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Fuel Pump


delta4242

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912iS

I typically leave the Aux Pump on from takeoff till cruise, then it’s off till I start my decent.

I don’t see the reason in turning it off low to the ground as if I did have a primary fuel pump failure I would have limited time.

I do perform a aux check with the main off at every run up.

Is leaving the aux pump on that long a bad thing?

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Perspective....

I like "What If's".

They haven't had any pumps just fail in flight.

Now what if: you have a mid air collision, what if you blow the pump fuses, what if you throw a prop blade, what if you throw a rod, what if the stab cable breaks, ect.. ect...

The odds are 99.9% in your favor with your single pump or Rotax wouldn't have set it up to run solely on one.

If a pump did fail you should have altitude to restart. If it fails in the pattern you should still be able to land. If it fails on take-off you should be able to either land because you should already be in that configuration until a safe altitude is reached.

And last but not least what if your mechanic made a mistake that caused a failure like a recent incident.

Any of these things stand a better chance of failure than losing your pump.

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Here's a question:  Is there an indication (other than fuel pressure) that the mechanical pump has failed?  For example, if the pump fails and the aux pump is on, would you know it before turning off the aux pump and stopping the engine?  It seems like without some pump indication either way you are going to get an engine stoppage (though if you have the aux pump on you might be in  a better situation when the engine quits and not in a takeoff climb out).

I'm not taking a side on this, just asking the question.

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Hi Andy,

It's a 912iS. He has two electric pumps and no mechanical pump. Fuel pressure for most are between 42 - 45 psi. If you lose a pump or both the engine immediately shuts down. That said Rotax hasn't had any issues with pump failures in flight. They did have a few pumps leak some, but that was a SB fix.

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1 hour ago, Roger Lee said:

Hi Andy,

It's a 912iS. He has two electric pumps and no mechanical pump. Fuel pressure for most are between 42 - 45 psi. If you lose a pump or both the engine immediately shuts down. That said Rotax hasn't had any issues with pump failures in flight. They did have a few pumps leak some, but that was a SB fix.

Ah, I knew the iS had two pumps, but I thought one was mechanical.  My question still stands though, with both pumps on, if the primary pump failed would you know it?

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I can't get comfortable with both pumps depending on electrical. I have only had one mechanical pump fail in 50 years and it was just a seal leak so it kept running in flight but not on the ground. It was a 1947 bonanza which had a hand pump for boost pump, not too thrilled with that either😄

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6 hours ago, Roger Lee said:

Hi Andy,

It's a 912iS. He has two electric pumps and no mechanical pump. Fuel pressure for most are between 42 - 45 psi. If you lose a pump or both the engine immediately shuts down. That said Rotax hasn't had any issues with pump failures in flight. They did have a few pumps leak some, but that was a SB fix.

In terms of preventative maintenance and operator piece of mind, the tiny hoses get changed every 5 years and the pumps are mechanical, they will wear out sometime, probably not at the same moment. How much are those things to replace, every-so-often, before the fact? 
Maybe ROTAX/FD has a ‘Mean Time Between Failure’ certification factor to forecast and plan replacement.  The Si pumps have already been operating without failure since 2012, roughly, based on this dialog.10 years and still going strong. What if that kind of factor was known…? Maybe we could rest knowing an expected thing to anticipate.

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Seems my post didn't get posted from this morning.

As a data point, many other aircraft want the boost pump on during takeoff. Usually it's the low wing aircraft. For those low wings that can operate without it running even during takeoff, the idea is that if the engine pump fails, at least something is still delivering fuel through the engine fuel pump bypass.

Now that said; if the main pump fails on the CTLSi, does it have enough pressure to run the engine at takeoff power?

I do agree with madhatter, mechanical fuel pumps are dead simple devices that just use shaft power instead of an electric motor, and I wish it was driven that way on iS engines.

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32 minutes ago, airhound said:

In terms of preventative maintenance and operator piece of mind, the tiny hoses get changed every 5 years and the pumps are mechanical, they will wear out sometime, probably not at the same moment. How much are those things to replace, every-so-often, before the fact?

I would not replace one before it fails unless there is a time limit from the manufacturer...that is just asking for a maintenance-induced failure.

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That’s the question, time between failures? We are all guessing which pump in an aging fleet fails first…and if any ole  pilot will react without fail in a preventable scenario.

If I knew the factor, I’d option to replace a perfectly good XX year old pump or two. Replacement process doesn’t appear to be too risky, less complex than an oil change for an A&P, pretty straight forward. I’m sure price would be sobering to know.

 

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11 hours ago, Anticept said:

Seems my post didn't get posted from this morning.

As a data point, many other aircraft want the boost pump on during takeoff. Usually it's the low wing aircraft. For those low wings that can operate without it running even during takeoff, the idea is that if the engine pump fails, at least something is still delivering fuel through the engine fuel pump bypass.

