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Plane engines Vs. Car engines. "Alarms"

flying dog

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OK, sticking my neck out here.....


I think it is better to ask so anyone else wondering the same and not wanting to ask can also learn.






On their dashboard they have lights for things which can "fail".


Altenator, oil pressure, breaks, park break <ON>, seat belts not fastened, etc.


Now, in planes, you usually have OIL, T&P, CHT, and some others.


Yeah, guess where I am going.


Ok, a LONG LONG LONG time ago, I had my first car - which was by default a "bomb". An OLD Toyota corolla. It had lapped the odometer and WAS on it's last legs.


I used to do a lot of long distance driving when I was younger and was a bit "neglegent" (spelling?) about looking after it. But under the circumstances, it wasn't worth much.


Anyway, I was on a long trip of about 400+Kms. After dropping someone off about 130K's into the trip, I was getting back on track and was going down a hill. Suddenly and only quickly I saw a flash of the OIL PRESSURE light. :confused:


Hmmmm.... So when I pulled in to refuel, I looked at the dipstick and sure enough I (or more so the car) needed oil.


So it got me thinking:


Usually cars only have a light which goes out once the pressure is above the "required ammount", where as planes have actual pressure gauges.




Someone told me once you lose oil in the pump, there is bugger all time before things are damaged. So, ok, if you see the pressure start to drop, you start to do a precautionary landing and investigate. In a car, you go into neutral and coast to a quick stop and tend to the problem.


Be it a light or a pressure gauge displaying a low value, the end result is action is needed. Why do planes (small ones in particular) continue to have a pressure gauge and not simply something like cars have?


I'll stop digging there and see if someone would please help me learn something.







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Guest Sharp End

Hi FD,


The clue is in this statement:


... if you see the pressure start to drop, you start to do a precautionary landing and investigate. In a car, you go into neutral and coast to a quick stop and tend to the problem.


By their nature oil pressure lights come on at a predetermined low value, for example let's say 15psi and go out again as the pressure climbs to say 25psi. Whereas a gauge can indicate all values both up and down. So if you're flying along with 70psi and the gauge slowly drops to 60 you can see a trend and do something about it, i.e land asap! Give me a gauge over a warning light anytime. :thumb_up:





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Thanks Sharpend.


I just wanted someone else to "confirm" what you said.


Sometimes "you" get things in your head and it is "true" until it is disproven.


So, when someone (like you) explain the why, I can now understand it with a bit more confidence of it being right and it isn't just what I "think" is the right answer.



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Hi F.D,


One that has always got me with most cars and now that you made me think about it, for aircraft is why no 'low coolant fluid level' alarm. I had an old datsun 620 series ute once (similar condition to your corolla i think) that was happily blowing coolant out through a hole in the radiator and rapidly cooking its self all the while the temp gauge was saying "don't worry the temps fine" (The temp guage measures air temp as well as fluid temp).It was only that I smelt it that I saved the engine.


Regards Bill



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Guest Andys@coffs

So the benefits of a guage is that you can see subtle movements. The draw back is the movement your looking for is subtle and may not be seen. The warning light is either on or off so nothing subtle about it, butwhen it goes on or off it wont be missed......Easy, you need both. There is a complexity penalty and probably a few grams of additional weight per light for the light and the sensing and hysterisis circuitry. In fact most new electronic guages etc generally always have the ability to drive a warning light. On a smart singles EMS guage I have for the 582 I used the warning light to output for EGT. On a 2 stroke an over EGT situation can sieze a motor very quickly. A warning light to me gave me the best chance of avoiding that very ugly and costly situation.





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Attention getting indicators.


Regardless of what indicator you have they can fail. (particularly electric ones.).They are only as reliable as the power source and the sender unit.


As stated, a pressure gauge (instead of a warning light) gives a range of indications. This allows you to do a bit of interpreting. eg. If a low pressure is accompanied by a high temp indication, they might be linked. The oil quantity might be low but the engine could perhaps be operated at low power,to reach a nearby aerodrome, if the power reduction helped lower the oil temp. An oil quantity gauge would be a good idea on long trips.


Older cars and trucks had all the indicators. The lights are a bit of "dumbing down" that has come with modern attitudes. Most drivers only stop when the knocking noises and smoke make themselves apparent. Probably needs a gloved fist to come out of the dashboard as well as the light coming on.


All warnings that I have had have been false indications, but you have to assume that the indicator is correct. I recall an interesting situation years ago when I was rumbling along in a four engined radial engined thing, and the skipper said that since #4 engine had low oil pressure, that I should "feather" it. So I did. About 10 minutes later he said.. # 3 engine has low oil pressure and would I feather it, please? I had a look at the other engine indications and said that #1 and #2 engines had low oil pressure also, so should I feather THEM as well? (bit cheeky) That would make things very quiet. The lesson there was that there must be a common factor and it was that the power supply (28V DC. bus) had failed. There was nothing wrong with any engine. Nev



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Guest check-in

It's always nice to have a gauge AND a warning light, each backed up by a separate sender unit. Normally the light would be set to come on at the minimum pressure allowed for continued flight. The idea is to draw your attention to the gauge before it is too late and an additional benefit is that you can troubleshoot a gauge that may fail.


