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Old 10th Apr 2008, 09:20   #1 (permalink)
Martyn hobbs
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Piper Tomahawk fuel system


I'm a University student currently doing an Engineering Dissertation.

I have done 12 hours flying before and I'm of to a FTO in July/September to start the long road (Cant wait!)

So, for my project as I have a passion for aviation, I decided to implement a water in fuel sensor to the existing fuel system of a Piper Tomahawk to give the pilot a 'real time' display of any possible contamination in his/her fuel.

I figured this was a good idea as many accidents have happened in aviation in the past due to fuel contamination, and any preventative methods would be therefore welcomed.

The last thing I need to look at to complete the project is the Wiring Diagram of the Tomahawk as I can't find one in books or on the internet.

If anyone could point me in the right direction I would be very grateful!
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Old 10th Apr 2008, 12:27   #2 (permalink)
Join Date: Jul 2007
Location: Italy
Age: 36
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Hi Martyn,

I believe your idea is pretty nice, but I honestly think that the problem of fuel contamination is not only water, but also the little dirty particles that deposit on the bottom of the tank.
Not only this, also the bad complacency that a system like that would lead the pilot, in the "faulty" atmosphere of General Aviation. (it is like the fuel indicator accuracy of little airplanes, you can never thrust it)

Consider that on little planes there is not a so deep and massive control procedure as for bigger ones. Thus, with aging of the instrument..... (it is like with the typical anti ice devices of PA34 or other ones, no one wants to spend money to keep it efficient and working).

Anyway, I believe this "water detector" could be more reliable if the it is solely mechanical.

Happy research!!!!
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Old 10th Apr 2008, 12:50   #3 (permalink)
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Good project better than some of the rubbish undergrads do.

It take it you did your 12 hours in a PA38?

The easy way would be to go and visit the maint organisation for the club you used to fly with.

They will have a heap of books and microfiche on everything to do with the systems. If you go cap in hand with packets of biscuits and or a big bag of doughnuts you will find them reasonabely helpful.

The books etc are taking the wee expensive.

Comments like "you can teach a monkey to ride a bike, but not to fix it" will go down well.

Something you could also consider is a handheld fuel/water sensor to allow people to check the quality before it goes in the tanks. No point knowing about a problem after you have bought and payed for a fuel uplift. Generally by the time the bowser has driven to the aircraft any water is in solution so even if you check the tanks after filling up you won't spot it.
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Old 10th Apr 2008, 14:40   #4 (permalink)
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An a matter of interest, how long would it take water in solution to 'settle out'?

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Old 10th Apr 2008, 22:16   #5 (permalink)
Join Date: May 2006
Location: uk
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If a tomahawk had an engine failure due to water at about 3000ft. it would take about 6 mins for it to glide back to earth.

most toma's burn about 20 lts an hour. or 1 lt per 3 min's.

so, by my maths, you would need to have 2 lts of water in the tanks for the plane to crash (force land) due to water in the fuel.

I've got about 400 hrs on pa38's. and never have i had more than i few bubbles in the bottom of the normal tester.

I think a far simpler method of preventing engine failures due to water is by conventional tools, such as a fuel tester. I think if you also research what would be required to get such a devise approved by the manufacture and the likes of the CAA/FAA, you will find, financially it makes no sense.

remember the basic's of design -

K.I.S.S. = keep it simple stupid!

Also, for your project. check the POH (pilot op handbook) im sure there is a blueprint in there!

How about inventing a fuel tester which you can use with out getting covered by bloody avgas!
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Old 10th Apr 2008, 22:23   #6 (permalink)
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Adverse, I'm sorry gut your reasoning is fatally flawed. All that would be required to cause a forced landing would be sufficient water to stop the engine, plus a lttle bit more to thwart one, maybe two restart attempts.

Your reasoning assumes that the engine carries on processing the contents of the fuel tank at an unchanged rate after an engine failure, which is patently not going to be the case. Even with a windmilling prop and a normally aspirated carbureted engine, there is far less suction to cause fluid to be drawn from the carburettor. In fact the amount of fuel/water drawn would be tiny.
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Old 10th Apr 2008, 22:33   #7 (permalink)
Join Date: May 2006
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If you didnt touch anything, you wouldnt have to attempt to restart the engine, as once the water has passed the fuel would go bang and off you go.

I admit, the maths is not perfect, but that is not the point i was trying to make.

My point is this devise is simply not needed. a much similar way to prevent water contamination is to remove the water before flight. as water is heavier than fuel it simply falls to the bottom of the tank, were it can be removed by conventional means. I have flown different types where water contamination is a real threat, i have seen several fuel testers full of water, a touch of airmanship is all that is needed to prevent any problems, not a big mod to the fuel and electrical system.
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Old 10th Apr 2008, 22:42   #8 (permalink)
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Just out of interest martyn, which uni / flying school have you been with?
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Old 11th Apr 2008, 11:28   #9 (permalink)
Martyn hobbs
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Thanks for all your views.

