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  #31  
Old 05-10-2004, 07:34 AM
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I think it only operates in the horizontal plane. A bounce shouldn't do anything.
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  #32  
Old 05-10-2004, 02:20 PM
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Default deniable palpability....

So, what were the reasons for this fuel shut off?

1. High pressure fuel lines rupturing in cockpit after impact?

Think this out a little more: What circumstances would lead you to shut off fuel to your engine?
Let's call this the up/down rule: Engine on = airplane up, engine off = airplane down.

If you are flying along minding your own business and something hits you so hard that it ruptures your fuel lines in the cockpit (what are high pressure lines doing IN the cockpit...but I will let that one go...) you may have more problems than turning off fuel. But OK, let's say you develope a leak. Not a problem unless you have a spark. Do you have sparks as a common occurence in the cockpit of your aircraft (and you are a non-smoker)? If you cut the fuel, see up/down rule.

Normally, if there is a crash it is because of the down side of the up/down rule. In which case, the engine should already be off. You do what your emegency check list says and turn off fuel and power before impact. If you have constant fuel pressure (some pressure will always remain in the fuel rails if it is injected, but the pressure will be between the pump valve and the high pressure bleed off valve - which should NOT be in the cockpit) and your engine is off, then you have an engineering/installation problem.

Auto fuel cut-off mechanisms are designed for autos because after an impact which may sever lines while the engine is running, the auto will come to rest somewhere and when it does, you don't want fuel spraying all over the place while you wait to be rescued.
In an airplane, while the engine is running and you have an impact (called a mid-air collision) the airplane will eventually come down. If fortunate, it will still be flyable and the pilot, if conscious, will be able to land it safely. If not,
see up/down rule. These airplane are flying gas containers anyway. shutting the fuel off will help as much as shutting the landing light off when you are flying into a tree.

The only place it may help is while taxiing.

IMHO Burt's rule and my desire to keep the up side of the up/down rule by not allowing anything that is not ABSOLUTELY necessary to come between fuel and engine makes me give the cut-off device a thumbs down.
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  #33  
Old 05-10-2004, 02:32 PM
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I think we are looking at the inertia switch to shut off the fuel pump automajicaly in case of crash. I would belive that most all crashes have a mostly vertical component. As well a hard bounce could have a significant horizontal deaccelderation.

A warning and reset sounds good, but would the added complexity, when netted out add or remove saftey to the system?
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  #34  
Old 05-11-2004, 05:30 AM
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StRaNgEdAyS StRaNgEdAyS is offline
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Quote:
Originally Posted by Clutch Cargo
Normally, if there is a crash it is because of the down side of the up/down rule. In which case, the engine should already be off. You do what your emegency check list says and turn off fuel and power before impact.
I'd say only a small number of possible incidents that could damage the fuel lines would allow you the time to do this. E.g. hit a hole on a bush strip during landing, loosing your undercarriage, sudden loss of power, or shift of wind and you come down in a less than favorable attitude, bird strike breaks' bends prop and severe cowl/engine compartment damage results, these sort of things can spray fuel all over the place before you can shut it off, and spraying fuel ignites VERY VERY easily. Far easier than leaking fuel if for instance your tank ruptures.
Quote:
Originally Posted by LargePrime
A warning and reset sounds good, but would the added complexity, when netted out add or remove saftey to the system?
Most of these sensors have an easily accessable reset switch, and in any case one would be simple to install, and the warning light on the N/O side of the switch would also be a simple job.
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  #35  
Old 05-11-2004, 07:59 AM
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I know two canard flyer's personally who might not be here if they'd installed a switch like this.

1. Dave Domier. Exhaust muff departed airplane. Impacted prop. Broke blade. Resultant vibration sufficient to make canard tips invisible. He made it to a field. I know he killed the engine anyway, but only when he was ready to do so.

2. Tim Ragonese. Gear door went through prop on take off (first flight). Broke blade. Very severe vibration, but he made it around the pattern and put her back down. Imagine if he'd been fighting a reset switch as well as trying to fly the airplane.