Without the light, what to do if the oil pressure suddenly drops to zero? In this situation it is handy to be able to recognise 'gauge zero' versus 'electrical zero'. I have had gauges fail, but in the absence of a change in any other parameter such as oil temperature it was usual to continue to operate the engine, but not continue past the next landing point. If backed up by a light, it was considered no problem to keep flying to wherever the next maintenance could be performed. These days such decisions are governed by a Minimum Equipment List, but for our bugsmashers I like the idea of a gauge plus a light if feasible to fit both. If the choice is only one, the gauge has it because if you did have to rely just on the light would you set it to the point where you HAD to land and would you know if damage had been done? With a gauge you will at least know if some oil pressure remains.


Oh, and be aware that if you have a good oil cooler, as you run low on oil the temperature may DECREASE, not increase as you would expect. I have personal experience of this on several types including my present Jabiru-powered setup. With the gauge still indicating minimum pressure and temperature abnormally low, I knew that it was a quantity problem not a mechanical failure. Based on that I made the decision to nurse it to a proper airfield rather than make a forced landing.



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...and rapidly cooking its self all the while the temp gauge was saying "don't worry the temps fine" (The temp guage measures air temp as well as fluid temp).

As someone who has seen a lot of car and truck engines on the up side of "cooked to buggery", engine temperature senders are WATER temperature senders not AIR temperature senders.


Otherwise there would be not a lot of advantage of being set in the water jacket.



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"Watching the gagues...."


(Just discussing)


Aircrash investigation comes to mind.


Wasn't it a 727 with a blown "Gear down" light. All three crew were so busy working on the light, they crashed the plane?


I appreciate you see fluctuations on the needle to indicate the loss of pressure.


Thing is, this may lead to people spending too much time watching the instruments and not enough outside.


Again: I am only discussing it. Wanting people's thoughts/experiances. What have YOU learnt?



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Yep, I have oil pressure light too, with separate sender as I had the sender for the guage fail, some senders have 2 outputs but this only helps if you arent watching the guage


Its pretty common to find oil pressure problems coming back to sender problems


Ive also got charge, oil temp and plan on fuel pressure OR header tank level lights. All separate to main EFIS wiring and setup. Wasnt that hard to setup



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You can buy low coolant sensors - expensive though around $180


Nissan still today have "Non linear" guages, ie sit in the one spot for wide range of temp then rise suddenly for the last 10 deg. They are designed that way to make you feel everything is normal under the bonnet



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l would be looking at a guage that can also drive a warning light.


You should be checking guages regulary, a light just brings it to your attention more quickly and thus may avoid a bigger problem.


regards Bruce



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Crezzi, thanks. I stand corrected. (Bad memory block)


I guess it is a constant trade off between gauges/lights/systems and "all up weight".


I agree that it would be better to use seperate systems/wiring for dual systems, to reduce the possibility of a problem with one piece of cable/wire causing all systems monitoring ONE thing to agree - as they are all using the same piece of wire to get their data.


And putting the warning light above the associated gauge is a good idea too.


On a slight tanget - and hey isn't it fun doing that....


Once my car had a failed oil pressure sensor and it wouldn't go out. The "dorks" at the garage who installed it messed me about with it and... oh the fun getting it resolved.


Then there was the faulty themostat which always gave a signal even when open so the electric fan never came on. (Short to "earth")


Yeah, the interesting things which happen.



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Guage senders are the weak link,


Mine decided to slowly read down pressure mid flight until it was just above yellow, slow revs and it would improve, I was on a long X country and a long way from suitable landing site so diverted towards a bush strip and kept going after a while it all came good, then started to fall again. By this time I had diverted from strip to strip and was near destination and landed all OK. Probably lasted 1 hour but was very stressful


Rang Jabiru, first thing to check was sender, which I did and it was buggered, according to them its a regular failure, even brand new. Replaced it and no more trouble.


To put in separate oil press switch, wiring and light would be Id guess less than 80 grams and $50. Jabiru have the port on the front of the engine for the second sender, and these switches are smaller and cheaper (and reliable) than guage sender type.



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Low water alarm is an extremely sensible thing to fit, and has saved many engines that I know of. The ones that are common here are usually around $90 dollars from memory. If you are a bit of an electronic guru, they are pretty easy to make yourself, just make it so when it is "open" circuit the alarm squeals, and closed circuit - using the water to act as an earth conductor, requiring a probe in the top tank of radiator.


Anyway, that won't be much help on an aircraft....


Like others have said, the electronic gauge senders are prone to have problems some time in their life, they are basically a variable resistor that moves with pressure. They are very handy though if you don't like, or are unable to run oil feed to the back of a mechanical gauge.


