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Servo Failure

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steve2(at)sover.net
Guest

Posted: Fri Apr 13, 2007 5:59 am Post subject: Servo Failure

Had an odd failure the other day I thought I would share, mainly because it 

was a failure mode I hadn't ever thought of and if I had I might have 

recognized it (maybe not). It might help another 500 series driver someday.



We replaced a couple bladders (and found out from filling the airplane twice 

parked two different directions it holds 150 gallons, not 156). Also 

recalibrated the gauge with 20 gallons aboard. (twice). I'd like to think 

I've got fuel management pretty well figured for this aircraft, but for our 

work there is sometimes a lot of climbs and descents, but overall it's never 

varied much from predicted.



First trip out from replacing bladders was about 4.5 hours. We were at 7500' 

or so W.O.T. & L.O.P. Rougher than hell as usual dropping into 'happy 

valley' here back home. Pulling throttles back as we descend. Gas (pumps) 

Mixture (up) Gear (down) props still to go and getting close to turning 

about a four mile final (there's a mountain in the way of a shorter final). 

Something ain't right. Feels an awful lot like fuel starvation. Now we're in 

the real serious bumps down low, and of course the gauge is bouncing off 

empty and you figure maybe you blew your math, and the gauge was wrong all 

along. Definately feels like we're loosing an engine but at this point 

getting knocked around like we were and in the descent for landing, its hard 

to tell yet exactly which one. Beans, we loose one engine from fuel 

starvation, that means we're about to lose another and this is a piss poor 

place to be loosing it, just a big tree covered hill between me and the 

airport.  Quit descending, and went for some height, waiting to stow one one 

if it asked to be and trying to tell just what was going on. With a couple 

inches of throttle travel the right engine has no juice, but we're going to 

make the airport long as the left one keeps running which we're not at all 

sure of.



Long story short, the past two rebuilds on the fuel servo from two different 

shops we believe to be not very good. I had exactly the fuel I thought I 

had. Turns out (if we understand it right) there was mechanical wear 

internally to the servo that stuck wide open throttle fuel levels at near 

idle throttle settings. So when power came back on the lever, the fuel 

remained full tilt and essentially flooded the engine to the point of 

killing it.



Couple things:



I wasn't ever in the trouble I thought I might be.



Never needed to firewall it, raise the gear, flaps. If I did, there was a 

good chance the right engine would have cleared itself after a bit and 

recovered to develop full power in short order.



A lot of my focus has been identifying failed engines on departure, in a 

climb. Its a different critter to identify a failed engine in a descent. 

Even harder if it's only partly failing going downhill in moderate 

turbulence.



This engine was probably controllable by using the mixture control, but it 

would be asking a lot to have made that quick a diagnostic and remedy on the 

fly during final.



My last trip was 5 hours and I roughly figured I'd put 120 gallons back into 

the thing to fill it. It was good to see it took 118.



I hope this posting was appropriate. I'm always interested in failure modes, 

and perhaps one or two others might take something interesting from the 

above and file it away.....



Steve

hcourier(at)cox.net
Guest

Posted: Fri Apr 13, 2007 7:46 am Post subject: Servo Failure

Very interesting Steve.  THANKS.



R. Mansfield

N91ES

---- Steve <steve2(at)sover.net> wrote: 

 	  Quote: 	
		   

 

 Had an odd failure the other day I thought I would share, mainly because it 

 was a failure mode I hadn't ever thought of and if I had I might have 

 recognized it (maybe not). It might help another 500 series driver someday.

 

 We replaced a couple bladders (and found out from filling the airplane twice 

 parked two different directions it holds 150 gallons, not 156). Also 

 recalibrated the gauge with 20 gallons aboard. (twice). I'd like to think 

 I've got fuel management pretty well figured for this aircraft, but for our 

 work there is sometimes a lot of climbs and descents, but overall it's never 

 varied much from predicted.

 

 First trip out from replacing bladders was about 4.5 hours. We were at 7500' 

 or so W.O.T. & L.O.P. Rougher than hell as usual dropping into 'happy 

 valley' here back home. Pulling throttles back as we descend. Gas (pumps) 

 Mixture (up) Gear (down) props still to go and getting close to turning 

 about a four mile final (there's a mountain in the way of a shorter final). 