I have now got the wiring diagrams from the maintenance facility at my local flying club so I can continue to integrate the sensor into the existing a/c system.

adverse-bump- I agree with you totally, good airmanship is the key to having a sustainable safe flight but from the research I have done (Reading many NTSB,AAIB,ATSB reports) It can be concluded that good airmanship and maintenance alone doesn't always prove to work.
(I will add one example in my next post).

The system measures the dielectric properties of the material that passes through it and so will pick up any indication of contamination by dirt, water or any other contaminant.

The proposed system is to be used before take-off but after the fuel drain procedures have been conducted and therefore acts as a back-up or fail safe tool in the event of a pilot not noticing any water in the drained sample.

The sensor can measure contaminants as low as 10,000 parts per million and so would notice any trace of water that the human eye wouldn't see. Even these small amounts of water over time can cause internal corrosion or aid fatigue of components leading to a failure without warning.
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Old 11th Apr 2008, 11:31   #10 (permalink)
Martyn hobbs
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An example of the problem:

Previous accident caused by fuel contamination:

Year: No date

Article Title: Accident caused by fuel contamination. Federal Aviation Administration (FAA)(Online)

Available at: www. lexington.injuryhelpline.com

Accessed: 27 , January, 2008.

Certain sections have been highlighted in bold because they are highly relevant to the report:

On January 1, 2005, at 2:45 pm eastern standard time, a Cessna 172L, N7180Q, was substantially damaged during collision with a tree and a fence following a loss of engine power after takeoff from Duchess County Airport in Poughkeepsie, New York.
Visual meteorological conditions prevailed for the local flight that originated, at 1740. No flight plan was filed for the flight conducted under 14 CFR Part 91.
According to the pilot, the airplane was serviced with fuel about 1 week prior to the accident, and the tanks were filled at that time.
During the preflight inspection before the accident flight, the pilot said that drained 10-12 ounces of fuel from the gascolator. According to the pilot, the samples contained no water.
After takeoff from runway 06, the pilot completed one traffic pattern, performed a touch-and-go landing, and was on climb-out when the engine stopped producing power about 200 feet above ground level.
As the pilot maneuvered it back towards the airport, the airplane struck a tree and a fence on the airport perimeter. The airplane came to rest upright, entangled in the fence.
On January 3, 2005, a Federal Aviation Administration (FAA) aviation safety inspector examined the airplane. In a telephone interview, he said the gascolator drain valve would not drain fuel.
The gascolator bowl could not be removed due to corrosion, and required two hands to break it free from its mount. The bowl contained water, with a thin film of fuel on top. The bowl also contained rust and debris, and the gascolator plunger gasket/grommet was torn.
The carburetor was drained, and the mixture collected was about 50 percent fuel, and 50 percent water. The carburetor also contained dirt and debris. Fuel drained from the right tank contained water.
The auxiliary tank contained about 4 ounces of fluid. The fluid was drained, and the mixture collected was about 70 percent water. The other 30 percent was an unidentified fluid that was yellow in color.
The pilot held a private pilot certificate with a rating for airplane single engine land. He also held a commercial pilot certificate with a rating for rotorcraft helicopter. His most recent FAA second-class medical certificate was issued October 15, 2003.
The pilot reported 3,446 hours of flight experience, of which, 1,869 were in single engine airplanes. He reported approximately 50 hours of flight experience in make and model.
The tachometer reading at the crash site was 2,083.5 aircraft hours whereas, annual inspection was completed October 10, 2004, at 2083.2 aircraft hours.
According to the Cessna Pilot Safety and Warning Supplements, rocking the wings, and lowering of the tail, should only be accomplished after contaminants have been discovered in routine fuel sampling.
On February 1, 2005, an FAA inspector plumbed a can of aviation gasoline into the airplane's fuel system. The engine started immediately, and ran continuously without interruption. According to the inspector, all instrument readings and the magneto checks were perfect.
The previous report of the accident concludes that fuel contamination was the cause of the engine malfunction which resulted in the crash of the aircraft.
The important parts of the report to focus on have been underlined. The report shows that gross negligence lead to the amount of water being in the fuel system which at the carburettors equated to 50% water. This accident happened even after the aircraft had been serviced one week before.

The above report provides sufficient evidence that servicing alone cannot be trusted to make sure no contaminants are present in the fuel system.
It may be, as the FAA points out that gross negligence lead to this accident.
This is negligence by many people because both engineers that serviced the aircraft and the Pilot in command that day had failed to differentiate between Aviation Gasoline and water which resulted in the accident.

The pilot had said that he had drained 10-12 ounces of fuel from the aircrafts tank during pre-flight inspection and the sample had contained NO water. In reality, this check was most probably overlooked by the pilot that day as an FAA inspector had said that the gascolator and fuel draining tap would not drain fuel in tests made.

This shows that procedures and checklists before flight can be overlooked and also points out the fact that sometimes, even with maintenance, aircrafts fuel tanks can corrode and contain water.
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