Fuel Flow vs Pressure gauge

[Ed Haywood]

BLUF: can I replace the fuel pressure gauge in my 78 Decathlon with a fuel flow totalizer?

I would like to get rid of the mechanical fuel pressure gauge. I don't like having a pressurized fuel line in the cockpit, especially in an aerobatic aircraft. Also the gauge is worthless because the needle jumps so much due to engine-driven fuel pump pulses. If I drain the gauge line to introduce an air buffer, the pulses are damped out, but the problem returns after a few aerobatic flights.

There are few good options for a stand-alone electric gauge and sender. Most of the gauges available are limited to 15 or 30 psi, and my AFM specifies a green arc to 45 PSI. UMA does make a TSO gauge/sender with a range of 60 PSI, but it is quite pricey at $500.

I'm not ready to drop $4K for a full blown engine monitor, but there are a few smaller bar graph monitors for under $2K that include fuel flow, most notably the Insight G2. There are also several fuel flow totalizers available in the $500 range.

FAR 23 says about required powerplant instruments:

(4) For each pump-fed engine, a means:

(i) That continuously indicates, to the pilot, the fuel pressure or fuel flow; or

(ii) That continuously monitors the fuel system and warns the pilot of any fuel flow trend that could lead to engine failure.​

A fuel pressure gauge is listed in my AFM as required equipment. However, the TCDS has been updated over the years to include other options. Here is what the factory shared as current as of 2015:

 

[flyingron]

If that's the correct information for your particular airframe, it appears that any of the listed units are valid replacements. EI's Dave Acosta is a regular here and probably can give you the direct poop as they know it for your airframe. The quote you have in Part 23 means NOTHING for you. It's binding on the manufacturer or those applying for an STC. You can't just substitute what you want just because you assert it meets the Part 23 requirementrs.

 

[Dan Thomas]

Also the gauge is worthless because the needle jumps so much due to engine-driven fuel pump pulses. If I drain the gauge line to introduce an air buffer, the pulses are damped out, but the problem returns after a few aerobatic flights.

View attachment 102390

You'd have to check your Decathlon parts catalog to see if the pressure line fitting at the pump is supposed to be a restrictor type to stop that pulsating. Someone might have chewed it up and installed a standard AN fitting.

 

[Ed Haywood]

You'd have to check your Decathlon parts catalog to see if the pressure line fitting at the pump is supposed to be a restrictor type to stop that pulsating. Someone might have chewed it up and installed a standard AN fitting.

Can't tell from looking at it. Maybe if I pulled it off and took a look?

 

[Stewartb]

Is a Decathlon fuel injected? If so? I’d want a pressure gauge and a flow gauge. If not? I’d want a flow gauge. My injected 4-banger has both.

 

[Dana]

You'd have to check your Decathlon parts catalog to see if the pressure line fitting at the pump is supposed to be a restrictor type to stop that pulsating. Someone might have chewed it up and installed a standard AN fitting.

It would be very poor design not to have one as it not only damps the pulses but also restricts how much fuel could flow if tube between the restrictor and the gauge fails (or the gauge itself fails due to the constant pulsing, a real risk). Same thing is used with a mechanical oil pressure gauge.

It can be as simple as a standard fitting soldered shut and a tiny pinhole drilled through the solder. I could definitely see somebody not knowing that and saying, "What's the matter with this fitting, it's all plugged up," and replacing it with a standard fitting.

 

[Ed Haywood]

It would be very poor design not to have one as it not only damps the pulses but also restricts how much fuel could flow if tube between the restrictor and the gauge fails (or the gauge itself fails due to the constant pulsing, a real risk). Same thing is used with a mechanical oil pressure gauge.

It can be as simple as a standard fitting soldered shut and a tiny pinhole drilled through the solder. I could definitely see somebody not knowing that and saying, "What's the matter with this fitting, it's all plugged up," and replacing it with a standard fitting.

Design includes it. Question, as you point out, is whether someone swapped it out at some point in the 44 year life of this aircraft.

I'll give the ACA factory a call.

But I'd really like to get rid of the gauge entirely.

 

[Jim Carpenter]

It would appear that the EI FP5-L is STC'ed for primary use/replacement in the Decathalon.
Downloads – Electronics International | Aircraft Engine Instruments (iflyei.com)
Scroll down to the FP5-L LxxHyy Primary STC/AML listing.

 

[Bell206]

BLUF: can I replace the fuel pressure gauge in my 78 Decathlon with a fuel flow totalizer?

Yes. However, what level of approval is required will depend on your approach.

FAR 23 says [...] However, the TCDS has been updated

FYI: You're better off following existing alteration guidance vs cherry picking bits and pieces of data. Look at AC 43-210 and use the flow-charts… then answer those questions with your research.

As to the pulsating indication there are several tricks/options to reduce that provided the internal indicator mechanism is not worn. The restrictor fitting is only there to reduce oil/fuel leakage in the event of an upstream line/indicator failure and not to reduce pulsations as its a certification requirement for that specific task.

 

[Ed Haywood]

Yes. However, what level of approval is required will depend on your approach.

I want to keep level of installation approval at my IA. That means I should be looking for an STC device with my aircraft on AML, correct?

