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Rotax 914 - how to avoid an overboost?

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Roland

Joined: 30 Nov 2009
Posts: 334
Location: EDLE

Posted: Sun Jul 15, 2012 9:09 pm Post subject: Rotax 914 - how to avoid an overboost?

Friends,



I had bad luck with my Rotax 914 some weeks ago. The TCU unit had a comoplete electricity failure due to a damaged automatic circuit breaker. All normal during run up - no electricity - no TCU warning lights. Select full T/O-power, suffered an overboost, cracked crankcase  - 10k Euro damage - ouch!



If the TCU-lights don't illuminate by switching the master on - okay, then you have a chance to be aware of the dawning danger. At any other time no chance at all from my point of view.



Do I get something wrong here or is it a design fault of the Rotax? Argument of sufficient power all time not applicable due to no power at all with cracked crankcase - lucky me I was not airborne yet....



Any comments most welcome.



Grounded Roland    



PH-ZTI

XS Trigear 914

frans(at)privatepilots.nl
Guest

Posted: Mon Jul 16, 2012 1:15 am Post subject: Rotax 914 - how to avoid an overboost?

On 07/16/2012 07:09 AM, Roland wrote:



 	  Quote: 	
		   Do I get something wrong here or is it a design fault of the Rotax?


	


I'm afraid you did something wrong...

You are not supposed to take off when the warning lights do not come on.

Furthermore you are supposed to watch the manifold pressure gauge when

you select T/O power.



Note that what you experienced was the worst scenario: Apparently the

last known state of the engine was at low power or switched off and thus

with the waste gate fully closed, and you applied full power without the

TCU available to open the waste gate, with an extreme overboost as a result.



During flight nothing dramatic will happen when the TCU suddenly fails,

as the waste gate will stay on its most recent setting, which is correct

for your selected power and atmospheric pressure. Depending on the

throttle setting and altitude you will either experience some loss of

power or a mild overboost when changing the throttle setting or changing

altitude. It should get you home safely.



Sorry to hear your misfortune...



Frans

Roland

Joined: 30 Nov 2009
Posts: 334
Location: EDLE

Posted: Mon Jul 16, 2012 4:59 am Post subject: Re: Rotax 914 - how to avoid an overboost?

Frans,



thanks for your sympathies and comments on this which of course are much appreciated.



Principally I agree with all you are saying,



But: electricity can be lost at any time, so everything might be looking good on start up. You can encounter the same problem during taxi or run up (like me) or in the descend, when the waste gate is also fully closed. Select full power on a go around and you have it as well - with no warning lights at all. 



Watching the manifold can be an option, but believe me that you have an overboost within a second or two with T/O-power and the gate fully closed - so you have to be very quick. I also doubt, that you have your eyes on this the first seconds in a go around situation.



Regards 

Roland



PH-ZTI 

XS Trigear 914

Gilles.Thesee(at)ac-greno
Guest

Posted: Mon Jul 16, 2012 6:12 am Post subject: Rotax 914 - how to avoid an overboost?

Roland a écrit :

 	  Quote: 	
		   Watching the manifold can be an option, but believe me that you have an overboost within a second or two with T/O-power and the gate fully closed - so you have to be very quick. I also doubt, that you have your eyes on this the first seconds in a go around situation.



   

Roland and all,
	




As a flight instructor and Rotax 914 homebuilder and flyer, I would 

strongly insist on monitoring engine parameters (viz manifold pressure 

and rpm) when setting takeoff/go around power. It is naturally easier 

with conventional gauges with clear legal redlines : one gets quickly 

used to watch the needles reaching the redline from the corner of one's eye.

Also, what is definitely needed in a "electrically dependant" aircraft, 

is a conspicuous under voltage warning light.

My best sympathy for your mishap,



Best regards,

-- 

Gilles

http://contrails.free.fr

rlborger(at)mac.com
Guest

Posted: Mon Jul 16, 2012 6:38 am Post subject: Rotax 914 - how to avoid an overboost?

914 folks,



I'm not sure another buzzer/warning light is sufficient.  I have the GRT EIS400 and you can set an acceptable operating range for just about every parameter the EIS measures and displays.  When any parameter goes out of range you have a noise in the headset, the RED master warning light illuminates and the EIS display shows a flashing out of range parameter value.  

So you apply throttle on go-around for some reason that already has your attention and you get buzzing and flashing, oh crap, what's wrong...  You look to see what the display is flashing and finally figure out that it's showing 45" of boost and you reach for the throttle.  By that time something has already broken.