 Something ain't right. Feels an awful lot like fuel starvation. Now we're in 

 the real serious bumps down low, and of course the gauge is bouncing off 

 empty and you figure maybe you blew your math, and the gauge was wrong all 

 along. Definately feels like we're loosing an engine but at this point 

 getting knocked around like we were and in the descent for landing, its hard 

 to tell yet exactly which one. Beans, we loose one engine from fuel 

 starvation, that means we're about to lose another and this is a piss poor 

 place to be loosing it, just a big tree covered hill between me and the 

 airport.  Quit descending, and went for some height, waiting to stow one one 

 if it asked to be and trying to tell just what was going on. With a couple 

 inches of throttle travel the right engine has no juice, but we're going to 

 make the airport long as the left one keeps running which we're not at all 

 sure of.

 

 Long story short, the past two rebuilds on the fuel servo from two different 

 shops we believe to be not very good. I had exactly the fuel I thought I 

 had. Turns out (if we understand it right) there was mechanical wear 

 internally to the servo that stuck wide open throttle fuel levels at near 

 idle throttle settings. So when power came back on the lever, the fuel 

 remained full tilt and essentially flooded the engine to the point of 

 killing it.

 

 Couple things:

 

 I wasn't ever in the trouble I thought I might be.

 

 Never needed to firewall it, raise the gear, flaps. If I did, there was a 

 good chance the right engine would have cleared itself after a bit and 

 recovered to develop full power in short order.

 

 A lot of my focus has been identifying failed engines on departure, in a 

 climb. Its a different critter to identify a failed engine in a descent. 

 Even harder if it's only partly failing going downhill in moderate 

 turbulence.

 

 This engine was probably controllable by using the mixture control, but it 

 would be asking a lot to have made that quick a diagnostic and remedy on the 

 fly during final.

 

 

 

 My last trip was 5 hours and I roughly figured I'd put 120 gallons back into 

 the thing to fill it. It was good to see it took 118.

 

 I hope this posting was appropriate. I'm always interested in failure modes, 

 and perhaps one or two others might take something interesting from the 

 above and file it away.....

 

 Steve

kamala(at)MSN.COM
Guest

Posted: Fri Apr 13, 2007 8:37 am Post subject: Servo Failure

sc, tnx, this shud be a help to all. mason

 	  Quote: 	
		  From: <hcourier(at)cox.net>

Reply-To: commander-list(at)matronics.com

To: commander-list(at)matronics.com

Subject: Re: Servo Failure

Date: Fri, 13 Apr 2007 11:45:54 -0400






Very interesting Steve.  THANKS.



R. Mansfield

N91ES

---- Steve <steve2(at)sover.net> wrote:

 > 

 >

 > Had an odd failure the other day I thought I would share, mainly because 

it

 > was a failure mode I hadn't ever thought of and if I had I might have

 > recognized it (maybe not). It might help another 500 series driver 

someday.

 >

 > We replaced a couple bladders (and found out from filling the airplane 

twice

 > parked two different directions it holds 150 gallons, not 156). Also

 > recalibrated the gauge with 20 gallons aboard. (twice). I'd like to 

think

 > I've got fuel management pretty well figured for this aircraft, but for 

our

 > work there is sometimes a lot of climbs and descents, but overall it's 

never

 > varied much from predicted.

 >

 > First trip out from replacing bladders was about 4.5 hours. We were at 

7500'

 > or so W.O.T. & L.O.P. Rougher than hell as usual dropping into 'happy

 > valley' here back home. Pulling throttles back as we descend. Gas 

(pumps)

 > Mixture (up) Gear (down) props still to go and getting close to turning

 > about a four mile final (there's a mountain in the way of a shorter 

final).

 > Something ain't right. Feels an awful lot like fuel starvation. Now 

we're in

 > the real serious bumps down low, and of course the gauge is bouncing off

 > empty and you figure maybe you blew your math, and the gauge was wrong 

all

 > along. Definately feels like we're loosing an engine but at this point

 > getting knocked around like we were and in the descent for landing, its 

hard

 > to tell yet exactly which one. Beans, we loose one engine from fuel

 > starvation, that means we're about to lose another and this is a piss 

poor

 > place to be loosing it, just a big tree covered hill between me and the

 > airport.  Quit descending, and went for some height, waiting to stow one 

one

 > if it asked to be and trying to tell just what was going on. With a 

couple

 > inches of throttle travel the right engine has no juice, but we're going 

to

 > make the airport long as the left one keeps running which we're not at 

all

 > sure of.

