Can a fuel servo failure cause a temporary fuel starvation

[peter-h]

I was discussing this with a colleague the other day, who had a approx 10 sec fuel flow interruption, with an IO540 engine and the RSA5AD1 servo. It was all over very fast and the engine ran perfectly afterwards, following the pulling of the alternate air and switching on the electric fuel pump. The OAT was about -15C.

Is such a fuel servo failure mode possible?

Supposedly, one can get icing on the little jets in the servo, and perhaps this ice could melt by the alternate air which is perhaps 20C warmer (having passed through the cylinders). But that aircraft is about 20 years old and has never exhibited this.

Or is such a failure possible in the engine driven fuel pump? Later on the flight, the electric pump was turned off and all was OK.

Assuming the cause is never found, what would you replace, just in case?

 

[JHW]

replacing parts "just in case" is a great way to have a failure. On most complex pieces of machinery (and airplanes too) maintenance is the #1 cause of unreliability

 

[tsts4]

I seriously doubt pulling alt air had anything to do with it -- it's not like carb heat although the induction air will be wamer like you said. Was there also a 10 sec fuel pressure drop that coincided with the fuel flow drop?

 

[peter-h]

I don't think anybody knows because nobody was looking at it. All that was noticed during the brief time was that the EGTs all fell right down, and then came back up to normal as if nothing happened.

 

[tsts4]

Was there any engine roughness? Also are there any other instrument indications that you can give us besides EGT?

 

[peter-h]

No - apparently just a straight cut. Then a quick and smooth restore, seconds after the electric pump plus alt air were selected.

No other indications noticed (or logged electronically).

The pilot reported it being just like somebody pulled the mixture to zero and then put it right back.

 

[tsts4]

Well I'm no expert, but it sounds like to me to be an issue with, or a line blockage at the mechanical fuel pump that the boost pump was able to overcome.

What phase of flight were you in (ie level cruise, etc.)?

 

[yakdriver]

Sounds to me like a slug of water went through. You start messing with a fuel servo and your talking big dollars. Don

 

[tsts4]

Sounds to me like a slug of water went through. You start messing with a fuel servo and your talking big dollars. Don

Wouldn't that have caused some roughness at some point prior to turning on the boost pump? Based upon the description there didn't appear to be any.

I'm asking to educate myself because I don't know the answer.

 

[peter-h]

It was near the operating ceiling, about FL180. But these airplanes routinely fly to FL190-200.

I have a more recent one myself but it has the same engine and fuel system, so I would be keen to find out what this was (if they ever find out).

There is a very fine (but large area) stainless steel mesh fuel filter after the fuel selector but before the electric pump. So there is really very little potential for a piece of rubbish to block the system after that filter. Before it, yes, but not IMHO at it or after it.

An exchange fuel servo is under $2k i.e. nothing really, and a mech pump is about $300.

Is there anything of relevance inside the fuel distribution spider?

 

[peter-h]

The fuel was tested and was found to have a GUM content of 32 which is about 10x the spec... it was dumped and the fuel system is being cleaned out, with a new fuel servo and engine pump fitted.

 

[Geico266]

The fuel was tested and was found to have a GUM content of 32 which is about 10x the spec... it was dumped and the fuel system is being cleaned out, with a new fuel servo and engine pump fitted.

Old fuel?

I have never heard of a GUM content in fuel. Can you elaborate?

I just read a little bit via Google. Interesting. I've heard the term used before but had no idea you could measure it. It seems to be caused by old fuel. Something when burning mogas yoiu must avoid....that means more flying!

 

[peter-h]

I know no more.

A google indeed suggests old fuel, but the airport claims they have "cycled" the whole bowser several times this year.

don't think it is anything to do with the aircraft not flying enough.

 

[airguy]

I know no more.

A google indeed suggests old fuel, but the airport claims they have "cycled" the whole bowser several times this year.

don't think it is anything to do with the aircraft not flying enough.

Cycling could mean ordering more when it's at 20% capacity, and the bottom layer never gets sucked out. If heavier junk collects down there and finally a chunk of it gets pulled into the suction line to the dispenser pump, well there ya go...

 

[Dan Thomas]

Cycling could mean ordering more when it's at 20% capacity, and the bottom layer never gets sucked out. If heavier junk collects down there and finally a chunk of it gets pulled into the suction line to the dispenser pump, well there ya go...

There should be filters in the delivery system that stop such stuff. And new fuel into old would dissolve any gummy stuff anyway.

Of course, the system that the fuel in question came from might be seriously deficient in many ways.

Dan

 

[N801BH]

There should be filters in the delivery system that stop such stuff. And new fuel into old would dissolve any gummy stuff anyway.

Of course, the system that the fuel in question came from might be seriously deficient in many ways.

Dan

I am going with Dons guess of a " slug" of water......

 

[Dan Thomas]

I am going with Dons guess of a " slug" of water......

The only problem I have with that is that it should have been an ongoing issue. For any water to reach the engine, the strainer (gascolator) has to fill right up and overflow through the screen into the outlet; that means that any further water at all will kill that engine dead. And the screens in those strainers are so fine that the water's surface tension often prevents it getting through the screen as long as the screen is wet with fuel. It hs the same effect, though: a dead engine.

Dan

 

[N801BH]

The only problem I have with that is that it should have been an ongoing issue. For any water to reach the engine, the strainer (gascolator) has to fill right up and overflow through the screen into the outlet; that means that any further water at all will kill that engine dead. And the screens in those strainers are so fine that the water's surface tension often prevents it getting through the screen as long as the screen is wet with fuel. It hs the same effect, though: a dead engine.

Dan

Not all the time.... I was in some pretty good turbulance 35 years ago while leaving Tamiami Airport and had the exact same thing happen to me.. Plane ran perfect for the previous 300 hours and 400 hours after... Except for the ONE time on the downwind departure leg.... Scared the crap out of me and the wife.. The ONLY theory after tearing the entire plane down when I got home was a "slug" of water got displaced somewhere in the fuel system and the carb got a gulp of it for about 5 seconds.. Or less... It seemed like forever though... YMMV

 

[yakdriver]

I was in my RV7 cruising at 12,500ft an hour into the flight the engine quit cold for about 5 seconds then stumbled for about another 5 seconds then resumed running smooth and never missed a beat after that. Fuel injected IO 360 with Airflow Performance servo. Sometimes our airplanes do weird things for no reason. Don