Giles has the best remedy.  Any time you advance the throttle, you should be watching manifold pressure.  The throttle is used to control MP on an aircraft with a C/S prop.  When you reach for the throttle, you should be doing it to set a specific MP so you should be watching MP.  Now that could be difficult to do when you are already preoccupied with another problem situation that has required an increase in power, but it should still be Throttle-MP.

Just my 2¢.  Take it for what it's worth.



 Blue skies & tailwinds,

Bob Borger

Europa XS Tri, Rotax 914, Airmaster C/S Prop.

Little Toot Sport Biplane, Lycoming Thunderbolt AEIO-320 EXP

3705 Lynchburg Dr.

Corinth, TX  76208-5331

Cel: 817-992-1117

rlborger(at)mac.com (rlborger(at)mac.com)

 

 

On Jul 16, 2012, at 8:25 AM, Peter Zutrauen wrote:

Sounds like a separate/dedicated  high-manifold pressure alarm cct with buzzer/indicator may be warranted?



Cheers,

Pete

frans(at)privatepilots.nl
Guest

Posted: Mon Jul 16, 2012 7:20 am Post subject: Rotax 914 - how to avoid an overboost?

On 07/16/2012 04:11 PM, Gilles Thesee wrote:



 	  Quote: 	
		   As a flight instructor and Rotax 914 homebuilder and flyer, I would

 strongly insist on monitoring engine parameters (viz manifold pressure

 and rpm) when setting takeoff/go around power. It is naturally easier

 with conventional gauges with clear legal redlines : one gets quickly

 used to watch the needles reaching the redline from the corner of one's

 eye.


	


I have quite a few instruments in my modified full width panel. But the

upper three holes are occupied by three conventional gauges: 1) the

manifold pressure, 2) the fuel pressure, 3) the RPM gauge.



These are the only instruments I monitor briefly during the T/O. And as

you say, a needle in some colored zone in a familiar angle can be

absorbed in the blink of an eye, much faster than reading a number and

interpreting its value.



Other instruments are located further down in the panel. Issues like

overtemperatures or even loss of oil pressure can wait until at least a

survivable altitude has been reached.



Further I agree that during a go around it is the worst time to have a

failure of something. I don't think you can safeguard against anything.

A warning light or buzzer would be meaningless in such a situation,

unless of course the go around is optional, but then what was the

purpose of the go around anyway?



The only time I executed a go around was when I was on an announced

short final upwind, while suddenly a twin engine aircraft emerged over

the top of the hill and decided to perform a downwind landing on the

same runway. Not sure what I would have done during an overboost, but

probably I would have opted to get out of there anyway. I don't remember

looking at the gauges I have to admit. Just what I said, some risks are

unavoidable.



Frans

Gilles.Thesee(at)ac-greno
Guest

Posted: Tue Jul 17, 2012 4:25 am Post subject: Rotax 914 - how to avoid an overboost?

PHILLIPS I a écrit :

 	  Quote: 	
		  

  



 I normally switch on my dedicated CB for the waist gate servo as part 

 of my check list as soon as I get in the cockpit, But on one occasion 

 because I was distracted by



 Ground crew I totally forgot to check,



 Half way down the runway gets a loud over boost warning from the Dynon 

 EMS 10




	


Ivor,

Your message seems to imply you disconnect the TCU/turbo CB when on the 

ground. Just out of curiosity, is that so, and what purpose exactly is 

that for ?

Doesn't the master switch suffice to turn off everything in the ship ?



Thanks



Best regards,

-- 

Gilles

http://contrails.free.fr

Ivor Phillips

Joined: 16 Jan 2006
Posts: 253
Location: London UK

Posted: Tue Jul 17, 2012 7:40 am Post subject: Rotax 914 - how to avoid an overboost?

Hi Gilles  

I decided early on in my wiring to make sure that the waist gate servo and ECU always had power, So I have wired it direct bi passing the battery contactor,  

It has a klixon CB that is always pulled after shutdown,  

   

Normally the first thing I do when climbing aboard is to push home the CB  

And listen for the servo to go through its start sequence and check the warning  

Lights,  

   

I also have a toggle switch for the same circuit but that is normally left on with  

A further single action push switch dedicated to the servo,  only used in case of hunting,  

   

The moral of the story is when interrupted for any reason doing your checklist  

Go back to the start,  



On 17 July 2012 13:24, Gilles Thesee <Gilles.Thesee(at)ac-grenoble.fr (Gilles.Thesee(at)ac-grenoble.fr)> wrote:

[quote] --> Europa-List message posted by: Gilles Thesee <Gilles.Thesee(at)ac-grenoble.fr (Gilles.Thesee(at)ac-grenoble.fr)>

 

 PHILLIPS I a écrit :

  	  Quote: 	
		   

  

 I normally switch on my dedicated CB for the waist gate servo as part of my check list as soon as I get in the cockpit, But on one occasion because I was distracted by

 

 Ground crew I totally forgot to check,

 

 Half way down the runway gets a loud over boost warning from the Dynon EMS 10

 

 	
 



 Ivor,

 Your message seems to imply you disconnect the TCU/turbo CB when on the ground. Just out of curiosity, is that so, and what purpose exactly is that for ?