 >

 > Long story short, the past two rebuilds on the fuel servo from two 

different

 > shops we believe to be not very good. I had exactly the fuel I thought I

 > had. Turns out (if we understand it right) there was mechanical wear

 > internally to the servo that stuck wide open throttle fuel levels at 

near

 > idle throttle settings. So when power came back on the lever, the fuel

 > remained full tilt and essentially flooded the engine to the point of

 > killing it.

 >

 > Couple things:

 >

 > I wasn't ever in the trouble I thought I might be.

 >

 > Never needed to firewall it, raise the gear, flaps. If I did, there was 

a

 > good chance the right engine would have cleared itself after a bit and

 > recovered to develop full power in short order.

 >

 > A lot of my focus has been identifying failed engines on departure, in a

 > climb. Its a different critter to identify a failed engine in a descent.

 > Even harder if it's only partly failing going downhill in moderate

 > turbulence.

 >

 > This engine was probably controllable by using the mixture control, but 

it

 > would be asking a lot to have made that quick a diagnostic and remedy on 

the

 > fly during final.

 >

 >

 >

 > My last trip was 5 hours and I roughly figured I'd put 120 gallons back 

into

 > the thing to fill it. It was good to see it took 118.

 >

 > I hope this posting was appropriate. I'm always interested in failure 

modes,

 > and perhaps one or two others might take something interesting from the

 > above and file it away.....

 >

 > Steve

 >

 >

 >

 >

 >

John Vormbaum

Joined: 21 Aug 2006
Posts: 273
Location: SF Bay Area, CA

Posted: Fri Apr 13, 2007 12:58 pm Post subject: Servo Failure

Steve,



Count yourself fortunate that you were safe the whole time. I'm sure I would 

NOT have recognized that failure mode at all...and it would be extra tough 

to diagnose in bumps, on a descent, when you think you might have to land 

short.



One thing I do differently in my 500B is to NOT use the fuel boost pumps on 

approach, even though I was initially trained to. I've heard a couple of 

stories of not-great fuel systems where the pumps allowed an engine to flood 

& die when the power was pulled back. Have you checked to see if the pumps 

have any effect on the engines under low power settings? Just my $0.02 and 

probably not related to your servo issue in any way.



Cheers & glad you got down safely,



/John

---

rsrandazzo(at)precisionma
Guest

Posted: Fri Apr 13, 2007 2:39 pm Post subject: Servo Failure

John-



Interesting you mention not operating the fuel pumps on descent.  I've heard

mixed messages as well.



Would be fascinated to hear the opinion of "more experienced" commander

pilots.



Robert 



--

steve2(at)sover.net
Guest

Posted: Fri Apr 13, 2007 3:59 pm Post subject: Servo Failure

John, I notice a slight richen-ing of the mixture at idle with the pumps on, 

but I am usually agressively leaned on the ground. I learned some time ago 

that I liked ground leaning, and if you were going to do it at all, more is 

better. Kind of hard to forget you're leaned during a take-off roll if 

pushing the handles up results in no noise.



Pumps also go on pre-takeoff, although I'm carrying a little more power than 

arrival, but again I've not noticed anything irregular.



Worst case scenario has been a hurried arrival and turn-off a runway, with 

pumps left on. I'll notice a lumpier idle than normal and at lower RPM.



Steve Welebny

Col-East, Inc.





Subject:  Re: Servo Failure

From:    John Vormbaum (john(at)vormbaum.com)

Date:    Fri Apr 13 - 1:58 PM



Steve,



Count yourself fortunate that you were safe the whole time. I'm sure I would

NOT have recognized that failure mode at all...and it would be extra tough

to diagnose in bumps, on a descent, when you think you might have to land

short.



One thing I do differently in my 500B is to NOT use the fuel boost pumps on

approach, even though I was initially trained to. I've heard a couple of

stories of not-great fuel systems where the pumps allowed an engine to flood

& die when the power was pulled back. Have you checked to see if the pumps

have any effect on the engines under low power settings? Just my $0.02 and

probably not related to your servo issue in any way.