And while where on the subject, Mechanical oil pressure gauges are the best for reliability, self contained system, and overall simplicity, but like everything they have failings.


- The pressures do vary with oil temps/viscosity of oils; you will have a higher pressure when the oil is thicker (cold) than when it warms up; not much, but it is noticeable if you take careful notice.


- You run the risk of the oil pressure tube melting/rubbing through/busting/etc, which then causes hot oil to be pumped everywhere, (can cause a fire), and also empties the engines life maintaining source... Having it leak at the gauge end isn't very pleasant either, since it goes inside.


- They are however, great if you loose power, since they keep telling you the correct answer.


On the subject of lights, they are great for the initial notice that somethings wrong, but how bad is unknown, which in an aircraft is nice to know if you can continue at a lower power setting etc to an appropriate landing spot. Rather than a light telling you somethings wrong, to which, technically speaking you should turn the engine off then and there if you want to save it in anyway shape or form. Not particularly appropriate in an aircraft sometimes.


Lights however in road vehicles are great, because if it goes on, you can pull over and stop the engine straight away, hop out and have a look. No need for expensive and space taking gauges to asses the situation.


Light oil senders are very simple and quite cheap, and a light is, very easy also, so an ideal thing for automotive manufacturers to do in big quantities.


Lights in an aircraft, would probably be a good thing, I'd like to see a gauge with a light sensor in the actual gauge itself, so there is only the one sender needed on the engine. When the gauge gets below a certain pressure it activates the light. Would be a simple and pretty easy thing to incorporate into a gauge. (who knows, there might already be something like this)


Anyway, I'm getting carried away.... :ah_oh:



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Hi everyone,


Slightly off topic, but how about audible alarms?


Our pool filter has a pressure guage on it which starts beeping quite loudly when the pressure goes over a certain value. The guage has a switch in the face with three positions. Off, on and test the switch springs back to on when in test position. from what i can tell the tip of the needle is magnetic and on the other side there is another magnet which follows the needle tip. When the guage reaches the limit the other magnet comes into contact with a piece of metal (highly technical term), completing the circuit.


Perhaps this system could be used say for example on the oil pressure guage?


I can't see why not, the all up extra weight is that of a 9V battery!







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"Murphy" make these for engine applications, very expensive for what they are, and all are mechanical in the guage type so have to run oil lines etc to it.

Yes, that's right.... very good though for what they are.



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I would be very careful about using a single sender to operate both a gauge and warning light. If the sender / wiring / gauge fails then you are left with nothing.


The most reliable way is for 2 separate circuits for redundancy and with an LED as the light. An oil pressure lamp will usually start to flash on and off as the oil pump tries to scavenge what oil is available so is usually a fairly reliable indicator. Most aero engines are designed to operate at high power for about an hour after the oil runs out so there is also some mechanical redundancy - completely stuffed when you get it on the ground though!


Temperature senders located in the water jacket are great while imersed in water but as handy as a wheel on a walking stick when the water runs out. The most reliable way is to have a sender inserted into the engine head for water cooled engines.


A simple test button can be put in the panel to test lights at anytime similar to those in Gazelle's. If you are trying to protect a $20,000 engine then a $180 gauge and $20 lamp is fairly cheap



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Guest Sharp End

which engines can run for hour after exhausting their oil?...


...some turbines if left at a constant throttle setting. Not recommended however.


In my opinion, reciprocating four stroke piston engines have buckley's of making high power once out of oil, let alone high power for an hour, maybe two strokes with caged roller big ends etc can run oilless for a while, but an hour??? 049_sad.gif.af5e5c0993af131d9c5bfe880fbbc2a0.gif



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Just a bit interested as to which engines can run for an hour at high power, after the oil is exhausted and just how you would design an engine to do that. Nev

Thanks for picking me up there Nev, suffered from a large dose of oops!! 088_censored.gif.2b71e8da9d295ba8f94b998d0f2420b4.gif


A bit of topic but part of the engine certification process is that the engine must maintain output for for a stated minimum time with no oil pressure. I have seen the results of a particular aircraft engine that was run to destruction in this way and it lasted better than most would think.


I'll try not to let my mind wander again - it gets lonely 087_sorry.gif.8f9ce404ad3aa941b2729edb25b7c714.gif



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There is an engine that has run for sometime without any oil (as a test), the oil additive they put in contains magnetically charged hydrocarbon (or something to that effect), so it does have a self lube effect for quite a while. I don't know much about it myself though.


An engine will run for longer than you think without oil, though the damage in the meantime can be spectacular. From con-rod's sticking through the engine block, melted pistons, broken crankshaft etc... not ideal if you want to use some components again anyway.


Just for interests sake, I know of a fella who rebuilt his engine, but he thought he should assemble everything as clean as a whistle, which resulted in no oil being put anywhere. So when it came to turn it over, the poor guy couldn't turn it over. Oil is amazing stuff, without that little film of oil, it ain't going no where.


Now how did I get into this subject... :ah_oh:



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