Now that said; if the main pump fails on the CTLSi, does it have enough pressure to run the engine at takeoff power?

I do agree with madhatter, mechanical fuel pumps are dead simple devices that just use shaft power instead of an electric motor, and I wish it was driven that way on iS engines.

I don't think head pressure alone is sufficient to run the fuel injectors.  They need high pressure fuel to do their thing.  And I agree with both of you, a mechanical pump with an aux boost pump seems a better setup.

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10 hours ago, airhound said:

That’s the question, time between failures? We are all guessing which pump in an aging fleet fails first…and if any ole  pilot will react without fail in a preventable scenario.

If I knew the factor, I’d option to replace a perfectly good XX year old pump or two. Replacement process doesn’t appear to be too risky, less complex than an oil change for an A&P, pretty straight forward. I’m sure price would be sobering to know.

 

The problem is we don't know what failure rates are.  For a lot of things like electric fuel pumps, "infant mortality" -- meaning the number of devices that can fail right out of the box or shortly after -- can be quite high.  And devices that survive that initial period might actually have higher reliability.  In other words, once you get the thing working for a while without failure, the chances of a failure are quite low for a *long* time.  This is what is known as the "bathtub failure curve":

ht21_1.gif

So once the device survives the break-in high failure period and until you get to the very end of life, the device is highly reliable.  Until you get to the very end of the graph, replacing the device incurs higher failure risk than just leaving it in service.  And we don't know where that point is unless we work for the company making them.

I don't know for a fact that the pumps in question have this kind of failure curve, but it applied to most mechanical and electrical devices.  That is the only reason I'd hesitate to replace a pump that is working well and shows no sign of failure.  Of course if you start seeing signs like fluctuating fuel pressure or voltage fluctuations at the pump, that is certainly a good time to consider replacement.

I think the safest course is to use both pumps during critical phases of flight (takeoff, landing, maneuvering at low altitude, etc), and just replace a pump if it fails.  And be prepared if the engine stops for any reason to engage the secondary pump and attempt a re-start.

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1 hour ago, FlyingMonkey said:

I don't think head pressure alone is sufficient to run the fuel injectors.  They need high pressure fuel to do their thing.  And I agree with both of you, a mechanical pump with an aux boost pump seems a better setup.

I meant with the aux pump running. Officially, head pressure alone isn't enough for the injectors, which is one of the reasons for the main fuel pump.

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Last fall at 970 hours on my 2013 CTLS iS (I purchased new in spring of 2014) the engine died suddenly in flight. I immediately engaged the secondary pump and the engine instantly came back to full power. I ran it a couple minutes on the secondary pump and then tried turning that pump off and engine immediately started to die. Obviously it seemed wise to be finding a place to land, and fortunately we were only 20 minutes from my home base (3OH0), and I elected to continue there on the secondary pump. After hours and hours of troubleshooting we finally discovered a faulty pin connection on the bottom bundle of wires going into the computer module (red arrow in photo below). With a little adjustment to the pin and a small application of dielectric grease the primary pump has run faultless now for 20 hours. My standard procedure is to run both pumps on takeoff until around 800 ft AGL and then disengage secondary pump, and then reengage secondary pump when around 800 ft AGL when in landing pattern.

IMG_0760.thumb.jpg.07c667d5b14cddae3634e7350334ce5c.jpg

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45 minutes ago, 678Papa said:

Last fall at 970 hours on my 2013 CTLS iS (I purchased new in spring of 2014) the engine died suddenly in flight. I immediately engaged the secondary pump and the engine instantly came back to full power. I ran it a couple minutes on the secondary pump and then tried turning that pump off and engine immediately started to die. Obviously it seemed wise to be finding a place to land, and fortunately we were only 20 minutes from my home base (3OH0), and I elected to continue there on the secondary pump. After hours and hours of troubleshooting we finally discovered a faulty pin connection on the bottom bundle of wires going into the computer module (red arrow in photo below). With a little adjustment to the pin and a small application of dielectric grease the primary pump has run faultless now for 20 hours. My standard procedure is to run both pumps on takeoff until around 800 ft AGL and then disengage secondary pump, and then reengage secondary pump when around 800 ft AGL when in landing pattern.

IMG_0760.thumb.jpg.07c667d5b14cddae3634e7350334ce5c.jpg

Thank you for the enlightenment! Glad you had a little altitude and presence of mind to hit the switch. So, whatever worked at that one pin took 7-8 years/ -900 hours, was it the wire/pin crimp connection, or the pin connector seating? Or…how do you think whatever it was with that pin it came about? Wonder if those critical connections should be ACI items? I’m guessing the other pump connections are in one of the black plugs as well?

Thanks again for the education. Any further insight appreciated.

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