Cheers & glad you got down safely,



/John

John Vormbaum

Joined: 21 Aug 2006
Posts: 273
Location: SF Bay Area, CA

Posted: Fri Apr 13, 2007 4:42 pm Post subject: Servo Failure

Yeah, I lean aggressively on the ground too....best thing in the world for 

the engines. I also use my boost pumps on takeoff. I have turbos too, so the 

pumps go on over 15,500' too. I'm just not a big fan of them on final 

approach.

---

steve2(at)sover.net
Guest

Posted: Fri Apr 13, 2007 6:21 pm Post subject: Servo Failure

I'd be interested in other's opinion, but the only difference I can see 

between pumps on and pumps off could be a slight rise in pressure to the 

servo with the pumps on at idle, or a slight rise in pressure to the servo 

if the engine's pumps were set lower than the boost pump pressure at other 

RPM.



At idle (no boost pump) I see the lowest fuel pressure of the engine pump. 

Add a couple hundred RPM and the pressure kicks up and stays put. Once out 

of a dead idle, I'm not sure I notice any difference in performance of the 

engine which stands to reason and is backed up by the EGT.



Since the servo is working with a lot of differential pressures, at a dead 

idle the boost pump IS enough to enrichen the mixture a bit because the idle 

mixture was set based on the lower engine pump only pressure. I'm theorizing 

here that the extra boost pump fuel pressure at extreme low idle is throwing 

the balance of the servo's idle mixture off, but only a little and only near 

dead idle when set off engine pump only.



I suppose the amount of 'extra' pounds of fuel pressure with a boost pump at 

idle could be unique to each aircraft, or maybe not at all. I'll have to 

play with the thing next time out, but higher than about 1200 RPM or so, 

maybe lower, I didn't see any difference boost on or off.



Any of that sound legit?



Steve W



---

rsrandazzo(at)precisionma
Guest

Posted: Fri Apr 13, 2007 6:48 pm Post subject: Servo Failure

Steven-



In THEORY (which is an economist's way of saying, "this is someone elses

idea but if it works you should give me credit for it") the fuel boost pumps

on our commanders are there to provide positive fuel flow from the tank to

the Engine Driven Pump.  The EDP should meter fuel upline from there based

on it's demand calibration- but the main fuel pumps are primarily to ensure

you have positive fuel pressure from the tank source to the engine.



So in THEORY what you describe is true...  You should see no difference in

performance with the fuel pump LOW or OFF- because the tank based pump isn't

the one providing the performance calibration... Set to HIGH I'm guessing

you will see a performance change because it appears that the HIGH setting

is strictly an override mode so it likely invalidates the normal metering

process (dislaimer: I'm outside my knowledge band in HIGH mode...but this

performance change is what I see when setting my pumps to HIGH when testing

them pre-flight.)



Just my opinion...



Robert S. Randazzo



--

John Vormbaum

Joined: 21 Aug 2006
Posts: 273
Location: SF Bay Area, CA

Posted: Fri Apr 13, 2007 7:41 pm Post subject: Servo Failure

Also Robert, keep in mind that you're flying a Continental-powered 

Commander, so you have low & high boost pump settings. Us Lyc-powered guys 

only have "on" and "off" .



And Steve, the theories sound good. I wonder what effect the failure of an 

engine-driven pump would have on the fuel pressure? Is the servo 

self-regulating or is the EDP always the first link in the fuel pressure 

management process? Being just the guy who pushes the levers around in the 

cockpit, I'm probably light on the more esoteric points of the IO-540's 

systems. Perhaps it's time to rectify that.



I'm going to dig around and see if I can find any articles or other 

documentation on boost-pump-related engine floodings on approach. I've seen 

ONE story along those lines related to Commanders, but I don't remember 

which model the author was flying. I want to assume a 500A or 685 because of 

all the Bonanza-specific issues I've heard about with the hi/lo boost pump 

settings & engine floodings, but I don't know that the Lycs are immune to 

similar occurence.



/J

---

rsrandazzo(at)precisionma
Guest

Posted: Fri Apr 13, 2007 10:21 pm Post subject: Servo Failure

John-



Good "Head's Up."  (and good memory, too!)  I wasn't aware that the 685/TCM

powered commanders have different pumps.  Learn something every day!