I still think it's much better to minimise the problem by keeping fuel lines short and out of the cockpit where possible. Nothing should decide for you that the engine needs to be stopped.

But then - I also think the invention of the automatic choke on cars was a really bad idea.
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  #36  
Old 05-11-2004, 09:35 AM
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mtorzews mtorzews is offline
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I don't think an inertia switch would not have shut the fuel off in either of these cases. Vibration doesn't cause the switch to trip. It requires substantial g - loading. Driving down a rough road at high speed will not cause it to trip, but hitting a tree after losing control will. Also some switches are designed to trip in a roll over accident.

Here is a decent article of how a inertia switch works.
www.therangerstation.com/InertiaSwitch.html

Just my 2 cents.
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  #37  
Old 05-11-2004, 10:23 AM
Aaron Aaron is offline
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Quote:
Originally Posted by mtorzews
I don't think an inertia switch would not have shut the fuel off in either of these cases. Vibration doesn't cause the switch to trip.
Mtorzews, I know you know your stuff, so I'm sure you have heard of bags deploying in less than optimum conditions.

I just read about a Mini Cooper who hit a minor pothole and the bags deployed (activating the engine cut,etc) This happens fairly regularly with all makes of cars, just sometimes the suspension binds and you get g loading high enough to trigger the device. He was startled and coasted to a stop, no biggie. Outliers like this are acceptable in a car, not so in most planes I think that's what John is trying to say.

OTOH, not to bring up triple nickel, but sometimes people go too far when trying to avoid fuel lines in the cockpit, and introduce new failure modes.
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  #38  
Old 05-11-2004, 10:32 AM
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I am sorry, i just think a problem is being made of nothing.

i have a fuel line and a return line from the tank to the valve at my hand, from each side, i have ADDED a fitting to this so that i can take out and clean the finger strainers at the bottom of the tank that i have added to each tank and feed and return line to the engine compartment.

End of story, but not the book, this argument will not stop, some of us will go this way and some of us will go the other way and as always
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  #39  
Old 05-11-2004, 12:08 PM
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Aaron,

I really wasn't trying to give an oppinion about whether or not to use one. I just thought I would share a little reading material on how they operate.

I'm no where near having to worry about this, but reading the material below I would probably not use the switch. Mostly because of Dust's reasoning that there are other effective methods of reducing the risk. Although I would add an electrical switch for the fuel pumps (assuming electric fuel pumps are used).

Here's my thought process
Benefit Vs. Risk
The benefit is fuel flow is shut down in the event of a crash reducing the risk of a secondary fire. However it is only a benefit in situations where there isn't time to manually shut down the pumps or if the pilot forgets under stress. To me this is a limited benefit. Others may think different.

Risk
Inertia valve fails stopping fuel flow. Greatest risk would occur at low altitudes or if far from airport. Risk is tempered by the fact that the valve can be reset if their is enough time and pilot remembers under stress.

I have personally "saved" a stranded motorist (my mother) by reseting the valve when she was stuck on the side of the highway so I know that they do fail. (Although I think most failures occur due to causes other than vibration). In a car the failure mode is benign. In a plane it is severe. This would be a 10 on the old FMEA. The engineer in me thinks that it is best to limit the number of severity 10 components in order to reduce risk unless the benefit is substantial.
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  #40  
Old 09-02-2005, 07:50 AM
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Quote:
Originally Posted by mplafleur
This system that I have knowledge of is seperate of the ECU and is located in the rear quarter panel of the car. I takes a pretty good wallop to turn off the pump. Think of how many accidents you get into and are able to drive away.

Hmmm, wonder if it works at all then. Maybe only in the really BAD accidents.

I'll find out more at work.
i reset the switch all the time,but last year i was driving and slowing to make a turn(5 mph) and never saw it coming,small Honda SW hit the ass end of me. it was bent so bad you could see day light from under the rear window. but the fuel was cut and i could not reset because the trunk needed crow bar. i think my neck hit 8+ G's i was out for haft a block just rolling
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