I'm learning to read the STC details. Many of the common fuel flow computers and bar graph monitors are not approved as primary instruments for fuel pressure or flow, and are for secondary or reference only.

Here are the options I have found.

1. UMA sells a TSO electric gauge and sender. Cost $500. Link.
2. EI sells a version of the FP-5 that is STC approved for primary fuel pressure and flow. Cost $1K.
3. Aerospace Logic sells various combo gauges that include fuel pressure and are STC as primary indicators. Cost $1200ish.
4. EI CGR-30P and JPI EDM-900 engine monitors are STC approved for primary fuel pressure. Cost $4K.

If I go with the TSO electric gauge from UMA, can the IA approve the install? Does the fact that the OEM mechanical gauge is an automotive gauge open the possibility of replacing it with another non-TSO gauge?

I'm really stuck in analysis paralysis on this issue. It puzzles me that there are so few low end gauge options for fuel pressure.

 

[Eric Pauley]

Don't forget the (newer) Garmin GI-275 EIS, which can be primary for most everything.

 

[Bell206]

I want to keep level of installation approval at my IA.

FYI: Your IA only approves the aircraft for return to service after a major alteration or repair. He cannot “approve” the data used for those items. But the key here is to finalize with your IA if the fuel indicator swap is a major or minor alteration first.

That means I should be looking for an STC device with my aircraft on AML, correct?

Maybe. An AML-STC would provide the easiest path for the alteration. But if approved data is needed a PMA’d indicator or FSDO field approval are other options.

If I go with the TSO electric gauge from UMA, can the IA approve the install?

As mentioned IAs cannot approve data, i.e., installation, in your context. But provided this indicator swap is a major alteration your IA could pursue a field approval with the local FSDO. Now will those costs be close to the $1000 FP-5, probably.

Does the fact that the OEM mechanical gauge is an automotive gauge open the possibility of replacing it with another non-TSO gauge?

It’s not a matter that the original is a “automotive gauge” but rather where that alteration of the indicator fits into the guidance/rules of Part 43. Keep in mind regardless where the OEM obtained their original parts once that collection of parts rolled off the assembly line and was TC’d and given an AWC all those parts became "approved" parts per the type design. See the difference?

I'm really stuck in analysis paralysis on this issue. It puzzles me that there are so few low end gauge options for fuel pressure.

Define “low end gauge options.” In my book you have two right in front of you: the UMA and FP-5. That’s pretty low end when changing aircraft type designs. I always enjoyed the challenge of trying to put different parts on aircraft especially on helicopters. Sometimes my customers didn't when I gave them the bill but they were along for the challenge as well.

 

[Ed Haywood]

FYI: Your IA only approves the aircraft for return to service after a major alteration or repair. He cannot “approve” the data used for those items. But the key here is to finalize with your IA if the fuel indicator swap is a major or minor alteration first.

OK, let me rephrase. I want to avoid spending time and money having my IA ask the FSDO for approval. I want a solution where the IA can install the part, make a logbook entry, and maybe file a 337.

On the subject of major vs minor alteration, replacing a mechanical fuel pressure gauge with a TSO electrical gauge and sender seems like something that (1) would not appreciably affect weight, balance, strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; and (2) could be done according to accepted practices or elementary operations.

Soooo ... minor?

Define “low end gauge options.” In my book you have two right in front of you: the UMA and FP-5.

I suppose you are right. I am just comparing the limited selection of TSO analog fuel pressure gauges to other gauges, such as oil temp or oil pressure. Part of the problem is that my pressure range is apparently unusual. There are numerous gauges for 0-7,0-15, and 0-30 PSI. But according to my AFM the max radial is 45 PSI. UMA is the only company that makes one of those. And oh BTW, it is currently out of stock.

But the FP-5 does seem like a decent option, and I do like the idea of having a fuel computer. Since it is listed in the current aircraft specifications, it would not be a major alteration, correct? Or, we could install using the STC approved data and file a 337, correct?

 

[Checkout_my_Six]

If an STC or PMA exists for your aircraft then any old IA can do it easy peasy.

 

[Bell206]

Soooo ... minor?

Isn’t this fun? This is why your mechanic gets paid the big bucks as it falls only to him to make that call. But the catchall statement “other qualities affecting airworthiness“ usually requires a bit more research. And this is where your previous Part 23 reference comes into play… provided your aircraft was certified under Part 23 vs CAR 3. However, given the Part 23 reference you provided has several options I don’t believe that would be an issue. But you need to make sure there are not any other similar conflicts to be 100% sure.

Since it is listed in the current aircraft specifications, it would not be a major alteration, correct?

What are you referencing as “aircraft specifications?” If that reference is not provided by your aircraft TCDS holder it is not considered “aircraft specifications” for the purposes of major alterations. Why not talk to ACA about an option? I'm sure you're not the only one to bring this up. Maybe offer to use your aircraft to approve something?

 

[Ed Haywood]

What are you referencing as “aircraft specifications?” If that reference is not provided by your aircraft TCDS holder it is not considered “aircraft specifications” for the purposes of major alterations. Why not talk to ACA about an option?

I am referring to this document, provided by ACA when I asked them for options:

 

[Bell206]

provided by ACA

Then you're good to go provided your IA accepts that also. Perhaps contact ACA to see if there's an installation instruction to help your cause with your IA.