heat exchangers, and despite the filter bypass. Boeing introduced a “rollback” procedure as a result. Fuel contamination on modern jets tends to wreak havoc with the fuel control units, it can lead to flameout also or very erratic engine control. In some cases engines have gone to a high power setting and were unable to be reduced.