Anyone familiar with the 685 have good fuel pump management techniques?

I've gotten a range of opinions from folks who don't really have much direct

experience with them- so I'd love to hear experienced opinions.



Robert S. Randazzo 



--

steve2(at)sover.net
Guest

Posted: Sat Apr 14, 2007 3:09 am Post subject: Servo Failure

John,



I've got a pretty good article on the Bendix servo. Monday I can scan the 

thing and post it to our company's website.



If the fuel boost pump pressure and the engine fuel pump pressure were about 

the same, I can't see that the servo should care which one is feeding it. 

(For the single speed IO-540 pumps).



 	  Quote: 	
		  From what I understand (and it's limited) the servo is set up on the bench 

for normal aircraft fuel pressure.
	




All things being equal, if you change the ship's fuel pressure, you change 

the fuel flow. For example at W.O.T, setting the engine driven fuel pump 

pressure up and down will have a direct relationship to the fuel flow. The 

last 'rebuild' on our servo came back twice with a wide open throttle fuel 

flow lower than what we hoped to see. The second time was better, but we 

compensated somewhat by running slightly higher fuel pressure on the right 

side.



With this last rebuild it's finally right, but with us running the right 

engine fuel pressure toward the high end of the green arc, the WOT fuel flow 

was now too high, and the ship's fuel pressure was dropped back to the 

middle of standard.



On the IO-540's the 'fuel flow' isn't really a measure of flow, but another 

measure of pressure, this time at the fuel distribution spider. Plug an 

injector and it would appear you've got more flow, when all you've got is 

really more pressure.



Also keep in mind with these servos that with differential pressures being 

used, some of the failure modes are fuel crossing to places it shouldn't 

ought to be.



I think everything I said is true.



Steve

Also Robert, keep in mind that you're flying a Continental-powered

Commander, so you have low & high boost pump settings. Us Lyc-powered guys

only have "on" and "off" .



And Steve, the theories sound good. I wonder what effect the failure of an

engine-driven pump would have on the fuel pressure? Is the servo

self-regulating or is the EDP always the first link in the fuel pressure

management process? Being just the guy who pushes the levers around in the

cockpit, I'm probably light on the more esoteric points of the IO-540's

systems. Perhaps it's time to rectify that.



I'm going to dig around and see if I can find any articles or other

documentation on boost-pump-related engine floodings on approach. I've seen

ONE story along those lines related to Commanders, but I don't remember

which model the author was flying. I want to assume a 500A or 685 because of

all the Bonanza-specific issues I've heard about with the hi/lo boost pump

settings & engine floodings, but I don't know that the Lycs are immune to

similar occurence.



/J

steve2(at)sover.net
Guest

Posted: Mon Apr 16, 2007 2:28 pm Post subject: Servo Failure

Well I was going to email the Bendix servo article out to anyone that wanted 

it, but after having to use a chainsaw, tow strap and my truck this morning 

to get pine trees out of the way to get to work after this last Nor'easter, 

I think up in Vermont we're going to be without power for a while. I did put 

it on our company's Downloads page, under Bendix.zip.



http://www.col-east.com/index.php



Let me know if anyone has any trouble with zip files. I'll check messages on 

Tuesday.



Steve Welebny





---

steve2(at)sover.net
Guest

Posted: Thu Apr 19, 2007 4:06 am Post subject: Servo Failure

John,



If you're reading this, did you see the servo data is up on the web-site? 

www.col-east.com



We got power back, but no email at home and I believe you had sent me mail 

interested in a copy. Just wanted to follow-up and I don't know when we'll 

have our phone back for the modem.



Aside from the computer, we don't really miss the phone much.



Steve Welebny



---

John Vormbaum

Joined: 21 Aug 2006
Posts: 273
Location: SF Bay Area, CA

Posted: Thu Apr 19, 2007 8:19 am Post subject: Servo Failure

Steve,



Thanks for the article, I downloaded it but haven't read it yet.



Sure hope you guys get some relief today...the last few days have been 

rough! My 89-yr-old grandmother lives on the north coast of Long Island and 

she had 18" of water in the basement on tuesday....



/John

---

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