 Doesn't the master switch suffice to turn off everything in the ship ?

 

 Thanks

 

 Best regards,

 -- 

 Gilles

 http://contrails.free.fr

 

 



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[b]

Gilles.Thesee(at)ac-greno
Guest

Posted: Tue Jul 17, 2012 8:18 am Post subject: Rotax 914 - how to avoid an overboost?

PHILLIPS I a écrit :

 	  Quote: 	
		  

 Hi Gilles



 I decided early on in my wiring to make sure that the waist gate servo 

 and ECU always had power, So I have wired it direct bi passing the 

 battery contactor,



 It has a klixon CB that is always pulled after shutdown,



  



 Normally the first thing I do when climbing aboard is to push home the CB



 And listen for the servo to go through its start sequence and check 

 the warning



 Lights,



  



 I also have a toggle switch for the same circuit but that is normally 

 left on with



 A further single action push switch dedicated to the servo,  only used 

 in case of hunting,



  



 The moral of the story is when interrupted for any reason doing your 

 checklist



 Go back to the start,


	


Ivor,



Understand, thank you for your response.

We followed the same line of thought, but our TCU is fed through an 

Essential/Endurance bus. The auxilliary pump is fed direct from the aux 

battery (with a 'forget me not' buzzer when the engine does not run, to 

help prevent draining the battery after shutdown).



Best regards,

-- 

Gilles

http://contrails.free.fr

frans(at)privatepilots.nl
Guest

Posted: Tue Jul 17, 2012 1:29 pm Post subject: Rotax 914 - how to avoid an overboost?

On 07/17/2012 05:39 PM, PHILLIPS I wrote:



 	  Quote: 	
		   I decided early on in my wiring to make sure that the waist gate servo

 and ECU always had power, So I have wired it direct bi passing the

 battery contactor,


	


Just like some people like the stall warner, the trim servo, and some

other bunch of "recommended execptions" bypassing the master switch?

My thought here is that everything could be essential in some stage of

the flight, but then what is the meaning of the master switch if half

the ship is bypassing it?

I want the master switch do exactly what it is supposed to do, to power

down the ship completely. If I want to retain power on the TCU, the

stall warner, the trim servo, or whatever, I just don't power down. Am I

missing something here?



You have already experienced how such an exception can turn against you.

Everything that is wired apart from the master switch can be forgotten

to be switched on, and can fail just as easily as the master switch. In

an unavoidable crash situation, where you have to land in hostile

terrain, it is highly desirable that you can power down the electrical

system with just one switch, without having to think (or worry) about

all the exceptions which are bypassing the master switch and keep their

sparking abilities when ripping the fuel tank is a possibility.



 	  Quote: 	
		   It has a klixon CB that is always pulled after shutdown,


	


These things are not supposed to be used as a switch (please read the

manufacturer technical manual). They will fail, much sooner than an

ordinary switch, if you regularly pull them. There is a quite

complicated mechanism behind these things and repeated use will wear

them out quickly. So, there again, it might turn against you.



I never quite understood why folks want to bypass the master switch.

What is the reason behing it?



Is it just me thinking like this about the purpose of the master switch?



Frans

Fred Klein

Joined: 26 Mar 2012
Posts: 503

Posted: Tue Jul 17, 2012 2:10 pm Post subject: Rotax 914 - how to avoid an overboost?

On Jul 17, 2012, at 3:21 PM, Frans Veldman wrote:

 	  Quote: 	
		  Is it just me thinking like this about the purpose of the master switch? 	




	I'm w/ you Frans...what kind of "Master" allows for independent choices?...

	Years ago, when I added a 12 hp go cart engine to my foot-launched, tail-less biplane glider, the "Master" kill switch was in my mouth...opening the circuit was as simple as relaxing my jaw or opening my mouth...of course there were some intrepid flyers...who, when frozen w/ fear...just gritted their teeth...

	Fred

do not archive

 [quote][b]

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